Page 88 | Astronomy Magazine (2024)

We live in a world in which momentous decisions are made by people often without forethought. But some things are predictable, including that if you continually consume a finite resource without recycling, it will eventually run out.

Yet, as we set our sights on embarking back to the Moon, we will be bringing with us all our bad habits, including our urge for unrestrained consumption.

Since the 1994 discovery of water ice on the Moon by the Clementine spacecraft, excitement has reigned at the prospect of a return to the Moon. This followed two decades of the doldrums after the end of Apollo, a malaise that was symptomatic of an underlying lack of incentive to return.

A United States Navy video on the 25th anniversary of NASA’s Clementine mission.

That water changed everything. The water ice deposits are located at the poles of the Moon hidden in the depths of craters that are forever devoid of sunlight.

Since then, not least due to the International Space Station, we have developed advanced techniques that allow us to recycle water and oxygen with high efficiency. This makes the value of supplying local water for human consumption more tenuous, but if the human population on the Moon grows so will demand. So, what to do with the water on the Moon?

There are two commonly proposed answers: energy storage using fuel cells and fuel and oxidizer for propulsion. The first is easily dispensed with: fuel cells recycle their hydrogen and oxygen through electrolysis when they are recharged, with very little leakage.

Energy and fuel

The second — currently the primary raison d’être for mining water on the Moon — is more complex but no more compelling. It is worth noting that SpaceX uses a methane/oxygen mix in its rockets, so they would not require the hydrogen propellant.

So, what is being proposed is to mine a precious and finite resource and burn it, just like we have been doing with petroleum and natural gas on Earth. The technology for mining and using resources in space has a technical name: in-situ resource utilization.

And while oxygen is not scarce on the Moon (around 40 percent of the Moon’s minerals comprise oxygen), hydrogen most certainly is.

Extracting water from the Moon

Hydrogen is highly useful as a reductant as well as a fuel. The Moon is a vast repository of oxygen within its minerals but it requires hydrogen or other reductant to be freed.

For instance, ilmenite is an oxide of iron and titanium and is a common mineral on the Moon. Heating it to around 1,000 C with hydrogen reduces it to water, iron metal (from which an iron-based technology can be leveraged) and titanium oxide. The water may be electrolyzed into hydrogen — which is recycled — and oxygen; the latter effectively liberated from the ilmenite. By burning hydrogen extracted from water, we are compromising the prospects for future generations: this is the crux of sustainability.

But there are other, more pragmatic issues that emerge. How do we access these water ice resources buried near the lunar surface? They are located in terrain that is hostile in every sense of the word, in deep craters hidden from sunlight — no solar power is available — at temperatures of around 40 Kelvin, or -233 C. At such cryogenic temperatures, we have no experience in conducting extensive mining operations.

Peaks of eternal light are mountain peaks located in the region of the south pole that are exposed to near-constant sunlight. One proposal from NASA’s Jet Propulsion Lab envisages beaming sunlight from giant reflectors located at these peaks into craters.

These giant mirrors must be transported from Earth, landed onto these peaks and installed and controlled remotely to illuminate the deep craters. Then robotic mining vehicles can venture into the now-illuminated deep craters to recover the water ice using the reflected solar energy.

Water ice may be sublimed into vapour for recovery by direct thermal or microwave heating – because of its high heat capacity, this will consume a lot of energy, which must be supplied by the mirrors. Alternatively, it may be physically dug out and subsequently melted at barely more modest temperatures.

Using the water

After recovering the water, it needs to be electrolyzed into hydrogen and oxygen. To store them, they should be liquefied for minimum storage tank volume.

Although oxygen can be liquefied easily, hydrogen liquefies at 30 Kelvin (-243 C) at a minimum of 15 bar pressure. This requires extra energy to liquefy hydrogen and maintain it as liquid without boil-off. This cryogenically cooled hydrogen and oxygen (LH2/LOX) must be transported to its location of use while maintaining its low temperature.

So, now we have our propellant stocks for launching stuff from the Moon.

This will require a launchpad, which may be located at the Moon’s equator for maximum flexibility of launching into any orbital inclination as a polar launch site will be limited to polar launches — to the planned Lunar Gateway only. A lunar launchpad will require extensive infrastructure development.

In summary, the apparent ease of extracting water ice from the lunar poles belies a complex infrastructure required to achieve it. The costs of infrastructure installation will negate the cost savings rationale for in-situ resource utilization.

Water can be found near the Moon’s south poles.

Alternatives to extraction

There are more preferable options. Hydrogen reduction of ilmenite to yield iron metal, rutile and oxygen provides most of the advantages of exploiting water. Oxygen constitutes the lion’s share of the LH2/LOX mixture. It involves no great infrastructure: thermal power may be generated by modest-sized solar concentrators integrated into the processing units. Each unit can be deployed where it is required – there is no need for long traverses between sites of supply and demand.

Hence, we can achieve almost the same function through a different, more readily achievable route to in-situ resource utilization that is also sustainable by mining abundant ilmenite and other lunar minerals.

Let us not keep repeating the same unsustainable mistakes we have made on Earth — we have a chance to get it right as we spread into the solar system.

Alex Ellery, Professor, Canada Research Chair in Space Robotics and Space Technology, Carleton University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Supernovae are the most spectacular fireworks in the universe. Just one such explosion can briefly outshine the rest of the stars in a galaxy combined.
Astronomers have become adept at spotting these events across the cosmos, with particular attention to a specific class called type Ia supernovae.

Much of modern cosmology rests on these events. That’s because thanks to their intrinsic physics, these stellar blasts should always have the same intrinsic brightness. This property makes them excellent distance indicators, which allows astronomers to obtain their distance simply by observing how dim their light appears upon reaching Earth. Ultimately, the information gleaned by mapping type Ia supernovae across the cosmos has allowed astronomers to measure the expansion of the universe itself.

And yet, perhaps the most incredible thing about these particular supernovae is that despite their reliability as distance markers, we don’t know all the details of what exactly triggers their progenitors to explode in the first place. This gap in our understanding could affect the accuracy of our estimates of the universe’s expansion — and it leaves astronomers thirsting for answers.

“It’s just wild to me that we don’t know what causes a type Ia explosion when they’re so important to science,” says Tyler Holland-Ashford, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “It’s still a mystery, and one I would like to know the answer to.”

He’s not alone. Numerous researchers are striving to find that answer, using clues from the supernovae themselves and the remnants they leave scattered across the galaxy.

Classes of blasts

Supernovae are separated into two broad categories: type I and type II. Type I supernovae have little to no hydrogen in their spectra, meaning none of this element is present in their progenitor. Type II supernovae, which are created by massive stars exploding at the end of their lives, do contain hydrogen because these suns still have ample reserves of unfused hydrogen near their surface. This second class of blasts occurs when a massive star collapses into itself — leaving behind a neutron star or black hole.

A type Ia (pronounced “one-A”) supernova is generated through an entirely different process. It starts with a stellar remnant called a white dwarf. When a star that is not large enough to undergo a type II supernova can no longer keep fusion going at its core, it will shed its outer layers and leave behind a white dwarf. (This is the fate that awaits our Sun one day.) A white dwarf is not massive enough for additional usion; instead, it glows due to residual heat, which it very slowly loses as it cools.

However, there is an exception to this no-more-fusion rule: when a type Ia supernova happens. Stellar fusion occurs because a star is massive enough to squeeze the elements within it into new ones. Stars like our Sun convert hydrogen into helium, releasing energy in the form of light, and die once this fuel is exhausted. The more massive a star, the heavier the elements it can fuse.

White dwarfs aren’t stars anymore — they are simply the dead, leftover cores. Without the heat of fusion to hold it up, the white dwarf shrinks under its own gravity until it becomes not a ball made of atoms, but something called degenerate matter instead. Degenerate matter consists of atomic nuclei swimming in a sea of electrons. Now the force holding the white dwarf up against gravity is electron degeneracy pressure, which arises from the fact that the electrons cannot get any closer to each other without breaking the laws of physics dictating how many can occupy the same space at once.

Unless the white dwarf exceeds 1.44 times the mass of the Sun (an amount called the Chandrashekar mass), that is. As the white dwarf gains more mass, its temperature increases. And once it hits this critical mass, it will reach an ignition temperature to fuse heavier elements — notably carbon, which the star was not massive enough to fuse earlier. At this so-called critical mass, the white dwarf will reignite and tear itself apart within a matter of seconds. This is what gives rise to a type Ia supernova.

The usefulness of type Ia supernovae comes from two facts. First, they are more luminous than almost all other supernovae — about 5 billion times brighter than the Sun. This allows us to easily see them even billions of light-years away.

And second, because each type Ia arises from the same type of progenitor and thus has roughly the same amount and type of combustible material, their light curves — the way their brightness evolves over time — are incredibly uniform. After the peak, the rate at which their light fades follows a predictable pattern over a few months, as heavy elements at the core of the white dwarf radioactively decay.

This is why type Ia supernovae can be used to measure distance — even when it’s vast. Galaxies themselves can vary greatly in size and brightness, making it hard to tell whether, for instance, a galaxy that appears bright in the sky is intrinsically luminous or simply nearby. But type Ia supernovae are standard candles: The dimmer one appears, the farther away it lies. This property allowed astronomers to discover that the expansion of the universe is accelerating, thanks to mysterious dark energy — a find that was awarded the 2011 Nobel Prize in Physics.

You might think that such a fundamental cosmic distance marker must be perfectly understood. But astronomers still debate what actually triggers a gigantic type Ia explosion in the first place — a detail that could have repercussions for our understanding of the role of the dark energy these blasts have revealed.

Degenerate details

The trouble is this: Supernovae are rare, with just one every century in a galaxy like our Milky Way. Type Ia supernovae are rarer still because of the unusual circ*mstances that produce them. This means we have not been able to actually see what the system around the white dwarf is like when it blows — we only spot a bright new point of light from millions of light-years away.

And in astrophysics, the devil is in the details. “It doesn’t require extreme precision to know the universe’s acceleration is expanding,” explains Ken Shen, an astronomer at the University of California, Berkeley, who specializes in type Ia supernova theory. “But there are appropriate concerns that there might be systematic problems in observations.”

That’s because while most type Ias appear to reach a standard maximum luminosity before their predictable decay, a smaller subset appear too bright or too dim compared to what is typical. This discrepancy isn’t big enough to change the observation that the universe is expanding and accelerating, but it is enough to affect how precisely we can measure this expansion. And Shen is convinced that understanding the progenitors of type Ias is a key to doing that.

“Knowing what the progenitors of type Ia are is an important thing,” says Shen. “How did these supernovae happen? The physics there is what drives me.”

There are two leading theories on how to prompt a white dwarf to create a type Ia supernova, and both require the white dwarf to have a companion star that donates the extra mass to tip it over the limit and cause the explosion. The first is a single-degenerate (SD) system, where the white dwarf is in a close binary system with a fusion-burning star. (The “degenerate” here is the white dwarf, which, remember, is made of degenerate matter.) Over time, the white dwarf siphons material away from the star and, if it accretes enough material, it may reach the Chandrashekar mass and explode.

But in recent years, the SD scenario has run into some troubles — namely, it doesn’t appear to explain what we see in some type Ia supernovae. One notable example was supernova SN 2011fe, a quintessential type Ia explosion that occurred in the Pinwheel Galaxy (M101) in 2011, some 23 million light-years away. By type Ia supernova standards, it was right next door, and it was detected just hours after the explosion — the earliest a type Ia had ever been discovered. This allowed astronomers to search for evidence of a pre-existing companion star as well as any material in the system that was falling onto the white dwarf that exploded.

They found neither. According to a 2013 review of the supernova published in Publications of the Astronomical Society of Australia, written by Laura Chomiuk at Michigan State University, SN 2011fe’s environment was low density. So, there wasn’t considerable material available to collect onto the white dwarf and tip it over the weight limit into a supernova.

Instead, Chomiuk writes, “the data imply that SN 2011fe may have been the merger of two white dwarfs.” This is the second popular theory explaining type Ia supernovae: a so-called double-degenerate (DD) system, where there are two white dwarfs, presumably remnants from a stellar binary. Over many thousands of years — long enough for any gas and dust for the original system to disperse — the two are drawn together by gravity and eventually merge, creating the type Ia supernova.

The exact details are still debated — for one, it’s not clear whether a full merger into a single object is needed, or whether a contact touch between the two white dwarfs is enough. Some researchers even think such a system might not need to reach the Chandrasekhar mass — even if the two white dwarfs add up to less than 1.4 solar masses, the force of the collision could still trigger a runaway nuclear reaction and cause them to explode. Either way, it’s different enough from the SD scenario that a DD explosion would have a different signature, provided astronomers could see sufficient detail.

And this is where a twist of astronomical history comes in: While no one alive has seen a type Ia supernova with the level of detail required to crack their code, who’s to say astronomers haven’t in the past? What if we can use clues from the historic astronomical record to solve a modern astrophysical mystery?

Turning back the clock

Humans have been studying supernovae for thousands of years, though of course it was only recently that we understood what they are. If it’s close enough to Earth and with minimal dust along the line of sight, a supernova can be visible all over the world as a bright new naked-eye star for several months. And you can bet that people noticed — some with fear, some with wonder, some with confusion — which often led early astronomers to write down what they saw. Ancient Chinese astronomers were particularly careful record-keepers, detailing many bright “guest stars” over the centuries, along with their locations. The earliest such supernova record dates to A.D. 185 and was visible for eight months; in modern times, astronomers found the remnant from the explosion, RCW 86, and determined it was created by a type Ia supernova.

The most recent type Ia supernova seen with the naked eye (and the last supernova observed within our Milky Way) was first spotted in October 1604 and named Kepler’s Supernova, after astronomer Johannes Kepler. Kepler was not the first one to discover the supernova, but he took meticulous records of its position and its light curve for over a year and compiled his measurements with those of other astronomers for a book, De Stella Nova. The work is so meticulous that not only have modern astronomers identified the location of Kepler’s supernova remnant centuries later (some 20,000 light-years from Earth), they have even reconstructed the light curve to confirm it’s consistent with a type Ia supernova. Such historical records are so vital because they have guided modern astronomers to the remnants and allowed them to verify their ages — and such still-fresh remains are our best chance of distinguishing between the SD and DD scenarios.
Four hundred years may sound like a long time, but that’s a blink of an eye, cosmically speaking. “This is still the time where we’re probing what the actual explosion itself made,” explains Holland-Ashford, who is studying the remnant using data from the Japanese Suzaku X-ray telescope. The X-rays we see are still from the material ejected by the explosion itself, known as ejecta — some of which is speeding outward at a whopping 23 million mph (37 million km/h), even centuries later. Holland-Ashford is studying the elemental composition of this ejecta. Different types of explosions “would have different elements,” he says. So, by conducting the most detailed study of these elements to date, Holland-Ashford aims to find what event led to the “stella nova” that Kepler saw in the sky more than four centuries ago.

Supernova remnants are a promising way to unlock the clues of their progenitors, but they’re not the only potential clue hiding in our galaxy. Shen has proposed a DD scenario where both stars don’t get shredded apart: Instead, back-to-back explosions first end one white dwarf as a type Ia supernova and then fling outward the second white dwarf at a fantastic speed. The surviving white dwarf would travel at thousands of miles a second; such “hypervelocity white dwarfs” would theoretically be all over the galaxy. According to Shen’s idea, if the majority of type Ia supernovae are produced this way, there should be about 30 such hypervelocity white dwarfs within 3,000 light-years of Earth. But do such stars exist?

“We didn’t really know if they’d survive,” recalls Shen, but he and his team have used data from the European Space Agency (ESA) observatory Gaia to find proof that some do. Gaia has obtained precise positional data on approximately 1 billion astronomical objects, and Shen and his team led a search for local hypervelocity white dwarfs. After follow-up observations, they found three hypervelocity white dwarfs that fit the bill, each speeding along at a whopping 2.2 million to 6.7 million mph (3.5 million to 10.7 million km/h). What’s more, the team traced the path each white dwarf has traveled in the past. Two of the candidates show no sign that they originated in a nearby supernova remnant, which is perhaps not surprising, as the remnants could be faint or have dissipated over time. But one traced back to the location of a large, faint supernova remnant called G70.0–21.5, estimated to be from a supernova explosion approximately 90,000 years ago.
It’s not quite a smoking gun — for one thing, Shen’s study fell a bit short on finding the right number of hypervelocity white dwarfs. But there are many reasons Gaia might not have spotted them, Shen says. The white dwarfs the team did see were bright, but because these remnants cool over time, they also fade. Some may have dimmed below Gaia’s ability to see them, Shen says, though future surveys may pick them up.

Going to gravitational waves

The true origin of type Ia supernovae is unlikely to hide forever. One of the ESA’s primary future research missions is a gravitational-wave detector called the Laser Interferometer Space Antenna (LISA), a space-based observatory that will look for ripples in space-time itself. Gravitational-wave studies are still in their infancy — the first detection by the Laser Interferometer Gravitational-wave Observatory (LIGO) happened in 2016, and LIGO is not sensitive enough to study white dwarf binary pairs.

However, when it launches in 2037, LISA will be able to detect binary white dwarf pairs in our galaxy with very short periods and glean details such as how long it will take for them to merge and the rate of such events. Perhaps, if we are very lucky, LISA might detect a signal just before a type Ia supernova lights up the sky as a new guest star. Using LISA, astronomers will finally know whether such mergers explain all type Ia explosions or if more than one scenario is at play — and perhaps uncover a bit more about fundamental physics along the way. What’s clear is that in a universe filled with cosmic explosions as exotic as type Ia supernovae, there is still much to uncover.

Most of us can only name a few of the seven wonders of the world, but historians, archaeologists, and pub quizzers can rattle off the names of each and every one of them. Their faces will light up as they tell you all about the Temple of Artemis, the Statue of Zeus, the Mausoleum at Halicarnassus, the Colossus of Rhodes, and the Lighthouse of Alexandria, and so on.

But ask a roomful of astronomers to name the seven wonders of the solar system and you’ll get lots of different answers — and probably start an argument! That’s because there is no definitive, universally agreed-upon list of the seven most fascinating places in our cosmic neighborhood.

Maybe there will be one day, when probes and people have traveled to and explored every planet and moon, every asteroid and chunk of icy debris, every nook and cranny around the Sun. But that day is a long way off.

So, in the meantime, I’ve compiled my list of seven places in our solar system that I consider to be worthy of the status of wonders. You’ll have your own suggestions, and I’m not claiming these are the most wonderful wonders. These are simply the places in the solar system that I personally wish I could visit in person.

Mare Orientale (the Moon)

Mare Orientale is a giant, 560-mile-wide (900 kilometers) impact basin on the Moon, comprising two rings of mountains surrounding a dark mare — or ancient flow of lava — that gives it the appearance of an enormous cosmic bullseye. Unfortunately, it lies right on the eastern limb of the Moon, so we only ever see it obliquely. But this fascinating feature is still visible as a thin sliver of gray close to the Moon’s curved edge. However, if it were positioned so that it faced Earth, it would make the Moon look like a huge eye staring down at us. (Just imagine the creepy appearance that bloodshot eye would take during a lunar eclipse…) Either way, this gorgeous lunar site is definitely worthy of closer exploration.

Valles Marineris (Mars)

Valles Marineris is an epic martian rift valley, stretching some 2,500 miles (4,000 km) long and reaching depths of about 4.5 miles (7 km), that was first discovered by Mariner 9 in 1971. Dwarfing the famous Grand Canyon in the U.S., many planetary astronomers consider Valles Marineris to be the Grand Canyon of the entire solar system. It stretches around almost a quarter of Mars’ circumference along the equator, like an ancient axe wound in the planet’s rust-hued crust. It’s hard not to envy the astronauts who will one day fly over its long, narrow central canyon, staring down at its landslides, cliffs, and side canyons in all their glory.

The volcanoes of Io (moon of Jupiter)

Of the four moons of Jupiter discovered by Galileo, Io is by far the most colorful and dramatic. The first Voyager images showed the moon was a bewildering variety of shades of yellow, orange, and red, like a pizza painted by Picasso and Matisse. But Io isn’t covered in cheese, it is covered in sulfur that gushes, spurts, and sloshes from hundreds of volcanoes that are scattered across its surface. The largest, appropriately named Loki, is a depression more than 130 miles (200 km) wide that hosts a lake of liquid sulfur within it. Other volcanoes send fan-shaped plumes of material shooting hundreds of miles out into space —a surely stunning sight up close!

The ‘Tiger Stripes’ of Enceladus (moon of Saturn)

On May 20, 2005, the Cassini probe took images of the south polar region of Saturn’s icy moon Enceladus. These new views showed four long, dark lines crossing the moon’s crust, resembling wounds raked across the world’s face by the claws of a tiger. Later observations revealed them to be fractures in Enceladus’ crust flanked on each side by high ridges. These Tiger Stripes on Enceladus became even more fascinating when images taken during later fly-bys revealed huge plumes of water vapor hissing from the fractures, sending a mist of ice crystals skyward that would sparkle and glint in the sunlight.

Kraken Mare on Titan (moon of Saturn)

Near the north pole of Saturn’s largest moon, Titan, lies a vast sea named Kraken Mare, which covers some 150,000 square miles (400,000 square kilometers). That’s roughly five times the area of Lake Superior. Discovered by the space probe Cassini in July 2006, Kraken Mare was named after the legendary sea monster, the Kraken. But Kraken Mare does not host cool blue water. Instead, this body is made of liquid methane that sloshes and slurps in Titan’s dim orange sunlight. Tides up to 16 feet (5 meters) tall roll across it. Kraken Mare has also displayed bright, short-lived features, christened “Magic Islands,” which are thought to be huge bubbles of nitrogen that rise up from the alien sea’s shadowy depths and burst at the surface.

The icy plains of Pluto

The dwarf planet Pluto has so many fascinating features on it that I should probably call it a wonder in its own right. Lurking in the frigid depths of the Kuiper Belt — a ring of icy bodies beyond the orbit of Neptune — Pluto was first seen in detail by the New Horizons probe. This ambitious mission whooshed by the world in 2015, sending back breathtaking images of the Pluto’s heart-shaped icy plain (roughly the color of vanilla ice cream), glaciers rolling through gaps in towering mountains, and possible ice volcanoes, too. Plus, the north polar region of Pluto’s largest moon, Charon, is colored almost blood red, which is why it was informally named Mordor. Who could resist such a sight?

The greatest comet of all time

Somewhere out there in the dark, eternal winter wastelands of the outer solar system, is a huge chunk of dusty ice destined to become the greatest comet ever seen from Earth. One day, its tail will be painted across the sky like a banner as it passes near our planet, and people around the world will marvel at it until it fades from view. It has no name yet — in fact, its discoverer might not even have been born yet. All we know is that it’s out there, waiting to be found. At first, it may only look like an out-of-focus smudge on an image of another celestial target. But months or years later it will blaze in the sky, having grown into a truly Great Comet that will put comets Halley, Hale-Bopp, and all other comet throughout history, to shame.

And who knows, maybe you’ll spot it tonight!

When the first five images from the James Webb Space Telescope (JWST) were unveiled, one of them stared at me with two eyes. It was an image of the Southern Ring Nebula, NGC 3132, and smack in the middle were two bright stars.

Now, the fact that NGC 3132 houses a binary star system (two stars orbiting one another) has been known since the days of the Hubble Space Telescope.

But in those early images, the central star that ejected the nebula – a tiny, hot white dwarf – was so dim it was almost invisible next to its bright Sun-like companion. In effect, the nebula had one eye almost closed.

But the JWST reveals more than Hubble did. It can collect “cooler” photons (light particles) in the infrared range of the electromagnetic spectrum. In this cooler light, we saw both stars in the binary system shining as bright as one another: two glaring eyes!

This was surprising to any astronomer who understands this type of nebula; super-hot white dwarfs typically don’t shine brightly in infrared light. It made sense for the cooler star to be shining this way, but observing the same brilliance from its partner was unexpected.

Emails started to bolt coast to coast and across oceans as astronomers pieced the puzzle together. The central white dwarf star of NGC3132, they realised, is enshrouded in dust. The dust is warmed up by the star’s heat and therefore shines in the infrared, producing the light we observed.

It was this that led us on the trail to find out what was really happening in the Southern Ring Nebula. Our findings from a team of nearly 70 astronomers are published Dec. 8in Nature Astronomy.

At the heart, a hot white dwarf

The Southern Ring Nebula is a planetary nebula. That means it’s a gaseous nebula formed by a Sun-like star shedding most of its gas in the last act before its demise.

Once it shed much of its mass, the star became a hot white dwarf. This central star now sits in the middle of the nebula, cooling like a stellar ember, effectively dying.

The beauty of planetary nebulae is they can be looked at forensically: parts of the nebula farther from the middle were ejected earlier in time. In this way, the entire nebula functions a bit like a geological record.

With its dying white dwarf in the centre, our group approached NGC 3132 like a crime scene.

Two unknown suspects emerge

First, we quickly realised the dust making the central star shine so brightly was actually a disk wrapped closely around the central star that must have been forged by a companion. This orbiting companion star would have stripped gas away from the central star, hastening its demise.

We didn’t spot the companion, though. We think it’s either too faint to detect, or has potentially perished in the interaction and merged with the central star.

Then we noticed something else: broken concentric arches engraved in the extended halo of the nebula. These also betrayed the presence of an orbiting companion. Could this culprit be the same one that forged the disk of dust?

We don’t think so. Although the arches have suffered some “weathering”, our measurements of them betray the presence of yet another companion star. This one is placed a little too far from the central star to have created the dust disk.

And just like that, we had gathered evidence the Southern Ring Nebula contains not just two stars in a binary system, but four.

And we would gather more yet.

Tied up in a bumpy, gassy bubble

The “ring” that gives the nebula its name is actually the wall of an egg-shaped bubble containing hot gas, heated by the central star. This wall is marked with noticeable protuberances.

Combining the JWST image with data from the European Southern Observatory, our team created a 3D model that revealed these protuberances come in pairs, moving in opposite directions away from the central star.

One possible explanation is the interaction that created the dust disk didn’t involve just one close companion, but two. In other words, we’re looking at a potential fifth star in the mix – interacting chaotically with the central star to blow out jets that push out those protuberances.

This fifth star’s presence is still tentative. But we can say with a good degree of certainty the stellar system that created the Southern Ring Nebula comprises not just the binary star system (the two eyes of the nebula), but also a third star that ripped away the gas to form the disk, and another that inscribed a track of concentric arches in the gas bubble.

As for the second eye of the nebula – the one we’d always known about – it was definitely an innocent bystander. It’s too far from the central star to have participated in its demise.

One case closed, more to come

The case of the Southern Ring Nebula isn’t the only one demonstrating how stars work in packs. Much of stellar astrophysics is being revisited today in light of the realisation of just how gregarious stars can be. And we’re all the more excited for it.

A wealth of phenomena arise from stellar interactions, from supernova explosions, to the merging of black holes and neutron stars giving rise to gravitational wave events.

As the JWST delivers more detailed images of the universe, astronomers will be keenly dusting off their gloves to tackle more mysteries.

Orsola De Marco, Professor of Astrophysics, Macquarie University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

This month, all eyes will be on Mars as it reaches opposition on Dec. 8, within everyone’s favorite bull, Taurus. In addition to the Red Planet, Taurus is home to many striking deep-sky targets. You may be familiar with its two most arresting open star clusters, the Pleiades (M45) and the Hyades (Melotte 25). But what about others that are often overlooked?

Let’s begin our journey at Aldebaran (Alpha [α] Tauri), the Bull’s angry red eye. After you have enjoyed it, shift your attention 8½°, or about one and a half binocular fields, due east. There, you’ll find an arc of three approximately 5th-magnitude stars formed by 11 Orionis, 15 Orionis, and SAO 94377. Two NGC star clusters are ½° north of the latter (an orangish star) and appear less than a Moon’s diameter from each other. They form a pseudo-Double Cluster of sorts.
The western member of the pair, NGC 1807, contains about three dozen stars and has an apparent diameter of 17′. Due to the compactness of the grouping, binoculars may have a difficult time resolving individual members. The brightest stars, however, give the cluster a triangular shape tilted toward the southeast with 8th-magnitude SAO 94371 marking the triangle’s tip.

Although many references still list NGC 1807 as an open star cluster, some studies — including a paper published in Astronomy & Astrophysics in 2018 — suggest it is an illusion caused by several stars at varying distances lying along the same line of sight as seen from Earth. The authors determined that since the stars show different proper motions, they are not traveling as a group through the Milky Way.

The same paper proved that the eastern member of the duo, NGC 1817, is the real deal. However, it’s tougher to spot, despite containing more than 200 stars by some counts. Its three brightest, shining at only 9th magnitude, form a faint triangle along the cluster’s western edge that seems to point back toward NGC 1807. The rest, at 10th magnitude and fainter, blend into a feeble glow.

Scan east of the Taurus Double Cluster and you’ll notice a broad, arrowhead-shaped pattern of about half a dozen 5th- and 6th-magnitude stars aimed back at NGC 1817. Notably, the northern barb of the arrow is marked by orange 119 Tauri. Also known as CE Tauri, this semi-regular variable fluctuates slightly between magnitudes 4.2 and 4.5.

From the arrow’s base, look for a curved shaft of seven or more stars hooking southeastward toward Orion the Hunter’s raised arm. Combined, I think of this asterism as Orion’s Spear, since it looks as though he threw it at Taurus before raising his sword.

While the stars in the spear’s curved shaft bear no physical relation to each other, many in the spearhead belong to the widespread open cluster Collinder 65. The cluster has 11 members centered on 6th-magnitude 113 Tauri and spans the spearhead’s central 3°. It’s difficult to tell the true cluster members apart from the field stars, since there are about 20 stars brighter than 8th magnitude within the cluster’s borders. But together, they fill in the spearhead’s form.

Finally, let’s shift northward to Mars and use the Red Planet to revisit NGC 1746. We stopped by this object back in the January 2019 issue, but since Mars is standing close by all month, I thought it worth revisiting. The two will appear closest together on Dec. 3/4, when Mars is just 1° north of the cluster. Mars stays within a 7°-wide region — roughly a binocular field — centered on NGC 1746 from Nov. 26 until Dec. 12.

I’d like to close by tipping my hat to Glenn Chaple, whose final Observing Basics column appears in this issue. I have known Glenn for decades and we see each other every summer at Stellafane. It has been an honor and a pleasure to share these pages with him for so many years.

Questions, comments, suggestions for future columns? Contact me through my website, philharrington.net. Until next time, remember that two eyes are better than one.

This was it. Apollo 17 was set to be the last mission of the Apollo program, as well as the last time humans would land on the Moon for the foreseeable future.

In the previous three-and-a-half years, NASA saw five Lunar Modules (LMs) land 10 Apollo astronauts on the Moon’s surface. The knowledge gained from these missions — primarily from the rocks and soil the astronauts returned — was nothing short of revolutionary. And since the first lunar samples were returned to Earth in July 1969, the story of the Moon’s birth and evolution was being rewritten almost daily.

But as Apollo 17 approached, our understanding of the Moon’s history remained incomplete. The Moon rocks and soil collected so far were dominated by materials from lowland lunar maria (Latin for “seas”). However, the Moon is mostly composed of highland material, which accounts for nearly 85 percent of the lunar surface.

Scientists knew a complete picture of lunar evolution couldn’t possibly be developed without highland samples. And Apollo 17 would be the last chance for astronauts to gather such material. So, NASA decided Apollo 17 should land at a highland site. The only question was where?

Apollo 17 narrows down possible landing sites

During the early stages of planning for the Apollo 17 mission, Harrison “Jack” Schmitt, a geologist who would later be named the LM Pilot for Apollo 17 — ultimately becoming the first professional scientist to fly to the Moon — proposed that they should land on the farside of the Moon in Tsiolkovsky Crater.

Tsiolkovsky Crater would not be in the highlands, but it would give the astronauts access to mare material from the poorly understood lunar farside. It would also perhaps provide access to lunar material from deep inside the Moon’s crust that had been “splashed” up into Tsiolkovsky’s central peak during the impact that birthed the crater.

Schmitt even managed to line up a couple of spare TIROS weather satellites that could be launched into lunar orbit to relay voice and data between astronauts at a farside landing site and Mission Control on Earth. Unfortunately, the added cost of the satellites, not to mention the risk of a farside landing, was something NASA management couldn’t swallow. Such a spectacular mission to Tsiolkovsky Crater would have to wait for a later generation of lunar explorers.

The actual decision on where Apollo 17 would land was to be made by the Apollo Site Selection Board (ASSB). This group of scientists, engineers, and managers was responsible for weighing the scientific goals against the operational requirements of each potential Apollo landing site.

In the early days of the Apollo Program, the Board leaned toward sites that were safe and easily accessible, if not the most scientifically interesting. That’s why smooth mare areas were chosen for the first three Apollo landings.

However, the last three Apollo Moon landings (known as “J Missions”), were entirely focused on serious scientific exploration of the Moon. That led to the selection of landing sites that were scientifically intriguing, even if the operational aspects — rugged terrain, landing approach paths through mountain passes, etc. — were much more challenging.

The next-to-last meeting of the ASSB was held in June 1971, one month before the launch of Apollo 15, which was the first of the J Missions. At this meeting, the Descartes highland plains were selected as the landing site for Apollo 16, and the crater Alphonsus was selected as one of the prime candidates for the Apollo 17 landing.

The other candidate sites for Apollo 17 included the central peaks of the craters Copernicus and Gassendi; a “pure highland” site in the southwest corner of Mare Crisium; and a combination highland-volcanic site on the southeast edge of Mare Serenitatis (“Sea of Serenity”). The Serenitatis site was classified as a volcanic site based on photographs taken by Apollo 15 Command Module (CM) Pilot Al Worden, which seemed to indicate the presence of volcanic craters. At this site, astronauts would be able to gather both older highland samples and younger volcanic samples, a prospect which generated much excitement among the lunar science community.

The final meeting of the ASSB occurred on Feb. 11, 1972, at NASA headquarters in Washington, D.C. A couple of months prior to this meeting, geologist Noel Hinners, the Chairman of the ASSB, had sent a letter to 32 lunar scientists closely associated with the upcoming Apollo 17 mission. Hinners’ letter asked the scientists to objectively evaluate the candidate landing sites identified by the ASSB during the June 1971 meeting.

Copernicus Crater was quickly eliminated as a candidate because the Apollo 12 crew had already collected samples from one of Copernicus’ crater rays, making a landing there redundant. Mare Crisium was eliminated because it was accessible to one of the Soviet’s uncrewed Luna sample-return spacecraft. (In fact, Mare Crisium was visited by the Luna 20 spacecraft just a few days after the last ASSB meeting.)

That left Gassendi Crater, Alphonso Crater, and Mare Serenitatis as viable landing sites for Apollo 17. After weighing the scientific and operational aspects of each site, the consensus of the 32 scientists was that Mare Serenitatis topped the list of candidates.

Apollo 17 targets Taurus-Littrow valley

The ASSB unanimously decided that Apollo 17 would land in the southeast corner of Mare Serenitatis, near the Taurus mountain range and the crater Littrow. This charming little valley is nestled between two soaring mountain ridges named North Massif and South Massif. These Massifs are so tall that the valley floor between them is deeper than the Grand Canyon!

The landing site didn’t even have a name before it was selected. But the Apollo 17 crew quickly (and logically) named the area Taurus-Littrow.

Taurus-Littrow is not only a beautiful location, it also contains a unique mix of lunar geological features. Its huge massifs were probably formed by the impact that created the Serenitatis basin. And together, they create a classic graben, a geological feature that occurs on Earth when two parallel mountains rise up, forming a deep valley between them. A lunar graben has different origins than an Earth graben — the lunar mountains formed as a result of a giant impact — but the result was the same: a small, square valley tucked between the massifs.

The eastern end of Taurus-Littrow valley was partially closed off by another mountain range called the East Massif. And near the East Massif lay the Sculptured Hills, an odd-looking assortment of knobby hills whose rounded shapes looked very different from the blocky Massifs, hinting at an origin very different than that of the Massifs. Capping off the diverse geology of the site, the western edge of Taurus-Littrow hosts a large scarp that snakes across the valley, eventually rising about 1.2 miles (2 kilometers) above the floor.

The valley also contains a slew of interesting craters that appeared to be volcanic in nature. Additionally, there were “streaks” of dark material overlaid on the massifs, which many geologists interpreted as the products of volcanic outbursts that spewed material out onto the mountains. One particular crater — nicknamed “Shorty” by the Apollo 17 crew, after a character in the Richard Brautigan novella Trout Fishing in America (1967) — had many of the hallmarks of a volcano cinder cone. This meant it could possibly provide an opportunity to sample young, volcanic soil and rocks, checking off one of the major objectives of Apollo 17.

Also, NASA had tantalizing images of boulders near the bottoms of the massifs, located at the end of tracks descending from higher up on the mountains. These boulders could be pristine samples of the early lunar crust that had been excavated by the impact that formed Mare Serenitatis and the massifs. Finally, there was clear evidence of a landslide on the South Massif, where lighter material had spilled off the mountain and into the darker valley floor below, allowing for easy sampling.

All-in-all, the Taurus-Littrow valley offered the very real possibility of collecting both very young and very old lunar material. The landing approach would be difficult: During the latter part of the descent, the LM would literally fly below and in between the massifs’ peaks, which were only about 4.3 miles (7 km) apart. Plus, the landing trajectory and lighting conditions necessary for landing would require NASA to carry out a night launch of the Saturn V booster rocket, a first. Nonetheless, everyone agreed that the anticipated scientific payoff of Taurus-Littrow was more than worth the added complexity and risk.

So, Taurus-Littrow it was. The final landing site of the Apollo program absolutely held promise. But it wasn’t until astronauts Cernan and Schmitt landed and explored the beautiful valley that scientists knew for sure they were right.

Michael Engle retired from NASA in 2018 after a 38-year career, during which he worked as a Space Shuttle flight controller, an astronaut training engineer, and finally as the Chief Engineer for Flight Safety in the Astronaut Office.

Grimy, sweaty, and dust-streaked from the gray sand of the lunar desert, Apollo 17 Commander Gene Cernan looked every inch the explorer.

Fifty years ago, Cernan and Lunar Module Pilot Jack Schmitt spent three days on the Moon, making them the most recent human visitors to our nearest celestial neighbor. And beyond the triangular windows of the spider-like Lunar Module (LM), Challenger, lay the pristine lunar valley of Taurus-Littrow.

Peering outside, Cernan and Schmitt saw steep-sided mountains tumbling to a boulder-strewn, age-darkened valley floor. To their right, the North Massif rose higher than eight Eiffel Tower piled one atop the other. And to their left, the South Massif rose as tall as a half-dozen Empire State Buildings. Farther east, the dome-like Sculptured Hills — some more than a mile (1.6 kilometers) tall — offered a tease of ancient lunar volcanism. Meanwhile, the blue-and-white marble of Earth glittered in the velvet lunar sky, providing a reassuring reminder of home.

For those three days in December 1972, Taurus-Littrow was Cernan and Schmitt’s home.

A geologist’s dream: Taurus-Littrow valley

The Taurus-Littrow valley sits at the edge of the Moon’s Serenitatis basin, deep in the Taurus mountains and south of the eroded crater Littrow. When picked for Apollo 17’s landing, Taurus-Littrow was so far off the beaten track in terms of photographic coverage that nobody had even bothered to name it. But its double-barreled moniker has since earned notoriety, as this lunar valley is the last place visited by humans on another world.

Apollo 17 was a last-chance saloon for geologists seeking evidence of volcanism in the Moon’s infancy. Two Apollo missions had already been cancelled due to budget cuts. And President Richard Nixon, faced with an unwinnable war in Vietnam and social unrest at home, had little appetite for the spiraling costs of the lunar program championed by John F. Kennedy a decade earlier.

As Apollo 17 teetered on its own precipice of cancellation, one facet in the mission’s salvation was Schmitt: the first professional geologist to practice his craft on the Moon. Pressure from the scientific community saw Schmitt assigned to Apollo 17, joining Cernan and Command Module Pilot (CMP) Ron Evans. But as the crew trained, the managers fretted, for a failed mission might spell doom for NASA’s up-and-coming Space Shuttle Program.

One day, legendary flight director Chris Kraft pulled Cernan aside. “Geno, put away that fighter pilot’s silk scarf and just bring your crew home alive,” Kraft cautioned. “If you run into something you don’t like out there, and decide not to land, I’ll back you 100 percent.”

Landing on the Moon is hard enough. But getting there was equally taxing. Months before launch, Cernan suffered a prostate infection, which was treated by flight surgeon Chuck LaPinta. Then, during a softball game, something snapped in his right leg. Fears of a ruptured tendon proved unfounded, but LaPinta discreetly shielded Cernan’s recovery from wary managers who might have grounded him due to the injury. LaPinta, Cernan wrote, was “a great doctor, a terrific liar and an even better friend.”

Cernan, Evans, and Schmitt roared aloft from Pad 39A at Florida’s Kennedy Space Center (KSC) at 12:33 A.M. EST on Dec. 7, 1972, making them the first U.S. astronauts to launch at night. This enabled Apollo 17 to reach the Moon in the early lunar morning, with the Sun providing adequate shadows and good visibility over Taurus-Littrow.

“It’s lighting up the sky,” cried NASA commentator Jack King, as the Saturn V rocket powered uphill. “It’s just like daylight here at the Kennedy Space Center!”

Apollo 17: A final dance

The four-day transit to the Moon was uneventful. And on Dec. 11, Cernan and Schmitt boarded Challenger and undocked from Evans, who would remain in lunar orbit aboard the Command Module (CM), America. Descending like an express elevator between the forbidding lunar mountains, Challenger swept into the Taurus-Littrow valley at its eastern entrance and landed on sure ground at 2:54 P.M. EST.

For the next 75 hours, Cernan and Schmitt were the Moon’s only living occupants. Each of their three moonwalks exceeded seven hours. They drove a cumulative 22 miles (35 kilometers) in the battery-powered Lunar Roving Vehicle (LRV) and gathered 254 pounds (155 kilograms) of rock and soil specimens, the largest haul of any Apollo crew.

Setting foot on alien soil, Cernan compared the oddness of the Moon’s one-sixth gravity to walking on a bowl of Jello-O. Moonwalking and lunar dust’s abrasive clinginess proved dirty work: Within minutes, both men’s pure-white spacesuits were black from the knees down.

They unpacked the LRV from Challenger’s descent stage — eliciting a hearty “Hallelujah!” from Cernan — then test-drove it. And the glint of Earth, hanging like a dainty Christmas ornament over the South Massif, proved captivating.

Cernan called for Schmitt to look at their home planet, an iridescence of life in a sea of darkness: “Just look up there!”

“Ah,” drawled Schmitt, feigning disgust. “You seen one Earth, you’ve seen ’em all!”

Moving briskly, they set up the U.S. flag. Next was the Apollo Lunar Surface Experiments Package (ALSEP), a station of instruments used to explore the Moon’s environment and interior. They then bored holes into the lunar surface for a heat-flow study, which was physically strenuous work that left Cernan’s fingers bruised and bloodied.

The next morning, having awakened to Wagner’s Ride of the Valkyries, courtesy of Mission Control, the astronauts hopped down Challenger’s ladder for the second moonwalk. A broken fender on one of the LRV’s wheels — which had earlier sprayed rooster-tails of dust over the astronauts — was fixed with clamps and four maps taped together. Cernan and Schmitt headed for the South Massif, sampling boulders and inspecting craters Lara, Camelot, and trough-like Nansen, the latter of which reminded Schmitt of Alpine valleys on Earth.

But it was the 360-foot-wide (110-meter) Shorty Crater that yielded Apollo 17’s biggest surprise. Dark-rimmed with blocky interior walls, Shorty’s ubiquitous grayness was broken by the merest hint of orange. Schmitt thought his eyes were tricking him, but he was indeed seeing orange soil, a tantalizing hint of ancient volcanism. Cernan took a core sample, which turned out to be red along part of its length, before fading to purplish-black. Today, the Shorty soil is thought to have derived from rapidly cooled molten rock.

Leaving the Moon for the next 50 years

Cernan and Schmitt’s final moonwalk took in Sherlock Crater, swung past Turning Point Rock, then headed over the lower flanks of the North Massif. They completed one last sampling stop at the Sculptured Hills. But with oxygen supplies dwindling, the clock was forever their enemy on the Moon.

The ‘lastness’ of Apollo 17 was readily apparent to Cernan as he took his final steps on the surface. “America’s challenge of today has forged man’s destiny of tomorrow,” he said before climbing up Challenger’s ladder. “And as we leave the Moon at Taurus-Littrow, we leave as we came, and God-willing, as we shall return: with peace and hope for all mankind.”

A few hours later, Challenger headed back to lunar orbit to dock with America. An excited Evans had spent three solo days running the Scientific Instrument Bay (SIMBay) of cameras and sensors on America. The work proved so exhausting that one morning he overslept by an hour, despite Mission Control’s continued efforts to wake him. Evans also set a record for the longest time (147 hours) spent circling the Moon.

After 75 lunar orbits, Apollo 17 headed back to Earth. On the homeward-bound journey, Evans performed a 65-minute spacewalk to retrieve SIMBay film cassettes. His excitement was readily apparent in his first words as a spacewalker: “Hot diggety dog!”

Three days later, America splashed down in the Pacific Ocean, close to the recovery ship, Ticonderoga, wrapping up what would be the 20th century’s last human visit to the Moon.

For five decades, Taurus-Littrow, peppered with boot prints and LRV tire-tracks, and littered with remnants of a fleeting, long-ago human presence, remains the most recent human outpost on the Moon. Cernan and Evans are now gone. But the shared hope of Apollo 17 that we will someday return to this enticing gray-tan world has drawn inexorably nearer.

By the time Apollo 17 launched at the end of 1972, NASA was already looking ahead. It was planning a first-ever joint mission with the USSR, preparing to launch the Skylab space station, and soliciting proposals from contractors for a new, reusable launch system dubbed the space shuttle, which was already attracting criticism and being called an expensive boondoggle.

But even as public interest waned, Apollo 17 was NASA’s last chance to demonstrate to scientists, politicians, and the taxpaying public that the entire $26-billion Apollo program had been worthwhile. To that end, NASA pulled out all the stops it could afford, especially when it came to science.

The crew’s most prominent member was not its commander, but rather Lunar Module Pilot (LMP) Harrison “Jack” Schmitt. A geologist by training, Schmitt was hailed as the first Apollo “scientist-astronaut” — a novelty in an era when the U.S. astronaut corps had been the exclusive domain of military and test pilots.

His selection was the culmination of a long-running campaign from the scientific community to send a geologist to the Moon. Schmitt had been assigned to the backup crew of Apollo 15, who, per NASA custom, were unofficially in line to fly three missions later. But after Apollo 18 was axed due to budget cuts in September 1970, speculation swirled inside and outside of NASA that Schmitt would be moved up to Apollo 17. Finally, on Aug. 13, 1971, six days after Apollo 15 splashed down, NASA announced Schmitt would be on Apollo 17 — bumping Joe Engle.

Commander Gene Cernan was making his third spaceflight and second trip to the Moon. On Apollo 10, as LMP, he had come tantalizingly close to the lunar surface, descending to a height of just 47,000 feet (14,300 meters) in a dry run for Apollo 11 before throttling up and returning home. Command Module Pilot Ronald Evans rounded out the crew; he and Cernan were former naval aviators.

Schmitt would unleash his expertise in what he called a geologist’s paradise. The landing site was Taurus-Littrow, a valley on the southeastern rim of Mare Serenitatis (Sea of Serenity). It was hemmed in by mountains as high as 7,900 feet (2,400 m), which were heaved upward in the massive impact that formed the Serenitatis basin. Since then, volcanoes and impacts had further sculpted the area. Features included landslides, ejected boulders that had scraped tracks into the lunar soil, and a 260-foot-tall (80 m) scarp — the remains of a fault line — that cut across the 4.4 mile-wide (7 kilometers) valley.

It promised much — and the crew delivered, sending off Apollo with a remarkable swan song.

The final Apollo mission was the first to launch at night — and also the first to suffer a delay on the pad. Just 30 seconds from liftoff, the launch software ordered the proceedings to a halt. It turned out an oxygen tank had not been automatically pressurized — and though technicians manually pressurized it, the program had not gotten get the message. To bypass the checks, the ground crew rewired the launch computers. Two hours and 40 minutes later, Apollo 17 lifted off, witnessed by a crowd of half a million people.

Cernan, the only crew member to have flown to space before, warned his crewmates about the bumpy ride at staging, when the first stage would separate and the second stage would ignite.

Cernan: Inboard [engine] cutoff.

Robert Overmyer, Capsule Communicator (CapCom): Roger, inboard.

Cernan: OK, now hold on after staging, guys. […]

Cernan: Hold on!

(Unknown): OK.

Cernan: Five seconds. Which gs?

(Unknown): Four gs.

The astronauts received a jolt forward — from 4 gs nearly instantly to 0 gs — as the first stage engines cut out.

(Unknown): Stage cu — ooof —

Evans: Jesus Christ!

Cernan: I told you to hold on. Look at that son-of-a-b—-. Man!

Thanks to the nighttime launch, the crew got a spectacular view out the Command Module (CM) windows of the bright yellow fireball from the first stage overtaking and enveloping them. Then, when the second stage lit, they blasted back through the flame.

Evans: Jesus criminy! [High-pitched laughter.] Haha!

Cernan: OK, Bob. I guess we got all five [engines]. […]

Schmitt: By the way, the cabin’s sealed. [Laughter.] […]

Cernan: Let me tell you, this night launch is something to behold. […]

You guys believe me about that S-I staging now?

Evans: [High-pitched laughter.] Haha!

Schmitt: I can’t believe how smooth this is. […]

Cernan: OK, let’s keep this mother burning. We’ve got a long way to go.

The valley of Taurus-Littrow was the Apollo program’s most ambitious landing site. It runs roughly east-west between two massifs to the north and south that are taller than the Grand Canyon is deep. A third massif and a slighter range called the Sculptured Hills sit at the eastern end of the valley; the western end opens into Mare Serenitatis. In the Lunar Module (LM) Challenger, Cernan and Schmitt approached from the east, soaring over the hills and descending between the North and South massifs.

Cernan: OK, I got the South Massif.

Schmitt: OK, update the AGS [Abort Guidance System], Houston?

Cernan: Yes.

Gordon Fullerton (CapCom): That’s affirmative, update the AGS.

Cernan: OK, Gordo, I’ve got Nansen [Crater]; I’ve got Lara [Crater]; and I’ve got the Scarp. Oh, man, we’re level with the top of the massifs, now. […] OK, Gordo, we’re out of 11,000 [feet] at nine [minutes].

Schmitt: OK, stand by for pitchover.

Cernan: Oh, are we coming in. Oh, baby.

Schmitt: OK; through 9,000.

Cernan: Stand by for pitchover, Jack.

Schmitt: Eight thousand.

Cernan: I’ll need the PRO[CEED command entered into the computer].

Schmitt: I’ll give it to you.

Cernan: Pitchover.

Schmitt: There it is! Proceeded.

Cernan: And there it is, Houston. There’s Camelot [Crater]!

Schmitt: Wow!

Cernan: Right on target.

Schmitt: I see it.

Cernan: We got them all.

In classic carrier-landing style, Cernan brought Challenger in hot, at a faster rate of descent than planned — then leveled it out right before touchdown.

Schmitt: Moving forward a little. Ninety feet. Little forward velocity. Eighty feet; going down at 3. Getting a little dust. We’re at 60 feet; going down about 2. Very little dust. Very little dust, 40 feet; going down at 3.

Cernan: Stand by for touchdown.

Schmitt: Stand by. Twenty-five feet; down at 2. Fuel’s good. Twenty feet. Going down at 2. Ten feet. Ten feet. Co — contact!

The LM engine cut off and Challenger freefell the last several feet to the surface.

Schmitt: STOP, push. Engine STOP; Engine ARM; PROCEED; Command Override, OFF; Mode Control, ATT HOLD; PGNS, AUTO.

Cernan: OK, Houston. The Challenger has landed!

Fullerton: Roger, Challenger. That’s super. [Applause in background.]

Schmitt: OK, [cycling thruster] Parker valves.

Cernan: Boy, you bet it is, Gordo. [To Schmitt] Boy, when you said shut down, I shut down and we dropped, didn’t we?

Schmitt: Yes, sir! But we is here!

Cernan: Man, is we here! […]

Cernan: Jack, are we going to have some nice boulders in this area. […]

Schmitt: Ohhh!!! Man, look at that rock out there!

Cernan: Absolutely incredible. Absolutely incredible.

Schmitt: I think I can see the rim of Camelot [Crater].

Fullerton: Roger.

Cernan: Epic moment of my life.

After relaying their initial impressions of the landscape to Houston and taking care of some housekeeping items, the crew went straight into preparation for their first extravehicular activity (EVA). Cernan was first to clamber out.

Cernan: I’m on the footpad. And, Houston, as I step off at the surface at Taurus-Littrow, I’d like to dedicate the first step of Apollo 17 to all those who made it possible. [Pause.] Jack, I’m out here. Oh, my golly. Unbelievable! Unbelievable, but it is bright in the Sun.

The crew didn’t stray far from Challenger on their first EVA. First, they set up the Apollo Lunar Surface Experiments Package (ASLEP), a suite of scientific instruments, near the LM. They also drilled a deep core sample. But difficulties with the core sample meant the day’s planned 90-minute geology field trip to Station 1, roughly 0.6 mile (1 km) to the south, had to be cut short by half an hour, leaving Schmitt none too pleased.

The next day, Cernan and Schmitt set out across the valley in their lunar rover for the South Massif, roughly 5 miles (6 km) away, on the other side of the scarp. Their destination had a dynamic history: A landslide had swept across the lower slopes and the scarp, leaving a fan of lighter material that contrasted with the dark, volcanic deposits on the valley floor. Strewn atop that was talus — a field of even lighter rocks and boulders that had tumbled down the mountain, which provided the opportunity to sample older massif material from higher elevations.

As time wore on, Schmitt began to get annoyed with some of the requests that CapCom Robert Parker was sending up from the Mission Control back room, where a team of geologists huddled, glued to the TV pictures from the rover and strategizing a sampling plan on the fly. In one conversation, Schmitt took passive-aggressive objection with a call to sample further upslope.

Parker: [To Schmitt] OK, and if you’re going up the massif, why don’t we try and get the rake sample up there now — when you finish these rocks.

Cernan: [Already working upslope.] Hey, Jack — Jack, don’t come up here unless you bring the rake. It’s a long trip. No sense coming up here twice.

Schmitt: Well …

Cernan: I can go get this sample. I’d get the rake if I were you. Don’t walk back up twice.

Schmitt: Well, I don’t … uhrm uhrm uhrm … I don’t — I’m not sure they’re going to gain anything by coming up to the top. OK. [Schmitt starts walking uphill back toward the rover.] You’re not going to gain a thing, Bob.

Parker: Standby.

Schmitt: You’re still on the talus. [Pause.] You guys — oh, well. [He drops the subject and continues trudging up the slope.] The rims of the small craters in the talus are — are softer than the normal terrain. My foot goes in maybe 10 centimeters [4 inches] where normally it only goes in a centimeter.

Parker: [Interrupting.] OK. As long as it’s above the break of the slope, Jack, we don’t have to get very far up the slope.

Schmitt: [Annoyed.] That’s right! It’s too late! [He turns to leave the rover and walk further uphill.]

Parker: And, Jack, if you’re back at the rover, how about giving us a grav[imeter] reading before you leave.

Schmitt: Because I’m late sampling! That’s why. But I’ll do it anyway.

Parker: Uh, Roger.

While Cernan attempted to delicately change the topic with details of another boulder, Schmitt wasn’t ready to let the topic go.

Schmitt: Hey, look, Gene, on these rake samples, there’s just no point in carrying a rake all the way up here —

Parker: [Interrupting.] Negative, Jack, as long as you’re above the break —

Schmitt: — because all we needed was a break in the slope.

Parker: — as long as you’re above the break in the slope, that’s right.

Schmitt: Well, that’s all right. It’s being done. But let’s watch those kind of calls, please.

Cernan: [Diplomatically.] Yeah, they can’t appreciate the toughness of going up this slope, though. We can — we’ve got to tell them that.

Schmitt: Well, we did.

Parker: Yeah, that’s what we were saying. Don’t go above just at the base of the break in the slope, Jack. Don’t climb all the way up there with it.

Schmitt: Ah, relax!

Cernan: OK, we’re all set, Bob. No problem.

Station 3 at Lara Crater brought more frustration for Schmitt. Cernan was tasked with drilling a sample core, leaving Schmitt to take surface samples solo — a difficult task for a tired astronaut in a bulky suit. In one slapstick sequence, he managed to knock over the bag of samples, scattering them across the slope. He gathered them back up, only to drop the container again. In trying to pick it up, he fell over, rolling on the ground. He finally retrieved the container, only for his stack of empty bags to fall off their mount atop his camera.

Parker: Hey, Gene, would you help — would you go over and help Twinkletoes, please?

Schmitt: I tell you, you fix that camera bracket so the bags stay on and I’ll be a lot better off.

Parker: Roger.

Cernan: Want some help, Jack? I’ll be there.

Schmitt: No! I don’t need any help.

Cernan: OK.

Schmitt: I just need better bags.

But a couple minutes later, Schmitt offered an olive branch to Parker.

Parker: We’re watching you, Jack.

Schmitt: What’s that?

Parker: I said we’re watching you, but don’t let that inhibit you.

Schmitt: Bob, I don’t let anything inhibit me. And I don’t stay mad very long. [He neatly grabs his sampling scoop by stepping on the head, pivoting the handle up.]

Parker: [Impressed.] That was very good!

But in the end, Parker couldn’t resist one parting quip as they departed Station 3.

Parker: And be advised that the switch board here at MSC [the Manned Spaceflight Center] has been lit up by calls from the Houston Ballet Foundation requesting your services for next season.

Schmitt: I should hope so!

Station 3 was an emotional low point of the mission for Schmitt. But the crew’s next stop — Shorty Crater, 360 feet (110 m) wide and 46 feet (14 m) deep — would give him a shot of adrenaline and the mission’s highest of highs.

Cernan: This is an impressive one. Wait until you see the bottom of it. […]

Parker: OK, and the primary priority — the No. 1 and 2 priorities at this station will be samples from the crater rim and the pan from the crater rim. Over.

As the crew prepared to handle these tasks, Schmitt suddenly saw something curious.

Schmitt: Ohhh, hey!! [Pause. Doubting himself.] Wait a minute —

Cernan: What?
Schmitt: — where are the reflections? I’ve been fooled once. [Discarding his doubts.] THERE IS ORANGE SOIL!

Cernan: [Skeptically.] Well, don’t move it until I see it.

Schmitt: It’s all over! Orange!!!

Cernan: Don’t move it until I see it …

Schmitt: I’ve stirred it up with my feet.

Cernan: … HEY, IT IS! I can see it from here!

Schmitt: It’s orange!

Cernan: Wait a minute, let me put my visor up. It’s still orange!

Schmitt: Sure it is! Crazy!

Cernan: Orange!

Schmitt: I’ve got to dig a trench, Houston.

Parker: Copy that. I guess we’d better work fast.

Schmitt took command of the situation, sizing up the images they would need to obtain and digging a trench for samples. The crew had roughly 30 minutes before mission rules dictated they return to the LM.

Cernan: Hey, he’s not — he’s not going out of his wits. It really is.

Parker: [Half-joking.] Is it the same color as cheese? […]

Schmitt: It’s almost the same color as the LMP decal on my camera.

Parker: OK. Copy that.

Cernan: That is orange, Jack! […]

Schmitt: Fantastic, sports fans. It’s trench time! You can see this in your color television, I’ll bet you.

Cernan: How can there be orange soil on the Moon?! [Pause.] Jack, that is really orange. It’s been oxidized! Tell Ron to get the lunar sounder [in the CM pointed] over here.

Schmitt: It looks just like a — anoxidized desert soil, that’s exactly right.

Schmitt, Cernan, and the geologists immediately realized there was a possibility that they had stumbled upon a young volcanic vent, where escaping steam and gases had rusted the soil. Next, Schmitt readied a core sample.

Schmitt: Did you want that in the orange?

In Mission Control, voices shouted in unison from the geologists’ back room: “Yes!”

Parker: Roger, that’s affirm. We can put cores in gray soil all the time.

After the mission, analysis would show that the orange soil was not oxidized from volcanic venting; rather, it contained glassy beads that had formed from a fiery fountain of molten droplets and were encased in a lava flow some 3.5 billion years ago. Shorty Crater itself was formed by an impact — one that had excavated the beads from below.

As the crew prepared for their rest period, CapCom Joe Allen shared some late-night philosophical musings.

Allen: Might add, also, that there are a lot of us looking forward to that third EVA tomorrow. It’s going to be the last one on the lunar surface for some time.

Cernan: I tell you, if it’s anywhere near what the first two were like, we’re looking forward to it, also. [Long pause.]

Allen: Gene and Jack, we’re still marveling at the beautiful television pictures that we’re getting from your TV camera there. It’s fun, in fact, to watch the tracks that you’re leaving behind in the lunar soil, both footprints and rover tracks. And some of us are down here now reflecting on what sort of mark or track will, someday, disturb the tracks that you leave behind there tomorrow.

Cernan: That’s an interesting thought, Joe, but I think we all know that somewhere, someday, someone will be here to disturb those tracks.

Allen: No doubt about it, Geno.

Schmitt: Don’t be too pessimistic, Joe. I think it’s gonna happen.

* * *

The final lunar EVA of the Apollo program took the pair across the other side of the valley to the North Massif. They drove through the field of boulders strewn across the lower slopes of the massif, made an excursion east to sample the Sculptured Hills, and finished the day surveying the rocky crater Van Serg.

When the crew returned to the LM, they performed a brief closing ceremony in front of the rover’s TV camera. They unveiled a commemorative plaque at the base of Challenger and NASA administrator James Fletcher came on the radio to convey well-wishes from President Richard Nixon.

Then, before climbing up the ladder for the final time, Cernan delivered one last soliloquy on the most distant stage in human history.

Cernan:Bob, this is Gene, and I’m on the surface. And as I take man’s last steps from the surface back home, for some time to come — but, we believe, not too long into the future — I’d like to just [say] what I believe history will record: that America’s challenge of today has forged man’s destiny of tomorrow. And as we leave the Moon and Taurus-Littrow, we leave as we came, and, God willing, as we shall return: with peace and hope for all mankind. Godspeed the crew of Apollo 17.

In a 2007 NASA oral history, Cernan reflected on those final moments on the lunar surface.

Cernan:People kept saying, “What are you going to say, what are going to be the last words on the Moon?” I never even thought about them until I was basically crawling up the ladder. […]

I looked down, and there was my final footsteps on the surface […] I looked over my shoulder because the Earth was on top of the mountains in the southwestern sky. […]

I wasn’t coming back. This was it. I wanted, like in the simulator, I wanted to push the freeze button, stop time, stop the world. I just wanted to sit there and think about this moment for a few moments, and hopefully absorb more subconsciously than I had the ability to take in consciously.
But I couldn’t, there was no freeze button.

So up the ladder I went.

As the Moon orbits Earth, it skims through the sky, passing by the planets each month. But sometimes, things align just right and the Moon appears to pass in front of a planet from our point of view. Such an event is called an occultation, and there’s one coming to the sky Wednesday night, when bright, ruddy Mars is temporarily blocked from view by Earth’s lone natural satellite.

Enjoying the show

The Red Planet reaches opposition early Thursday morning — a day already marked on many observers’ calendars. But just a few hours earlier, late on Wednesday night, the nearly Full Moon will occult Mars for skywatchers in most of the U.S. (The only exceptions are those on the East and Gulf coasts, who will witness a near miss.) Additionally, observers in Canada, Greenland, Iceland, the U.K., northern Africa, and northwestern Europe can also see the event.

Binoculars or a telescope will be useful to watch the planet disappear and reappear. But even without any equipment, you’ll notice when bright, red-hued Mars (magnitude –1.9) — readily visible to the naked eye — winks out of view behind the lunar disk. It will take about an hour for the Moon to move far enough along the ecliptic to allow Mars to slip back out into sight. The moments of disappearance and reappearance — as well as the duration of the occultation — depend heavily on your location; below are the times for a few notable U.S. cities, but you can look up the crucial times for the city nearest you at www.lunar-occultations.com/iota/planets/1208mars.htm. (Note that the times given on this website are in UT, so you will need to convert to your local time. Below are the local times that Mars will disappear and reappear from behind the Moon for several major cities across the U.S.)

Chicago
Mars disappears: 9:10:58 P.M. CST
Mars reappears: 10:04:55 P.M. CST

Dallas
Mars disappears: 8:53:53 P.M. CST
Mars reappears: 9:27:35 P.M. CST

Dayton
Mars disappears: 10:22:25 P.M. EST
Mars reappears: 10:56:42 P.M. EST

Denver
Mars disappears: 7:44:56 P.M. MST
Mars reappears: 8:48:30 P.M. MST

Des Moines
Mars disappears: 8:59:48 CST
Mars reappears: 9:59:54 CST

Las Vegas
Mars disappears: 6:34:14 PST
Mars reappears: 7:35:43 PST

Los Angeles
Mars disappears: 6:30:23 PST
Mars reappears: 7:30:11 PST

Phoenix
Mars disappears: 7:32:23 MST
Mars reappears: 8:30:47 MST

Seattle
Mars disappears: 6:51:57 PST
Mars reappears: 7:50:35 PST

All eyes on Mars

An hour after sunset in the Midwest on Dec. 7, Mars and the Moon are already more than 10° high in the east and stand less than 2° apart, with Mars to the lower left (east) of the Moon. Over the next few hours, they will continue to rise higher above the eastern horizon, nestled among the stars of Taurus the Bull. The pair sit roughly halfway between the bright star Aldebaran (magnitude 0.9), which serves as the Bull’s eye, and Elnath (magnitude 1.7), which marks the tip of Taurus’ western horn. Although Full Moon does not officially occur until 11:08 P.M. EST, our satellite’s face will already appear fully illuminated to the naked eye.

Occultations occur over only a limited portion of the globe because the celestial geometry must be just right for the Moon to appear to cross in front of Mars. It’s just like an eclipse, which also traces a limited path over the globe. Even if you’re not in the path, it’s worth standing outside under the stars to enjoy the sight of our Moon skimming close to Mars. Again, a telescope or binoculars will show this best, but you should be able to see the bright planet easily without any optical aid.

Shortly before midnight EST, the Full Cold Moon will occur, the last Full Moon of the year. And shortly after midnight EST, Mars will officially reach opposition. Through a telescope, Mars appears 17″ wide — large enough for bigger scopes to show some of the planet’s most prominent geological features. Around midnight, Mare Cimmerium and Mare Elysium should appear near the center of the planet. Look also for its lighter-colored polar caps; the northern cap may even sport a darker region, called the north polar hood, which is now starting to shrink as winter in Mars’ northern hemisphere comes to a close.

So much else to see

About 16° west of the pair is the Pleiades (M45), a nearby open cluster of young stars that is easily visible to the naked eye. Many people see six or seven stars without any optical aid; pull out binoculars or a low-power telescope, however, and many more luminaries will pop into sight. This is a particularly good target for low powers, as larger telescopes will provide a field of view so small that not many stars are visible.

Just southeast of Taurus, closer to the horizon, is Orion the Hunter, familiar to many because of Orion’s three-star belt and bright red shoulder, Betelgeuse. Hanging below the easternmost belt star is Orion’s Sword, which contains the stunning Orion Nebula (M42), one of the nearest star-forming regions to Earth. Visible to the naked eye as a faint, extended glow, Orion’s Nebula is a target you’ll want a telescope for, which allows you to peer into the heart of thick dust clouds where baby stars are born. The stars of the Trapezium Cluster form a small box in the center of the nebula.

Close to the horizon is the brightest star in the sky: Sirius, the nose of the Big Dog in Canis Major. A blazing magnitude –1.4, this star will appear nearly as bright as Mars and may seem to twinkle or dance like mad, particularly earlier in the evening after it’s just risen. This is because you’re looking through more — and more turbulent — air closer to the horizon than the zenith, and all this air “scrambles” the light from Sirius, like looking at a light through water. While this star is clearly visible to the naked eye, binoculars or a telescope may show it twinkling and dancing like a diamond catching the light until it climbs high enough above the horizon.

And there’s one more obvious target in the sky: the Moon itself! The Full Moon is quite bright; using high magnification to look at just a small portion of its surface can help cut down on the glare. Additionally, you can try using a Moon filter or even wearing your sunglasses when looking through the eyepiece. During the Full Moon, the Sun appears directly overhead from the lunar nearside’s point of view. Shadows are at their smallest and contrast is low, but the dark maria (Latin for seas, although these regions never hosted water) still provide a wonderful landscape to enjoy.

Both the Moon and Mars will remain in the sky until dawn on Dec. 8. So, enjoy this stunning pairing all night, paying particular attention as they not only cross paths but slowly draw apart again — by the time they set, they’re just over 3.5° apart!

When Howard Chen was a teen, he discovered Carl Sagan’s classic book Pale Blue Dot (Random House, 1994), a rumination on humanity and its lonely, fragile planetary home.

“I loved spending time at the library and, out of chance, I found this book that changed my perspective of our place in the universe,” says Chen.

Now 29, Chen was born in Taipei, Taiwan, and spent his early years in Burnaby, British Columbia, before moving to the U.S. for high school. He studied creative writing during his first year at Boston University, but switched to physics the next year after taking an astronomy class. In the new program, Chen got to experience the best of both worlds: being creative and learning about science.

“I thrived in an environment where I could ask big, bold questions to try and figure out answers using the tools and methods of science,” he says.
Chen went on to complete his Ph.D. at Northwestern University, specializing in planetary science. He is currently a postdoctoral program fellow at NASA Goddard Space Flight Center, where he focuses his research on the atmospheres of exoplanets.

“I really enjoy pushing our limits to understand the atmospheres of smaller, rocky exoplanets that are within the habitable zone,” he says. These worlds, where liquid water could exist on the surface, are the obvious places to hunt for signs of life-friendly conditions or even life itself.

Chen uses computer simulations to predict the evolution and composition of such exoplanetary atmospheres. His work focuses on incorporating chemical reactions that occur between a host star’s light and a planet’s atmosphere into climate models, something that hasn’t traditionally been done. These reactions play an important role in determining which gases, and how much of each, are present in the atmosphere of an exoplanet. “Each gas can give us some clue of the climate evolution and the current state of the planet,” says Chen.

He anticipates that this, in turn, will help him answer big questions, such as whether water could be present on the surface of Earth-like planets, and if there are potential biosignatures that indicate the presence of life.

“I’m hoping my work will allow us to be able to consistently predict the atmospheric composition of terrestrial exoplanets and to compare this to what we might be able to eventually observe using telescopes,” says Chen.

Daniel Horton, a climate scientist and Earth system modeler at Northwestern University who was Chen’s thesis advisor, says, “Howard’s drive, independence, and creativity suggest he will be a pioneer in the planetary habitability research community for years to come.”

Make sure to explore our full list of 25 rising stars in astronomy. Check back each week for a new profile!

To get the latest astronomical news and observing content delivered directly to your door,subscribe to Astronomy magazine today!

The night sky is a shared wilderness. On a dark night, away from the city lights, you can see the stars in the same way as your ancestors did centuries ago. You can see the Milky Way and the constellations associated with stories of mythical hunters, sisters and journeys.

But like any wilderness, the night sky can be polluted. Since Sputnik 1 in 1957, thousands of satellites and pieces of space junk have been launched into orbit.

For now, satellites crossing the night sky are largely a curiosity. But with the advent of satellite constellations – containing hundreds or thousands of satellites – this could change.

The recent launch of BlueWalker 3, a prototype for a satellite constellation, raises the prospect of bright satellites contaminating our night skies. At nearly 700 square feet (64 square meters), it’s the largest commercial communications satellite in low Earth orbit – and very bright.

Pollution of the night sky

While spotting satellites in the night sky has been a curiosity, the accelerating number of satellites in orbit means pollution of the night sky could become a serious problem.

On a clear night, particularly near twilight, you can see satellites traveling across the night sky. These satellites are in low Earth orbit, just a few hundred kilometers above Earth and traveling almost 5 miles (8 kilometers) every second.

Apps and websites allow you to identify or predict the arrival of particular satellites overhead. And it is genuinely fun to see the International Space Station travelling by, realising that on that speck of light there’s a crew of astronauts.

But in the past few years, the pace of satellite launches has accelerated. SpaceX has made satellite launches cheaper, and it has been launching thousands of Starlink satellites that provide internet services.

Roughly 50 Starlink satellites are launched into orbit by each Falcon 9 rocket, and initially produce a bright train of satellites. These initially produced UFO reports, but are now sufficiently common to not be particularly newsworthy.

Once the Starlink satellites disperse and move to their operational orbits, they are near the limit of what can be seen with the unaided eye.

However, such satellites are bright enough to produce trails in images taken with telescopes. These trails overwrite the stars and galaxies underneath them, which can only be remedied by taking additional images. Short transient phenomena, such as a brief flash from a gamma ray burst, could potentially be lost.

BlueWalker 3

While Starlink is the largest satellite constellation in service, with thousands of satellites in orbit, others are planned.

Amazon’s Blue Origin plans to launch more than 3,200 Project Kuiper satellites, and AST SpaceMobile plans to launch 100 BlueBird satellites (and perhaps more).

The recently launched BlueBird prototype, BlueWalker 3, has produced genuine alarm among astronomers.

While BlueWalker 3 was initially quite faint, it unfolded a 690-square-foot (64 square meter) communications array – roughly the size of a squash court. This vast surface is very good at reflecting sunlight, and BlueWalker 3 is now as bright as some of the brightest stars in the night sky.

It’s possible the operational BlueBird satellites could be even bigger and brighter.

Large numbers of satellites this bright could be bad – very bad. If there were thousands of satellites this bright, sometimes you would be unable to look at the night sky without seeing bright satellites.

We would lose that sense of wilderness, with an almost constant reminder of technology in our sky.

There could be a big impact on professional astronomy. Brighter satellites do more damage to astronomical images than faint satellites.

Furthermore, many of these satellites broadcast at radio frequencies that could interfere with radio astronomy, transmitting radio waves above remote sites where radio observatories observe the heavens.

A precipice?

What happens next is uncertain. The International Astronomical Union has communicated its alarm about satellite constellations, and BlueWalker 3 in particular.

However, the approval of satellite constellations by the US Federal Communications Commission has had relatively little consideration of environmental impacts.

This has recently been flagged as a major problem by the US Government Accountability Office, but whether this leads to concrete change is unclear.

We may be on the edge of a precipice. Will the night sky be cluttered with bright artificial satellites for the sake of internet or 5G? Or will we pull back and preserve the night sky as a globally shared wilderness?

Michael J. I. Brown, Associate Professor in Astronomy, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

In 2010, I started making YouTube videos to help people glimpse interesting celestial objects, from the planets to galaxies and everything in between. All the while, I also tried to educate them about observing equipment and the issue of light pollution. The series, Eyes on the Sky, celebrated its 12th anniversary this November.

Since the program’s inception, I’ve been fortunate enough to both lead and participate in live outreach events across the greater Chicagoland area, despite its light-polluted skies. My outreach adventures have also taken me to suburban libraries, local forest preserves, community centers, and even to California’s redwood forest and Pennsylvania’s Allegheny National Forest.

Suffice to say, I’ve interacted with many amateur and experienced observers, both virtually and in person, during that time. And one popular question that always seems to crop up, no matter the observer’s experience or desired target: “How do you know where to find that?” Once I noticed, I started making a mental list of commonly requested targets, and eventually I started writing them down. Since 2012, I’ve been tweaking this master list based on in-person feedback.

With so many beginner observers turning to the skies during the pandemic, a related question has been popping up online: “What deep-sky objects should I hunt down after I’ve observed the Moon and bright planets?” The response I’ve often seen from others is to start with the Messier list. But I think that’s misguided advice.

Stay with me; let’s walk through this. A logical beginner might think: “OK, what’s the first object on the Messier list? Oh, Messier 1, the Crab Nebula. I’ve seen photos of that online — it is amazing! I just got a telescope for Christmas and this object is in my sky tonight. So, let’s start at the start.”

Now imagine the disappointment of that beginner after observing the Crab Nebula through a 60mm or 70mm refractor under heavily light-polluted skies — assuming they can even find it. I’ve had trouble viewing M1 with a 76mm reflector using averted vision under decent skies, and I’m a rather experienced observer.

Remember, Charles Messier and his assistant Pierre Méchain did not compile the famed list because they were looking for a bunch of great and bright nebulae for amateurs. Messier was a comet hunter and he kept finding annoying not-comets that he didn’t want to distract him from his search. True, he added some questionable entries — M44, M45, and the double star M40 come to mind — but most of the objects on the list could easily be mistaken for comets.

So, should we really expect beginners to be excited about low-surface-brightness objects like M1, M33, M74, M88, or M101? By recommending the entire Messier list, we are indeed recommending many such objects. But I think we can do better. And that’s by starting off newbies with targets that are both easily found and clearly visible.

Don’t get me wrong; I’ve included a fair number of brighter Messier objects in my beginner’s deep-sky list, which appears later in this article. And those do serve as prime examples for beginners to try observing first, and then continue to check back in on, like revisiting old friends over the years. But even a list of the prime Messiers would leave out many other perfect deep-sky targets for beginners.

When I was a young amateur …

Certainly, back in the day, many of us amateurs learned persistence while tracking down whatever we could find with the telescopes available to us. And, driven by a love of the cosmos and lots of reading, I developed a good sense of what deep-sky objects to seek out. Additionally, I was in an astronomy club where more experienced members were ready and willing to mentor me. The sky was also darker then, so fainter deep-sky sights were easier to locate and view.

Today, you can find a slew of suggestions with a quick search on the internet. But like Forrest Gump’s box of chocolates, you never know what you’re going to get. Typically, it’s an off-the-cuff list of both good (high surface brightness, visually impressive) and terrible (low surface brightness, dim or invisible) objects supposedly for beginners. But they are often simply the favorites of the amateurs suggesting them.

1) The object must be within 8° — a typical finder scope field of view — of a magnitude 3.5 or brighter star to help beginners easily locate the target.

2) It should overlap as many bright stars as possible when viewed through a red dot (zero magnification) finderscope, as these are increasingly found on new telescopes popular with beginners.

3) It must have a high enough surface brightness to be well received through a small telescope, despite light pollution.

The first criterion is important because, despite the roughly 9,000 potentially naked-eye stars visible from Earth, the vast majority of those are rendered invisible by light pollution. Only around 280 stars are magnitude 3.5 or brighter. By choosing these stars as signposts, the search for beginner-friendly deep-sky objects can start with a naked-eye star visible in moderately to significantly light-polluted skies. This is essential for those using non-magnified finders (point two), which is often overlooked in online lists.

As for the third criterion, my past outreach experiences helped me identify what beginners were impressed with at the eyepiece. That’s why my list contains a good number of brilliant open clusters, a few impressive globulars, and some of the brightest nebulae. Galaxies often received the least enthusiasm from beginners, but I’ve sprinkled in a spiral and dwarf to ensure a bit of everything.

Seeing double

Importantly, there is an entire class of objects that we miss altogether if we only focus on the Messier, NGC, and IC galaxies, clusters, and nebulae: double stars. Such targets are great for amateurs, especially under light-polluted skies.

The problem with more expansive deep-sky objects is that, even if they are very bright, their brightness is spread over a large area, often resulting in relatively low surface brightness. This is not the case for double stars. They are pointlike, so there are no surface brightness issues to deal with — visibility is only a question of visual magnitude. Plus, multi-star systems litter the night sky. With so many colorful and interesting stellar duos and trios (and more) in reach of beginners, a thorough list must also include those.

This compilation includes just 40 objects that are accessible to almost anyone. Such a relatively short list allows a beginner to enjoy multiple quick successes.

So, start to work your way through my official “Telescopes on the Sky” list, which includes 17 open clusters, 11 double or multiple stars, six globular clusters, four nebulae, two galaxies, and a lone variable star. You can also access the list at http://eyesonthesky.com/tutorials/telescopes-on-the-sky, where you’ll find further details on locating and observing each target.

Easy telescopic targets

I hope you feel the urge to share this new beginner’s list of deep-sky objects with your astronomically inclined friends. And, if you’re interested, consider joining me in advocating for darker nights. That won’t just help us all get a better night’s sleep, it will also help unlock views of the many beautiful distant objects that circle above our heads every night — most of which few beginners have had a chance to glimpse.

Keep your eyes on the sky and your outdoor lights aimed down. That way, we can all see what’s up!