Job title of the future: Space debris engineer

Stijn Lemmens has a cleanup job like few others. A senior space debris mitigation analyst at the European Space Agency (ESA), Lemmens works on counteracting space pollution by collaborating with spacecraft designers and the wider industry to create missions less likely to clutter the orbital environment. 

Although significant attention has been devoted to launching spacecraft into space, the idea of what to do with their remains has been largely ignored. Many previous missions did not have an exit strategy. Instead of being pushed into orbits where they could reenter Earth’s atmosphere and burn up, satellites were simply left in orbit at the ends of their lives, creating debris that must be monitored and, if possible, maneuvered around to avoid a collision. “For the last 60 years, we’ve been using [space] as if it were an infinite resource,” Lemmens says. “But particularly in the last 10 years, it has become rather clear that this is not the case.” 

Engineering the ins and outs: Step one in reducing orbital clutter—or, colloquially, space trash—is designing spacecraft that safely leave space when their missions are complete. “I thought naïvely, as a student, ‘How hard can that be?’” says Lemmens. The answer turned out to be more complicated than he expected. 

At ESA, he works with scientists and engineers on specific missions to devise good approaches. Some incorporate propulsion that works reliably even decades after launch; others involve designing systems that can move spacecraft to keep them from colliding with other satellites and with space debris. They also work on plans to get the remains through the atmosphere without large risks to aviation and infrastructure.

Standardizing space: Earth’s atmosphere exerts a drag on satellites that will eventually pull them out of orbit. National and international guidelines recommend that satellites lower their altitude at the end of their operational lives so that they will reenter the atmosphere and make this possible. Previously the goal was for this to take 25 years at most; Lemmens and his peers now suggest five years or less, a time frame that would have to be taken into account from the start of mission planning and design. 

Explaining the need for this change in policy can feel a bit like preaching, Lemmens says, and it’s his least favorite part of the job. It’s a challenge, he says, to persuade people not to think of the vastness of space as “an infinite amount of orbits.” Without change, the amount of space debris may create a serious problem in the coming decades, cluttering orbits and increasing the number of collisions.  

Shaping the future: Lemmens says his wish is for his job to become unnecessary in the future, but with around 11,500 satellites and over 35,000 debris objects being tracked, and more launches planned, that seems unlikely to happen. 

Researchers are looking into more drastic changes to the way space missions are run. We might one day, for instance, be able to dismantle satellites and find ways to recycle their components in orbit. Such an approach isn’t likely to be used anytime soon, Lemmens says. But he is encouraged that more spacecraft designers are thinking about sustainability: “Ideally, this becomes the normal in the sense that this becomes a standard engineering practice that you just think of when you’re designing your spacecraft.”

Inside the quest to map the universe with mysterious bursts of radio energy

When our universe was less than half as old as it is today, a burst of energy that could cook a sun’s worth of popcorn shot out from somewhere amid a compact group of galaxies. Some 8 billion years later, radio waves from that burst reached Earth and were captured by a sophisticated low-frequency radio telescope in the Australian outback. 

The signal, which arrived on June 10, 2022, and lasted for under half a millisecond, is one of a growing class of mysterious radio signals called fast radio bursts. In the last 10 years, astronomers have picked up nearly 5,000 of them. This one was particularly special: nearly double the age of anything previously observed, and three and a half times more energetic. 

But like the others that came before, it was otherwise a mystery. No one knows what causes fast radio bursts. They flash in a seemingly random and unpredictable pattern from all over the sky. Some appear from within our galaxy, others from previously unexamined depths of the universe. Some repeat in cyclical patterns for days at a time and then vanish; others have been consistently repeating every few days since we first identified them. Most never repeat at all. 

Despite the mystery, these radio waves are starting to prove extraordinarily useful. By the time our telescopes detect them, they have passed through clouds of hot, rippling plasma, through gas so diffuse that particles barely touch each other, and through our own Milky Way. And every time they hit the free electrons floating in all that stuff, the waves shift a little bit. The ones that reach our telescopes carry with them a smeary fingerprint of all the ordinary matter they’ve encountered between wherever they came from and where we are now. 

This makes fast radio bursts, or FRBs, invaluable tools for scientific discovery—especially for astronomers interested in the very diffuse gas and dust floating between galaxies, which we know very little about. 

“We don’t know what they are, and we don’t know what causes them. But it doesn’t matter. This is the tool we would have constructed and developed if we had the chance to be playing God and create the universe,” says Stuart Ryder, an astronomer at Macquarie University in Sydney and the lead author of the Science paper that reported the record-breaking burst. 

Many astronomers now feel confident that finding more such distant FRBs will enable them to create the most detailed three-dimensional cosmological map ever made—what Ryder likens to a CT scan of the universe. Even just five years ago making such a map might have seemed an intractable technical challenge: spotting an FFB and then recording enough data to determine where it came from is extraordinarily difficult because most of that work must happen in the few milliseconds before the burst passes.

But that challenge is about to be obliterated. By the end of this decade, a new generation of radio telescopes and related technologies coming online in Australia, Canada, Chile, California, and elsewhere should transform the effort to find FRBs—and help unpack what they can tell us. What was once a series of serendipitous discoveries will become something that’s almost routine. Not only will astronomers be able to build out that new map of the universe, but they’ll have the chance to vastly improve our understanding of how galaxies are born and how they change over time. 

Where’s the matter?

In 1998, astronomers counted up the weight of all of the identified matter in the universe and got a puzzling result. 

We know that about 5% of the total weight of the universe is made up of baryons like protons and neutrons— the particles that make up atoms, or all the “stuff” in the universe. (The other 95% includes dark energy and dark matter.) But the astronomers managed to locate only about 2.5%, not 5%, of the universe’s total. “They counted the stars, black holes, white dwarfs, exotic objects, the atomic gas, the molecular gas in galaxies, the hot plasma, etc. They added it all up and wound up at least a factor of two short of what it should have been,” says Xavier Prochaska, an astrophysicist at the University of California, Santa Cruz, and an expert in analyzing the light in the early universe. “It’s embarrassing. We’re not actively observing half of the matter in the universe.” 

All those missing baryons were a serious problem for simulations of how galaxies form, how our universe is structured, and what happens as it continues to expand. 

Astronomers began to speculate that the missing matter exists in extremely diffuse clouds of what’s known as the warm–hot intergalactic medium, or WHIM. Theoretically, the WHIM would contain all that unobserved material. After the 1998 paper was published, Prochaska committed himself to finding it. 

But nearly 10 years of his life and about $50 million in taxpayer money later, the hunt was going very poorly.

That search had focused largely on picking apart the light from distant galactic nuclei and studying x-ray emissions from tendrils of gas connecting galaxies. The breakthrough came in 2007, when Prochaska was sitting on a couch in a meeting room at the University of California, Santa Cruz, reviewing new research papers with his colleagues. There, amid the stacks of research, sat the paper reporting the discovery of the first FRB.

Duncan Lorimer and David Narkevic, astronomers at West Virginia University, had discovered a recording of an energetic radio wave unlike anything previously observed. The wave lasted for less than five milliseconds, and its spectral lines were very smeared and distorted, unusual characteristics for a radio pulse that was also brighter and more energetic than other known transient phenomena. The researchers concluded that the wave could not have come from within our galaxy, meaning that it had traveled some unknown distance through the universe. 

Here was a signal that had traversed long distances of space, been shaped and affected by electrons along the way, and had enough energy to be clearly detectable despite all the stuff it had passed through. There are no other signals we can currently detect that commonly occur throughout the universe and have this exact set of traits.

“I saw that and I said, ‘Holy cow—that’s how we can solve the missing-baryons problem,’” Prochaska says. Astronomers had used a similar technique with the light from pulsars— spinning neutron stars that beam radiation from their poles—to count electrons in the Milky Way. But pulsars are too dim to illuminate more of the universe. FRBs were thousands of times brighter, offering a way to use that technique to study space well beyond our galaxy.

NASA/NCSA UNIVERSITY OF ILLINOIS VISUALIZATION BY FRANK SUMMERS, SPACE TELESCOPE SCIENCE INSTITUTE, SIMULATION BY MARTIN WHITE AND LARS HERNQUIST, HARVARD UNIVERSITY

There’s a catch, though: in order for an FRB to be an indicator of what lies in the seemingly empty space between galaxies, researchers have to know where it comes from. If you don’t know how far the FRB has traveled, you can’t make any definitive estimate of what space looks like between its origin point and Earth. 

Astronomers couldn’t even point to the direction that the first 2007 FRB came from, let alone calculate the distance it had traveled. It was detected by an enormous single-dish radio telescope at the Parkes Observatory (now called the Murriyang) in New South Wales, which is great at picking up incoming radio waves but can pinpoint FRBs only to an area of the sky as large as Earth’s full moon. For the next decade, telescopes continued to identify FRBs without providing a precise origin, making them a fascinating mystery but not practically useful.

Then, in 2015, one particular radio wave flashed—and then flashed again. Over the course of two months of observation from the Arecibo telescope in Puerto Rico, the radio waves came again and again, flashing 10 times. This was the first repeating burst of FRBs ever observed (a mystery in its own right), and now researchers had a chance to determine where the radio waves had begun, using the opportunity to home in on its location.

In 2017, that’s what happened. The researchers obtained an accurate position for the fast radio burst using the NRAO Very Large Array telescope in central New Mexico. Armed with that position, the researchers then used the Gemini optical telescope in Hawaii to take a picture of the location, revealing the galaxy where the FRB had begun and how far it had traveled. “That’s when it became clear that at least some of these we’d get the distance for. That’s when I got really involved and started writing telescope proposals,” Prochaska says. 

That same year, astronomers from across the globe gathered in Aspen, Colorado, to discuss the potential for studying FRBs. Researchers debated what caused them. Neutron stars? Magnetars, neutron stars with such powerful magnetic fields that they emit x-rays and gamma rays? Merging galaxies? Aliens? Did repeating FRBs and one-offs have different origins, or could there be some other explanation for why some bursts repeat and most do not? Did it even matter, since all the bursts could be used as probes regardless of what caused them? At that Aspen meeting, Prochaska met with a team of radio astronomers based in Australia, including Keith Bannister, a telescope expert involved in the early work to build a precursor facility for the Square Kilometer Array, an international collaboration to build the largest radio telescope arrays in the world. 

The construction of that precursor telescope, called ASKAP, was still underway during that meeting. But Bannister, a telescope expert at the Australian government’s scientific research agency, CSIRO, believed that it could be requisitioned and adapted to simultaneously locate and observe FRBs. 

Bannister and the other radio experts affiliated with ASKAP understood how to manipulate radio telescopes for the unique demands of FRB hunting; Prochaska was an expert in everything “not radio.” They agreed to work together to identify and locate one-off FRBs (because there are many more of these than there are repeating ones) and then use the data to address the problem of the missing baryons. 

And over the course of the next five years, that’s exactly what they did—with astonishing success.

Building a pipeline

To pinpoint a burst in the sky, you need a telescope with two things that have traditionally been at odds in radio astronomy: a very large field of view and high resolution. The large field of view gives you the greatest possible chance to detect a fleeting, unpredictable burst. High resolution  lets you determine where that burst actually sits in your field of view. 

ASKAP was the perfect candidate for the job. Located in the westernmost part of the Australian outback, where cattle and sheep graze on public land and people are few and far between, the telescope consists of 36 dishes, each with a large field of view. These dishes are separated by large distances, allowing observations to be combined through a technique called interferometry so that a small patch of the sky can be viewed with high precision.  

The dishes weren’t formally in use yet, but Bannister had an idea. He took them and jerry-rigged a “fly’s eye” telescope, pointing the dishes at different parts of the sky to maximize its ability to spot something that might flash anywhere. 

“Suddenly, it felt like we were living in paradise,” Bannister says. “There had only ever been three or four FRB detections at this point, and people weren’t entirely sure if [FRBs] were real or not, and we were finding them every two weeks.” 

When ASKAP’s interferometer went online in September 2018, the real work began. Bannister designed a piece of software that he likens to live-action replay of the FRB event. “This thing comes by and smacks into your telescope and disappears, and you’ve got a millisecond to get its phone number,” he says. To do so, the software detects the presence of an FRB within a hundredth of a second and then reaches upstream to create a recording of the telescope’s data before the system overwrites it. Data from all the dishes can be processed and combined to reconstruct a view of the sky and find a precise point of origin. 

The team can then send the coordinates on to optical telescopes, which can take detailed pictures of the spot to confirm the presence of a galaxy—the likely origin point of the FRB. 

CSIRO

Ryder’s team used data on the galaxy’s spectrum, gathered from the European Southern Observatory, to measure how much its light stretched as it traversed space to reach our telescopes. This “redshift” becomes a proxy for distance, allowing astronomers to estimate just how much space the FRB’s light has passed through. 

In 2018, the live-action replay worked for the first time, making Bannister, Ryder, Prochaska, and the rest of their research team the first to localize an FRB that was not repeating. By the following year, the team had localized about five of them. By 2020, they had published a paper in Nature declaring that the FRBs had let them count up the universe’s missing baryons. 

The centerpiece of the paper’s argument was something called the dispersion measure—a number that reflects how much an FRB’s light has been smeared by all the free electrons along our line of sight. In general, the farther an FRB travels, the higher the dispersion measure should be. Armed with both the travel distance (the redshift) and the dispersion measure for a number of FRBs, the researchers found they could extrapolate the total density of particles in the universe. J-P Macquart, the paper’s lead author, believed that the relationship between dispersion measure and FRB distance was predictable and could be applied to map the universe.

As a leader in the field and a key player in the advancement of FRB research, Macquart would have been interviewed for this piece. But he died of a heart attack one week after the paper was published, at the age of 45. FRB researchers began to call the relationship between dispersion and distance the “Macquart relation,” in honor of his memory and his push for the groundbreaking idea that FRBs could be used for cosmology. 

Proving that the Macquart relation would hold at greater distances became not just a scientific quest but also an emotional one. 

“I remember thinking that I know something about the universe that no one else knows.”

The researchers knew that the ASKAP telescope was capable of detecting bursts from very far away—they just needed to find one. Whenever the telescope detected an FRB, Ryder was tasked with helping to determine where it had originated. It took much longer than he would have liked. But one morning in July 2022, after many months of frustration, Ryder downloaded the newest data email from the European Southern Observatory and began to scroll through the spectrum data. Scrolling, scrolling, scrolling—and then there it was: light from 8 billion years ago, or a redshift of one, symbolized by two very close, bright lines on the computer screen, showing the optical emissions from oxygen. “I remember thinking that I know something about the universe that no one else knows,” he says. “I wanted to jump onto a Slack and tell everyone, but then I thought: No, just sit here and revel in this. It has taken a lot to get to this point.” 

With the October 2023 Science paper, the team had basically doubled the distance baseline for the Macquart relation, honoring Macquart’s memory in the best way they knew how. The distance jump was significant because Ryder and the others on his team wanted to confirm that their work would hold true even for FRBs whose light comes from so far away that it reflects a much younger universe. They also wanted to establish that it was possible to find FRBs at this redshift, because astronomers need to collect evidence about many more like this one in order to create the cosmological map that motivates so much FRB research.

“It’s encouraging that the Macquart relation does still seem to hold, and that we can still see fast radio bursts coming from those distances,” Ryder said. “We assume that there are many more out there.” 

Mapping the cosmic web

The missing stuff that lies between galaxies, which should contain the majority of the matter in the universe, is often called the cosmic web. The diffuse gases aren’t floating like random clouds; they’re strung together more like a spiderweb, a complex weaving of delicate filaments that stretches as the galaxies at their nodes grow and shift. This gas probably escaped from galaxies into the space beyond when the galaxies first formed, shoved outward by massive explosions.

“We don’t understand how gas is pushed in and out of galaxies. It’s fundamental for understanding how galaxies form and evolve,” says Kiyoshi Masui, the director of MIT’s Synoptic Radio Lab. “We only exist because stars exist, and yet this process of building up the building blocks of the universe is poorly understood … Our ability to model that is the gaping hole in our understanding of how the universe works.” 

Astronomers are also working to build large-scale maps of galaxies in order to precisely measure the expansion of the universe. But the cosmological modeling underway with FRBs should create a picture of invisible gasses between galaxies, one that currently does not exist. To build a three-dimensional map of this cosmic web, astronomers will need precise data on thousands of FRBs from regions near Earth and from very far away, like the FRB at redshift one. “Ultimately, fast radio bursts will give you a very detailed picture of how gas gets pushed around,” Masui says. “To get to the cosmological data, samples have to get bigger, but not a lot bigger.” 

That’s the task at hand for Masui, who leads a team searching for FRBs much closer to our galaxy than the ones found by the Australian-led collaboration. Masui’s team conducts FRB research with the CHIME telescope in British Columbia, a nontraditional radio telescope with a very wide field of view and focusing reflectors that look like half-pipes instead of dishes. CHIME (short for “Canadian Hydrogen Intensity Mapping Experiment)” has no moving parts and is less reliant on mirrors than a traditional telescope (focusing light in only one direction rather than two), instead using digital techniques to process its data. CHIME can use its digital technology to focus on many places at once, creating a 200-square-degree field of view compared with ASKAP’s 30-degree one. Masui likened it to a mirror that can be focused on thousands of different places simultaneously. 

Because of this enormous field of view, CHIME has been able to gather data on thousands of bursts that are closer to the Milky Way. While CHIME cannot yet precisely locate where they are coming from the way that ASKAP can (the telescope is much more compact, providing lower resolution), Masui is leading the effort to change that by building three smaller versions of the same telescope in British Columbia; Green Bank, West Virginia; and Northern California. The additional data provided by these telescopes, the first of which will probably be collected sometime this year, can be combined with data from the original CHIME telescope to produce location information that is about 1,000 times more precise. That should be detailed enough for cosmological mapping.

ANDRE RECNIK/CHIME

Telescope technology is improving so fast that the quest to gather enough FRB samples from different parts of the universe for a cosmological map could be finished within the next 10 years. In addition to CHIME, the BURSTT radio telescope in Taiwan should go online this year; the CHORD telescope in Canada, designed to surpass CHIME, should begin operations in 2025; and the Deep Synoptic Array in California could transform the field of radio astronomy when it’s finished, which is expected to happen sometime around the end of the decade. 

And at ASKAP, Bannister is building a new tool that will quintuple the sensitivity of the telescope, beginning this year. If you can imagine stuffing a million people simultaneously watching uncompressed YouTube videos into a box the size of a fridge, that’s probably the easiest way to visualize the data handling capabilities of this new processor, called a field-programmable gate array, which Bannister is almost finished programming. He expects the new device to allow the team to detect one new FRB each day.

With all the telescopes in competition, Bannister says, “in five or 10 years’ time, there will be 1,000 new FRBs detected before you can write a paper about the one you just found … We’re in a race to make them boring.” 

Prochaska is so confident FRBs will finally give us the cosmological map he’s been working toward his entire life that he’s started studying for a degree in oceanography. Once astronomers have measured distances for 1,000 of the bursts, he plans to give up the work entirely. 

“In a decade, we could have a pretty decent cosmological map that’s very precise,” he says. “That’s what the 1,000 FRBs are for—and I should be fired if we don’t.”

Unlike most scientists, Prochaska can define the end goal. He knows that all those FRBs should allow astronomers to paint a map of the invisible gases in the universe, creating a picture of how galaxies evolve as gases move outward and then fall back in. FRBs will grant us an understanding of the shape of the universe that we don’t have today—even if the mystery of what makes them endures. 

Anna Kramer is a science and climate journalist based in Washington, D.C.

How to safely watch and photograph the total solar eclipse

On April 8, the moon will pass directly between Earth and the sun, creating a total solar eclipse across much of the United States, Mexico, and Canada. 

Although total solar eclipses occur somewhere in the world every 18 months or so, this one is unusual because tens of millions of people in North America will likely witness it, from Mazatlán in Mexico to Newfoundland in Canada.

“It’s a huge communal experience,” says Meg Thacher, a senior lab instructor in the astronomy department at Smith College in Massachusetts. “A total solar eclipse is the Super Bowl of astronomy.” Here’s how to safely watch—and photograph—the natural phenomenon.

Fail to prepare, prepare to fail

It pays to have a plan of action for the day. 

Before you decide on a spot to watch the eclipse, whether it’s in your own backyard, in a national park, or at a viewing party, it’s worth checking the weather forecast to see how likely clouds are to spoil the show. Currently the majority of the eclipse’s path of totality—areas where onlookers will see a full eclipse, as opposed to a partial one—is forecast to have some degree of cloud cover.

However, even if visibility turns out to be poor, you still have options. NASA and the National Science Foundation are broadcasting livestreams, and many eclipse viewing parties will broadcast unobstructed views as part of their festivities. The American Astronomical Society has a state-by-state list to help you find your nearest event.

Safety first

You need proper eye protection to look at the eclipse, because the sun’s light can cause long-term damage to your vision. Be sure to purchase either specially made eclipse glasses or handheld solar viewers. Glasses might be the best option if you plan to take photos, as they’ll keep your hands free. Eclipse glasses are thousands of times darker than regular sunglasses and contain a polymer designed to filter out harmful light. 

You should also make sure any cameras, binoculars, or telescopes through which you plan to look at the sun have been fitted with a solar filter. You don’t need to double up and wear eclipse glasses if you already have a solar filter, though.

Once the moon fully obscures the sun, it’s safe to remove your eye protection for the duration of the totality, which is projected to last around four minutes during this eclipse.

A proper camera is your best bet …

Photographing an eclipse is pretty simple, says Randall Benton, a professional photographer who has been capturing them since 1979. Although cameras have changed vastly since then, the fundamentals remain the same. (If you plan to use your phone to take photos, skip to the next section.)

He recommends fixing a DSLR or mirrorless camera (equipped with a solar filter to protect both your eyes and the camera itself) to a tripod. A short exposure, which is designed to capture movement, is more likely to capture the details of the sun’s corona—the plasma surrounding it. A longer exposure, which keeps certain elements of pictures in focus while blurring others, is likely to stretch the corona further out. The exposure you choose will depend on the kind of shot you’d like to capture.

Before the eclipse begins, take the time to focus the camera exactly where you want the sun and moon to appear in your shot, and turn off any autofocus function. While some mounts come with an automated tracking feature that will follow the eclipse’s progression, others will require you to move your camera yourself, so make sure you’re familiar with the mount you’ve got to prevent the eclipse from drifting out of your frame.

Then, “when there’s just a sliver of sun left and it’s a few seconds away from disappearing, take the filter off the camera lens,” Benton says. “At the very last moment, there’s a phenomenon called the diamond ring effect, when the last speck of visible sunlight resembles a ring—that’s a great dramatic photo. Once the sunlight reappears, it’s time to put the filter back on.” 

… but smartphones work too

Despite the rapid advances in smartphone cameras over the past decade or so, they can’t really rival DSLR or mirrorless cameras when it comes to capturing an eclipse. 

Their short lenses means the sun will appear very small, which doesn’t tend to produce great photographs. That said, you can still capture the best photo possible by cutting out the plastic lens from a pair of spare eclipse glasses, taping it over your phone’s camera lens (or lenses), and securing the device in a tripod (or propping it up against a cup).

Don’t try to hold the phone, and use your phone’s shutter delay to decrease vibrations, says Gordon Telepun, an amateur enthusiast who has been photographing eclipses since 2001 and has advised NASA on how to capture them. “During totality, take the [eclipse glasses] filter off and take wide-angle shots of the corona in the sky and the landscape,” he says. “Automatic mode will work fine.”

Something smartphones are great at capturing is video of the moment the moon glides over the sun, says Benton: “That transition from daylight to nighttime is dramatic, and smartphones can handle that pretty well.”

Don’t be afraid to get creative

During the eclipse, there are plenty of other things to photograph besides the sun and moon. Foliage will create a natural version of a pinhole viewer, casting thousands of crescent images of the sun dancing around in the shade as the light streams through the trees. 

Another natural phenomena is shadow bands—flickering gray ripples that appear on light-colored surfaces like sheets or the sides of houses within a few minutes of totality. “It’s almost like a stroboscopic effect,” Benton says, referring to the visual effect that makes objects appear as though they are moving more slowly than they actually are. “Videos of that could be interesting.” 

“Take pictures of the faces of the people around you, too,” he adds. “Twenty years from now, your photo of the eclipse is going to be pretty much the same as anyone else’s. These other pictures are going to be a little more powerful in reminding you what your day was like.”

Take a moment to look around

Finally, when you’re looking up at the moon covering the sun during totality, let yourself enjoy the moment free from your technology. The next eclipse the US can expect to experience on this scale is in August 2044—so try hard to stay present.

“During totality, if you’re really concentrating on getting your photo, at some point let go of everything. Turn around—take a look with your eyes,” says Benton. “Whatever you’re seeing in the viewfinder or on the screen, it isn’t the same thing as seeing it with your own eyes. And it will change your life.”

The search for extraterrestrial life is targeting Jupiter’s icy moon Europa

We’ve known of Europa’s existence for more than four centuries, but for most of that time, Jupiter’s fourth-largest moon was just a pinprick of light in our telescopes—a bright and curious companion to the solar system’s resident giant. Over the last few decades, however, as astronomers have scrutinized it through telescopes and six spacecraft have flown nearby, a new picture has come into focus. Europa is nothing like our moon. 

Observations suggest that its heart is a ball of metal and rock, surrounded by a vast saltwater ocean that contains more than twice as much water as is found on Earth. That massive sea is encased in a smooth but fractured blanket of cracked ice, one that seems to occasionally break open and spew watery plumes into the moon’s thin atmosphere. 

For these reasons, Europa has captivated planetary scientists interested in the geophysics of alien worlds. All that water and energy—and hints of elements essential for building organic molecules —point to another extraordinary possibility. In the depths of its ocean, or perhaps crowded in subsurface lakes or below icy surface vents, Jupiter’s big, bright moon could host life. 

“We think there’s an ocean there, everywhere,” says Bob Pappalardo, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Essentially everywhere on Earth that there’s water, there’s life. Could there be life on Europa?” 

Pappalardo has been at the forefront of efforts to send a craft to Europa for more than two decades. Now his hope is finally coming to fruition: later this year, NASA plans to launch Europa Clipper, the largest-­ever craft designed to visit another planet. The $5 billion mission, scheduled to reach Jupiter in 2030, will spend four years analyzing this moon to determine whether it could support life. It will be joined after two years by the European Space Agency’s Juice, which launched last year and is similarly designed to look for habitable conditions, not only on Europa but also on other mysterious Jovian moons. 

Neither mission will beam back a definitive answer to the question of extraterrestrial life. “Unless we get really lucky, we’re not going to be able to tell if there is life there, but we can find out if all the conditions are right for life,” says planetary geologist Louise Prockter at the Johns Hopkins Applied Physics Laboratory, a co-­investigator on the Clipper camera team. 

“Essentially everywhere on Earth that there’s water, there’s life. Could there be life on Europa?”

Bob Pappalardo, planetary scientist, NASA’s Jet Propulsion Laboratory

What these spacecraft will do is get us closer than ever before to answers, by identifying the telltale chemical, physical, and geological signatures of habitability—whether a place is a suitable environment for life to emerge and thrive.

The payoff for confirming these signs on Europa would be huge. Not because humans could settle on its surface—it’s far too harsh and rugged and cold and irradiated for our delicate bodies—but because it could justify future exploration to land there and look for alien life-forms. Finding something, anything, living on Europa would offer strong evidence for an alternate path through which life could emerge. It would mean that life on Earth is not exceptional. We’d know that we have neighbors close by—even if they’re microbial, which would be the most likely life-form—and that would make it very likely that we have neighbors elsewhere in the cosmos.

NASA/JPL-CALTECH

“With the prospects of life—the prospects of vast oceans—within reach, you just have to go,” says Nicholas Makris, director of MIT’s Center for Ocean Engineering, who uses acoustics and other innovative methods to observe and explore big bodies of water. He once led a team of scientists who proposed a mission to land a spacecraft on Europa and use sound waves to explore what lies beneath the ice; he still hopes to see a lander go there one day. “You have to find out. Everyone wants to know,” he says. “There isn’t anyone who doesn’t want to know.” 

From a spot in the sky to a dynamic moon

Long before it became the cosmic destination of the year, Europa played an outsize role in transforming our understanding of the solar system. That began with its discovery, when one night in January 1610, the Italian astronomer Galileo Galilei fixed his occiale—an ingenious homemade telescope—on Jupiter and noted three bright little dots near the side of the gas giant. 

Galileo assumed it was an illusion, that they were distant stars that only appeared to be close. But the next night, he observed those same three bright little stars now on the other side of the planet. Further observations revealed yet another bright light, also wandering nearby but refusing to leave Jupiter’s side. In a short treatise called Sidereus Nuncius (Starry Messenger), published in March 1610, Galileo reported that he’d found four worlds orbiting Jupiter, similar to how Mercury and Venus orbit the sun. (Astronomers still regard Jupiter and its satellites as a kind of mini solar system.) Galileo named the worlds I, II, III, etc., and referred to them as the “Medicean planets,” though they’re now called the “Galilean moons.” His discovery was the first time scientists had directly observed small worlds orbiting something other than Earth or the sun, giving strong evidence to the argument, still controversial at the time, that planets circled the sun and not the other way around. 

NASA/JPL/UNIVERSITY OF ARIZONA (IO); NASA/JPL-CALTECH/KEVIN M. GILL (CALLISTO); NASA/JPL-CALTECH/SWRI/MSSS/KEVIN M. GILL (GANYMEDE¡)

Naming rights for these four Jovian moons ultimately went to the German astronomer Simon Marius, who claimed (but couldn’t prove) that he’d actually discovered them a few weeks before Galileo. In 1614, on a suggestion from Johannes Kepler, Marius proposed naming the moons Io, Callisto, Europa, and Ganymede—after four “irregular loves” pursued by Zeus (Jupiter) in ancient mythology. It took 200 years for those names to gain widespread adoption, but they were definitely an upgrade. Had Galileo’s naming scheme stuck, you’d now be reading about the “II Clipper,” which doesn’t have the same ring.

These moons were only the first to be discovered orbiting Jupiter. As of December 2023, astronomers had officially confirmed the existence of 91 others—and there are likely many more. Where the first four are round and follow stately, simple orbits, the more recent discoveries are more diverse. Some orbit in erratic swarms or go the opposite way around; some were asteroids captured in passing; others resulted from collisions. There are so many objects around Jupiter, in fact, that the International Astronomical Union no longer confers names on Jovian satellites unless they’re deemed to have significant scientific value.

It turned out that Ridgway, who describes himself as a tinkerer as well as a physicist, was well positioned to answer those questions. Using an old mathematical trick, he had devised an innovative instrument that could capture the spectrum of a distant light source, and he was using it during nighttime observations at a telescope at Kitt Peak Observatory, in Arizona. Every element and molecule absorbs and emits a unique collection of wavelengths of energy, and astronomers can read these spectra as fingerprints that reveal the composition of cosmic bodies. Pilcher suggested that he use the instrument to observe Europa.

They thought it would take a week to get a useful spectrum of one of Jupiter’s moons. “I went and got it in one night, maybe two,” Ridgway recalls. Ridgway showed the data to Pilcher, who showed it to his advisor, Tom McCord. Their analyses, published in Science in December 1972, suggested that water ice covered at least half, and possibly all, of the surface of Europa. (They also confirmed that the Jovian moons Ganymede and Callisto, both of which are larger than Europa, also had ice on their surfaces.) 

In a 1980 paper, scientists reported that Europa looked “cracked like a broken eggshell” and compared it to a white pool ball fouled by a felt-tip pen.

One year later, the Pioneer 10 spacecraft, which had launched in March 1972, passed close enough to Europa to take a photo. The grainy image was provocative enough to justify sending Pioneer 11—which launched in 1973—to swing by on its way to Saturn and then out of the solar system. 

NASA/JPL/UNIVERSITY OF ARIZONA/UNIVERSITY OF IDAHO (TITAN); NASA/JPL£CALTECH/SPACE SCIENCE INSTITUTE (ENCELADUS)

But Europa really started to come into focus in 1979, after the Voyager 2 spacecraft sped past the moon on July 9. (Voyager 1 also passed near Europa, but Voyager 2 had better photos.) The photographs the spacecraft beamed back revealed a smooth, bright surface, crisscrossed by long linear marks and low ridges; they might have been cracks or cliffs. In a 1980 NASA paper describing the observation, scientists reported that Europa looked “cracked like a broken eggshell” and compared it to a white pool ball fouled by a felt-tip pen. A 1983 Nature paper fueled interest in Europa by proposing that those features were consistent with liquid water and regular resurfacing, like the work of a natural Zamboni machine.   

Among other things, Galileo’s magnetometer revealed a wildly varying magnetic field. Ice is a poor conductor, but liquid salt water isn’t, and Europa’s magnetic oscillations pointed to something moving beneath the surface. Its readings fit the idea of a global ocean being pushed, pulled, and heated by the tidal forces of Jupiter and its moon companions. They also lined up with earlier theoretical predictions of liquid water near the surface of icy moons. “We are pretty certain there’s an ocean there,” Prockter says, “but there is a chance that it might be something really exotic we don’t understand.” The only way to know for sure, she says, is to go back. 

Other images from Galileo confirmed what telescope observations had long suggested: that Europa sports a youthful appearance despite its advanced age. It likely formed at the same time as Jupiter and the rest of the solar system, about 4.5 billion years ago, yet its surface—as dated by the oldest craters—is less than 100 million years old. “That’s a long time for us mere mortals,” says Prockter, “but in geological terms, it was born yesterday. The surface is very, very young.” The cracks and crevices on Europa suggest that giant ice plates on its surface collide, break apart, shove under and over each other, and refreeze. 

The longer scientists stared at Europa, the more mysteries emerged—like the questions around those ubiquitous dark ridges, often in pairs, that splatter the surface like a Jackson Pollock painting. Theorists have been busy devising explanations. Perhaps they’re made by ice volcanoes or geysers, or cracks where liquid water from subsurface pools rose, froze, and crumbled as the opening closed again. Maybe they resulted from subduction, which occurs on Earth in plate tectonics, as one giant sheet of ice slid and crumpled under another. “I’ve lost count of the number of different models for forming those landforms, but we really don’t know how they form,” Prockter says. “Part of the reason is that geology is based on Earth geology, but it’s not like Earth.” 

One particularly striking image of Europa, captured in September 2022 by a camera on the Juno spacecraft, which is currently exploring Jupiter, reveals many of the features that are driving scientists to want to take a closer look. It shows the side of Europa that always faces Jupiter, bathed in sunlight. The moon’s surface is covered with cracks, streaks, and ridges where water may rise from the ocean beneath, or where irradiated surface material may sink lower. It also shows the “chaos terrains”—remarkably messy areas suggesting that giant pieces of ice have broken off, moved around, and refrozen, bolstering the case for geological activity similar to plate tectonics on Earth. 

However, Juno’s brief two-hour flyby failed to answer questions about how those features formed or to confirm the existence of a buried ocean. For planetary scientists and astrophysicists, Clipper’s data can help fill in the missing knowledge. It will also push our relationship with Europa into new, unexplored territory. 

What all those previous missions did do was help build enthusiasm for the plan to get to Europa, a plan that has evolved dramatically over the last 20 years. Originally, scientists wanted orbiters and landers, and NASA and ESA were working together on a joint mission with multiple spacecraft. Those plans fizzled, but in 2013—as a result of the 2011 Decadal Survey, a report that sets the priorities for space exploration for the next 10 years—NASA approved a plan to send an orbiter. By 2015, the agency had selected the instruments on board. Independently, the ESA moved forward with its own mission, with a broader goal of studying Jupiter’s icy moons. 

“The Voyager mission transformed Europa from a light in the sky to a geologic world, and then the Galileo mission did the transformation to an ocean world,” says Diana Blaney, a JPL geophysicist who leads the Clipper team charged with using a mapping image spectrometer to identify molecules on Europa’s surface. “Hopefully, Clipper will bring the transformation to a habitable world.” 

Getting in close

Researchers have long searched for signs of habitability in the solar system. Landers and rovers on Mars have found evidence of liquid water, mostly long gone, and organic molecules, which contain carbon, often in chains or rings. The building blocks of biological organisms—including nucleic acids and proteins—all contain carbon, which is why scientists get excited when they find organic molecules. Their presence could indicate that it’s possible for the precursors of life to form. 

But it’s not enough just to have promising pieces in place. Any alien species would also have to find a way to grow and survive. That far from the sun, photosynthesis is likely impossible. Organisms would necessarily be fueled by chemical energy, much as microbial extremophiles near the black smokers and hydrothermal vents on the seafloor live off the minerals and methane. 

The possibility for Europan life is at the mercy of the moon’s geophysics, says Lynnae Quick, a planetary geophysicist at NASA’s Goddard Space Flight Center. In fact, she argues that you can’t have one without the other. Europa seems to host the necessary ingredients for life. But ingredients alone, on Europa as in the kitchen, won’t spontaneously combine in the right way on their own. Other forces have to intervene: the moon needs to shift and squeeze, with heat, to mix the minerals from the seafloor with the salt water and any irradiated particles that seep down from the icy surface. “We need something to stir the pot, and I think the geophysical processes do that,” says Quick, whose graduate work on cryovolcanism in alien worlds led to her recruitment to join Clipper. She’s particularly excited about the possibility of finding pockets of warm salty water, trapped just beneath the surface, that could be abodes for life. 

 “Europa is my favorite body in the solar system,” Quick confesses. But she notes that other ocean worlds also offer promising places to look for signs of life. Those include Enceladus, a small moon of Saturn that, like Europa, has an icy crust with an ocean beneath. Images from the Cassini mission in 2005 revealed that geysers on the south pole of Enceladus spew water and organic molecules into space, feeding Saturn’s outermost ring. 

However, Europa is bigger than Enceladus and is more likely to have a surface covered in icy plates that move in a way similar to Earth’s plate tectonics. This sort of activity would help combine the ingredients for life. Ganymede, another Jovian moon and the solar system’s largest, also likely has a liquid ocean, but sandwiched between two ice layers; without an interface between water and minerals, life is less probable. Other possible places to look include Titan, Saturn’s biggest moon, which also probably hides a liquid-water ocean beneath an ice crust. (Quick is an investigator on Dragonfly, a mission to explore Titan, scheduled to launch in 2028.) 

To look for the signs and signals of habitability, Clipper will use nine primary instruments. These will take pictures of the surface, look for water plumes, use ground-penetrating radar to measure the icy shell and search for the ocean below, and take precise measurements of the magnetic field. 

The spacecraft will pass close enough to the moon to sample its thin atmosphere, and it will use mass spectrometry to identify molecules in the gases it finds there. Another instrument will enable scientists to analyze dust from the surface that has been kicked into the atmosphere by meteorite collisions. With any luck, they’ll be able to tell if that dust originated from below—from the enclosed ocean or subsurface lakes trapped in the ice—or from above, as fragments that migrated from the violent volcanoes on the nearby moon Io. Either scenario would be interesting to planetary geologists, but if the molecules were organic and came from below, they would help build the case that life could exist there.

ESA’s Juice mission has a similar suite of instruments, and scientists from the two teams meet regularly to plan for ways to jointly exploit the data when it starts coming in—five or six years from now. “This is really very good for scientists in the planetary community,” says Lorenzo Bruzzone, a telecommunications engineer at the University of Trento who leads the Juice mission’s radar tool team. He’s long been involved in efforts to get to Europa and the rest of the Jovian system. 

Because Juice will visit the other ocean-bearing Galilean moons, Bruzzone says, data from that mission can be combined with Clipper’s to generate a more comprehensive picture of the geological processes and potential habitability of all the ocean worlds. “We can analyze the differences in subsurface geology to better understand the evolution of the Jupiter system,” he says. Those differences may help explain, for example, why three of the Galilean moons formed as icy worlds while the fourth, Io, became a volcanic hellscape. 

Jupiter’s radiation has the potential to interfere with every measurement, turning a meaningful signal into a mess of digital snow, like static on a television screen.

To make sure those instruments work when they get there, engineers and designers for both missions have had to contend with a raft of challenges. Many of them revolve around energy: Europa receives only a fifth as much sunlight as Earth. Clipper addresses the problem with gargantuan solar panels, which will span 30 meters when fully extended. (An earlier proposal for a mission to Europa included nuclear batteries, but that idea was expensive, and it was ultimately scrapped.) 

In addition, Jupiter’s magnetic field is more than 10,000 times more powerful than Earth’s, accelerating already-­energetic particles around the planet to create an intense radiation environment. The radiation has the potential to interfere with every measurement—turning a meaningful signal into a mess of digital snow, like static on a television screen—and can threaten the integrity of the instruments. 

To slow the accumulation of radiation damage, Clipper won’t orbit Europa when it reaches the moon in 2030; instead, it will make about 50 flybys over four years, swooping nearer and farther from the destructive radiation field. At its closest, it will pass just 16 miles above the surface. The name points back to fast 19th-century sailing vessels, but it also describes the journey. The craft will sail past the world, over and over. In between passes, its distance from Jupiter will give it openings to transmit data back to Earth. 

Those first transmissions will have been generations—if not centuries—in the making. Some of the people who laid the groundwork for the mission, decades ago, have already died. Makris, at MIT, says that when scientists were first discussing how to get to Europa, he was told by Ron Greeley, a planetary geologist and NASA advisor who proposed and fiercely advocated for missions to the moon, that space travel spans generations: “He likened it to building a cathedral.” Prockter notes that by the time Clipper’s data comes in, she’ll be in her late 60s. “I will have spent my entire career on Clipper,” Prockter says. Quick, at 39, is one of the youngest members of the science team. 

ESA/ATG MEDIALAB

Many of the scientists involved in Clipper—including Pappalardo, Prockter, and Quick—are already planning ways to use its insights for future missions to other worlds. But it’s Europa that holds the most promise, at least for the moment. 

Pappalardo thrills at the prospect of finding a Europan neighborhood that might be just right for life. “What if we find a place that’s kind of an oasis, where there are hot spots or warm spots that we detect with a thermal imager?” he says. 

Ultimately, Pappalardo says, his hope is that Clipper finds enough evidence to make a strong case for sending a lander someday. The mission’s observations could also tell scientists where to land it: “That would be a place where we’d say, well, we really need to go and scoop up some of that stuff from below the surface, look at it with a microscope, put it in a mass spectrometer, and do the next step, which is to search for life.” 

Stephen Ornes is a science writer based in Nashville, Tennessee.