Johns Hopkins Magazine - November 1994 Issue

Comet Watch: An Inside Story

By Elise Hancock


As Comet Shoemaker-Levy 9 plunged toward its collision with Jupiter, a national team of astronomers gathered at the Space Telescope Science Institute (STScI), which is headquartered on Hopkins's Homewood campus. This collision could be the show of shows--five and a half days of heavenly fireworks, beginning on July 16, 1994--and STScI's Hubble Space Telescope, in orbit far above Earth's blurring atmosphere, would have by far the best view.

If there was going to be a show, that is. California astronomer Carolyn Shoemaker (teamed with David Levy and her husband Eugene) first saw the comet on March 16, 1993, and astronomers were wildly excited after learning the collision was due. One could wait a thousand lifetimes and not see such an event. But after the Hubble had taken a closer look at the comet, they began to fear, in the words of Hopkins astronomer Paul Feldman, that the big hit would fizzle into a "meteor shower on the far side of Jupiter."

Jupiter is a gassy planet, big enough to swallow a comet without so much as a burp. And Shoemaker-Levy, as predicted by Murphy's law of research ("If you can only observe one, it won't be typical"), turned out to be a strange sort of comet. First of all--and this was very odd indeed--even as it dove deeper in the warmth of the solar system, the object did not outgas any water. That was a mystery, if it really was a comet, because comets are icy, and ice in sun evaporates. Yet it did not seem to be an asteroid, a solid rocklike object, because it was breaking into pieces. The structure was so weak "you could have torn it apart with your bare hands," says Hal Weaver (PhD '82), the STScI astronomer leading the Hubble team that observed the comet.

At one time, the comet may have been as big as Comet Halley, which is a solid, potato-shaped object about 10 miles long and five miles across. But Shoemaker-Levy, by the time it was nearing collision, comprised 21 much smaller chunks. These pieces-- quickly dubbed "a string of pearls"--bobbed through the heavens in a row that got longer and more ragtag all the time, each bright golden glow trailing its own haze of reddish dust.

Were any of the fragments big enough to make an observable impact? No one was sure. Furthermore, the hits would occur just out of sight of Earth, on the dark side of Jupiter. By the time the hit-sites rotated into view, after some 10 to 30 minutes, there might be nothing left to see.

Publicly, then, and even among themselves, astronomers tended to downplay the event. As a group, says Weaver, "we tended to emphasize that jeez, we might not see anything, don't get your hopes up." Quite apart from their scientific doubts, researchers feared "Comet Kohoutek syndrome," the 1974 storm of disappointment after the much-vaunted Comet Kohoutek devolved into Cosmic Dud, a distant smudge. "We didn't want newspaper stories about how here we give them a $2 billion telescope and they can't even see a comet hit a planet," says Melissa McGrath, a former research scientist at Hopkins, now at STScI, who headed one of the Shoemaker-Levy research teams.

So on the one hand, the astronomical community got ready. Observatories all over the world would be trained on Jupiter. In the United States--after considerable discussion (not to say argument)--it was decided to free up precious time on the newly repaired Hubble. NASA asked for research proposals, and by January 1994, six research teams (all led by people under age 40) were busy on the hard, grinding work of preparation: thinking through every possible turn of events, then assembling materials, developing computer programs, and so on to meet each one.

At the same time, however, the astronomers' behavior shows traces of elementary subconscious magic: It will happen if I act as if it won't. Warren Moos, who chairs Physics and Astronomy at Hopkins, stuck with plans to be in Germany on the day of the first hit, Saturday, July 16. When Caltech astronomer Andrew Ingersoll bought his tickets to Baltimore, he got return tickets for Tuesday. ("Refundable tickets," he emphasized later. "I wasn't that stupid.") Melissa McGrath, who had bought two bottles of celebratory champagne "just in case," served one before the event, to make sure it wouldn't be wasted.

Fragment A, the first of the series (and rather a dim one), was calculated to hit Jupiter on July 16, at approximately 4 p.m. Eastern Daylight Time, 465 million miles from Earth. After processing, Hubble's first pictures would be available at 8 p.m.

July 16 was a day of keeping busy at STScI, until 5 p.m., when the "science team," meaning all the astronomers on the various scientific projects, had one last preparatory meeting. Then most people just hung around the offices. With two hours to go, the lobby was swarming with reporters from all over the world. "Every time we went anywhere," says McGrath, "someone would stick a microphone in my face and say, 'How do you feel? ' "

Astronomers in South Africa saw nothing, according to National Public Radio. An observatory in the Canary

Islands reported likewise. But according to e-mail on McGrath's computer, the Calar Alto, an infrared observatory in Spain, had reported a fireball. McGrath ran down the hall to tell a colleague. "'Hey, people see it!' I said. He didn't believe me." (McGrath always stays right in the middle of the action. Quietly intense, with speaking eyes, she often makes a suggestion that others follow, then forget where the idea came from.)

Antsy, some ventured out to eat. ("How do you feel?" asked the reporters. "We feel hungry. We're going out to dinner.") Weaver and a friend grabbed a quick bite at Roy Rogers, then hurried back to STScI.

Soon some 25 scientists huddled in concentric rings around a bank of computer monitors in the basement room where real-time data come in from the Hubble. At 8 p.m., the first image came.

Nothing--just Jupiter.

Three minutes later came the second: again, just Jupiter--or, wait a minute. Heidi Hammel, a principal research scientist from MIT who was crouched right near the screen, pointed out a tiny bright speck, floating near the edge of Jupiter. It was in the correct general area, but_ but floating. It didn't seem to belong to Jupiter. It might be a Galilean satellite, someone suggested.

McGrath and Weaver rushed to the astronomical almanac, and had just determined that, no, there shouldn't be any Galilean satellites there, when the third image came.

People gasped and leaned forward. This time, the tiny bright speck had become, in McGrath's description, "a bigger, bright spot." The speck was spreading out, as you would expect if a mountain-sized object plunged into a planetary atmosphere at some 130,000 miles an hour, exploding. Could it be? Hammel gazed, entranced, her open hand splayed on her chest as if to slow her heart. "Look at THAT!" said someone through the babble.

The fourth image showed an obvious plume of flame, and the room exploded in shouts, cheers, hooting and hollering. Bodies erupted from the floor, from the chairs, hugging and jumping. "We just saw this LITTLE BALL!"says Weaver, who is an ebullient, smiling sort of person. Even remembering, his voice rises near a shout.

He thought, "My God! We really have something here! It's amazing. And jeez, here's only the first one, it isn't particularly big, so we have a real fireworks display coming up. This whole week is going to be fantastic!"

McGrath ran upstairs and came back with champagne.

The Hubble wouldn't send another set of pictures for an hour, but for many in the room, the uproar lasted all of five minutes. It was time to go to work--urgently time.

Says John Clarke (PhD '80), a research professor at the University of Michigan, "We went back to our workstations, got our data, tried to figure out what we could in an hour. I made a lot of typos."

No one else had seen the pictures yet, but the world soon would. That part of the press conference would begin at 10 p.m., and coherent explanations would be needed. Upstairs, the Shoemakers, David Levy, and others were backgrounding reporters about the comet, about Jupiter, about the various instruments that would observe their collision. Meanwhile, three vans pointed large white satellite dishes at the sky. They were waiting to send out images of what one paper had billed as "the celestial event of the millennium."

Clarke is a specialist in the atmospheres of non-Earth planets and was principal investigator for the Hubble team working on UV imaging of Jupiter's clouds and aurorae after the impacts. Since no UV data would come in until Sunday, he pitched in now to help Hammel's group figure out how deep the explosion had plunged into Jupiter's atmosphere.

What we see with a backyard telescope is not so much Jupiter as the clouds of ammonia and brightly-colored smog that surround it. Just underneath the ammonia comes another layer of cloud, which theoretically ought to be ammonium hydrosulfide (ammonia mixed with hydrogen and sulfur), often called the sulfur clouds. The sulfur clouds, in turn, are thought to be underlain with a third cloud layer, water-ice. Jupiter has no abrupt shift from gas to solid, atmosphere to planet, as Earth does; rather, it is a massive ball of gas, with a volume 1,300 times bigger than Earth's. (One thousand Earths could fit inside, says Keith Noll, STScI astronomer and Hubble team leader for spectroscopic studies of Jupiter.) The planet is roughly 89 percent hydrogen and 11 percent helium--the lightest elements in the universe. However, explains Noll, at the innards of the planet, atoms are packed so tightly that hydrogen behaves like a metal, conducting stored heat outward and generating a magnetic field of extraordinary power. At the cloud layer, this magnetosphere is 25 times stronger than Earth's, and it reaches out 2.5 million miles.

So: How high in Jupiter's layers of gas did Fragment A's impact occur? In the months before July 16, Clarke had developed computer programs to help him answer such questions. Now, hastily, he pulled up the latest image on his screen and set to work. First he had to find the edge of Jupiter's shadow, then overlay a grid and place the plumes with respect to the planet. "It was clear even before we went back down and swapped data," he says, "that this plume was 600 miles or more above the edge of Jupiter. The explosion had gone right back through the top of the atmosphere and out into space."

There, that was one tidbit of fact to put in the pot. At 10 p.m., a group including Heidi Hammel went into the press conference, smooth, informative, and convincing. Indeed, Hammel became the press's darling.

She had won their hearts earlier, though, in the first moments after the images came in. A thin woman with large glasses and a who-cares frizzle of hair, Hammel radiates competence and an endearing enthusiasm about her work. She could be an MIT professor from Central Casting. Not only that, when McGrath broke out her champagne, it was Hammel who took the first swig, right from the upended bottle--a great picture. When McGrath suggested that Levy and the Shoemakers (having discovered the comet) shouldn't have to wait till 10 p.m., Hammel led the pack that ran upstairs and burst into the press conference, bringing the bottle and the first images to them. Again, it was a great moment and a great picture.

Across the street, deflected to the Bloomberg Center for Physics and Astronomy by polite but immovable guards, astronomy buffs watched much of the action on "NASA Select," the space agency's internal TV program. The Maryland Space Grant Consortium had telescopes set up on the roof for public use, as well as NASA Select running in a student lounge. "We were expecting about 15 people," said the hapless graduate student in charge, before he rushed off to make more copies of the hand-out. At least 100 had come, some of them bringing children. People stood in patient lines to gaze at Jupiter, then scrunched into the lounge to see NASA Select. With people standing behind sofas, sitting on tables, huddled on the floor, the room grew steamy, but no one cared. It was worth it, even at one remove, to see all that could be seen: that tiny plume and the scientists explaining it--words restrained, faces glowing. To see Hammel swigging, the Shoemakers' jovial laugh as the champagne approaches.

After all the doubts and fears, this was the show of shows.

Between July 16, 4 p.m. and July 22, 3:55 a.m., chunk after chunk of Shoemaker-Levy hit Jupiter at an angle of 45 to 48 degrees, going 130,000 miles an hour. Each chunk would tunnel inward, heating the atmosphere with the speed of its travel, until the temperature reached 20,000-30,000 degrees and the material exploded, atomizing everything nearby. An inferno of burning gases then jetted back out through the tunnel, cooling within a minute or so to only 2,000 or 3,000 degrees. The fireball towered and mushroomed.

About 12 minutes after each explosion, the smoke would begin to fall back toward the planet, dropping a trail of debris. After 20 minutes, the smoke settled into an asymmetric pancake resting on top of the clouds. Then, as the days passed, the smoke spread so that dark blotches bigger than Earth could be seen even through backyard telescopes. In the infrared, the string of sites glowed molten gold.

"It was frankly kind of scary," says the forthright Clarke, "to think of an explosion that big." A week after impact, Fragment G's cloud of debris looked like a black eye 16,000 miles across. "People ask whether this can happen [to Earth], and I tell them to look at the moon, which is just pockmarked with craters," says Clarke. "This tells all of us, the whole human race, what can happen. It has happened, in the past, and it will happen again."

Excitement bubbled worldwide, and for all those five and a half days, almost every observatory on Earth was turned to Jupiter at just after sunset, local time, when the planet can best be seen. Satellites, sounding rockets, and a modified C-141 aircraft--whatever the world's astronomers could muster--all gathered data from radio emissions, visible light, the infrared, the far infrared, or the ultraviolet. What, scientists hoped to learn, would be the effect on Jupiter's atmosphere? How deep did each fragment plunge? What was the chemistry of each event? What did that chemistry say about comets? About Jupiter? Later, some of the information may have implications for Earth.

While the research effort was international, the hub remained in Baltimore. One reason, of course, was the Hubble. Its large primary mirror newly corrected, the Homewood-based telescope had finally achieved the capability it was designed to have: 10 times the resolution of any instrument confined to the surface of the planet.

The other was the Internet "exploder" organized last January by researchers at the University of Maryland. Essentially a sophisticated bulletin board, the exploder assembled any incoming e-mail about the event and zipped reports right back out again to all subscribers, some 250 observing groups and theorists all over the world. The result, scientists agree, is one of the biggest advances in scientific methods to come along in decades.

The exploder's novelty was not the technology but its use--an uncontrolled international flow of information, including actual raw data such as the key Hubble images. Scientists normally share only preliminary reports, if that, hoarding data until they can complete and verify their analyses. But here the time-scale was so short, says McGrath, that "it would be kind of crazy to keep anything secret. The science would suffer."

As a result of the exploder, scientists all around the world could coordinate their work, with no loss of time. For instance, astronomers in Arizona could know what exposures and filters had yielded good pictures near the Indian Ocean. And when one of the Hubble groups detected molecular sulfur on Jupiter, an important but unexpected observation, groups elsewhere knew of it fast enough to confirm the finding. "Without the exploder," says McGrath, "that never would have happened, because we wouldn't have known who to call. It was really an incredible experience-- it was just so effective."

What the scientists learned:

Comet Shoemaker-Levy 9 (9 because it was the ninth discovered by that team) may not have been a comet at all. "Either this is a very strange comet, or it's something else," says Weaver. Perhaps it was more like a former comet, one somehow stripped of all its ice. Or perhaps, despite its spectacular disintegration, the object was an asteroid of sorts.

Often described as "dirty snowballs," comets are huge agglomerations of ice and other frozen gases, mixed about half-and-half with solid particles. They drift around the chilly edges of the solar system by the billions. But occasionally, its orbit disturbed by a passing star, one chances deeper into the system, and as the comet spins closer to the sun, the (relative) warmth starts to evaporate the ice. Soon the comet is surrounded by a cloud of dust and gases.

It is these molecules of dust and gas, pushed into the comet's flaring "tail" by the solar wind, that fluoresce when they absorb the radiation of the sun. Then we on Earth can see the comet--and identify what it's made of, by means of a spectrograph.

Crudely speaking, a spectrograph measures the wavelengths of light reflected from an object. Because different elements absorb different wavelengths, to know what wavelengths are missing is to know what elements are present to do the absorbing. For that reason, spectrographs are a major tool for astronomers. Of the four instruments that comprise the Hubble, only two take photographs; the other two are spectrographs. Researchers use them to pinpoint the materials, temperature, ongoing chemistry, velocity, and more of planets, stars, comets, and other celestial objects.

Looking at "Comet" Shoemaker-Levy, the Hubble's instruments had found no signs of water or oxygen. Earth-bound instruments concurred. So whatever Shoemaker-Levy was, by the time it fell into orbit around

Jupiter it was not an iceball.

What was it, then? What was the nucleus like at the heart of each pearl? No one knows. Telescopes can only see a comet's hazy coma of dust and gas. (Halley's nucleus, the only one ever viewed close-up, was photographed during the spacecraft Giotto's fly-by in 1986.) However, to some astronomers, Shoemaker-Levy's spectacular plumes at impact seem to imply that the fragments had to be fairly solid chunks, perhaps as big as a mile and a half in diameter. To get a sense of why, think of throwing rocks into a pond--big rock, big splash.

Other astronomers argue that what's called a "traveling junk heap," a collection of remnant sand and boulders, could have caused that much combustion if its pieces stayed close together. And if the separate chunks of Shoemaker-Levy were junk heaps, this school of thought points out, that would also help explain why the impacts on Jupiter were shallow.

Primary evidence for shallow hits: calculations like Clarke's on Fragment A, and--again--the lack of water. If the Shoemaker-Levy fragments blasted as deep as the water-ice layer of Jupiter, the astronomers expected that the multiple spectrographs would show a lot of the breakdown products of water, especially oxygen-bearing compounds. These were seen, however, by only one observatory, NASA's high-flying Kuiper (the one mounted on a C-141). Many astronomers, because so little water was seen, think the explosions took place in the planet's upper atmosphere--perhaps as high as the stratosphere, above the ammonia clouds (though near enough to disturb them, bringing up helium that Hopkins astronomer Doyle Hall and his team observed with the Extreme Ultraviolet Explorer). Observing the impact site of Fragment G, Caltech's Ingersoll saw two dark rings spreading, like ripples from a pebble thrown in a pond. One ring appears in the ammonia clouds, but since the stronger ripple is above the cloud deck, Ingersoll argues that is where the actual explosion took place.

Whatever it was, Shoemaker-Levy's composition must have included some minerals, because Hubble's Faint Object Spectrograph picked up a flare of magnesium ions from Fragment G about a week before impact. Magnesium is a common element in minerals, and apparently Weaver's team, by some astounding piece of luck--the flare only lasted half an hour, Weaver estimates-- just happened to be observing at the moment when G hit Jupiter's magnetosphere. Furthermore, when Keith Noll's team later began observing the planet, the spectrograph detected magnesium at the site on Jupiter where both G and S had hit. This magnesium--as well as silicon and iron, also observed at the site--must have ridden in on Shoemaker-Levy, because none of them had ever been observed on Jupiter before. "Metallic emissions!" says McGrath. "We were astonished."

Another surprise seen at the G site, large quantities of sulfur, may have much to say about Jupiter (not the comet, because none was seen in the comet). Sulfur-bearing compounds in the middle deck of clouds would explain the tawny coloring of Jupiter, so the simple presence was expected. "But no one had guessed that sulfur might dominate the spectra," says Weaver. It was a highly unusual form of sulfur, too, one that Weaver's team took hours to identify.

"They called me up at 10 p.m. on Sunday night," says Feldman, to borrow spectroscopy literature. That call was natural, not only because Feldman had been Weaver's mentor in graduate school at Hopkins, but because Feldman had experience with peculiar sulfurs and Weaver knew it. In 1983, in Comet IRAS-Araki-Alcock, Feldman observed what turned out to be diatomic sulfur, S2, a molecule made up of two sulfur atoms. Far more usual is the supercooled form, S8 ("the sort of sulfur you find in chemistry sets"). In fact, S2 had never been observed in a celestial object before or since--until Shoemaker-Levy hit Jupiter. Sulfur was also seen at the G site in the form of carbon disulfide, CS2, another mystery molecule that had never been seen except in the laboratory.

One implication of the sulfur is that fragment G, for one, had exploded not in the stratosphere, as Ingersoll argued, but in the sulfur clouds, creating S2 and CS2 by superheating the clouds in a kind of celestial furnace.

Another implication derives from the sheer quantities of sulfur--"so much," says Noll, "that you cannot explain it as dust from the comet." No comet or asteroid of reasonable size could possibly yield such a glut, so it "tells us the sulfur cloud is there--we were pretty certain, but now we see it! Now we know!_. Sulfur's been sort of a holy grail of planetary astronomy," he adds. "We thought it should be there, people had looked for it very hard--and now this comet comes in, stirs up the atmospheres, and conveniently places some of this stuff high up so we can see it."

After the explosions, over hours, days, and weeks, the sites cooled, changing their chemistry again. Ammonia increased steadily until mid-August, but heavy molecules, including sulfur, dropped quickly out of sight. Clarke explains that initially, while still high in the atmosphere, the smoke had moved north and south as well as east and west. North-south winds were a surprise because, as the horizontal streaks of Jupiter's clouds indicate, its winds blow mostly east and west. By early September, Jupiter wore a long, lumpy band at 45 South, some 10 of latitude wide. A few wisps of matter had made it all the way around Jupiter, but individual sites could still be seen, perhaps even with some home telescopes.

No structure on the surface of a gas planet can be permanent, not even the so-called Great Red Spot, actually a sort of mammoth typhoon first seen by Galileo. But Noll thinks the new belt on Jupiter may last as long as two years. "If you have a telescope," he says, "you'd better take a look." Jupiter sets early in the fall, so look as early as you can, examining the side away from the Big Red Spot.

On Earth, the legacy of Shoemaker-Levy's clash with Jupiter includes one empty bottle of champagne, now part of the growing "astronomy bottle" collection of Melissa McGrath, as well as reams of data. Astronomers will be working with this material for years.

The legacy also includes, perhaps, two warnings--one about science, and one about what happens when a celestial object hits a planet.

For the scientists involved, this event was a high point of their professional lives--but not because of the results, interesting though they were. The scientists say the week was great because of their community. Ingersoll, for instance, says that the high point was "the interaction of the scientists: high-level ideas slopping, theories rising and falling, getting shot down faster than we could throw them up--and that is science! Except it just came at such a great rate."

What may be worrisome: Among the younger researchers, similar comments are tinged with open astonishment. They do not say of free interaction, like the older man (a veteran of the Voyager missions), "that is science." They speak like McGrath, who says, "It was just unbelievable." And like Hal Weaver, who says with surprise, "scientists tend to be competitive. But in this particular group, we put all our data on a common location on a computer. We trusted each other not to take each other's data and publish it, we just trusted each other."

Academic scientists would like to work in cooperative mode all the time, and in the past they often did. Sharing material with colleagues was a matter of routine courtesy. As recently as 1983, the Frenchman Luc Montagnier sent a sample of his newly isolated virus, now called HIV, to American Robert Gallo. This example represents both the best- and the worst-case scenarios, as it happened. The crucial early work on AIDS--including the tests used to clean the blood banks--advanced with astonishing speed, in good part because of such international cooperation. Yet Montagnier's generosity also led to a prolonged international lawsuit over who had first isolated the AIDS virus, and which nation was therefore entitled to lucrative patents.

These days, sharing raw data is hardly common. "If we didn't have to get tenure," says McGrath, "if we didn't have to compete with each other, it would work that [cooperative] way. But we have no choice. You have to get credit. You have to do better than other people or you're not going to succeed."

Paul Feldman, now of the mentor generation, says such concerns are widespread among the younger scientists, "and I think they're correct." Science has become a more dog-eat-dog world. "There's always been competition to be the first to discover." But now, he says, "it's dog-eat-dog for jobs. We have overproduced young scientists."

Question: What synergies are prevented when cooperation grows rare?

As for the planet Earth, could something like Shoemaker-Levy hit us? That question from reporters was almost as common as "How do you feel?" and rightly so.

The answer is, Yes, of course, it happens all the time. However, most of the heavenly intruders disintegrate harmlessly in Earth's atmosphere, like the meteors we call falling stars. The biggest impact site on Earth, a large crater near and under the Caribbean, apparently does mark a crash that killed off all large species on the planet, including the big dinosaurs. But there is only one such humongous site known. The planet's most serious recent encounter took place in 1908, when a meteor exploded in the atmosphere above Tonguska, Siberia, leaving a circle of charred trees 30 miles across. Luckily, the area was unpopulated.

The astronomers generally counsel a lack of worry. "I don't intend to lose any sleep over it," declared Heidi Hammel to a reporter for an Arabic newspaper.

Jupiter, it seems, is a special case. As McGrath crisply explains (perhaps for the dozenth time), "First of all, Jupiter is really near the asteroid belt, also much closer to the belt where the comets are. The second thing is, Jupiter is much more massive than Earth. It has a much stronger gravitational attraction, much more power to capture those small kinds of bodies. And the third thing is, Jupiter is so much bigger--it's a bigger target. So the probability of Earth being hit compared to Jupiter_" she trails off and laughs. "It's like John Clarke said in one of the press conferences: 'Just be glad you don't live on Jupiter.'"

That said, meteors deserve a second thought--not planet-killers, which are worry-food only for the paranoid, but meteors of, let us say, about 140 feet across. That's the size that exploded near Tonguska, and objects like that may come along every hundred years. "Your odds of being killed by a meteor are bigger than of being killed in a plane crash--and much larger than your odds of winning the lottery," says Noll.

A meteorite weighing 2,300 pounds landed in Kansas in 1948, a two-tonner in Mexico in 1969. As the world becomes more populated, we humans more often see what's been happening all along.

What if something like Tonguska had happened not in 1908, but in 1968? Noll wonders. "What if it happened now, over Ukraine? Would people have the wherewithal to know that oh gee, that was just a 50-meter asteroid hitting Earth's atmosphere?" Or would they think it was a nuclear explosion--and retaliate?

Asked whether we would know if a Tonguska-like collision were impending, Hal Weaver says no. "Right now we wouldn't because we haven't catalogued the objects in the near-Earth environment that well. That's why, in fact, Gene Shoemaker and others are pushing to study these objects, so we can tell what might happen."

Noll predicts "a highly automated telescope that sits in the sky and finds most or all of the objects, even down to the size of 50 meters." That way, if collision threatened, perhaps people could be evacuated. At least no one would reach for the nukes.

Elise Hancock is the magazine's senior editor.


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