If you want to see what's inside a comet, you've got to break some spacecraft.
- By Tony Reichhardt
- Air & Space magazine, May 2005
MIKE A’HEARN THINKS BINOCULARS should be enough. When his 800-pound, copper-tipped spacecraft collides at six miles a second with an unsuspecting comet called Tempel 1 in July, no one, not even A’Hearn, knows exactly what will happen. There almost certainly will be a smash, and a splash, and a flash, as tons of icy grit freed from the heart of the comet spray outward into sunlight. The whole drama, he reckons, shouldn’t take more than 800 seconds. A’Hearn expects the brightening will be visible on Earth with a small telescope or binoculars, perhaps even the naked eye, and that millions of people will be watching. God knows he will. So will half the telescopes on Earth, amateur and professional alike.
Closer to the action, the impactor’s mothership will be observing with its cameras and spectrometers from a safe distance of 310 miles, having separated and veered off from the smaller impactor 24 hours earlier. A’Hearn, a planetary scientist at the University of Maryland, and the rest of the team that designed the mission, which they call Deep Impact, could have placed the mothership even closer. But they chose the distance partly to protect it from dust impacts and partly to ensure that the instruments take in everything that comes flying out from the comet.
There are many ways to study comets, and scientists have tried most of them. The U.S.-European Solar and Heliospheric Observatory—SOHO—routinely watches comets fall into the sun from its vantage point a million miles from Earth. In the 1980s, European, Russian, and Japanese spacecraft flew close to Halley’s Comet, taking pictures and sampling the dust and gas boiling off the nucleus as the comet rounded the sun. Deep Space 1 visited Comet Borrelly in 2001, and Stardust came within 149 miles of Comet Wild 2 last year, grabbing dust samples that will return to Earth next January.
Deep Impact will be the first spacecraft to crack open a comet’s nucleus to see what’s inside. But if that makes it sound all big and bad, it’s not. The comet runs into the spacecraft, not the other way around. The TV-size impactor will wait in Tempel 1’s path like a bug on a highway, and when the four-mile-wide comet comes crashing into it, the spacecraft will vaporize instantly. Then, if all goes as planned, a 100- to 150-foot-deep crater will form, exposing the comet’s pristine interior—material that has been sealed up since it formed at the edge of the solar system billions of years ago. For A’Hearn and his team, this is the payoff of the mission: finding out precisely which elements make up Tempel 1’s nucleus.
As appealing as most people find the idea of smashing up a multimillion-dollar spacecraft, A’Hearn says that when the impactor was first proposed more than a decade ago, “the instinctive reaction was ‘That’s dumb—why would you want to do that?’ ” In fact, there’s no better way to see deep inside a comet; no present drill could go into space and make a hole that deep. Anyway, the destructive element never bothered A’Hearn: “If you think about what you know and what you don’t know, this is a way to find out things you don’t know.”
This isn’t just fun, people. It’s science.
Peter Schultz, a co-investigator for Deep Impact, has spent much of his career thinking about the physics of cratering. As a planetary geologist at Brown University in Rhode Island, he has investigated impact sites from Argentina to Canada, trying to reconstruct in detail what happens when a small body like an asteroid slams into a planet at ten times the speed of a rifle bullet. To predict the effect of Deep Impact’s collision, he’s been making his own craters at the Vertical Gun Range at NASA’s Ames Research Center near San Francisco. There, inside a bland, industrial-looking building, he shoots at different target materials with projectiles traveling at velocities up to four miles per second—not as fast as Tempel 1 will be moving, but fast enough so the results can be used to predict what might happen to the comet.
Schultz has experimented with pumice, a lightweight volcanic rock, as the target. He’s also tried pumice dust. He’s used perlite, a crushed rock commonly found in gardens; perlite with ice; and silica microbeads, the material used to make stop signs reflective. He varies the speed and angle of impact; he varies the porosity and density of the target. He’s done hundreds of test firings to master the nuances of cratering.
Comet nuclei are not easy to imagine. Having seen pictures of what look like solid objects, we think of them as rocks. But some, says Schultz, may be as fluffy as cotton candy. A’Hearn likens their insides to “very good powder snow for skiing.” The degree of fluffiness, porosity, lumpiness or smoothness—all these factors affect how deep a crater will form, and what shape it will take.
The speed of the impactor also makes a difference. If you stood over a deep pile of pumice dust, the kind Schultz sometimes uses to simulate cometary material, and dropped a metal rod pointing down into the pile, the rod would fall straight through. But at four miles a second, a rounded impactor forms a crater instead, shattering and melting in the process.
In one of Schultz’s scenarios for Deep Impact—the most likely one, he thinks—material will spray out from the crater in a nice conical pattern. In others it also shoots straight back out the hole like sparks from a Roman candle. Some scenarios have the impactor getting embedded in the comet, and in one it goes right through the nucleus and comes out the other side. The last outcome, says Schultz, is so improbable that it is mentioned “almost tongue-in-cheek. But it shows you what we know about comets.”
Schultz conducts his gun tests with a projectile made of Pyrex; that material shatters at the slower velocity of the simulations, just as the copper impactor will shatter when Tempel 1 hits it at a higher speed. When A’Hearn first started working on Deep Impact, some people, no doubt hoping for the biggest possible boom, suggested the impactor be made of the heaviest materials they could think of, including uranium. But to dig a crater most effectively, says A’Hearn, you really want something about the same density as the comet. In fact, the engineers have carved little pockets from the copper projectile to reduce its density in order to more closely match the density estimated for Tempel 1.
And because the projectile will vaporize on impact, it has to be made of an element that won’t chemically combine with water from the comet, confusing the spectrometer readings taken by the mothership. That requirement ruled out aluminum, for example. Ball Aerospace, which built the spacecraft, had gotten a good deal on electronics boxes made of magnesium, but A’Hearn had to nix that deal. The best materials turned out to be noble metals, like gold, silver, platinum, and copper. Having only $267 million to spend on their mission, the team went with copper.
Whatever transpires when copper strikes comet, it will happen in slow motion. When an asteroid smashes into Earth, a crater forms in a few seconds of unimaginable violence. On a tiny comet nucleus, with its extremely weak gravity—you could jump off the surface and never come back down—you’d expect the explosion to go faster. But exactly the opposite happens. “It is very counterintuitive, and it took me a long time to think my way through it,” says A’Hearn. Right after impact, displaced material starts coming out from the interior. The more time passes, the slower the material exits. The crater stops growing only when the stuff from the interior is moving so slowly that gravity pulls it back before it reaches the rim. But in low gravity, even stuff moving very slowly can make it to the rim, so the whole process takes longer.
Schultz predicts that Deep Impact’s crater will take 200 seconds to form, maybe longer, though not more than 500 seconds. To give themselves some margin, the science team has planned to have the mothership’s cameras and spectrometers observe closely for 800 seconds. “We don’t want to fly by until it’s all over,” says A’Hearn.
Low gravity also makes the crater end up much bigger. If the Deep Impact projectile hit an airless body with the mass of Earth, it would gouge a hole maybe 20 feet wide. Schultz thinks the hole in the comet nucleus will be 10 or even 20 times larger.
The drama may not end with cratering. One important question about comets, particularly old ones like Tempel 1, is whether centuries of swinging in toward the sun has caused their volatile components, like water, to have boiled away. If not, reservoirs may be bottled up inside that will vent once the hard crust is breached. If Deep Impact opens such a vent, says A’Hearn, “my guess is that it will come within minutes. It could certainly be hours. Days I think is unlikely.”
The venting could be violent, with large jets of gas spewing into space. And if the nucleus contains lots of water vapor, Schultz says, “we may cause an explosion inside the comet,” one powerful enough to break Tempel 1 apart. That’s unlikely, says Schultz, “but as an experimentalist, you never say never.”
Some of Schultz’s simulations show big plates of crust flying off after the impact. By tracing the ballistic arc of the plates, the scientists could determine the comet’s gravity, and therefore its mass—a fundamental property that has never been measured for a comet.
During and after the explosion, the mothership’s cameras and spectrometers will be busily scanning the crater and the icy dust that comes flying out. The pristine material A’Hearn hopes to see—ices that haven’t been crunched, melted, or altered by sunlight since they first formed—could be dozens of feet deep, or right below the surface. The important clues about the early solar system will be the relative abundances of water, carbon monoxide, and carbon dioxide. From the proportions, the scientists will be able to deduce the temperature at which the compounds formed. That in turn will help them understand the conditions under which the solar system was created.
Another uncertainty is driving the scientists crazy: They don’t know the exact shape of the elongated comet nucleus—where it bulges and where it’s thinner. And the uncertainty makes navigating Deep Impact trickier.
Because the orbit of Tempel 1 is well known, the team will be able to put the craft on a trajectory that comes reasonably close to the comet. But to ensure that it actually makes contact, the impactor will, in the last 24 hours of its journey, rely on onboard software that makes small course corrections based on images the probe takes as it closes in. The navigation system will direct the craft’s hydrazine thrusters to guide Deep Impact to the brightest area on the comet. But if the slowly turning comet is shaped like a dumbbell, its brightness will be constantly changing, and that could fool the system into charting a course that misses the comet completely. The team has been tweaking the software to account for the uncertainty, though, and A’Hearn is now “99 percent confident” that the craft will hit the target.
The world will be watching. Most big observatories in Hawaii, like the twin Keck Telescopes on Mauna Kea, will be trained on Tempel 1 at the critical moment. A’Hearn and some of the team members will watch from the Jet Propulsion Laboratory in Pasadena, California, where Deep Impact’s data will be received. They will all be anxious to see if years of work will produce 13-plus minutes of unique data. If the team pulls it off, Deep Impact will make humanity’s first direct contact with a comet nucleus.
Did we mention that all the fireworks take place on the 4th of July? A perfect date for making history.