Deep Impact will race toward comet Tempel 1 after its launch on December 30, 2004. One day before its comet encounter on the following July 4, the parent ship will release a "smart impactor"—an 820-pound mass of copper and aluminum, equipped with a camera and small thrusters to help steer toward the target. The flyby vessel will slow down and record the crash through telescopes as the nucleus and the impactor slam together at 6.2 miles per second, generating an explosive force of 4.5 tons of TNT. "At that instant, we won't know what's going to happen," says Green. "It's like a real-life experiment with a spacecraft."
Project director Michael A'Hearn, a jovial and bearded man who favors shorts and Hawaiian-print shirts on Maryland's sweltering summer days, has an educated guess. "My gut feeling is that we will excavate a crater, get down to where there is a lot more volatile ice than near the surface, and create a new active area that will persist for months," he says. "It also would not amaze me if we split the comet and broke off a small piece."
The most likely model predicts that the impactor will create a crater 400 feet in diameter and nearly 100 feet deep. The actual size and the speed at which the crater forms—gauged by cameras on the mother ship—will tell researchers about the comet's internal strength. The instruments on the flyby vessel also will measure the composition of material sprayed out of the crater as sunlight illuminates the gas and dust.
Again, getting the aim right is critical. Hitting a dark, irregular, three-mile-wide target on a sunlit part of its surface is a stiff engineering task, but thousands of simulations of the encounter have had a success rate of 90 percent to 99 percent. "That gives me a lot of confidence," A'Hearn says. "On the other hand, the military has tried to hit things when they knew exactly what it was they were trying to hit, and they failed."
Deep Impact faces another challenge. NASA, viewing the mission as a prime opportunity to build public interest in space science, has asked for an extensive outreach program, so the public will be watching closely. If the smart impactor succeeds too well, a visible chunk of the comet might break off. That would be a scientific boon, but laypeople might perceive it as reckless demolition of a celestial object. Astronomer Lucy McFadden and her Deep Impact colleagues have heard such concerns at their public talks, so they've come up with a creative spin: "We're actually not hitting the comet. We're putting the impactor in front of the target and letting it hit us. The comet is destroying the impactor. When they hear it in that way, people don't protest."
As for the scientific doubts, A'Hearn and his team have worked steadily to make their case. Gradually, they're winning converts. "The 21st century will be about a lot of actual active science on other worlds," says Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. "Deep Impact is great comet science. It was gimmicky to me at first. Now, I'm nothing but a fan. It has a chance to just blow our doors off."
"In many ways, this is a really dumb, simple experiment," A'Hearn says, folding his hands across his stomach with a smile. "If we knew a lot about comets, it would be a stupid thing to do. The only reason it's a smart thing to do is because we know so little." Within a decade, that statement may seem as quaint as the 11th-century belief that Halley's comet was a malicious omen heralding the Battle of Hastings.
More Comets, More Missions
Last August NASA's $159 million Contour comet mission broke apart as it was boosted from Earth orbit. Mission planners hope NASA will launch a replacement to visit at least two comets later this decade. A new mission would likely head for Encke, a comet described as "evolved, degassed, used-up, and as old as possible" by Contour mission leader Joseph Veverka, a planetary scientist at Cornell University. Another potential destination, comet Schwassmann-Wachmann 3, fragmented into pieces in 1995, suggesting it is a young, fragile object. "We don't have to blast that one apart to study the inside," says Veverka.
Not to be outdone, the European Space Agency hopes to launch an ambitious 12-year mission in February called Rosetta. "We're going to get a holistic picture of a comet, and then we're going to see how it evolves over time," says Alan Stern of the Southwest Research Institute. Rosetta's target is a comet called Churyumov-Gerasimenko. When Rosetta reaches its destination in 2014, the comet will be near Jupiter's orbit and not very active. Over the following 18 months, Rosetta will orbit at a safe distance as the comet gets closer to the sun and begins to boil, bubble, and blow off gas and dust. The most scientifically exciting part of the billion-euro mission is a 220-pound lander designed to scoop up comet chunks and analyze them in onboard labs. A penetrator will stab about a foot deep into the nucleus to test temperature and hardness. But so far, Rosetta has had trouble getting off the ground; its original launch date, last January, had to be postponed because one of the new French-built Ariane 5 rockets, a close cousin of which was to blast Rosetta into space, blew up during another launch.
— R.I.
Rocks From Hell
Comets are remarkably simple and remarkably mysterious. A comet is simply a primitive rock (the nucleus) that is thought to contain ice. Comets orbit the sun and originate from beyond Pluto. Most are about a mile in diameter, but there may be thousands that are more than 50 miles across. A comet produces two visible tails when it gets close to the sun and heats up. Its dust tail is made of tiny particles of matter. Its ion tail is formed of electrons and charged molecules that fluoresce when sunlight strikes them. An aura of gases called a coma also forms a cloud around the nucleus.
Killer Comets
Comet Shoemaker-Levy 9, May 17, 1994
Photograph by H.A. Weaver, T.E. Smith
(Space Telescope Science Institute), and NASA.
People once feared comets, then came to regard them as beautiful astronomical oddities, then learned to fear them all over again. "They are in fact agents of destruction," says Don Yeomans, who leads NASA's Near-Earth Object Program at the Jet Propulsion Laboratory. Although about 100 times as many asteroids as comets approach Earth, comets pack a bigger punch—they plunge toward the sun several times faster than asteroids. That means a comet could hit Earth with about 10 times as much energy as an asteroid with the same mass. Furthermore, astronomers can locate and project the orbits of most asteroids, but comets are not easy to spot until they get inside Jupiter's orbit, where the sun heats them enough to create a visible tail. A new comet gives us only a nine-month lead time. "These things have the potential for very little warning time, whereas most asteroids give us a decade or more," says Yeomans. He believes we should have a plan for a doomsday comet. But no one is quite certain what to do if one does show up. Small nuclear bursts in space near the comet might nudge it and change its orbit, a vast solar sail anchored to one would tug it off course, and coating part of the surface with a white powder that reflects solar energy might cause a change in orbit. Blowing it up could make things worse. In 1994 Jupiter's gravity shredded comet Shoemaker-Levy 9 into 21 visible chunks, which then plunged into the gas giant piece after piece. A typical piece detonated with the force of about 25,000 megatons of TNT. A chain of blasts around Earth might wreak more havoc than a single impact. Yeomans hopes that exposing the ingredients and the structure of comets will show us how to alter their course. Until then, he says, "We're a long way from being able to deal with an impactor."
— R.I.
For more, visit stardust.jpl.nasa.gov, deepimpact.jpl. nasa.gov, www.esa.int/export/esaMI/Rosetta, www.contour2002.org, and sci.esa.int/home/giotto.
