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
(Page 3 of 3)
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.