Comet Cracker

If you want to see what’s inside a comet, you’ve got to break some spacecraft.

The Deep Impact mothership will capture information on the comet's freshly exposed primordial interior. (Pat Rawlings/NASA)
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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.

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