Preparing for Impact

SPACE SCIENCE

After six months of space flight, an 800-lb spacecraft carrying 317 lb of copper is preparing to slam into the comet Tempel-1 on July 4. The resulting impact crater—expected to kick up material from the comet’s still-mysterious nucleus—could be as small as a house, or as big as a football stadium.

Scientists hope that this unprecedented access to a comet’s innards will help them understand the solar system’s formation. Right now, all they know is that Tempel-1’s nucleus, which is about the size of Washington, D.C., is “a jet-black, pickle-shaped, icy dirt ball,” Donald K. Yeomans, coinvestigator for the Deep Impact mission at NASA’s Jet Propulsion Laboratory (JPL), said at a press briefing at National Aeronautics & Space Administration headquarters in Washington, D.C.

Space-based telescopes such as NASA’s Hubble, Chandra, and Spitzer and the European Space Agency’s Rosetta spacecraft, also en route to a comet, will track the event. One hundred or so professional ground-based telescopes and a small army of amateur astronomers will also be watching the show, hoping to spot signs of the impact blast.

Comets, with elliptical orbits that can reach to the outer edges of the solar system, are ancient icy bodies that condensed during the solar system’s genesis. “They formed cold and stay cold for most of their lives,” noted Michael F. A’Hearn, principal investigator for Deep Impact at the University of Maryland, College Park.

The pristine ices of the comet’s nucleus, which are normally hidden by a cloak of warmer gas, or “coma,” that surrounds it, should be a time capsule preserving the early conditions that led to planet formation in the outer solar system.

Comets are thought to have brought Earth much of its water and carbon-based molecules, and also might be a source of water and fuel during future attempts to colonize the solar system, Yeomans said.

The Deep Impact spacecraft has two components: the impact craft with its copper slug designed to deliver a “maximum wallop” to the comet, according to Rick Grammier, Deep Impact project manager at JPL, and a second flyby craft that will monitor the explosion from a safe distance. Just after the impact, the flyby craft will swing past the comet; it has only 800 seconds to collect all key data with its suite of cameras and spectrometers.

Although one of Deep Impact’s cameras was recently found to be out of focus, scientists say they can solve the problem with image processing. The team assured reporters that Deep Impact’s deep impact would have a negligible effect on the comet’s structure and orbit.