Rarely do astronomers get a chance to reach out and touch what they study. One of those rare opportunities came on July 4, when the National Aeronautics and Space Administration’s Deep Impact probe smacked into a comet with the force of nearly 5 tons of TNT. The impact blew a crater in the comet 200 yards across and 30 yards deep.

NASA organized this cosmic assault to study the structure and composition of a comet nucleus—a Manhattan-sized chunk of ice and dirt that gradually vaporizes. As it turns to gas it creates the glowing head and tail we see from Earth. Comets hold material left over from the formation of the solar system, providing clues to how the planets formed.

When Deep Impact hit, astronomers at the Smithsonian Astrophysical Observatory were watching with the Submillimeter Array telescope in Hawaii and the Submillimeter Wave Astronomy Satellite. Both instruments seek cosmic radio signals from gases, such as water vapor and carbon monoxide.

The debris cloud created from the blast contained fine dust similar to talcum powder. Surprisingly, little water was detected, considering that comet nuclei are often described as “dirty snowballs.”

“It’s pretty clear that this event did not produce a gusher,” SAO Astronomer Gary Melnick says.

The target, Comet Tempel 1, travels through the inner solar system every 5½ years, making it a “short-period” comet. Scientists expected that solar heating during its previous passages had vaporized ice close to the comet’s surface. However, the drought extended deeper inside the comet than predicted.

“Theories about the volatile layers below the surface of short-period comets are going to have to be revised,” SAO Astronomer Charlie Qi says.

Melnick, Qi and colleagues will continue to analyze the comet debris in coming months. “The big picture will emerge once astronomers meld data from different observatories at different wavelengths,” Melnick says.