Deep Impact

Comet Tempel 1, discovered in 1867 by the German astronomer Wilhelm Tempel, orbits the sun every 5.5 years. In the summer of 2005, when it returned from its travels in the outer solar system, a host of astronomical observatories were waiting to study it. Comets are largely composed of water ice; because comets can strike the earth, astronomers suspect that much of the young earth's water may have originated in comets. Furthermore, comets contain primordial material from the early solar system that scientists want to measure to determine the prevailing
conditions of that era. To mark the return of Tempel 1, therefore, NASA prepared the Deep Impact Mission, a spacecraft designed not only to observe the comet, but also to shoot a 364 kg copper impactor into it in order to release material beneath the surface for study.

Deep Impact succeeded, and two SAO-led missions were waiting to study the gases and particulates expected from the release. Writing in the latest issue of the journal Icarus, these teams report their findings. SAO astronomers Frank Bensch, Gary Melnick, Brian Patten, and Volker Tolls, together with three colleagues, pointed the Submillimeter Wave Astronomy Satellite (SWAS) at Tempel 1 to study the release of water. SWAS is a NASA Small Explorer Project proposed, designed, and operated by an SAO team; it was designed to study the chemical composition of interstellar gas. SWAS had completed its mission in July, 2004, and had been turned off, but it was brought out of hibernation a year later (still in perfect condition) in order to measure the water released during the dramatic impact on the comet. The team reports that it observed the comet before, during, and after the impact over a period of four months.
Surprisingly, the team did not see any major release of water due to the collision. Although they did measure the normal evaporation of water due to sunlight -- a total of about 1.3 billion kilograms during this period -- the amount released in the impact itself was less than about 1% of this amount. They conclude that most of the frozen water in Tempel 1 lies deeper below the surface than the impact could affect.

SAO astronomer Scott Wolk was part of a team of thirteen scientists that used the Chandra X-ray Observatory to study Tempel 1. With the surprise discovery of X-rays from comets in the mid-1990's, astronomers realized that charged particles in the solar wind could produce significant diagnostic X-ray emission upon interacting with neutral gaseous material in a comet's tail. Chandra observed the comet regularly over a period of one month before and after the impact, and continuously during the impact itself. Although the controlled impact on Tempel 1 released about ten million kilograms of fresh neutral material, Chandra did not see evidence for an X-ray flash due to this event. However, the team reports that it did see X-ray emission from the comet throughout the period, with some flaring in the emission corresponding to increases in the solar wind. They also saw a gradual overall increase of about 18%
in emission over one day, presumably due to the impact. The basic conclusions from these two sets of data were corroborated by other observations of the comet, all of which have led to an improved undertanding of the composition of comets, and a better appreciation of the complexity of its internal structure.