Hours after the Deep Impact space probe flew within about 435 miles of comet Hartley 2 on Thursday morning, images beamed back to Earth revealed a body shaped rather like a peanut or an overturned bowling pin, with two bulbous, roughened edges and a smooth band in between.
The images coming in were "just amazing," team scientist Jessica Sunshine of the University of Maryland said at a news conference.
The ice and debris that make up a comet are thought to be leftovers from the solar system's early development, when the planets were still coalescing, said astronomer Michael A'Hearn of the University of Maryland, the mission's principal investigator. Thus, learning about their composition could help reveal what our early solar system looked like.
The fly-by was only the fifth time a spacecraft has had such an up-close and personal look at a comet — and each of those encounters has provided new information, the scientists said.
Hartley 2 was not the spacecraft's original target. The Deep Impact spacecraft, operated by engineers at the Jet Propulsion Laboratory in La Cañada Flintridge, shot an 820-pound copper impactor into the comet Tempel 1 five years ago in an effort to determine its composition. After the mission was completed, NASA officials decided to send the intact spacecraft to another comet.
Hartley 2 was chosen because its small size — its total volume is about 100 times smaller than that of Tempel 1 — and its propensity to shoot out relatively large amounts of gas and dust made it quite different from other closely observed comets.
Scientists have been observing Hartley 2 for several weeks, and it has already surprised researchers by spitting out cyanide for eight days. What may have caused the outburst remains a mystery.
"Trying to interpret any of the data from a fast fly-by will invariably take a lot longer than the fly-by," A'Hearn said.
The spacecraft will continue to collect data about Hartley 2 until Thanksgiving, when scientists expect to have taken 120,000 images.