NASA/JPL-Caltech/Cornell

Flyby. The nucleus of comet Tempel 1, as seen by the Stardust spacecraft just before its closest approach. (NASA/JPL-Caltech/Cornell)

Valentine's Day Encounter Yields Fresh Comet Views

Valentine's Day was doubly romantic for Steve Chesley. Not only did the NASA Jet Propulsion Laboratory (JPL) planetary scientist propose to his girlfriend, astronomer Jana Pittichova of the University of Hawaii, Honolulu, but he also served on the science team that successfully orchestrated a rendezvous between NASA's Stardust spacecraft and comet Tempel 1. Zooming by the object at a distance of a mere 181 kilometers, Stardust obtained its planned 122 images, including 72 high-resolution science photos.

"Everything worked perfectly," says principal investigator Joseph Veverka of Cornell University. "This is our first chance to learn something about the way comets change over time."

Unfortunately, Veverka and his colleagues had to be patient to see the highest-resolution images. Although Stardust was supposed to download the closest-approach photos first, it actually sent them in the order in which they were taken. Because each photo took about 15 minutes to transmit, the best views of Tempel 1 weren't available until early Tuesday afternoon U.S. Eastern Standard Time.

Comet Tempel 1, a chunk of ice some 6 kilometers across, orbits the sun every 5 ½ years between Mars and Jupiter. NASA's Deep Impact spacecraft, another cometary flyby mission, first saw it in close-up in July 2005. In passing, Deep Impact also fired a 370 kilogram impactor into the comet's nucleus (the actual icy body) to study its composition.

Tempel 1 has now completed one full orbit, and scientists are eager to see which parts of the frozen surface have been modified by the warmth of the sun. Their goal is to learn more about the evolution of comets.

"We know some areas on a cometary nucleus must have changed recently, for instance, by eruptions of subsurface pockets of gas, while others remain relatively undisturbed," says Veverka. However, he adds, until now astronomers have never known exactly which areas were young and which were old. They are also curious about when and how fast such changes occur. Scientists hope to get a better understanding by comparing the new Stardust images of Tempel 1 with the pictures shot over 5 ½ years ago by Deep Impact.

Comets are frozen leftovers from the very early days of the solar system. They probably brought much of Earth's water to our newborn home planet. At the same time, they pose a threat: Impacts on Earth could wipe out cities or even complete continents. Studying the properties of comets in detail may help astronomers understand the formation of planets like Earth and tell scientists how to deflect dangerous objects from space.

Project manager Timothy Larson of JPL in Pasadena, California, says another potential bonus of the Stardust mission is high-resolution imaging of the crater that must have been produced by the Deep Impact projectile. Determining its size and depth will reveal more information about the composition and internal structure of the comet. Deep Impact itself couldn't image the crater because of the huge amount of fine, powdery material that was blasted into space during the impact.

The $300 million Stardust spacecraft was launched just over 12 years ago on a trip to another comet, Wild 2. Since then, it's been on a whirlwind tour of the nearby solar system. It flew by a small asteroid called Annefrank in November 2002, it collected dust particles from Wild 2 on 2 January 2004, and it returned a parachuted capsule with this precious sample to Earth on 15 January 2006. It was assigned its next cometary mission (called Stardust-NExT, for Next Exploration of Tempel 1) in July 2007. By the time the ageing spacecraft reached this second comet, its mileage had soared to a staggering 5,673,464,575 kilometers.

Unfortunately, Stardust almost ran out of fuel, so there won't be an opportunity for an orbital maneuver that sets the spacecraft on course for yet another comet.

Posted in Space