Stardust-NExT: Second Mission to Deep Impact Comet
This pair of images shows the before-and-after comparison of the part of Comet Tempel 1 that was hit by the impactor from NASA’s Deep Impact spacecraft. The left image is a composite made from images obtained by Deep Impact in July 2005. The right image, taken by the Stardust-NExT spacecraft, has arrows identifying the rim of the crater caused by the impactor. Photo courtesy NASA/JPL-Caltech/University of Maryland/ Cornell.
In July 2005, NASA’s Deep Impact spacecraft left a big piece of metal in the path of Comet Tempel 1 so that scientists could observe the collision and, they hoped, see the inside of a comet. While the data from this mission significantly changed their ideas about how comets form and evolve, scientists were unable to see the crater created by the collision because the material ejected from the comet blocked the view.
On February 14, 2011, they received a second chance to view that crater. NASA’s Stardust spacecraft, which had completed its original mission of taking a dust sample from Comet Wild 2 and returning it to Earth, had been redirected to come within 111 miles (178 km) of Tempel 1 and take dozens of high-resolution images. The new mission, called Stardust-NExT (for "New Exploration of Tempel 1") also gave scientists their first opportunity to get a close look at a comet on successive trips around the Sun.
IfA astronomer Karen Meech was the co-investigator in charge of all the Earth- and space-based observing support for the mission, a role she also performed for Deep Impact. This effort used data from 25 telescopes throughout the world, especially the UH 2.2-meter telescope on Mauna Kea, to understand the comet and its rotation. Meech explained just how difficult this was: "It was very important to predict the rotation so that we could see the impact crater made by the Deep Impact mission in 2005. The challenge was to know the rotation speed so precisely that we could predict the side we would see a year later. The last time we could adjust the spacecraft arrival time was during February 2010, when it would take the least amount of energy to change the spacecraft velocity, because the spacecraft had very little fuel left. Ordinarily, this would be daunting, but for this comet it was especially difficult because we knew that the comet’s spin rate was not only slowing, but also the rate at which it was slowing was changing. During each close passage to the Sun, uneven outgassing from the comet’s nucleus acts like rocket thrusters, both speeding up and slowing down the rotation. The Stardust-NExT encounter occurred during a period of near maximum outgassing—before the spin rate would settle down. So not only did the team have to get the spin rate right, we had to understand how it was changing while the comet was active. Our predictions were verified during the flyby—a remarkable achievement that would not have been possible without the UH 2.2-meter telescope."
"We achieved all of our science objectives," said Joe Veverka, Stardust-NExT principal investigator and a professor at Cornell University, at the post-encounter press conference. He noted that they had accomplished their three imaging goals: First, to look at areas of Tempel 1 imaged with the Deep Impact spacecraft to see "what changes occur on a comet when it comes close to the Sun," second, to see the Deep Impact crater, and third, to "see areas on Tempel that we had not seen before." They also obtained data about the dust in Tempel’s coma, the cloud that is a comet’s atmosphere.
Because this mission involved a "recycled" spacecraft, it cost only $29 million, much less than the $500 million needed for a mission that builds and launches a new spacecraft. In November 2010, the Deep Impact spacecraft also visited a second comet, Hartley 2 (see Na Kilo Hoku no. 37). Meech also oversaw the ground-based operations for that mission.
Co-investigator Peter Schultz (Brown University), the impact cratering expert for the Deep Impact and Stardust-NExT missions, characterized the Deep Impact crater as "more subdued than expected" with a small central mound. The crater is 150 meters across (about 500 feet) and indicates "the surface of the comet where we hit is fragile."