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The Two Faces of Tempel 1

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In January 2007, NASA chose Veverka's plan to revisit comet Tempel 1 with NASA’s already in-flight Stardust spacecraft. Stardust had just completed the mission it was designed for – flying to comet Wild 2, collecting samples of the coma as it hurtled by, and then flying back to Earth to drop off a sample return capsule so scientists could study pieces of comet in their labs.

Ask any spacecraft project manager -- re-tasking a spacecraft designed for a completely different mission is a challenge. To be in the right place at the right time to see changes in surface features on a small celestial body that seemingly changes its rotation rate on a whim and is out of view from observers for most of its five-and-a-half-year orbit about the sun -- that’s something else entirely. But that was the assignment given to Stardust-NExT team members Mike Belton, Steve Chesley and Karen Meech.

"As comets sweep though the inner solar system, they come alive," said Belton, a Stardust-NExT co-investigator from Belton Space Initiatives in Tucson, Ariz. "They belch gas and dust, and this outgassing can not only change their orbits, it can also change their rotation rate."

Determining the comet's rotation rate and which side will be illuminated when is tricky, because the comet had only been seen up close for a short time in July 2005 during the Deep Impact encounter. From then on, the comet nucleus, about 6 kilometers (3.7 miles) wide, appeared to observers to be little more than a point of light in the sky for even the best telescopes -- including NASA's Hubble Space Telescope. (Tempel 1's orbit takes it as far out as Jupiter's orbit and almost as close as Mars’ orbit.) But even points of light can bear scientific fruit for astronomers and space scientists. The flattened, oblong Tempel 1 nucleus was no exception.

"Its shape is central to what we could learn about its rotation," said Belton. "A comet reflects the sun's light. When one of its two broad regions is facing us, it gives off more light. When one of its skinnier sides is pointed toward the telescope, it gives off less light. So we felt we could develop an accurate model for the comet's rotation."

The plan was for Belton and Chesley to generate comet rotation models independently. What both needed was data (and a lot of it) on the amount of sunlight Tempel reflected and when. Both knew the source for that information: fellow Stardust-NExT co-investigator Karen Meech. Meech, an astronomer from the University of Hawaii, reached out to her network of fellow astronomers around the world.

"They came through (in spades)," said Meech. "In total, 25 telescopes at 14 observatories around the world allocated about 450 whole or partial nights to this project. With telescope time at a premium, it was a massive effort on their parts."

With the Tempel 1 light curve data acquired by Meech in hand, Belton and Chesley independently worked on determining the rotation rate for Tempel 1. As it turned out, the data revealed it was not so easy.

"The comet doesn't just rotate at a specific rate -- it speeds up and slows down its rotation depending on what part of its surface is heated by the sun," said Steve Chesley, Stardust-NExT co-investigator from JPL. "Overall, the comet’s spin is speeding up over time. We expect its average rotation rate to go up progressively as it continues its orbits around the sun, but it is hard to define just how much."

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