On Valentine's day, while we were all cooing over your loved ones or lamenting the obvious negligence of the postman, scientists at Denver's NASA station were cooing over something rather larger. On February 14th this year, NASA's Stardust probe made its second visit to the comet Tempel 1 at 8.40pm PST, shaving the comet at a distance of 111 miles (178 km) and traveling at a relative speed of 24,300 mph (10.9 km per second). This is the first time scientists have been able to get a second look at a comet, which allows them to compare data from the first visit in order to learn more about these icy inhabitants of our solar system.
"Here's a chance where we can see what has changed, how much has changed" on Tempel 1, so we'll start unraveling the history of a comet's surface," said the mission's lead investigator Joe Veverka of Cornell University. "That could help us better understand the life cycle of a comet. We have no idea whether we're talking about things that have been there for a hundred years, a thousand years, a million years."
The Stardust probe was initially launched in 1999, and spent the first five years traveling towards its mission target, another comet called Wild 2. Having collected dust samples and returned them to Earth in 2006, it was still fully functional so NASA recommissioned it to visit Tempel 1, which had first been visited by the Deep Impact spacecraft in 2005. On this occasion Deep Impact slammed a projectile into the comet to collect cometary ice and dust, and scientists were particularly interested to examine the crater left in the surface, and hopefully learn more about the comet.
The scientists are intrigued by the crater scar, which shows a small mound in the center, indicating that some ejecta exploded upwards and immediately back down again. "This tells us this cometary nucleus is fragile and weak based on how subdued the crater is we see today," said Pete Schultz of Brown University, Rhode Island.
The Stardust-NExT mission had three particular goals; to observe changed surface features; to get images of new terrain; and to view the crater generated by Deep Impact's projectile. All three were completed successfully, and Stardust's NavCam instrument took 72 high-resolution images of the comet and collected 468 kilobytes of data about its 'coma' dust – the cloud that constitutes a comet's atmosphere.
"This mission is 100 percent successful," said Veverka. "We saw a lot of new things that we didn't expect, and we'll be working hard to figure out what Tempel 1 is trying to tell us."
Stardust sustained some damage during the closest approach, with a dozen impacts of disintegrating cometary particles penetrating more than one layer of its protective shielding. "The data indicate Stardust went through something similar to a B-17 bomber flying through flak in World War II," said Don Brownlee, Stardust-NExT co-investigator from the University of Washington in Seattle. "Instead of having a little stream of uniform particles coming out, they apparently came out in chunks and crumbled."
As the mission winds down, the departure phase will include snapping an image of the receding comet every five minutes for five days and then every 12 minutes for the following six days. Several weeks after this, the Stardust spacecraft will finally be retired. "This spacecraft has logged over 3.5 billion miles since launch, and while its last close encounter is complete, its mission of discovery is not," said Tim Larson, Stardust-NExT project manager at the Jet Propulsion Laboratory (JPL). "We'll continue imaging the comet as long as the science team can gain useful information, and then Stardust will get its well-deserved rest."
JPL, part of the California Institute of Technology, manages Stardust-NExT for the NASA Science Mission Directorate. Lockheed Martin Space Systems built the spacecraft and manages day-to-day mission operations.
"This little spacecraft has really been around the block. Even through the odometer is high and the fuel is low, it did everything we asked of it and the results are visually amazing," said Allan Cheuvront, Lockheed Martin Space Systems Company program manager for Stardust-NExT.