A philosopher once asked, "Are we human because we gaze at the stars, or do we gaze at them because we are human?" Pointless, really... "Do the stars gaze back?" Now that's a question. -Narrator, "Stardust"
Late last night, the NASA mission Stardust flew within 178 km (110 miles) of the nucleus of the comet Tempel 1, seeing it up close for the first time since July 2005! Here's one of the better images from closest approach:
[Click to embiggen.]
To give you an idea of what you're seeing here, the comet is roughly 7.6 x 4.9 kilometers (4.7 x 3.0 miles) in size.
So, why did NASA fly Stardust past this comet? Ah, set the way-back machine for 5.5 years ago... In July 2005, NASA's Deep Impact space probe flew past the comet Tempel 1. As it did, it slammed an 370 kilogram (800 pound) block of copper into the comet at a speed of 10.2 km/sec (6.3 miles per second). The purpose was to create a crater on the surface and excavate deeper material, allowing scientists to see what lay beneath. When the impactor hit, the energy released was equivalent to nearly 5 tons of TNT exploding! And while some amazing images were returned, the expanding cloud of debris blocked the best views, and so it wasn't possible to get all the data they wanted.
The Stardust probe, meanwhile, was originally launched to collect samples from the comet Wild-2 and return them to Earth using an ejectable return container. The main probe sailed on, and engineers repurposed it to swing by Tempel 1 to see if any new images could be taken. And now, nearly six years later, the renamed Stardust-NeXT (Stardust-New Exploration of Tempel) mission has succeeded!
The whole point here was to see the impact crater from 2005, and Stardust was able to do that. It's difficult to see in these images here, but Pete Schultz, an impact specialist with the mission, said the crater is about 150 meters across and has a central peak, indicating material fell back to the comet. The crater wasn't as obvious as expected, but is about the right size given the impactor speed, mass, and angle of impact.
Here's a comparison of one of the new Stardust images with an image taken from Deep Impact in 2005:
On the right is an image from Stardust near closest approach, and on the left is one from Deep Impact, which I rotated to match up the orientation. The angle of observation was different for the two spacecraft but you can see some of the same features. For example, on the DI image you can see two large craters on the left, and those are also in the Stardust picture fairly well centered. It so happens that the impactor from DI hit between those two craters, very close to the one on the left. As I mentioned, the new crater is extremely difficult to see -- during the press conference, I couldn't see it even when the location was pointed out!
Comets don't have weather like planets do -- no atmosphere! -- but their icy surfaces get modified as they approach and recede from the heat of the Sun. Other Stardust images do show some changes to the comet's surface! As the scientists get more of a chance to compare before-and-after shots, they'll be able to see even better how this tiny world has changed since we visited it last.
Image credit: NASA/JPL-Caltech/Cornell; NASA/JPL-Caltech/UMD