Comet Deep Impact
© NASA/JPL-Caltech/UMD
Deep Impact's impactor hit Comet Tempel 1, spewing debris, but the mission was not able to see the resulting crater
NASA's Deep Impact mission pounded a comet in 2005, but failed to see the resulting crater. Now, scientists will get a second chance to glimpse the damage when a second spacecraft flies by the comet on 15 February.

In an unprecedented experiment, NASA smashed a 372-kilogram impactor into Comet Tempel 1 on 4 July 2005 as part of its Deep Impact mission.

A flyby spacecraft recorded images of the impact from a safe distance, but the cloud of impact debris and a flawed camera made it impossible to see the crater itself. Studying the crater could have revealed more about the interior composition and structure of comets.

Now, another spacecraft is about to make its own fly-by of the comet, offering a second chance to image the structure.

Called Stardust, the spacecraft collected material from Comet Wild 2 in 2004 and sent it in a capsule back to Earth, where scientists have been studying it ever since.

Lucky sighting

Since the Stardust spacecraft was still in good health after its mission to Wild 2, NASA decided in 2007 to send it to Tempel 1 as a follow-up to the Deep Impact mission.

At 0437 GMT on 15 February (2037 PST on 14 February), the Stardust spacecraft will complete its long journey to Tempel 1, flying within 200 kilometres of the comet's solid core, or nucleus.

In a press briefing on Wednesday, the mission team sought to dampen expectations for imaging the crater itself, which might be inconveniently located on the wrong side of the comet during the spacecraft's brief encounter.

The comet is slowly rotating once every 41 hours and scientists do not have enough information to guarantee that the side with the Deep Impact crater will be in view at the time of the fly-by.

"I hope we get to see the impact crater, but that's going to be a bonus," said Pete Schultz of Brown University in Providence, Rhode Island.

Then and Now

Team members said much would be learned from the fly-by even if the crater remains elusive.

For example, scientists will look for changes since the Deep Impact spacecraft's visit in 2005 that may be unrelated to the collision. Comets are constantly shedding gas and dust, which may have caused noticeable changes to Tempel 1's nucleus.

"We know that comets lose material, but the question is, how much does the surface change and where does the surface change?" said Joe Veverka of Cornell University in Ithaca, New York, the chief scientist for Stardust's mission to Tempel 1. "We'll be able to answer that question by comparing our images with those taken by Deep Impact in 2005."

Like the Stardust spacecraft, the Deep Impact spacecraft was also sent to visit a second comet. It flew by Comet Hartley 2 on 4 November, where it saw basketball-sized snowballs shed by the comet.