Hartley 2
Once upon a time a spacecraft really did have a five-year mission.
It is NASA’s EPOXI spacecraft, which paid its second visit to a comet since 2005 when it flew past and photographed the Hartley 2 comet on November 4.
The EPOXI mission is recycling the Deep Impact spacecraft, whose probe intentionally collided with comet Tempel 1 on July 4, 2005, revealing, for the first time, the inner material of a comet. In fact, I have been able to find a whole riff I did about comets, with a preview of Deep Impact’s first mission for the late, lamented Trufen.net (scroll down).
CNN’s coverage of today mission used the word “survived” in its lead, dramatically implying the spacecraft had gone in harm’s way:
A spacecraft survived the closest encounter ever with a comet on Thursday, tracking it just 435 miles (700 kilometers) from the comet’s nucleus.
Since 435 miles is farther than Los Angeles is from San Francisco my initial reaction wasn’t to gasp in amazement. Yet people in San Francisco give the impression they’d like to be even farther away, so who can say? NASA also says comet Hartley 2 is “much more active” than Tempel 1, the previous comet visited by Deep Impact, despite being smaller. Smaller and much more active – there’s San Francisco all over again.
Amateur skywatchers may be able to see Hartley 2 in a dark sky with binoculars or a small telescope.
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CNN’s coverage of today mission used the word “survived” in its lead, dramatically implying the spacecraft had gone in harm’s way…
Look at all those jets. The vicinity of the comet’s nucleus is filled with tiny dust particles (maybe some not-so-tiny– look at all those boulders), each of them moving at terrifying speed with respect to the spacecraft.
Presumably the EPOXI/Deep Impact crew made a careful decision that 700 kilometers was an appropriate distance. But the risk of damage or destruction was not negligible.
Thanks for the explanation Bill!
Looks rather like 25143 Itokawa, an asteroid that was met by Japanese probe Hayabusa in rendezvous last year. The both have peanut shapes, and jagged bolders sitting on the surface. Some of the rocks are upright on their narrow ends because the extremely weak gravitational field doesn’t exert much force to pull them over. And both bodies have extensive smooth areas. I’d say they are probably “snow” fields in the case of the comet, but perhaps pools of sand and dust in the case of the asteroid.
In both cases, the extended shape makes me think that two or more pieces are involved. The smooth areas may well be where fine material has gathered between them, “cementing” them together. I doubt they are well cemented, though. I bet you could separate them with a stick of dynamite, and send a cloud of fine material into space in all directions.
But I suppose that, fundementally, Itokawa and Hartley 2 are very different. The asteroid doesn’t spew ice and dust into space, to form a tail. The comet seems to have more volatile components, hintng at a very different nature.