Deep Impact punches large hole in comet
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 4, 2005
Elated scientists studying a treasure trove of data from NASA's Deep Impact mission said today the 820-pound probe that slammed into comet Tempel 1 at some six miles per second early today excavated a large crater in its icy crust, blowing enormous amounts of dust and gas into space.
Before the collision, the Deep Impact science team started an informal betting pool to guess the size of that crater. Pre-impact estimates ranged from a crater the size of a house to one the size of a football stadium.
Ignoring the effects of the atmosphere, the impactor would have blasted a 100-foot-wide crater had it hit a solid surface on Earth, releasing the energy of 4.5 tons of TNT. The energy release when it hit Tempel 1 was roughly the same, scientists believe, but the crater almost certainly was much bigger.
"The difference is the gravity is so low it's like the Energizer bunny, it doesn't stop, it just keeps on flowing out," Pete Schultz, a cratering expert at Brown University and a member of the Deep Impact team, said in an interview. "Density and the low gravity, both of those things would make this thing much bigger."
Asked at a news conference to speculate on just how much bigger, Schultz declined, joking that "this is not fair because I'm in the pool, too, so I have to be careful not to show any bias."
"Rather than do that, I think I'll just simply say I think it's big and leave it at that," he said. "I don't think it's house sized, I think it's bigger than that."
The impactor was vaporized in the collision. But images from the Deep Impact flyby spacecraft that ferried the impactor to Tempel 1 showed a sudden incandescent flash when the washing machine-sized probe slammed into the comet's crust, a flash that went through stages as it quickly intensified. The images reveal a clear ejecta plume shooting back out into space and the shadow of the developing plume on the surface of the comet.
"At the moment of impact, you heat materials to extremely high temperatures," Schultz said. "Some of that is heated vapor, some of it is hot, molten droplets from dust from within the crater itself and it's kind of like firing a flash gun. It's material that is glowing so brightly that without a light, you could actually take your own picture. It is very similar to that. And it tells us a lot about the cratering process."
Thirteen minutes after impact, the flyby spacecraft fell behind Tempel 1, passing just 300 miles below the city-sized comet. After re-orienting itself, the probe began shooting so-called "look back" images showing the receding comet and the debris plume expanding into space.
One spectacular shot, taken at a distance of 16,700 miles, showed the debris cloud from the crater formation being lit by the sun, "which is exactly what we wanted to accomplish," said project manager Rick Grammier. "This just speaks a thousand words. It's an absolutely incredible picture."
The ejecta cloud in that photograph extends "thousands of kilometers at least," A'Hearn said. "Whether they go further requires more image processing to know. It is certainly largely dust. We see mostly dust in those images because that's what's reflecting the sunlight and by then, that early flash has cooled off. On the other hand there is probably water vaporizing from ice that is driving some of that. So there is gas with it, but that's not what you actually see in the image."
A'Hearn said the debris was "still coming out for at least several hours after the impact event.
"This will be important for understanding the nature of the material at the bottom of the crater," he said. "If there's a lot of volatiles there, the outgassing will continue to go on for a long time. It could go on for weeks. So that spectacular image is really important to the science."
The Deep Impact photos also provide the best views yet of a comet's nucleus. Prior to the collision, Tempel 1 was thought to be pickle shaped, but the reality proved much different.
"It obviously does not look like a pickle or a cucumber or the various things we've talked about before. It looks closer to a loaf of bread or a muffin or something like that," A'Hearn joked.
"There is a lot of topographic relief. There are things on this nucleus that really do look like impact craters to many of us. It looks very different from either comet Wild 2 or comet Borrelly, the two comets we've had closeup imaging of in recent years. We don't understand what this means yet.
"When Wild 2 looked different from comet Borrelly, we thought it might be due to the different orbital histories of the two comets," A'Hearn said. "But this comet's had an orbital history that we think its pretty much similar to comet Borrelly and yet it looks totally different. So there's something more going on here that we haven't understood yet."
In close-up images shot by the impactor before its demise, "there are lots of bright spots," A'Hearn said. "Most of them are just steep slopes facing the sun, so they get a lot of solar illumination. A couple of them don't seem to be that, we don't understand them.
"There are some really smooth areas there that we also don't understand. It's not even a flat smooth area like there was on Borrelly, it clearly curves around the side into darkness. We've got to understand what that is. Almost certainly there is layering and that's got to be part of what's causing the differences in in the nature of topography at different elevations.
"So there are craters, there are differences in topography due to layering of the surface materials and there are jets coming from somewhere which we hope to trace out."
Based on the amount of the dust blown away from the comet, Schultz said he believes Tempel 1 is relatively fluffy compared to a terrestrial bodies like Earth. Whatever crust is present probably isn't very thick.
"It indicates to me, at least, that this was probably a soft surface, a dusty, soft surface at the very top and then the stuff below," he said. "I don't think, at least from what we can see, (the comet has) a very thick, hard crust. Now that doesn't mean that doesn't exist. We are looking from a distance and there's a lot of material that's being obscured by the material coming out of the event itself. So there still may be things in there that will tell us much more yet."
MISSION STATUS CENTER