[meteorite-list] Deep Impact Punches Large Hole in Comet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Jul 4 20:14:05 2005
Message-ID: <200507050013.j650DHi16756_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/deepimpact/050704crater.html

Deep Impact punches large hole in comet
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
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.

Twelve hours after the spectacular July Fourth impact, researchers were
still unable to directly see the newly formed crater because of
streaming clouds of dusty debris blown out into space. But principal
investigator Michael A'Hearn said additional image processing is
expected to reveal the impact site, allowing measurements that will shed
new light on the comet's structure and composition.

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."

Images from the Hubble Space Telescope, a European probe in transit to
another comet and scores of ground-based observatories recorded a
five-fold increase in Tempel 1's brightness when the impactor hit.

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 Borelly, 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 Borelly, 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 Borelly 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 Borelly, 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."
Received on Mon 04 Jul 2005 08:13:16 PM PDT