Deep Impact is a smashing success
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 4, 2005
PASADENA, Calif. (CBS) - A scientific smart bomb crashed into Comet Tempel 1 early today, blasting a shower of icy debris into space in a 23,000-mph Fourth of July spectacular 83 million miles from Earth.
"Jeez, and we thought it was going to be subtle!" marveled comet expert Donald Yeomans as images of the impact were received at the Jet Propulsion Laboratory. "We've had a far bigger explosion than we anticipated. ... I can't imagine how this could go any better."
"It's illuminating some extremely interesting surface features," Yeomans said, describing the pictures as they as they came in. "That's going to keep the scientists going for a long time. The navigation was perfect, it couldn't have been any better. The impact was bigger than I expected, bigger than most of us expected. So this is going to tell us a great deal about how this comet is put together."
The 820-pound impactor spacecraft, programmed to place itself directly in the comet's path, collided with Tempel 1 at 1:52 a.m. EDT (10:52 p.m. PDT Sunday), releasing the energy equivalent of 4.5 tons of TNT as it vaporized in a sudden, spectacular flash.
Looking on 5,250 miles away, the Deep Impact mothership that ferried the impactor to Tempel 1 trained two telescopes and an infrared spectrometer on the impact site, studying the subsurface ices blown into space by the collision.
The crater that almost certainly was formed was not immediately visible
in the obscuring cloud of debris that sparkled in the sunlight against the
black of deep space. Images stored aboard the flyby spacecraft will be
downlinked later today, presumably including shots looking into the crater
"I'm at a loss to explain just how on Earth our little washing machine-sized impactor caused such a disturbance some 83 million miles away," Yeomans said. "This is going to take some work to explain, but it's sure taken me by surprise. And I suspect some of my colleagues up in the science area are equally surprised."
The flyby spacecraft had just 13 minutes to collect and transmit its highest-priority data before re-orienting itself, bringing protective dust shields to bear as it fell behind the comet, passing just 300 miles below the Washington, DC-size nucleus at closest approach.
Back on Earth, meanwhile, astronomers in virtually every major observatory in the world with a view of the collision aimed their telescopes at Tempel 1 to monitor the flash of the impactor's destruction. The Hubble Space Telescope, the Spitzer Infrared Telescope and the Chandra X-Ray Observatory also monitored the collision from Earth orbit.
The comet was just above the southwestern horizion for observers in the extreme western United States, but it was not immediately known how many amateurs might have been able to detect the sudden brightening that would have signaled the impactor's crash.
But it was the flyby spacecraft that had the best seat in the house, making its own observations while relaying final close-up images from the impactor taken just a few minutes before its kamikaze-like destruction.
In its final few shots, Yeomans estimated the impactor's camera was able to detect features as small as about eight inches across. The final picture was snapped just 3.7 seconds before impact.
To give the small impactor enough mass to excavate a significant crater, it was loaded with enough copper to make 45,000 pennies. Copper was chosen for the bulk of the impactor's mass because it is not present in comets, allowing astronomers to ignore the remains of the spacecraft in spectroscopic studies of the resulting debris cloud.
While it will take time to fully analyze the downlinked data, astronomers almost certainly will learn more than enough to re-write their textbooks about cometary structure and evolution. On a more fundamental level, the $333 million Deep Impact mission will shed light on the composition of the cloud of gas and dust that coalesced to form the solar system 4.6 billion years ago.
And while it was not a primary goal of the mission, the deep space fireworks display will give scientists and engineers valuable insights into what might be needed someday to divert or destroy a comet on a collision course with Earth.
"It's considerably brighter, there's considerably more material coming off than I thought," Yeomans said, watching the initial impact images come in. "The predictions on the science team were all over the map. Someone won a fairly large-size pool here with a long-shot prediction of a rather extraordinary impact.
"We've got an object the size of a washing machine going in here creating a crater and ejecta that's just enormous. At least that's the way it looks like now. ... One of our science team members actually predicted the impact would release sub-surface pressure and we'd have a far bigger explosion than they anticipated. That may be what happened, I don't know."
Deep Impact was launched from Cape Canaveral, Fla., Jan. 12. The impactor was released from the flyby spacecraft early Sunday, roughly 24 hours before the collision, at a point in space about 500,000 miles in front of Tempel 1.
As the much-faster comet overtook the two spacecraft, a steady stream of images was transmitted to Earth, revealing more and more surface detail as the distance between the hunters and their quarry closed.
The final few pictures revealed a strange-looking, tapered body resembling a misshapen avocado, one scientist joked. At one end, a feature that looked like a deep depression was evident, along with sharp, icy ridges.
Tempel 1 originated in the Kuiper Belt, a broad flattened disk of icy debris extending from the orbit of Neptune to well beyond Pluto. Disturbed by gravitational interactions, primarily involving Jupiter and Saturn, a Kuiper Belt comet can fall into the inner solar system and become captured in a so-called short-period orbit.
In the early solar system, gravitational encounters also threw large numbers of comets into a vast, spherical shell known as the Oort Cloud. Comets that eventually fall back into the inner solar system from the Oort Cloud typically have orbits measured in millions of years.
"Comets formed in the outer part of the solar system and preserve clues to its formation," said principal investigator Michael A'Hearn. "They formed from Jupiter on out to beyond Neptune four-and-a-half billion years ago, together with all the planets. The inner ones got ejected to the Oort Cloud, which extends halfway to the next star, whereas the ones that formed in the Kuiper Belt are probably still in the Kuiper Belt.
"We are examining comets that come in from the Kuiper Belt with Deep Impact. The problem in understanding the comets is, each time the comet goes close to the sun, the surface layer gets heated and this changes the surface layers. So it's only the interior that preserves the clues to the formation of the solar system."
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