Scientists eager for Deep Impact's final chapter
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 3, 2005
PASADENA, Calif. (CBS) - NASA's Deep Impact mission moved into its final stages today, with one spacecraft hurtling toward a suicidal July Fourth comet collision and another positioning itself to study the crash and relay close-up pictures from the doomed probe back to Earth.
The Deep Impact mothership, known as the flyby spacecraft, released its
820-pound copper-clad impactor probe earlier today, placing it on a
trajectory that will allow comet Tempel 1 to run it down from behind at 1:52
The flyby spacecraft, 5,350 miles away at the moment of impact, will monitor the crash and its aftermath for 13 minutes, training two telescopes and an infrared spectrometer on the crater and the ejected debris before passing 300 miles below and then behind Tempel 1.
Before moving into a protective orientation for close approach, the flyby spacecraft will relay pictures from the doomed impactor as it hurtled toward its target, images that could show surface features as small as eight inches across.
That's a best-case scenario, however, and just how sharp the pictures turn out is anybody's guess. It depends in large part on how thick the gas and dust in the comet's atmosphere might be as the impactor closes in and whether the probe's optical system is pitted or damaged.
The issue is complicated by the fact that Tempel 1 has been undergoing unexpected outbursts in recent weeks, suddenly spewing ice or water vapor into space in eruptive blasts. Such outbursts may be normal or evidence of some unusual process going on.
Regardless of the close-in image quality, the final impactor images and analysis of the ejected debris and the crater itself by the flyby spacecraft will give astronomers potentially revolutionary insights into the structure, composition and evolution of comets.
On a more fundamental level, Deep Impact is expected to shed light on the birth and evolution of the solar system by exposing pristine subsurface material that is representative of the raw materials used for planet building 4.6 billion years ago.
"You're witnessing with us one of the most daring and risky space missions we have ever undertaken," Charles Elachi, director of the Jet Propulsion Laboratory, said today. "Last night we successfully pulled the trigger ... and the impactor is heading at 23,000 mph toward the comet.
"You'll be witnessing tonight for the first time ever the unveiling of what comets' interiors are made of. Tomorrow, our knowledge of the solar system will be different than what it is today and a new trail will have been laid out for exploration that other people will follow in the future."
Here is a timeline of upcoming events (all times in EDT):
TIME...........EVENT July 3 05:45 p.m......Ephemeris update 11:30 p.m......NASA television coverage begins 11:53 p.m......Start impactor auto-navigation imaging July 4 12:21 a.m......Impactor trajectory maneuver No. 1 01:17 a.m......Impactor trajectory maneuver No. 2 01:39 a.m......Impactor trajectory maneuver No. 3 01:52 a.m......Impact 02:05 a.m......Flyby reorients to shield mode for close approach 02:06 a.m......Flyby closest approach (311 miles) 02:51 a.m......Flyby first post-impact image 04:00 a.m......Post-impact news briefing on NASA TV 02:00 p.m......Deep Impact news conference on NASA TV
To reach the desired impact zone on the sunlit side of Tempel 1, the impactor spacecraft was programmed to home in on the comet using star trackers and observations from an on-board 4.7-inch medium-resolution telescope. Because radio signals need more than seven minutes to cross the 83-million-mile gulf between JPL and the spacecraft, human intervention is not possible. The impactor must do the job on its own.
So far, the impactor has been flying freely in a quiescent mode, on the same trajectory it was placed on at release from the flyby spacecraft. But two hours before impact, the auto-nav system will begin actively homing in on Tempel 1, maintaining target lock like a smart bomb.
Three autonomous trajectory correction maneuvers are planned, the first at 90 minutes before impact; the second at 35 minutes out; and the third just 12.5 minutes before the crash.
The same navigation routines and an identical medium-resolution telescope are aboard the flyby craft, along with an 11.8-inch telescope intended for detailed high-resolution observations. While this morning's successful separation gave the flight control team a morale boost, the most difficult stages of the Deep Impact mission are still to come.
"We're going to a body in the solar system that we haven't even resolved yet from a spatial viewpoint," said project manager Rick Grammier. "I'm sure it still holds a lot of surprises for us, we still have yet to perform the actual auto navigation on the impactor flight system itself. That doesn't cut in until two hours out.
"We're still watching the performance converge on the attitude control system, so we've still got quite a few challenges left to go. Now, having said that, you still have to have the flyby spacecraft, operating on the identical targeting algorithms, to be able to look and see that impact occur and that's also dependent on what the shape of the comet nucleus is."
Donald Yeomans, a comet expert at JPL and a Deep Impact co-investigator, has described Tempel 1 as a "jet-black pickle-shaped icy dirtball the size of Washington DC."
"We think it's misshapen," Grammier said. "We're trying to target a lit portion of that nucleus but if there's a geographical feature that could possibly block the flyby from seeing (the impact), that's another issue we have to deal with. So there are all sorts of challenges still out there to meet."
MISSION STATUS CENTER