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Planetary News: Asteroids and Comets (2005)

Deep Impact Successfully Splits in Final Hours Before Comet Encounter

By Emily Lakdawalla
July 3, 2005
The Deep Impact impactor after separation
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The Deep Impact impactor after separation
This image, captured by the Medium Resolution Imager on the Deep Impact flyby spacecraft, shows the impactor flying away from the craft, toward the comet. The separation of the two spacecraft happened nominally at 23:07 on July 2, PDT (06:07, July 3, UTC). Credit: NASA / JPL-Caltech / UMD

JET PROPULSION LABORATORY, PASADENA, CALIFORNIA -- Mission controllers reported this morning that the Deep Impact flyby and impactor spacecraft separated successfully and that "everything is green" for the comet encounter scheduled for 22:52 this evening (05:52 July 4, UTC).

"Starting yesterday, about 36 hours before the impact, we were getting all the systems ready for performing the final targeting maneuver," said Rick Grammier, Project Manager for Deep Impact at JPL. "After traveling a little over 200 million miles [320 million kilometers], this final targeting maneuver was less than 1 mile per hour [0.5 meter per second] of adjustment. We ensured that all the systems onboard both spacecraft were healthy, because soon after the trajectory correction maneuver we would be transitioning to internal power on the impactor. The burn executed nominally and was within about 0.4% of the expected design of the burn. This has been a well-behaved spacecraft, all the burns have executed within less than 1% of our expectations."

The trajectory correction maneuver has placed both spacecraft "within one kilometer [0.6 mile]" of the intended aim-points for the two spacecraft, Grammier said. The impactor is on target for Tempel 1, and the flyby craft is on target for a point 500 kilometers (320 miles) to one side of the comet. However, that doesn't yet guarantee an on-target impact for the impactor. "The comet is still very tiny to the impactor targeting sensor," Grammier said. He explained that the ephemeris, or orbit description, for the comet is still uncertain enough that further trajectory correction maneuvers will be necessary for both the impactor and the flyby craft to set everything up for tonight's encounter. The impactor's last maneuvers will be performed 91 minutes, 35 minutes, and 13 minutes prior to the expected time of encounter.

With all engineering activities aboard the spacecraft happening apparently nominally, mission workers' concerns have shifted from potential engineering problems to potential science problems. The biggest science concern is something that the mission cannot control. "We are heading for a body that we haven't resolved yet," Grammier said. "The comet could be misshapen. The impactor is heading for a lit portion of the nucleus, but if there is a geographic promontory that could block our view, that could be a problem." Principal Investigator Mike A'Hearn concurred. "We don't want a mountain between the flyby spacecraft and the crater. We have no control over that."

Another potentially bad outcome would be for the flyby spacecraft to suffer serious damage as it passes through its closest approach point to the comet. Such damage could jeopardize the transmission of the priceless science data to the Earth. Because of the hazardous environment around the comet, the mission has adopted a "live for the moment" approach, explained System Engineer Jennifer Rocca. "We take the highest priority images and send them to the ground first. Our intent is to send all of these high priority images to the ground before closest approach. That way, if the flyby craft is damaged, we will have all the highest priority data on the ground." And in order to make sure the data is not lost due to bad weather or other mishaps on the Earth, the encounter will be observed simultaneously by three different Deep Space Network ground stations.

Some press questions focused on when the waiting science team and public would find out the size of the impact crater produced in this daring experiment. A'Hearn said he hoped to be able to describe the crater at a press briefing to be held at 10:00 local time on Monday morning (17:00 July 4, UTC).

A'Hearn confirmed that the science team has a betting pool but wouldn't state what the team's consensus was; he said only that "None of us really expects either of the true extremes. One extreme would be tunneling in with a really narrow, deep crater. There are ways that could happen but we don't expect it. At the other extreme, you go in, you make the crater, the comet reacts, ices vaporize and the comet pushes itself apart. I don't think any of us realistically believes either of those extremes." Impact scientist Peter Schultz wouldn't say what his actual guess was either. He did say that "as a betting man, I wouldn't guess what everybody else is guessing. I put a guess in for what I would really really like to see, a big boom. It could happen." Schultz said that the impact would generate temperatures of up to 8000 Kelvin [7700 degrees Celsius, or 14,000 degrees Fahrenheit], which could make things pretty exciting on the surface of Tempel 1.

Members of the public who want to make their own guesses can enter The Planetary Society's Great Comet Crater Contest right up until the moment of impact, at 05:52 UTC on July 4. It's still a wide open question what events will unfold over the next 24 hours!