Emily Lakdawalla • Jul 03, 2005
Quotes from Deep Impact "Pre-Impact Update" press conference at JPL
The panel consists of: Andy Danztler, Solar System Division Director at NASA HQ; Rick Grammier, Deep Impact Project Manager, JPL; Jennifer Rocca, Deep Impact Systems Engineer, JPL; and Mike A'Hearn, Principal Investigator, University of Maryland.
But JPL Director Charles Elachi gets to talk first. "You are witnessing with us one of the most daring and risky space missions we have ever undertaken. You will be witnessing tonight for the first time ever what the interiors of comets are made of. So tomorrow, our knowledge of the solar system will be different from what it is today."
Danztler: "I'm as ready as the spacecraft is. I'd like to emphasize how quickly this mission has played out. We launched less than 6 months ago. Everything looks green."
Grammier: "Starting yesterday, about 36 hours before the impact, we were getting all the systems ready for performing the final targeting maneuver, TCM-5. After traveing a little over 200 million miles, this final targeting maneuver was less than 1 mile per hour of adjustment. We ensured that all the systems onboard both spacecraft were healthy, because soon after TCM-5 we wold 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%.
"We took one final poll, determined everything was nominal, transitioned the impactor to its own power. We turmed to the release attitude, and released the impactor. Then we performed the divert burn on the flyby spacecraft, the largest burn we've done on the system. Not too long after that, we did verify that the impactor had detumbled and turned on its S-band communications system, and we established communication with the impactor. Data has started flowing to the MSA. I'm happy to report, this is a picture we captured, the impactor on its course right after the burn. That's how it looked from space." Then he showed a video of happy controllers in the MSA. "We were holding our breaths." Both the flyby and impactor spacecraft are within 1 km of their aim-points.
Rocca: "After successful separation, the flyby and impactor are beginning their independent missions. Both spacecraft communicate via S-band radio. It's working absolutely nominally. The impactor spacecraft, all of its data and images are sent across to the flyby, which sends all of its own images and data and the data from the impactor to the Earth. Our ground antenna coverage is extremely important, so we have triple coverage, two 70-meter stations plus an array of 4 images.
"We take the highest priority images sent 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." Rocca outlines the timeline immediately before the impact. "We anticipate a little bit of a rough ride. But we are taking images all the way in. As long as no debris goes into our camera, they should be pretty outstanding shots. We are using every single bit of bandwidth that we have to download those images to the ground." After the flyby, the spacecraft will look back for 60 hours. The lookback geometry is such that the High Gain Antenna can't point to Earth during that time; they'll be on Low Gain antenna, then after the 60 hours will turn to return all the data.
A'Hearn: "There have been outbursts by the comet. These were not expected, they haven't ever been observed for this comet from the ground, though they were seen for other comets." Shows a spectrum. "What we've done is combine a whole bunch of these squiggly lines and made a picure of the comet in water molecules. The first two images are before one of the outbursts, the third is in the middle, then in the lower right, the water has spread out. What we are trying to understand now is whether the water came out as water vapor, or as ice crystals that sublimed into water vapor." Showed a long movie of outburst activity over a month.
"What's going to happen tonight? We're going to get tremendous data from the spacecraft, and we're also going to get a tremendous amount of data from m any other observatories." Shows a Chandra X-ray image. "What we expect will happen is that after the outburst will put much more material into the coma, and that X-ray region will broaden toward the Sun."
Time for press questions.
How bright was the outburst? A'Hearn: "Several tens of percent of the brightness of the nucleus. But that rapidly dissipates. Within an hour they are entirely gone."
How long to get images of the crater? A'Hearn: "I'm going to say two days, so that I don't get nailed if we don't have them out in six hours."
Approaching the comet, how are the images going to be released, what are we going to see? When will we see things obvious to a nonscientist? A'Hearn: "You won't see a recognizeable nucleus in the MRI, which are the ones that are being a released." HRI must be deconvolved and will be released in "a couple of hours," both on the Web and TV.
How much detail will there actually be in the images? A'Hearn: "This depends on how comets work, which we don't actually know. But if you get a really big crater, you'll be able to see the crater as a crater even in the MRI images quite clearly. What you see in the ejecta cone, if it's fine material, it may look like a smooth continuum. If there's a crust near the surface then you may see large chunks as in the animation."
"The outbursts dissipate so rapidly that AutoNav can recover from any confusion. It turns out that several of the outbursts seem to have come from the same point in the rotation on the comet. If that one goes off again, that is something like 4 to 6 hours before the impact, which is before the AutoNav on the impactor really starts working."
Size and shape of comet? A'Hearn: "We'll have a great idea of it tomorrow. We know roughly the size, we know it's elongated."
How do we know what's going on on the impactor? Grammier: "The impactor is running on what we call the 'critical sequence engine.' It does not involve real-time intervention from the ground. It includes all imaging settings. At 2 hours out it is commanded to start its AutoNav processing. The data itself, the telemetry, it reports back to the flyby spacecraft based on a scheme we have given it, which is cyclical. Then the flyby spacecraft forwards those down to the ground. We are using the op nav data to make sure we have a good fix on where it is. Every hour, we get a status report from the attitude control folks. Every two hours, we get an update of the optical navigation solution."
Science? A'Hearn: "A lot of scientists think that a lot of the original water and organic matter on the Earth came from comets, 4 billion years ago. We want to know what those materials were, because the conditions that the planets formed under are not well determined. The ices in the comet's nucleus are very sensitive to the conditions of the early solar system." In essence, this mission should help create a "ground truth" for remote observations of comets. "We should find out whether we are all comets, or if we were partially asteroids."
Amateur telescopic observations? Grammier: "Depending on the type of crater that is formed and the amount of material that is thrown up, you can see a brightening of the comet for a few hours to several days."
An imaging question. A'Hearn: "We started separating the light of the nucleus from the light of the coma 2 weeks ago. It was still a point source, but you could separate it. We had some images from this morning from the HRI that suggest we're beginning to resolve the nucleus -- but you wouldn't believe it if I showed it to you. If we keep working hard, in the next few hours" they should separate it, and images of the nucleus will start appearing.
Rocca: "We are currently about 225,000 miles away. We're 12 hours out, we move about 550,000 miles a day. As far as we know, the impact time hasn't changed." Grammier: "Anywhere from 20 minutes to 45 minutes" is when they should be able to tell us if things worked well. How do we know it hit? Grammier: "Based on the images we get back from the flyby plus the images we get back from the impactor prior to impact. We don't consider loss of S-band signal sufficient because that could result from geometry, or from dust impacts. If, in the meantime, we get a report from one of the observatories, that'll help resolve it."
Targeting of impactor? Grammier: "The comet is very tiny in the targeting sensor." They are on target within their understanding of the comet ephemeris, but maneuvers will be necessary to improve the targeting as they approach.
Could anything send it off target? Grammier: "Particle hits could turn it around or send it off course, but the attitude control system is designed to understand that."
Can the flyby spacecraft be used for other comets after this mission is over? Danztler: "We're looking at all the options. Deep Impact was funded to do exactly what you've been hearing. What happens after that is another mission, so we have to look at funding levels. The flyby spacecraft, as long as it stays healthy, is out there to use."
What have been the biggest challenges? Grammier: "I still think we have our biggest challenges to go. We are heading for a body that we haven't resolved yet. The comet could be misshapen. We are 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."
Biggest concern? Grammier? "That the flyby spacecraft remains pointed at the impact site." A'Hearn: "We don't want a mountain between the flyby and the crater. We have no control over that."
How long until we understand what type of crater it is? A'Hearn: "I hope we will be able tell you at the press briefing tomorrow morning at 10 am. I won't be awake enough at the 1 am one."
Risks in this mission? Danztler: "Unlike many of the missions that we do, there's been a lot of learning as we go, learning after launch. For example, we didn't know the rotation rate of this comet."
Late in getting telemetry from impactor: Grammier: "I wouldn't call 30 seconds late. Especially considering the one-way light time."
So what kind of crater do you expect -- where are the betting pools among the team? A'Hearn: "None of us really expects either of the true extremes: one being tunneling in with a really narrow deep crater. There are ways it 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 pushes itself apart. I don't think any of us realistically believes either of those extremes."
International astronomers? A'Hearn: "We have a private email network that goes to every major observatory in the world. As soon as there are results, we tell them what we're seeing, they tell us what they're seeing. We also do it by PolyCom, which is coordinated by Karen Meech, who is in Hawaii." How many people? "150-200. A few per telescope. That doesn't include all the support staff, but the actual scientists."
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