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Stardust Heads Home After Successful Comet Encounter

By Don Brownlee

On January 2, 2004, Stardust flew within 236 kilometers of Comet Wild 2 and captured thousands of particles in its aerogel collector for return on Earth in January 2006. In addition to collecting the samples, which is its primary goal of the mission, the 13,000 miles an hour flyby was quite an event with unexpectedly exciting scientific payoffs. Going into the encounter, the team was confident that we had done everything that we could do to make the flyby a success, but nonetheless everyone was nervous. Space is very unforgiving and there were many things that could have gone wrong.

It's a Matter of Survival

Comet Wild 2

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The greatest fears were that either that the spacecraft would be severely damaged or that it would not collect enough samples. Comets are not well-understood bodies, particularly their production of dust and rocks, and there was a disturbing range of possible surprises that the comet might throw at us. We wanted to get close enough to collect our dust samples but not close enough to have a reasonable chance of getting hit with a small rock. Anything larger than a grape would bust through Stardust's protective shield, and potentially cause catastrophic damage. Telescopic observations showed that the rocks were there but we didn't know for sure how many there would be at the place and time of the flyby. A month before the encounter, putting the best data into the Wild 2 dust model, the prediction was that Stardust would have about a 4% chance of hitting a 1cm rock at the planned flyby distance of 150km. The risk was too high and after several reviews and much hand wringing we decided to back off the closest approach to 300km and thus cut the chance of a lethal impact in half to 2%. This would make things safer but reduce the number of collected particles in half.

Shortly before the flyby, the comet moved from the daytime sky into the nighttime sky and could just barely be observed with telescopes. With the telescopes tilted close to the horizon, critical observations of the comet's location and brightness were made from Mauna Kea in Hawaii with the world's largest telescope and also from Mount Palomar in California. The comet was found to be slightly fainter than expected and its location was better determined. On the basis of this information we re-targeted the miss distance to 250 km. We hoped that the decision was the right one.

During the encounter, there was a large wall-mounted display in the JPL control room showing a green spot that slowly traced a line from left to right. The display marked the reception of Stardust's radio signal coming all the way from the other side of the Sun almost a half an hour light travel time away. As the dot moved and traced out a line it indicated that the spacecraft was still alive and transmitting. It was a great thrill when we reached the estimated time of closest approach and the spot was still there and moving. Stardust was alive and steady as it flew through the unknown and dangerous region around the comet. There was still risk of destruction but reaching the halfway point showed, at the very least, that we had not greatly underestimated the impact threat.

Artist Depiction of Stardust Flying Through Comet Wild 2's Jets

Stardust Animations

When the dust impact data was analyzed, the peak dust impact rate measured on the sensor was over a thousand impacts a second implying the rate over the full area of Stardust was a million a second. Over a dozen particles actually penetrated the first layer of the impact shield and the largest impacting particle was about half a centimeter across, the size of a small grape. The flyby was Stardust's D Day and it survived flying through a cloud of bullets. In spite of all the fears, estimates were believed to have been right on. Stardust survived without serious damage and it collected the needed samples plus some.

Stardust's dust impact detectors produced an unexpected surprise. As the comet was approached, it was expected that the number of dust impacts would smoothly increase, reach a peak and then smoothly decline. The expectation was that dust would be smoothly distributed around the comet and only decreases with distance. Instead the dust impacts came in bursts. This is partly explained by the images that show sharp jets of gas, dust and rocks ejected from the surface into space. When Stardust passed through a jet it would see a burst of particles. Another factor producing bursts is that larger particles (rocks) ejected from the comet can break up at any time producing small clouds of debris. Using the D-day analogy, it truly was like flying through flack. If we had known about this behavior before the flyby it would have greatly concerned us because is adds another unpredictable element, making it harder to predict the likelihood of survival at a certain flyby distance.

Finding the Comet

Comet Wild 2

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Surviving the encounter was a challenge but another great concern was navigating to the comet from afar and then when finally up close, tracking the comet with Stardust's Navigation Camera (Navcam). The Navcam optical navigation task was done by taking images of the comet and measuring its position relative to background stars. This work started two months before the flyby when the comet was first seen in Navcam images. Comets are unusually difficult to target with a spacecraft because they are surrounded by gas and dust and the actual body cannot be seen from Earth. Another complicating factor is that comets do not follow perfect orbits around the Sun. Unlike other bodies, comets have a non-gravitational force, the rocket effect that acts on them. Comets lose mass to space in jets and this produces a small force that causes them to deviate from a purely gravitational orbit. To get close to the comet, Stardust used the Navcam as well as radio tracking data from NASA's Deep Space Network and astronomical observations. Right up to the encounter this was a major concern because we wanted to get close but not so close that would have a serious chance of hitting a centimeter or larger rock. There was a large team of navigators that worried about this right up to the last minute.

The Navcam was carried on the mission primarily for navigation but it also provided a means of taking high-resolution science images of the comet as the spacecraft flew by. Taking science images required that the camera center on the comet at close range and then track it at rates above one degree a second. Close to the comet, the spacecraft is held in a fixed attitude so that the impact shields - called Whipple shields - are kept up front to protect everything downstream. The spacecraft is fixed but the camera is able to track the comet because it looks through a mirror that rotates as the comet goes by. This is very much like taking a series of pictures of a roadside attraction from a rapidly moving car. The rotation axis of the camera mirror has to be precisely lined up the comets motion and this is done six minutes before the closest approach. An image was taken, the comet's center was determined and the entire spacecraft was rolled about its long axis to place the image in the center of view. This was all done without help from the navigation team on Earth because the round trip light travel time was nearly an hour. After the autonomous roll maneuver, the camera used the center of the comet image to predict how much the mirror should move to take the next image. This was quite a feat because the images used for tracking were taken 20 seconds apart. With the spacecraft's speed, the comet moved over 5 fields of view between some images. Tracking the fast moving comet was a major effort but it worked perfectly. During the time that the close-in images were being taken, dust and rocks but the craft's 16 rocket engines kept things well aligned were pelting Stardust.

When it counted, the camera worked to perfection but things were less than rosy just a week before the encounter. The camera was found to be foggy and its images looked like they had been taken through fogged glasses on a winter morning. During the whole mission we had dealt with a mysterious contamination issue where volatile material was condensing on or near the imaging part of the camera. We had developed procedures to deal with the problem and we though it was under control. Close to encounter it was realized that the problem had returned and was so severe that we might not be able to see background stars for the final stages of navigation. This was surprising because we had been fortunate to image the comet early, almost two months before the flyby. To reduce the fogging problem we rolled the spacecraft 180 degrees to place the camera cooling mechanism into direct sunlight. This maneuver heated the camera image detector enough to drive of the contamination and during the flyby the fogging problem was almost undetectable. This move was done just a week before the flyby and it placed the solar cell panels in the shade for nearly an hour. It was a somewhat daring but prudent and highly successful move.

The First Pictures

Comet Wild 2

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The camera was programmed to take 72 images during the flyby and then send them back when the flyby was over. With a special effort by the team in Denver, a scheme was developed to send back 5 images in time for a press conference scheduled only few hours after the flyby. There were a number of concerns about the quality of the images and if there would even be any images at all. Would the mirror track, was the imaging sequence timed right, would the images be severely blurred, would be exposure times be adequate and lastly would the comet be interesting? Would people be disappointed? When the first picture came down, it was one of the closest approach images and it was a wonder. Our jaws just dropped - we were awed. Instead of a body with subdued features, Wild 2 had huge kilometer depressions that looked like footprints. Some looked like sink holes. Detailed study showed that the comet has crater-like depressions, pinnacles, vertical cliffs and jets of high-speed gas spewing out into space. We were stunned that a 5 km body could have such a wealth of information, features caused by processes that were both ancient and recent. The comet differed from any other body imaged in the solar system. With its jets reaching out into space, our comet was clearly a spectacular body- a classic. Some of the Wild 2 features were made when the comet resided far from the sun near Pluto and others were made recently as it orbits inside the orbit of Jupiter. We were thrilled with the images and other data and we were pleased that things went so well. We were also thrilled that Wild 2 turned out to a scientific bonanza that retains features made over a vast range of space and time.

The first scientific results of this encounter are published in a special June 18, 2004 issue of the journal Science. A second round of papers will be published in the Journal of Geophysical Research later in the year.

Last Updated: June 17, 2004
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