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PI Update - October 2003

The impactor's mass has been decreased from 370 kg to 360 kg. Some late-arriving components for the impactor, mostly in the computer box which is at the back end of the impactor, turned out to be lighter than expected. We then had to take some additional mass from the front end of the impactor, in order to maintain the center of mass in the right place for balancing the thrusters. This small change in the mass has negligible effect on our expectations for the crater that we will form but it shows the irony that creeps into the decision-making. Normally one is fighting to keep the weight of a spacecraft under the limit for the launch vehicle, so when some component is too light one would normally expect that mass to be used somewhere else but in this case losing mass in one area required us to take away mass in another area.

Mission Scenario Testing on the spacecraft has been slowed down because of continuing problems that are being uncovered in the spacecraft computer. Since we have plenty of schedule margin and since we are confident that we will eventually solve all the problems with the computer and its memory, all parties (Ball, JPL, UM) agreed that it would be more efficient to slow down the pace of testing. Nevertheless, we have carried out several additional tests on the spacecraft, including the test of the Trajectory Correction Maneuvers, in which we simulate all the commands involved in changing the spacecraft trajectory, something we will do several times during the mission. We have also completed many stress tests, tests in which the software is pushed to do as many things as possible for as long as possible.

The Cometary Environment
Several members of our team participated in a review of models for the cometary dust that was organized by the Stardust project. This was aimed at understanding the confidence we should have in the models that are used by all cometary flight projects to assess survivability of the spacecraft when operating in the head of a comet. Our own conclusion is that the models that we have been using for Deep Impact are conservative in the way in which they interpret and extrapolate from the available data.
We have also continued our analysis of the rotation of Tempel 1 and it now appears that the comet may be more elongated than we had previously thought, perhaps more like a cucumber than a potato in gross shape. Because the rotational period is much longer than a single night of observations, there is some uncertainty in the amplitude of the light variation and because the orientation of the spin axis is not known, there is uncertainty in how to relate the observed amplitude to the actual shape of the nucleus. Our observational program is continuing in hopes of resolving the rotational period (it could be about 21 hours as well as about 42 hours, the latter being our previous result) and in hopes of determining the orientation of the spin axis.

Deep Space 1
At a science team meeting in early October, we heard presentations from some of the participants in the encounter of Deep Space 1 with comet Borrelly. The main goal was to understand what software tools they had used that might be useful for interpreting the results of Deep Impact. These discussions were valuable for planning our work between now and encounter.

Mike A'Hearn, University of Maryland

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