PI Update - April 2004
The last month has been a busy one for the Deep Impact project. Our biggest achievement, just days before this was being written, was to have completed the final mating of the impactor and flyby spacecraft. This implies that mechanically the system is ready for its environmental tests. The two spacecraft will be separated once more, after vibration tests, in the thermal-vacuum chamber, in order to test the thermal properties of the impactor when it is separated from the flyby, i.e., in order to be sure we know how it will react thermally after we release it from the flyby on the day before impact. Meanwhile, we are continuing our extensive program of testing the software that operates the spacecraft and the instruments.
The project also carried out a major review of all the previously identified risks facing the project. After considerable homework by various people, we were able to eliminate at least half the risks as being negligibly small, thus allowing us to concentrate our attention on the risks that are not negligible. These risks cover a wide range of topics - from failure to release the impactor from the flyby (rated at negligible chance based on our designs and tests), through failure of the spacecraft computer at some key point (different ratings for different aspects of failure), to the uncertainties in the environment of the comet (which we can't change, but which we are still studying and for which we think our models are conservative).
The science team has been active in analyzing all our ground-based data on the comet to study the rotational state of the nucleus. There are still some ambiguities in the ground-based data (due to sampling only during the night at any observatory) but we have had great cooperation from the staff of the Spitzer Space Observatory, which took observations for us just recently (although the calibrated data are not yet available) and from the staff of the Hubble Space Telescope, with which we expect to obtain a series of observations in early May. The combination of these data with the extensive ground-based data from the last 4 years should enable us to considerably improve our estimate of the size, shape, and rotational state of the nucleus of the comet. Knowing the orientation of the nucleus at the time of impact could simplify our approach to targeting for the impactor and will, in any case, allow us to much better estimate the reliability of all our targeting and pointing algorithms.