EPOXI Mission Status
14 Sep 2010
(Source: University of Maryland)
EPOXI Principal Investigator
We have now been observing the comet for about a week (we began on Sunday, 5 September). After the first day it was clear that portions of the spacecraft were getting hotter than we had predicted. The previously agreed procedure was executed to turn off the HRI, which generates a moderate amount of heat in the parts of the spacecraft near the telecommunications system. As we approach the comet, the geometry changes enough to make the instruments and the other parts of the spacecraft fit better into the shadow behind the solar panels so things will cool off and we can turn the HRI back on. Although we had planned on taking HRI images during this period, we had not scheduled any spectroscopy until the end of September and we expect to have the instrument turned back on by then.
We have begun preliminary analyses of the images from the MRI and we can already see variations in the brightness that appear to be correlated with the rotational period that has been previously announced by ground-based observers (Knight et al., IAU Circular 9163), namely 16.6 hours. At this time we only take observations every 6 hours because of the heating of the spacecraft, so we do not yet have enough temporal resolution to separate how much of the variation is due to the varying cross-section of the nucleus and how much is due to activity, but it seems clear that there is some variation in activity, possibly including at least one short-duration outburst.
In our images, the coma is seen to extend several tens of thousands of km (http://epoxi.umd.edu/3gallery/Hartley2_first_light.shtml and http://epoxi.umd.edu/3gallery/20100913_103P.shtml). As a point of comparison, at the time of the previously released image (5 Sept), the comet was 0.40 AU from the spacecraft but only 0.36 AU from Earth. We will be looking for any structure in the images and whether that structure varies with rotation.