Mission Status Report #56 Star Date: March 25, 2002
FUSE Operations are Nominal
The FUSE satellite continues to operate well after our recovery from Reaction Wheel problems, earnest on March 1, and science efficiency has been high since that time. With each passing week, as noted in my previous report. Science Operations restarted in we are also learning new things about our new control system and how it behaves both for slewing (moving between targets) and for static pointing stability.
Part of this process is developing predictive tools that help our planners figure out when and where the spacecraft can point and maintain stability. The primary disturbances felt by the satellite are due to gravity-gradient (GG) effects; that is, a differential pull of gravity on one end of the satellite relative to the other. (Amazing to think that gravity can do that!) When we point FUSE toward the north or south pole of its orbit, GG is minimized and we can point just fine. However, as we point off at some angle, the GG forces can get stronger than the corrective forces we can apply with the Magnetic Torquer Bars (MTBs), and we can lose fine pointing control temporarily. Also, since the strength of the magnetic field changes (both around an orbit and on a 24-hour cycle), the region of predicted stability changes size and shape as a function of time.
Below we show two plots, which represent the predicted region of stability for two different FUSE orbits (100 minutes each) on March 10, 2002. The region inside the heavy solid line shows the region of predicted stability for that particular orbit. See how they change in size and shape? This is the new paradigm for FUSE: we have to point at various targets when they are within the solid line in order to maintain stable pointing.
Caption: Two example skyplots from a new tool being used in FUSE Mission Planning. The plots show the southern celestial hemisphere and the small dashed circle shows the instantaneous Continuous Viewing Zone (CVZ). The white (left) side is the sky region available for viewing, and the pink (right) side is the region at high beta angle (and thus too close to the sun for FUSE to observe). The solid and dashed lines around the CVZ in each plot show two models of the region where the tool predicts pointing stability over an entire FUSE orbit (100 minutes) for two individual orbits out of a given day. (The solid contour is a conservative model being used in Mission Planning.) Note how the size and shape of the stable region change dramatically as a function of time. Mission Planners now have to take this variability into account as they plan objects for observation with FUSE.
(Click images above to see larger versions.)
Our predictions are still not perfect, and so we continue to test and refine our predictive ability as we have time. Also, these predictions are for "static pointing," and we have to understand more about how and when it is best to slew between targets as well. But we continue to give priority to science observations whenever possible to maximize the science return from FUSE while it remains healthy. As one example, the last 10 days was spent performing a wide variety of Cycle 2 and 3 science programs in the Large Magellanic Cloud, a neighboring galaxy to the Milky Way.
Other exciting news is that the third annual FUSE Science and Data Workshop was held at JHU last week. This year it was mostly "science", with a broad cross section of FUSE science results highlighted over the two and a half day meeting. It is always exciting to see these results first hand and talk to the people who are using FUSE data to learn about the universe in so many different ways!
Reported by: Bill Blair, Chief of Observatory Operations
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