Rob Matson wrote:
A third alternative would have been to let the orbit naturally decay to a lower altitude before doing that burn. The advantage of this approach is that once the orbit is very low (as it is now), that final burn can push perigee so low that reentry is guaranteed half an orbit after the burn. This allows spacecraft controllers to choose the reentry location judiciously (e.g. over the South Pacific Ocean). By burning years early as they did, they sacrificed the ability to choose the reentry location.
The problem here is, that it would have taken 30+ to 40+ years for UARS to naturally decay to a suitable altitude for such a burn.
This would have meant they would be obliged to keep the satellite operational for 40+ years. Apart from questions whether the satellite will not fail before that time (I am quiet sure it wasn't designed with 40+ years in mind), for a science satellite that is financially impossible to do.
Why does it need to be kept operational for these 40+ years in this case? The point is that without active maintenance, the satellite will start to lose attitude control quickly, pointing wildly to all places as it starts to tumble. Which means its receiver antennas could no longer point to earth at the moment, 40 years ahead, you want to contact it. But a bigger issue is the fuel in the tanks. Without active maintenance, the fuel will freeze in the tanks. Once that happens, it becomes impossible to use the engines for a deorbit boost.
UARS is not the only thing currently coming down by the way. In about a year, a 1.5 tons malfunctioned Japanese spy satellite (IGS 1B) will come down in an uncontrolled re-entry as well. That sat does still have some fuel onboard, unlike UARS, which is an added risk.
UARS is a nice bright object easily seen by the naked eye during a pass. Here is a picture I shot last year:
http://sattrackcam.blogspot.com/2011/09/watch-uars-its-dropping.html - Marco ----- Dr Marco (asteroid 183294) Langbroek Dutch Meteor Society (DMS) e-mail: d...@marcolangbroek.nl http://www.dmsweb.org http://www.marcolangbroek.nl ----- ______________________________________________ Visit the Archives at http://www.meteoritecentral.com/mailing-list-archives.html Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
