Hi Folks ... I'm one of the more quiet "lurkers" out here on SeeSat (and others). But, since I work on the CGRO satellite, I can offer some quick info for everyone on up-coming events ... 1). Gyro #3 had a hard failure back on December 6th ... interestingly, it was just a few hours into a formal Risk Management Class several of us started to attend ... 2). CGRO can handle 3-axis control with the 2 remaining gyros; there is, in fact, hardly any discernable difference in day-to-day operations. 3). CGRO must be de-orbited in a controllable state, at some point, because it is very large and massive. It is a virtual certainty that more than one part will make it to a ground/ocean contact. Statistics say CGRO has a 1/1000 chance of human casualty if it comes back UNcontrolled ... and THAT is a very real risk. 4). The threat in keeping CGRO up much longer is IF one of the 2 remaining gyros fail. In this case, there is no current capability at maintaining 3-axis control. Without 3-axis control, there is no way to continue science operations -- NOR do a controlled re-entry. 5). In addition to our gearing up for re-entry (doing sims, studying manuals, etc), there are parallel efforts going on to see if derived rates from the Sun Sensors or Magnetometers can be used in place of another gyro failure. This has been done on other satellites, but has not seriously been funded for CGRO. [NOTE: The IUE satellite successfully ran on 1- and 0-gyro control laws -- but IT was in geosync -- not LEO.] 6). It would certainly solve a lot of problems if a Shuttle were to either repair CGRO or retrieve it. However, there are also propulsion line problems with CGRO that probably eliminate STS involvement (safety waivers are also being examined in parallel efforts). 7). I know several of the CGRO scientists, and I also know they would love an extended mission. Despite having outlived its original mission objectives (and an internal clock that just rolled over the first time on December 22!) -- there are many more science observations they would like to make. 8). Official chores are for us to examine whatever is feasible, but between now and February 16 -- we are to prepare for re-entry. This involves a lot of work and testing, since none of us have experience with it. If there are no magical answers for successfully using derived rates for attitude control -- or some other options appear for keeping CGRO on-orbit and doing science operations -- it is likely NASA management will then make an official decision to de-orbit. 9). The goal is to have CGRO splash down east (and south) of Hawaii, whenever that day comes [currently scheduled for mid-March]. This will require many orbit-reduction burns over at least a 2 week period. 10). There are at least 2 critical propulsion-related problems with CGRO ... the A-side propulsion lines are probably unusable, and one of the attitude control thrusters has questionable integrity. Both of these problems were managed for the 1997 orbit re-boost, and we expect to be able to manage them for the de-orbit activities. But, what if we stay on-orbit a few more years, and we're wrong about how well things can be controlled? It's an extra risk that could have major consequences. All in all, we're gearing up for re-entry in mid-March -- but NO firm decision has been made yet. Steve Walter PS: If any of you send me an Email, please do NOT use the 'Erols' address, but: SteveWalter@worldnet.att.net ... my Erols account is disappearing VERY soon. ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Fri Jan 14 2000 - 11:21:52 PST