posted 08-10-2007 09:35 PM               

From the Flight Day 3 execute package sent to the crew of STS-118:

quote:

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FD 1 Conjunction during Payload Bay Door Opening Timeframe:

While you were busy opening the payload bay doors on FD 1, the MCC Ascent Team was monitoring a conjunction with a spent Delta rocket body that was in a 952 nm x 165 nm elliptical orbit with perigee just below that of the orbiter. The normal pre-launch screening process which evaluates the shuttle trajectory through two hours MET was executed per the normal MCC and USSTRATCOM procedures. This rocket body trajectory was cleared for launch and was outside the criteria that would have required the launch time to be adjusted to the next even minute.

After the OMS-2 burn was executed, the Ascent FDO passed the updated orbiter state vector to USSTRATCOM per standard procedures. After the standard USSTRATCOM evaluation process, which typically takes a minimum of 40 minutes to execute, a conjunction with the closest approach occurring at 0/01:34 MET was reported. The total miss distance was 2.3 km with a very close predicted radial miss of 0.1 km. The MCC received notice from USSTRATCOM approximately 14 minutes prior to this close conjunction. Because of the large uncertainty in the orbiter state vector in this post OMS-2 timeframe along with the limited time to plan and clear a maneuver, a burn was not performed.

On FD 2, a best estimate of the orbiter trajectory was developed using state vectors obtained from rev 2 orbit determination. Based on these state vectors the actual miss distance was 0.6 km in the radial direction with a total miss distance of 1.2 km. This reconstruction places the conjunction outside of the criteria for which the crew would be notified and for which a maneuver to clear the conjunction would be executed. This is not surprising based on previous studies of OMS-2 navigation accuracy, which were discussed by the Ascent Team on FD 1.

In fact, we have always known there is a vulnerability to changes in conjunctions in the post MECO and post OMS-2 timeframe because of the inherit navigation errors associated with accelerating the launch vehicle to orbital velocities and establishing the initial post OMS-2 orbit. Once a valid orbit state vector is available on rev 2, the orbit process is not subject to these errors, so this scenario is not applicable for the remainder of the STS-118 mission.

Steve Stich
STS-118 Ascent/Entry Flight Director

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That particular Delta lofted the European Space Agency's COS-B satellite from Vandenberg Air Force Base, California. COS-B was the first telescope to map the galaxy in the gamma-ray spectrum.