The primary payload, Gamma Ray Observatory (GRO), was deployed on flight day three. The GRO high-gain antenna failed to deploy on command; it was finally freed and manually deployed by Ross and Apt during an unscheduled contingency space walk, the first since April 1985. The following day, two astronauts performed the first scheduled space walk since November 1985 to test means for astronauts to move themselves and equipment about while maintaining planned Space Station Freedom. The GRO science instruments were Burst and Transient Source Experiment (BATSE), Imaging Compton Telescope (COMPTEL), Energetic Gamma Ray Experiment Telescope (EGRET) and Oriented Scintillation Spectrometer Experiment (OSSEE). Secondary payloads included Crew and Equipment Translation Aids (CETA), which involved scheduled six-hour space walk by astronauts Ross and Apt (see above); Ascent Particle Monitor (APM); Shuttle Amateur Radio Experiment II (SAREX II); Protein Crystal Growth (PCG); Bioserve/instrumentation Technology Associates Materials Dispersion Apparatus (BIMDA); Radiation Monitoring Equipment III (RME Ill); and Air Force Maui Optical Site (AMOS) experiment.
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