Following the decision not to use the first orbiter (OV-101 Enterprise) for orbital operations due to its excess weight, Structural Test Article OV-099 was upgraded to become the second orbiter to enter space. Originally planned for a 27 January 1983 launch, the discovery of serious problems with the SSMEs resulted in an exchange of engines and a launch delay until 4 April 1983. Had the engine problem not been detected, a disastrous launch pad explosion could have resulted. The primary payload of this flight was the first Tracking and Data Relay Satellite (TDRS), which would be launched from the payload bay using the Inertial Upper Stage (IUS).The TDRS would be the first of a series of communications satellites designed to expand the on-orbit coverage for direct contact with a Shuttle crew including, eventually, during re-
entry. The TDRS network would gradually replace the existing network of fifteen ground stations that gave only thirty per cent direct communication time with the Shuttle in any ninety-minute orbit. Three operational TDRS satellites placed 120 degrees apart in synchronous orbit would provide ninety-eight per cent real-time coverage each orbit. TDRS was also a vital element in the US/European Spacelab mission manifested for later in 1983 aboard STS-9 (Spacelab 1). This was a planned round-the-clock science mission over nine days. The second TDRS was manifested for launch on STS-8 and the third on STS-12.
The launch of STS-6 occurred on time and without major incident, and the
deployment of the IUS carrying the TDRS-A satellite was achieved successfully ten hours into the mission. However, although the crew's operations in the deployment of the payload went without a hitch, the pre-planning sequence to place the satellite into operational orbit did not. The IUS first stage burned for 2 minutes 31 seconds to place the combination into its transfer orbit. The second stage should then have burned for 1 minute 43 seconds to place the TDRS in a synchronous orbit at 56 degrees W. However, an oil-filled seal deflated, causing the steering mechanism to malfunction. Only prompt action by the ground controllers saved the satellite as they quickly separated it from the erroneous IUS stage, far short of its planned orbit. Over the next fifty-eight days, delicate manoeuvres eventually placed the satellite on station at 41 degrees W, but with 370 kg of the 598 kg of propellant used, its operational mission was seriously compromised. The STS-9/Spacelab 1 mission was secure, but future IUS launches were put on hold as the problem was investigated. The IUS from STS-8 and STS-10 (a planned military satellite deployment) were taken off the manifest until the problem could be rectified.
Aboard Challenger, after the TDRS deployment, the crew and vehicle performed flawlessly, including during the first EVA from the Shuttle in the payload bay of the orbiter. For over three hours, Musgrave and Peterson worked outside, clearly enjoying the experience. The crew also performed a number of mid-deck and secondary experiments and completed their mission with a 9 April landing on Runway 22 at Edwards AFB in California, after a flight of 5 days 0 hours 23 minutes 42 seconds and 81 orbits.
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