On 29 July 1985 STS-51F became the first mission to undertake an abort during the ascent. As Challenger continued to climb under its own power after jettisoning its SRBs, the centre engine in the SSME cluster overheated. The flight controllers monitoring the situation throughout the 104 per cent phase were relieved to see that it did not stray into the red line prior to being throttled back to 65 per cent for the final phase of the ascent. It continued to overheat, however, and the computer shut it off at T + 350 seconds. By this point, with two engines, the Shuttle would have

sufficient energy to attain a low orbit, so an Abort-To-Orbit was ordered, which involved mission commander Gordon Fullerton turning the selector to 'ATO' and pushing a button to enact his selection. The computer throttled up the two remaining engines to 91 per cent, and added 70 seconds to the burn to compensate. However, no sooner had this abort been initiated than a temperature rise was seen in one of the remaining engines. At this point the controllers began to suspect a faulty sensor rather than a genuine problem with the engine, and they recommended that the crew intervene to inhibit the computer from shutting off this engine. Unable to reach orbit on one engine, Challenger would have had to invoke a Transatlantic Abort Landing which, in this case, would be an emergency landing at Zaragoza in Spain. The final 220-kilometre circular orbit was not ideal for the observational programme planned for the telescopes in the payload bay, but any orbit was better than none, and the observing plan was hastily revised.

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