Despite a month of delays, Columbia's new launch target was jeopardised when another Eastern Test Range booking - that of a Delta rocket from Cape Canaveral Air Force Station, carrying a US Air Force navigation satellite - was postponed on 18 March due to high winds. In accordance with the range's launch policy to all customers, the US Air Force was given a second chance on the 21st, which pushed STS-55 back to the 22nd (plus its own 'second chance' on the 23rd). This then left the Atlas rocket, with its US Navy satellite, in third place at the end of March.
''Officially,'' Shuttle Test Director Mike Leinbach told journalists as engineers shifted their preparations to launch Columbia into high gear, ''we are on the range for Monday [22nd] and Tuesday [23rd]. Anything after that would have to be negotiated.'' The attempt on 22 March, which would have led to a liftoff at 2:51 pm, went remarkably smoothly. The seven astronauts - Nagel, Ross, Harris, Walter, Schlegel, Pilot Tom Henricks and Mission Specialist Charlie Precourt - boarded Columbia and moved crisply through their pre-launch checks in the cockpit. With 31 seconds to go, as planned, the Shuttle's onboard computers took command of the countdown.
Just as NASA had done in its previous 53 missions, any problems cropping up in this last half-minute would be dealt with by the computers. During this time, particularly in the last few seconds when the three main engines would be ignited at precise intervals of less than a fifth-of-a-second, it was realised that no human operator could have the same accuracy as the computers. The computers would monitor thousands of separate functions and decide whether or not Columbia was fit to launch. On this day, as circumstances would transpire, she was not.
Six-and-a-half seconds before liftoff, the ignition of the main engines got underway with a low-pitched rumble; within three seconds, however, dramatically, and to gasps from spectators at the press site 5 kilometres away, all three shut down. Subsequent analysis pointed to the incomplete ignition of the Number 3 main engine. It turned out that a liquid oxygen preburner check valve had suffered a leak in the final seconds of the countdown, causing the purge system to be pressurised above its maximum-allowable 50 psi. A small piece of rubber, caught in a main engine propellant valve, was identified as the culprit.
This moment, which relived the experiences of two previous Shuttle crews in 1984 and 1985, was one of the most dangerous and unwanted episodes in the entire programme. With unburnt hydrogen possibly hanging underneath the still-hot engines, the risk of an explosion or fire was very real. Although the astronauts had been trained to escape from such an on-the-pad abort and slide to a fortified bunker, the danger of them running through invisible hydrogen flames prompted NASA to keep them in the relative safety of Columbia's cockpit.
For 40 tense minutes, Nagel and his crew remained strapped into their now-motionless ship as ground controllers switched off all electronic components that could cause an explosion. Paramedics were rushed to Pad 39A as a precautionary measure. Having watched the clock tick down to the last few seconds and hearing the roar of the engines far below him, coupled with a violent shaking of the cabin,
Nagel would say later: ''I'd convinced myself and all my crew that we were going to fly.''
When the engines died and their noise faded away, he told his three crewmates on the middeck - Ross, Schlegel and Walter - what had happened. For Nagel, Henricks, Harris and flight engineer Precourt on the flight deck, they knew from looking at the instrument panel what was amiss. ''I wouldn't call it fear,'' Nagel said later. ''There's a couple of moments wondering what's happened because all you see on board are red lights [on the instrument panel], indicating an engine shutdown. So you know the computers shut down the engines, but you don't know why or exactly what went wrong with them.''
The situation was equally tense in the Launch Control Center, next to the VAB, from where Launch Director Bob Sieck, his face drained of colour, watched the proceedings. ''Your initial reaction'', he said after the abort, ''is to make sure there are no fuel leaks or that there's nothing that's broken that's causing a hazardous situation. Really, it was one of those nice boring countdowns until the last few seconds! What did work, and worked very well, were the safety systems on board. As a result, the crew is safe and the vehicle is on the pad and safe as well.''
Nevertheless, the entire crew emerged from Columbia looking visibly shaken by the incident, particularly Walter, who was experiencing his first countdown. It was becoming increasingly likely that a delay of several more weeks would be needed to remove, replace and retest the three main engines. On 30 March, NASA announced that the next Shuttle mission, STS-56 - an important atmospheric-research flight called ATLAS-2, using Discovery - would leapfrog STS-55 into orbit on 6 April. By the time of Columbia's abort, Discovery had already been on adjacent Pad 39B for a week and preparations for her flight were proceeding smoothly.
''Flying the missions in this order is the most effective use of all our resources,'' said Tom Utsman. ''The early April launch of ATLAS-2 will give scientists the opportunity to observe changes in Earth's ozone [layer] during the seasonal transition between spring and summer in the northern hemisphere. At the same time, the launch team will be working to get Columbia back to launch configuration for launch on 24 April. NASA is very pleased with the cooperation given by our friends in the German space agency. They have been involved as all possible options were considered. Their willingness to let the STS-56 mission have an early April launch will give the ATLAS folks the chance to collect some very important data on Earth's ozone.''
The delay to Spacelab-D2 did, however, require several time-critical experiments to be removed from the pressurised module in Columbia's payload bay, replaced or repaired. Often, the principal investigators for the experiments were present at the launch pad to supervise the operations. After a two-day delay of its own, STS-56 set off on 8 April and flew a highly successful nine-day mission.
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