46 SOCIAL SCIENTIST Challenger's computers send the critical command igniting the SRBs. Each of them provide 2.9 million pounds more of thrust. With this the combination ofshutde (orbiter), the SRBs and the external fuel tank breaks loose from the launch pad and propels itself into space bound trajectory. 35 seconds into the flight, with the SRBs providing maximum thrust, the Challenger undergoes severe aerodynamic stress. The orbiter's liquid fuel rockets are then automatically throttled down to-65 per cent thrust. Seventeen seconds later, Challenger's on board computers, then throttles the engines once again to provide 104 per cent thrust. At this stage, the SRBs would have burn-ed nearly a million pounds of fuel. On the computer screens at the mission control everything looked per-fecdy normal: "Challenger, go at throtded up" the mission control signalled and mission pilot Michael J Smith responded : "Roger, go at throtde up". Moments later a fireball engulfed the orbiter and the crew was lost in an enormous explosion. If the mission had been successful, the solid fuel boosters would have burned for two minutes and'10 seconds before the shuttle commander jettisoned them. They would have plunged into the sea at 150 km per hour. The impact at this speed destroys the nose cap and the avionics, but the steel segments of the casing survive to be recovered and re-used. This time, the mission control had to destroy the boosters because one of the boosters was going out of control and heading towards a thickly populated part of Florida. The self-destruct signal is common for both the boosters. Destruction of the boosters also destroyed with it the crucial physical evidence of what was wrong with them. The time when the flames were emerging from the solid rocket booster and licking the liquid fuel tank coincides with the stage of the lift off when the tank was subject to the greatest aerodynamic stress. At this stage, the shuttle is propelling through the densest part of the atmosphere and the air passing between boosters and tank as well as between shuttle and tank, is at very high temperature due to friction heating. To help cope with this aero-dynamic and thermal stress, the pilot throttles the shuttle engines down to 6596 of its full power. At the time of explosion, the shuttle commander was about to increase the thrust back to full power, after completing thrust reduction manoeuvre. This is done about 72 seconds after the ignition of solid rockets. Whereas the flames were seen escaping only 14 seconds before the explosion, the unusual black plume appears immediately after ignition. The Explosion Sequence The television pictures showed an orange glow at the base of the shuttle's fuel tank, which then spread. A second fainter glow was also seen behind the tank. The film taken from the opposite side and released a week after the accident revealed the glow as a jet of flame from the ruptured right hand booster. The glow finally spread towards the liquid hydrogen tank. According to New Scientist, the flame glowing like a blow torch burns into