A STUDY FOR NASA by New York-based Science Applications International research group puts the risk of a catastrophic failure of the Space Shuttle at one in 248 flights, compared with one in 78 in 1988 when the Space Shuttle returned to flight after the Challenger accident.

NASA's internal estimate of one in 120 will be improved to one in 160 in 1998, when second-generation main engines are operational, the agency says.

With 73 successful launches out of 74, the Shuttle has a success rate of 98.5%, but NASA concedes openly that another accident is possible. Before the Challenger accident on 28 January, 1986, the probability of an accident was placed at an unrealistic one in 10,000 flights (Flight International, 29 November-5 December, 1995).

The solid-rocket boosters (SRBs) and main engines represent 90% of the danger in ascent. SRB risk is dominated by the possibility of nozzle and igniter-joint malfunctions, leading to hot-gas leaks, rather than risks with the redesigned field joints, SAIC says.

NASA astronaut Leroy Chiao became the 100th person to conduct a spacewalk on 15 January, when he was the first to exit Shuttle Orbiter Endeavour on the first of two planned two-man excursions during the STS 72 mission, which was scheduled to end on 20 January. There have been 129 spacewalks, including 112 in Earth orbit; 66 by Russian cosmonauts and 63 by US astronauts. Of the several who have made more than one, two Russians have achieved nine spacewalks each.

Source: Flight International