Aviation Week, a usually authoritative industry magazine, reported yesterday that NASA investigators believe the explosion that finally destroyed the space shuttle Challenger was caused when its right-hand booster rocket broke loose from its lower attachment point, causing the booster to pivot and crash its nose into the external fuel tank.
Investigators believe the crash ruptured the tank, spilling highly explosive liquid hydrogen and oxygen, according to the magazine.
The magazine cited unnamed members of the National Aeronautics and Space Administration's in-house investigating board as its sources. NASA officials refused to comment on the report but a spokesman at the Kennedy Space Center in Florida said the scenario made engineering sense.
"The tip of the booster rocket could have acted as a spear to puncture the external tank but I don't know why anyone can be sure at this point that that happened," said Jim Mizell, a retired NASA engineer who has been acting as a spokesman. "This is just the theory of the day."
The magazine said the scenario is supported by data transmitted automatically by Challenger just before the explosion and by photographs that show the booster flying out of the fireball, its nose damaged, and trailing the parachutes normally used to lower the spent boosters for recovery.
The photographs also clearly show a large plume of flame emerging from a rupture in the booster's side at a point near the lower attachment struts. The cause of this rupture, which may have been linked to failures of seals between booster segments, was under scrutiny yesterday as the presidential commission investigating the accident questioned space agency officials in closed session.
NASA officials had told an earlier commission hearing that some of these seals failed on earlier shuttle flights but that backup seals kept the booster intact. Yesterday's closed session was believed to have dealt with internal documents from the space agency that warned of catastrophe if both seals, called O-rings, failed during a launch.
Commission Chairman William P. Rogers named Alton G. Keel, a PhD in engineering physics who is currently working in the Office of Management and Budget, as executive director of the commission. Keel, who has been assistant secretary of the Air Force for research, development and logistics, is currently OMB's associate director for national security and international affairs.
In a related matter, Rep. Edward J. Markey (D-Mass.) raised questions about NASA's stated confidence in the solid rocket boosters based on a 1983 Air Force-sponsored risk analysis of the shuttle.
The study, according to Markey, said that, based on the failure rates of past rocket launches, "the average failure rate of each solid rocket booster in the shuttle is one chance in 71, or approximately one chance in each 35 shuttle launches, given the fact that two boosters are apparently essential to each successful launch."
Markey said the study concluded that this was a rate "one or two orders of magnitude less" than that predicted by NASA.
The study included 14 potential "hardware failures" that could destroy the shuttle and its cargo, and found that "solid rocket booster case/nozzle failure" was the most probable, although the chances were reported as about two in 1,000.
The study also listed "inadvertent separation" of a solid rocket booster from the shuttle at its "aft attachment" as a less probable failure.
In a letter to acting NASA administrator William R. Graham, Markey said the Air Force study, done by Teledyne Energy Systems Inc., pictured risks that "strongly suggest that a catastrophic accident was a virtual certainty" given the number of shuttle launches planned.
According to the magazine, whose full name is Aviation Week & Space Technology, fire jetting from the rupture either melted the nearby metal struts that hold the booster to the external tank or caused such a wrenching, sideways thrust that the bottom of the booster tore loose. Once the struts were severed, the 149-foot booster, its rocket engine still firing, would quickly have pivoted around the only other point of attachment, which is near the booster's nose.
If the scenario is correct, the nose would have hit the tank just between the hydrogen chamber below and the oxygen chamber above, probably rupturing both and setting off the fireball that destroyed the orbiter and killed its crew of seven.
The magazine said data transmitted to Mission Control from the right-hand booster supported the idea that the lower part of the booster swung away in the split second before the explosion ended all transmissions. The data reportedly came from the gyroscopes that sense changes in the booster's position. They suggest that the base of the booster flipped out and away from the external tank to which it was fastened. The effect would have been to drive the nose into the tank.
The latest engineering data shows that the explosion occurred 73 seconds into the flight.
Meanwhile, Graham yesterday announced the indefinite postponements of several major scientific missions that were scheduled to fly aboard shuttles in the near future.
These include the European Ulysses mission to investigate the poles of the sun and the Galileo mission to orbit Jupiter and send a probe to that planet, both of which had been set for launch in May. A launch to Jupiter can be attempted only every 13 months because of the changing positions of Earth relative to the giant planet.
Graham also postponed the Astro-1 mission, which was scheduled to fly in March. Astro-1 is an ultraviolet astronomy lab mounted in the shuttle's payload bay that was to examine quasars and Halley's comet. Although the comet will be well out of prime viewing range long before it is likely that the lab can be flown, scientists say it can still carry out observations.
He said the principal reason for the decision not to carry out the missions was that the people most necessary to ensure a safe and successful launch are preoccupied with the Challenger explosion.
New launch dates have not been set.