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Seventeen years after the Challenger tragedy, NASA has sustained another blow which may have devastating consequences for the American and international space programs.

It is, of course, hard at this point to determine the reasons that caused space shuttle Columbia to disintegrate. NASA's investigators may have difficulties in the future as well, because much of the shuttle burned up and other parts were badly mangled when they smashed into the ground.

Based on the timing and position of the explosion, namely, the point of entry from outer space into the atmosphere, which is the most dangerous point on the way back, two scenarios are most likely. The shuttle moves from space, where there is no resistance or friction, into the atmosphere, where resistance and friction are massive, causing extreme heating. At this point, the temperature of the shuttle reaches 1,200 degrees celsius, which even prevents radio transmissions.

In order to keep the shuttle from incinerating, it is covered with ceramic tiles that are meant to absorb the heat and isolate the body of the craft. One possibility, theoretical at this point, is that many tiles fell off the shuttle as it was entering the atmosphere or even before. It is known that tiles came loose from the shuttle during take-off, although this has happened in the past.

In the event that too many of the ceramic tiles come off, then the intense heat is likely to cause serious damage to the shuttle and even to the disintegration of some of its parts.

Another scenario is that the shuttle was simply too old. Columbia, which was the first shuttle to fly into orbit in 1981, is the oldest one still in use. Quite a few experts have cautioned against using such an old shuttle, because of metal fatigue. As the shuttle was entering the atmosphere, one of its parts may have failed, causing terminal damage to the entire shuttle.

One less likely possibility is that the accident was caused by human error, namely, that the astronauts themselves had mistakenly taken the shuttle off its planned reentry course into the atmosphere.

The shuttle disintegrated more than 60 kilometers above earth, during the "silent time" because of the high temperature. Communication, which was to renew shortly after reentry, was never resumed. This will probably make it even harder to find the reason for the disaster, because the astronauts had no time to report any problem.

The disaster may have serious implications for the U.S. space program, and might even halt manned launches in the future. In any case, all space shuttles will be grounded for a long time, until the reason for the Columbia's breakup is ascertained.