Skylab Predictions

A striking, and somewhat confusing, aspect of Skylab's fall onto the Indian Ocean and Australia on July 11, 1979 was the multiplicity of reentry predictions we heard. Differences in the predictions emanating from different organizations gave one cause to wonder which, if any, of the predictions to believe and why different predictions should exist at all.

The reason for differences in the predictions being given out by NASA and NORAD (North American Air Defense Command) were that calculations of reentry times being performed by the two organizations were based upon different assumptions about the atmospheric drag on Skylab. One organization's calculations were made by assuming that the drag during future orbits would be the same as the drag experienced by Skylab during the past 24 hours. The other organization's calculation apparently was identical in every respect except for incorporating the assumption that the drag would increase with time in a particular fashion.

The atmosphere at lower altitudes is denser and gives greater resistance to an object moving through it. So in principle, the latter time-dependent calculation should give the most accurate prediction--at least it should if one could know exactly how the drag would increase as Skylab slipped lower into the atmosphere.

Regardless of which type of calculation one preferred, it was expected that the two would give results more exactly alike as the time of reentry was neared. Further, it was hoped that the convergence of the two predictions would occur early enough to permit with confidence a decision on whether or not to tumble the Skylab in order to adjust the reentry time.

In retrospect, it all worked out reasonably well. About 6 hours before reentry, the convergence in the predictions was sufficient to allow NASA to tumble Skylab somewhat earlier than first planned. The result was reentry on about the safest of Skylab's orbital tracks and in a portion of the orbit having minimal population, although NASA preferred to have Skylab fall just a little earlier than it did, so as to avoid Australia altogether.