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The crucial final countdown will begin when Skylab drops to about 120 miles above earth, roughly 48 hours before its re-entry into the atmosphere. At that point, a higher-level interagency team of experts, including NASA Administrator Robert A. Frosch, will take up positions in the Skylab Coordination Center on the sixth floor of NASA headquarters in Washington. Getting his information and recommendations from the Houston center, NORAD and the Marshall Space Flight "enter at Huntsville, Ala., Frosch will make the final decisions on what, if anything, should be done to try to influence Skylab's final fall.
The options are limited. On June 20, NASA's team members used up precious bursts of the spacecraft's dwindling propellant to turn its nose horizontally by 90° and into a sideways position, which exerts increased drag against its forward movement. That change gave NASA its best chance of some final control. Explained Cindy Major, 27, one of the Houston monitors: "There is more pressure now because the attitude of the spacecraft is more sensitive. There is no room for error."
The key decision is to be made when Skylab falls to a height of about 90 miles above earth, some twelve hours before estimated reentry. At that point the controllers could use some of the 6,000 remaining pounds of fuel to rotate the craft into various nose-forward, "low drag" positions, in the hope that this would prolong Skylab's life by anywhere from one to five more orbits. By contrast, a second option would be to send the vehicle into an early tumble, which would cut from one to three orbits from its natural, uncontrolled reentry. A third option would be to do nothing and let gravity take its course.
The choices were to be made on the basis of a complicated "hazard index," a computer calculation used to determine the final orbital paths that would take the spacecraft over the least densely populated areas. Frosch has already made one firm rule about reaching those last critical decisions: Skylab will not be sent into an orbit posing a high hazard in hopes of later reaching an orbit of lesser risk. That is because NASA is simply not certain that its efforts to select the precise final orbit will work. To do nothing in such a situation is preferable to taking a high-risk gamble and failing. Amid all those uncertainties, the engineers think the best final orbit would take the craft over the southern part of South America, across southern Africa, the Indian Ocean and India, then over China and the Pacific.
Even with all the planning, the margin of possible error is uncomfortably large. At about six hours before reentry, NASA'S projected impact points for Skylab's disintegrating parts occur somewhere along a path of 40,000 miles—nearly twice the circumference of the globe. At two hours, the final anticipated flight track still extends over a 13,000mile path. Testing its prediction on a falling Soviet Cosmos booster stage on April 29, NORAD made an estimate two hours before re-entry— and missed the actual impact points in the Pacific by 4,000 miles.