If it appears that Skylab is about to come down in the most populated regions of the Earth, the National Aeronutics and Space Administration may attempt to delay its fall to reduce the already slight risk of injury or damage.
"The largest peaks of population under Skylab's orbit are southeast Asia, India and China," Richard Smith, NASAdeputy associate administrator, told the House Committee on Science and Technology yesterday. "If it looks like there's a chance of re-entry over that part of the world, we may attempt a maneuver that will decrease the risk of reentry at that time."
Last week, NASAmaneuvered Skylab into a sideways position that maximizes the aerodynamic drag on it, meaning it is falling at a faster rate than it had been. But flying it sideways reduces the chance that Skylab will start to tumble out of control and prevent ground controllers from maneuvering it.
Smith said that controllers can reorient the space station to reduce the drag, thus delaying its reentry into the amosphere by as much as six hours - the time it takes Skylab to orbitthe Earth four times. Since the Earth is turning beneath Skylab, that much of a delay would be enough to bring it down over a different part of the world.
"There is only an 18 percent chance we'll attempt this maneuver," Smith said. "We have adopted what we call a 'Fail-No Worse' philosophy that means we will only attempt a delay maneuver if a delay looks like it will be a much safer thing to do than doing nothing."
Smith said the decision to delay will be made by NASA Administrator Robert Frosch, after consulting with engineers at the Marshall Space Flight Center in Alabama and flight controllers at the Johnson Space Center in Houston. Smith said this procedure was discussed with the State Department and the White House, which agreed that Frosch should be the one to make the decision.
The North American Air Defense Command has predicted that Skylab will fall to Earth sometime between July 11 and 19, with a 50 percent chance it will come down on or before July 16. The 77.5-ton space station, the largest spacecraft ever flown, is now 135 miles high and falling at a faster rate every day.
NASA has said that two-thirds of Skylab will burn up in the atmosphere and the 26 tons of metal left will break up into 500 pieces that will be scattered along a path 4,000 miles long and 100 miles wide. Of the 500 pieces, 38 will weigh more than 250 pounds and 10 more than 1,000 pounds.
"In 200 years of recording meteorite strikes, we've logged six injuries and no deaths," Smith said. "The chance of any of the four billion people under Skylab's orbit being injured are one in 150. The chance of me, Dick Smith, being hurt are one in 600 billion."