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Aviation History
1979
1979 - 1883.PDF
FLIGHT International, 2 June (979 1801 Skylab on final approach NASA's disused orbiting laboratory, Skylab, is likely to re-enter the Earth's atmosphere between June 20 and July 10, according to the latest estimates. Nasa has virtually no con trol over where Skylab will re-enter and at this stage is it impossible to calculate exactly where debris from the 80-tonne spacecraft will impact. However Skylab's orbital inclination limits it to somewhere between the latitudes 50.3°N and S. This area in cludes most of the populated world, but also a lot of ocean (see diagram below). No man-made object as large, as Skylab has ever re-entered before, but Nasa has estimated what will hap pen. As Skylab digs deeper into the Earth's atmosphere kinetic heating will begin to weaken the structure, until eventually atmospheric drag causes it to break up. Some of the resulting pieces will be small and burn up in the atmosphere, but others will easily be large enough to survive re-entry. Nasa estimates that about 500 pieces of debris will reach the Earth. They will range in size from one or two kilograms up to Skylab's airlock shroud, which has a mass of nearly 2-3 tonnes. The debris foot print on the Earth's surface will be roughly 160km wide and 6,500km long. Nasa scientists and engineers are currently working on ways to influ ence where the impact zone will occur. They talk in terms of prolonging Sky lab's re-entry, perhaps by as much as two thirds of an orbital revolution, rather than aiming for a particular target area. Radar tracking of Skylab should allow a rough prediction of the impact zone about a day or two be fore re-entry. Only if this is un favourable will engineers try to alter it. Skylab will impact somewhere in the shaded area, which represents the Earth's surface between latitudes 50°N and S. This region includes most of the populated world, but is about three-quarters ocean Skylab is in a solar inertial attitude at present (solar arrays continually pointing at the Sun), which is being maintained by the two remaining Con trol Moment Gyroscopes (CMGs) to preserve full electrical power. The orbiting laboratory's altitude is now about 295km and gradually decreas ing as atmospheric drag slows the craft down. If the cylindrical craft were left in its present attitude, it would begin to tumble end over end once it reached an altitude of 260- 280km, at which the CMGs would be unable to combat aerodynamic drag forces. Once Skylab begins to tumble, all control is lost and no further influence is possible, so Nasa has come up with two new attitudes which can be held to a lower height. Both attitudes are such that gravitational restoring moments work with the CMGs against aerodynamically generated forces. In one attitude Skylab is roughly parallel to the Earth's surface and in the other it is perpendicular to it. No choice has been made yet, but Skylab will have to be placed in its new atti tude by the second or third week in June. The craft's small gas thrusters will be used together with the CMGs to effect the attitude change. Skylab's new orientation does not in itself define the eventual impact zone, but it should allow Nasa engin eers to retain control of the space craft to a much lower altitude and shorter time to impact. It is at this late stage that Nasa will decide whether or not to try and influence the impact area. Maintaining a stable attitude to a lower height also en sures a more accurate prediction of the re-entry point. Once Skylab is down to a height of 130-150km, maintaining attitude will become impossible and the craft will begin to tumble. There is then nothing more Nasa can do, except to continue to track Skylab and try to predict where it will land. Skylab will begin to suffer serious kinetic heating at a height of about 120km. Aeritalia ships Spacelab parts AERITALIA has completed the second half of the cylindrical module which forms the inhabitable part of the first Spacelab flight unit The struc ture has now been sent to Erno's Bremen factory for integration with the first half, which was delivered in March. Aeritalia has yet to complete the thermal control and air-condition ing system for the module. Spacelab is due to fly during August 1981. An Atlas-Centaur carrying the US Navy's second fleet communications satellite, Fltsatcom 2, lifts off from pad 36A at Cape Canaveral on May 4. Fltsatcom 2 is currently undergoing tests in geostationary orbit
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