Anytime now, Rs 2,000 crore will go up in
flames when Skylab, the largest artificial earth satellite launched so
far, plunges back to earth. The objectives for which it was sent into
space have been achieved during the nine months of its operation (from
May 14, 1973 to February 8, 1974).
V.S. Venkatavardan
September 30, 1978 |
Anytime
now, Rs 2,000 crore will go up in flames when Skylab, the largest
artificial earth satellite launched so far, plunges back to earth. The
objectives for which it was sent into space have been achieved during
the nine months of its operation (from May 14, 1973 to February 8,
1974).
It was abandoned in space with its systems completely "turned off". Scientists had, however, hoped to revisit the space station in the next two years to recover material left in the spacecraft and discover the effect on it of long duration exposure to outer space conditions.
But the space station is in a critical condition today and may plunge down to earth anytime and at any place. It has started wobbling and is losing altitude every minute. The downward spiral is caused by the residual atmospheric drag on the satellite. Even though the atmosphere at the satellite orbit height is quite thin, it is sufficient to bring the satellite down to the lower atmosphere over a number of years where it will burn and eventually fall.
A satellite at low altitude thus has a lower lifetime in space compared to one at a higher altitude (spy satellites orbiting at very low altitudes have lifetimes of only a few days or weeks). If the satellite is small, it may be completely burnt out in the atmosphere owing to friction. If sufficiently heavy (like Skylab) a portion of it may fall on earth with a high impact speed.
Object in Space: On January 1, 1977, the number of man-made objects in near earth space was 4,141. On an average about five objects are placed into space during the launching of a single satellite. These include the discarded rockets and other hardware used in launching the satellite. Skylab revolutionized space station technology and set innumerable records in space science.
A total of 172 days were spent in the space station by three successive crews of astronauts. The missions conclusively proved that man can live for a prolonged period in space without any ill effects. Investigations were carried out in the physical sciences, bio-medical sciences and earth and space applications.
Among others, a detailed study of the sun was carried out which gave over 3,00,000 solar photographs during various phases of its activity. Further, thousands of photographs of the famous Comet Kohoutek were taken. Over 10,000 photographs of the earth and 70 km of magnetic tapes of information related to earth observations were made during the mission.
The Tata Institute of Fundamental Research, Bombay, in collaboration with scientists from the Physical Research Laboratory, Ahmedabad, exposed a stack of cosmic ray detectors for 73 days outside Skylab during the third manned mission and retrieved it for analysis of cosmic rays. These studies have given valuable information about cosmic rays - their origin and propogation in space. A new component of cosmic rays has been discovered by Indian scientists from Skylab studies.
The Fall: It was initially thought that Skylab would spiral down sometime during 1980. Its earlier anticipated fall is curiously due to the sun which controls the upper atmospheric pressure of the earth. The pressure is not a constant and varies by orders of magnitude, depending on the state of the sun. There are what are called sun spots - dark regions seen in the solar disc whose number varies periodically. From a near absence of sun spots during a "solar minimum", the number grows into a maximum and falls to a minimum again within a period of about 11 years which is called the "solar cycle".
The upper atmospheric pressure, and hence the air drag for the satellite, will be high during maximum solar activity. The sun is reaching the peak of its activity which is expected to be the maximum around 1979-80. That would be still 'safe' for Skylab if the solar activity during the present maximum is normal. But it appears that the present solar maximum will be extremely intense with the sun spot number exceeding 100 or so. If that is so, the drag on the satellite will be considerable, causing it to plunge downwards earlier than predicted.
Skylab can still be saved if a Space Shuttle is sent up and a booster rocket attached to the space station enabling it to be sent in a higher orbit. But this appears to be difficult because of a delay in the Space Shuttle project. Corrections have been carried out from the ground station at Houston, Texas, which has stabilized the spacecraft's wobble to some extent. But if the orbit is not sufficiently raised, it will quickly lose its altitude and will ultimately head towards the earth. A sudden increase in solar activity will quicken the process.
Food: The saving of Skylab is important because a number of materials were kept outside the spacecraft to discover the effects of prolonged exposure in space. Among the objects inside the space station are rehydratable thermostabilized bread pudding and beverages. A study of the retrieved material will be useful for future space programmes involving long duration flights.
Skylab left alone in space also poses a danger to the earth in that it may fall on some heavily inhabitated region. In principle its orbit could be changed so that it falls in an uninhabited area, such as the deep ocean or a desert. Thus there are two courses of action left open. Either it can be placed in an higher orbit so as to prolong its lifetime, or it can be destroyed in a pre-determined orbit with a minimum of danger to people and property on earth.
It was abandoned in space with its systems completely "turned off". Scientists had, however, hoped to revisit the space station in the next two years to recover material left in the spacecraft and discover the effect on it of long duration exposure to outer space conditions.
But the space station is in a critical condition today and may plunge down to earth anytime and at any place. It has started wobbling and is losing altitude every minute. The downward spiral is caused by the residual atmospheric drag on the satellite. Even though the atmosphere at the satellite orbit height is quite thin, it is sufficient to bring the satellite down to the lower atmosphere over a number of years where it will burn and eventually fall.
A satellite at low altitude thus has a lower lifetime in space compared to one at a higher altitude (spy satellites orbiting at very low altitudes have lifetimes of only a few days or weeks). If the satellite is small, it may be completely burnt out in the atmosphere owing to friction. If sufficiently heavy (like Skylab) a portion of it may fall on earth with a high impact speed.
Object in Space: On January 1, 1977, the number of man-made objects in near earth space was 4,141. On an average about five objects are placed into space during the launching of a single satellite. These include the discarded rockets and other hardware used in launching the satellite. Skylab revolutionized space station technology and set innumerable records in space science.
A total of 172 days were spent in the space station by three successive crews of astronauts. The missions conclusively proved that man can live for a prolonged period in space without any ill effects. Investigations were carried out in the physical sciences, bio-medical sciences and earth and space applications.
Among others, a detailed study of the sun was carried out which gave over 3,00,000 solar photographs during various phases of its activity. Further, thousands of photographs of the famous Comet Kohoutek were taken. Over 10,000 photographs of the earth and 70 km of magnetic tapes of information related to earth observations were made during the mission.
The Tata Institute of Fundamental Research, Bombay, in collaboration with scientists from the Physical Research Laboratory, Ahmedabad, exposed a stack of cosmic ray detectors for 73 days outside Skylab during the third manned mission and retrieved it for analysis of cosmic rays. These studies have given valuable information about cosmic rays - their origin and propogation in space. A new component of cosmic rays has been discovered by Indian scientists from Skylab studies.
The Fall: It was initially thought that Skylab would spiral down sometime during 1980. Its earlier anticipated fall is curiously due to the sun which controls the upper atmospheric pressure of the earth. The pressure is not a constant and varies by orders of magnitude, depending on the state of the sun. There are what are called sun spots - dark regions seen in the solar disc whose number varies periodically. From a near absence of sun spots during a "solar minimum", the number grows into a maximum and falls to a minimum again within a period of about 11 years which is called the "solar cycle".
The upper atmospheric pressure, and hence the air drag for the satellite, will be high during maximum solar activity. The sun is reaching the peak of its activity which is expected to be the maximum around 1979-80. That would be still 'safe' for Skylab if the solar activity during the present maximum is normal. But it appears that the present solar maximum will be extremely intense with the sun spot number exceeding 100 or so. If that is so, the drag on the satellite will be considerable, causing it to plunge downwards earlier than predicted.
Skylab can still be saved if a Space Shuttle is sent up and a booster rocket attached to the space station enabling it to be sent in a higher orbit. But this appears to be difficult because of a delay in the Space Shuttle project. Corrections have been carried out from the ground station at Houston, Texas, which has stabilized the spacecraft's wobble to some extent. But if the orbit is not sufficiently raised, it will quickly lose its altitude and will ultimately head towards the earth. A sudden increase in solar activity will quicken the process.
Food: The saving of Skylab is important because a number of materials were kept outside the spacecraft to discover the effects of prolonged exposure in space. Among the objects inside the space station are rehydratable thermostabilized bread pudding and beverages. A study of the retrieved material will be useful for future space programmes involving long duration flights.
Skylab left alone in space also poses a danger to the earth in that it may fall on some heavily inhabitated region. In principle its orbit could be changed so that it falls in an uninhabited area, such as the deep ocean or a desert. Thus there are two courses of action left open. Either it can be placed in an higher orbit so as to prolong its lifetime, or it can be destroyed in a pre-determined orbit with a minimum of danger to people and property on earth.
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