Communications satellites could be suspended above the Earth’s polar regions by the pressure of sunlight, according to an American physicist. At present, most communications satellites are crowded into the ‘geosynchronous’ orbit around the equator. Not only is this orbit limited to only a finite number of ‘slots’, but the satellites are difficult or impossible to use near the polar regions.
Robert Forward, while at Hughes Research Laboratories in California, proposed using the pressure of sunlight to hold a spacecraft in position. Rather than orbiting the Earth, the craft will remain essentially stationary in space with respect to the Earth as the Earth orbits the Sun. Such a spacecraft will stay constantly visible in the polar sky. And there is no limit to the number that can be used.
Forward has coined the generic name ‘statite’ for such a craft, because it will strictly not be a satellite of the Earth. He has given the name ‘Polestat’ to a statite that hovers above the polar regions.
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In January 1989, Forward filed a US patent application ‘Statite Apparatus and Method of Use’, serial number 07/294788. He is publishing a detailed technical paper in the May/June issue of the American Institute of Aeronautics and Astronautics’ Journal of Spacecraft and Rockets.
Communications satellites using the geosynchronous orbit were first proposed by the British engineer and writer Arthur C. Clarke in an article in Wireless World in 1945. Today there are many dozens of them in equatorial geosynchronous orbit, with many more planned. In the geosynchronous orbit, 36 000 kilometres above the equator, a satellite makes one revolution each time the Earth turns, so to an observer on the ground it remains fixed in the sky.
But the prime spaces along the geosynchronous arc (such as the midpoints over the oceans between two continents) are already filled to capacity. This means that nations that are not yet capable of space flight are arguing bitterly with the rich spacefaring nations in an attempt to obtain guaranteed future allocations of slots along this ‘limited natural resource’.
The geosynchronous orbit is most useful for communication between points on the Earth that lie near the equator. This is because the antennas of ground stations can point more or less straight upwards. In such a situation, the signal has little atmosphere to pass through so is hardly diminished. However, geosynchronous satellites are difficult or impossible to use near the polar regions because of local topography and attenuation through the atmosphere at low angles of elevation.
A Polestat will use a solar sail, not orbital motion, to counteract the Earth’s gravity. This will be in contrast to all the thousands of objects that are moving in orbits in which the Earth’s gravitational pull is exactly balanced by the centrifugal force generated as a result of their motion.
A solar sail is a large sheet of reflective material attached to the spacecraft (see ‘On the crest of a sunbeam’, New Scientist, 5 January). The light pressure from sunlight bouncing off a Polestat’s solar sail will result in a small but continuous force (about 9 newtons per square kilometre of sail area) that can provide constant propulsion. Already, many groups around the world, including ones at the Jet Propulsion Laboratory, Johns Hopkins University and the Massachusetts Institute of Technology in the US, and Cambridge Consultants in Britain, have carried out engineering studies of methods to build, deploy and control such large, flimsy structures.
A Polestat will use its solar sail to ‘levitate’ in space above the shadowed, or dark, side of the rotating Earth. The force of light pressure will exactly counterbalance the force of the Earth’s gravity (see Figure).
A Polestat does not have to be positioned directly opposite the Sun. It can be placed anywhere in a large volume over the shadowed side of the Earth. For this reason, a very large number of Polestats can be placed in the sky without interfering with each other’s broadcasts.
In its normal mode of operation, the Polestat is kept at a fixed angle to the polar axis. This angle will have to be greater than 23.5 degrees because the Polestat has to stay over the dark side of the Earth in order to catch the sunlight at the right angle. The tilt of the polar axis of the Earth takes each pole 23.5 degrees to the sunward side of the Earth during one of the two solstices.
In practice, statites will be at angles of 30 degrees to 40 degrees from the polar axis. From the viewpoint of an observer on the Earth, the statite will rotate around the pole once every solar day (24 hours). Ground stations will have to have their antennas on a polar mount with a 24-hour clock drive.
Polestats could provide services to the US, Europe, Alaska, Canada, the Soviet Union, northern China, Argentina, Chile, New Zealand, southern Australia, South Africa, Madagascar – and, of course, the Arctic and Antarctic, which cannot use equatorial geosynchronous satellites.
Using reasonable numbers for the size of a practical solar sail and the mass of the payload, the typical distance of a Polestat from the centre of the Earth will be between 30 and 100 Earth radii. For comparison, geosynchronous orbit is at 6.6 Earth radii and the Moon is at 63 Earth radii.
Because the altitude of the Polestats will be comparable to the distance to the Moon, their orbits will be perturbed by the Moon’s gravity. This will have to be compensated for by their control systems. There will be no danger of collisions, however, because the Polestats will be above the polar regions of the Earth while the Moon is always above the equatorial regions.
A Polestat could even be placed directly above the North or South Pole of the spinning Earth. At first glance, this seems to be impossible, because during the summer season the Polestat would have to be above the sunlit side of the Earth, and the force of solar light pressure and the Earth’s gravity would be in the same direction.
Colin McInnes of the University of Glasgow has shown, however, that a Polestat can hover stably above a pole at an altitude of 270 Earth radii. In his calculations, he has taken into account the increased gravitational attraction of the Sun and the decreased centrifugal force of the Polestat’s annual rotation about the Sun. To an observer on the Earth, the statite will stay fixed above the pole while the stars rotate around it. For these Polestats, the ground stations can use fixed antennas pointed parallel to the Earth’s polar axis.
Because of the difficulty of building large, lightweight solar sails, practical Polestats will always be at distances significantly greater than geostationary satellites. This means that they must compensate for the increased distance with larger antenna diameters or increased transmitter power.
For 30 Earth radii, the round-trip delay time for radio signals from Earth to the statite and back is 1.3 seconds; for 270 Earth radii, it is 11.5 seconds. These round-trip delay times make Polestats more suitable for direct television broadcast, data and facsimile communications than two-way telephone conversations. Polestats would also be ideal for monitoring weather and airline traffic over the polar regions, as well as maintaining a watch on the ozone holes.
Forward is paying for his patent through his company, Forward Unlimited. He expects it to be granted. The only question, he says, is how many of his patent claims will be accepted.
