Displaced Orbits Possible

Scientists at the University of Strathclyde in Glasgow, Scotland are claiming that maintainable displaced orbits – orbits above or below the standard geosynchronous orbits – may be possible with the help of solar sails. Scientist and science fiction writer Robert Forward first proposed this method in 1984 but the method was generally considered impractical if not impossible until this new study emerged.
 A displaced orbit shown next to a standard geosynchronous orbit. (Source: University of Strathclyde)
A displaced orbit shown next to a standard geosynchronous orbit. (Source: University of Strathclyde)

Satellites placed outside the natural geosynchronous orbits (called Keplerian orbits) without corrections applied by some outside force start to fall back toward the equator, deviating from their intended path. Making corrections with thrusters or other traditional means increases the size and limits the lifetime of a satellite because of the need to carry and consume fuel, making these displaced orbits impractical. As scientists launch more satellites, crowding of the Keplerian orbits became an increasing problem. Solar sails would use pressure from sunlight to provide the force needed to oppose gravity and keep the satellites in their planned orbits, removing the constraints of fuel-based corrections and making displaced orbits practical for the first time.

Graduate student Shahid Baig and Professor Colin McInnes, Director of the Advanced Space Concepts Laboratory at Strathclyde, devised a class of orbits displaced between 10 and 50 kilometers north or south from the band containing the Keplerian orbits sustainable with solar sails. Each of these orbits completes one circle of the Earth in 24 hours just like a standard geosynchronous orbit, thus remaining in a fixed position relative to the planet. This fixed position is important for communications satellites (and others) so satellite dishes do not have to swivel to track a moving position.

In the past, scientists argued that solar pressure was not sufficient to oppose the gravitational forces drawing satellites toward the equator. Baig and McInnes were able to formulate much more accurate computer models than available in the past to show that the solar sails would be sufficient to the task within a specific range of displaced orbits. Further, these orbits could be maintained indefinitely without the use of any thrusters or external power sources.

While the Strathclyde research is essentially just a proof of concept, several other groups are already experimenting with solar sails. The unmanned Japanese spacecraft Ikaros, currently on its way to Venus, uses a hybrid solar sail for its primary power source.

TFOT previously reported on other solar sail news including the launch of Ikarus earlier this year and NASA’s tests of its NanoSail-D solar sail this summer. TFOT has also examined one of the other causes of orbital crowding – space garbage.

Read more about the displaced orbits at the Advanced Space Concepts Laboratory research page or read Baig and McInnes’ paper on the subject (PDF).

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