Solar Sails Could Allow New Orbits for Satellites

Solar sails could be used on satellites to levitate them above the crowd of objects circling Earth and into orbital paths that have never been used, new research suggests.

First predicted by the American space scientist Robert L. Forward in 1984, these orbits are only now being shown to be viable, the researchers said.

By using the pressure of sunlight to produce thrust, solar sails would keep the satellites from falling back into the geostationary ring above Earth's equator, which is already chock-full of satellites and space debris, according to the study. That packed strip is 22,245 miles (35,800 kilometers) above the Earth. Any satellites receiving a solar "oomph" could instead travel in "displaced orbits" about six to 31 miles (10 to 50 kilometers) farther out.

The research is detailed in a recent issue of the Journal of Guidance, Control and Dynamics.

The orbital challenges

Geostationary orbits ? orbits in which a satellite can remain perched over the same patch of ground ?  are coveted for communications use because of their stability with respect to land-based antennas.

By matching the Earth's rotation, objects following these paths appear to always rest in the same part of the sky.

But true geostationary orbits are relatively rare: Only ones within the ring of sky 22,245 miles above the equator fit the bill. When dropped into one of the orbits in this strip, an unpowered satellite moves effortlessly along its curve.

Orbits that involve only the gravitational force are known as Keplerian.

"Satellites generally follow Keplerian orbits, named after Johannes Kepler, the scientist who helped us understand orbital motion 400 years ago," said the lead researcher of the new study, Colin McInnes, director of the Advanced Space Concepts Laboratory, at Scotland's University of Strathclyde.

While some families of orbits outside the equatorial band also let satellites complete an orbit in 24 hours, they present some notable problems.       

"Imagine a circular orbit whose plane doesn't pass through the center of the Earth. If the orbit plane was above the center of the Earth, the Earth?s gravity would exert a downwards pull and the forces would be out of balance," McInnes told SPACE.com.

In other words, the satellite would slip back toward the equator. But not if it had a solar sail, according to the new study.

Solar thrust

The first successful deployment of a solar sail in space occurred in July, by the unmanned Japanese spacecraft Ikaros, which is currently on its way toward Venus.

The idea that large space sails could levitate satellites above their natural resting spots at the Earth's equator was first proposed by Forward (1932-2002), a physicist and science fiction writer.  

Sunlight reflected off the sail could provide a vertical force that works against gravity and keeps the orbit pushed above the equator, the theory goes.

But the unusual dynamics of the problem made scientists consider it an impossible feat. Tilted in order to produce a vertical thrust, the sails would also experience an extra force in the horizontal direction, which could knock them out of orbit.

"The sail provides a thrust which pushes the orbit above the geostationary ring. However, there is also thrust in the direction away from the sun. This leads to a violation of the geostationary condition if it's not accounted for properly," McInnes said.

To correct for the pressure of photons hitting the sail and moving it away from the sun, researchers designed an orbit that would remain intact while accounting for this force.

"Other studies didn't do this and so could only find orbits which didn't have a 24-hour period," McInnes said.

Currently, researchers are studying the possibility of polar stationary orbits for climate-monitoring satellites. Spacecraft would require a continuous low thrust from a hybrid solar sail and electrical propulsion system in order to counteract for the inwards pull of gravity.

"In reality, we also need to compensate for the sun's gravity,"  McInnes said.

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