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Solar Sails Could Allow New Orbits for Satellites

Solar sailscouldbe used on satellites to levitate them above the crowd of objectscirclingEarth and into orbital paths that have never been used, new researchsuggests.

Firstpredicted by the American space scientist Robert L. Forward in 1984,theseorbits are only now being shown to be viable, the researchers said.

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

The researchis detailed in a recent issue of the Journal of Guidance, Control andDynamics.

Theorbital challenges

Geostationaryorbits ? orbits in which a satellite can remain perched over the samepatch of ground?  are coveted for communications use because of theirstability with respectto land-based antennas.

By matching theEarth'srotation, objects following these paths appear to always restin the samepart of the sky.

But true geostationaryorbits are relatively rare: Only ones within the ring of sky22,245 miles abovethe equator fit the bill. When dropped into one of the orbits in thisstrip, anunpowered satellite moves effortlessly along its curve.

Orbits thatinvolve only the gravitational force are known as Keplerian.

"Satellitesgenerally follow Keplerian orbits, named after Johannes Kepler, thescientistwho helped us understand orbital motion 400 years ago," said the leadresearcher of the new study, Colin McInnes, director of the AdvancedSpaceConcepts Laboratory, at Scotland's University of Strathclyde.

While somefamilies of orbits outside the equatorial band also let satellitescomplete anorbit in 24 hours, they present some notable problems.      

"Imaginea circular orbit whose plane doesn't pass through the center of theEarth. Ifthe orbit plane was above the center of the Earth, the Earth?s gravitywouldexert a downwards pull and the forces would be out of balance," McInnestold

In otherwords, the satellite would slip back toward the equator. But not if ithad a solarsail, according to the new study.


The firstsuccessful deployment of a solar sail in space occurred in July, by theunmannedJapanese spacecraft Ikaros, which is currently on its way toward Venus.

The ideathat large space sails could levitate satellites above their naturalrestingspots at the Earth's equator was first proposed by Forward (1932-2002),aphysicist and science fiction writer.  

Sunlightreflected off the sail could provide a vertical force that worksagainstgravity and keeps the orbit pushed above the equator, the theory goes.

But the unusualdynamics of the problem made scientists consider it an impossible feat.Tiltedin order to produce a vertical thrust, the sails would also experiencean extraforce in the horizontal direction, which could knock them out of orbit.

"Thesail provides a thrust which pushes the orbit above the geostationaryring. However,there is also thrust in the direction away from the sun. This leads toaviolation of the geostationary condition if it's not accounted forproperly,"McInnes said.

To correctfor the pressure of photons hitting the sail and moving it away fromthe sun,researchers designed an orbit that would remain intact while accountingfor thisforce.

"Otherstudies didn't do this and so could only find orbits which didn't havea 24-hourperiod," McInnes said.

Currently,researchers are studying the possibility of polar stationary orbits forclimate-monitoringsatellites. Spacecraft would require a continuous low thrust from a hybridsolar sail and electrical propulsion system in order tocounteract for the inwardspull of gravity.

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

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