ASCL/University of Strathclyde

Spacemaker. Assisted by solar sails, spacecraft could park in uncrowded geostationary orbits.

How to Alleviate an Orbital Traffic Jam

There's gridlock in orbit. More than 400 telecommunications satellites, plus an indeterminate number of retired, failed, and secret spacecraft, occupy a narrow band of space some 35,000 kilometers above Earth's equator. Now, researchers have found a way to alleviate the congestion: attaching solar sails to satellites that would propel them 10 to 30 kilometers north or south of the standard orbit. Space experts say that such sails could also open up other orbital positions that were previously considered unattainable.

Telecommunications satellites must remain in the same position above Earth at all times—in a so-called geosynchronous orbit—so that satellite dishes don't have to constantly swivel to track them. Thanks to the laws of gravity and orbital mechanics, the only way for a satellite to maintain a geosynchronous position has been to orbit above the equator.

But this prime real estate is growing scarce. The satellites could move north or south, but the extra rocket fuel needed to maintain these less-stable orbital positions would be expensive and would limit the life of the craft.

That's where solar sails come in. A large square of reflectively coated Mylar—say, a hundred meters on a side—could catch enough sunlight to propel a satellite above or below the plane of the equator and maintain enough thrust to hold it in geostationary position.

To see if the idea would work, aerospace engineer Colin McInnes and one of his graduate students, Shahid Baig, at the University of Strathclyde in the United Kingdom started with calculations developed by Robert Forward, a physicist and NASA consultant. Forward, who died in 2002, posited that the photons of sunlight streaming across the solar system contained sufficient energy to push a solar-sail–arrayed satellite out of geostationary orbit and maintain its new position without the need for heavy, liquid-fueled thrusters. The Japanese IKAROS spacecraft, which was launched last month, is testing the basic solar-sail concept.

Other scientists questioned Forward's calculations, because, they argued, his figures weren't precise enough. But using superaccurate computer models, McInnes and Baig have determined that Forward was indeed correct. The new calculations, published in the May/June Journal of Guidance, Control, and Dynamics, showed that sunlight hitting a solar sail would be sufficient to push the satellite into a geostationary orbit. And due to its continuous pressure, the sunlight could also hold the spacecraft in that orbit indefinitely without the need for thrusters.

Although the research provides only a theoretical proof of concept, experts see a bright future for solar-sail satellites. For example, says aerospace engineer Ben Diedrich of the National Oceanic and Atmospheric Administration in Silver Spring, Maryland, researchers could park satellites over Earth's poles to provide continuous climate monitoring at these latitudes. Or the sails could push solar-research spacecraft into more advantageous orbits to study the sun. Solar-sail–assisted orbits, he says, are "one more viable option for future mission planners to consider."

Posted in Physics