SOLAR sails could relieve traffic congestion in the skies above Earth.

For a satellite to hover above a point on Earth's surface, its orbit must encircle the equator. Such geostationary orbits allow uninterrupted communication or imaging, but room for new satellites is beginning to run out. "It's a finite natural resource, a unique circle around Earth," says Colin McInnes of the University of Strathclyde in Glasgow, UK.

A satellite orbiting slightly north or south of the equator would drift relative to the ground. Now McInnes and colleague Shahid Baig have shown how to counteract that drift by fitting satellites with solar sails, which generate thrust by absorbing or reflecting sunlight.

The idea of using solar sails in this way was first proposed in 1984, but with the sails at a fixed tilt, which more detailed calculations later showed cannot eliminate the drift.

However, McInnes and Baig show that the drift can be stopped by moving the tilt periodically back and forth, so long as the satellite's orbit is no more than 50 kilometres either side of the equator (Journal of Guidance Control and Dynamics, DOI: 10.2514/1.46681).

Issue 2772 of New Scientist magazine