GPS is handy for finding a route, but it might be able to solve fundamental questions in physics too. An analysis of GPS satellite orbits hints that Earth is heavier than thought, perhaps due to a halo of dark matter. Dark matter is thought to make up about 80 per cent of the universe's matter, but little else is known about it, including its distribution in the solar system. Hints that the stuff might surround Earth come from observations of space probes, several of which changed their speeds in unexpected ways as they flew past Earth. In 2009, Steve Adler of the Institute of Advanced Studies in Princeton, New Jersey, showed how dark matter bound by Earth's gravity could explain these anomalies.
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