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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.
Ben Harris at the University of Texas at Arlington wondered if dark matter might also affect satellites. "The nice thing about GPS satellites is that we know their orbits really, really well," he says. From nine months of data on the satellites in the GLONASS, GPS and Galileo groups, he calculated Earth's mass as "felt" by each one.
At a meeting of the American Geophysical Union in San Francisco in December, he reported an average figure that was between 0.005 and 0.008 per cent greater than the value for Earth's mass established by the International Astronomical Union. A disc of dark matter around the equator 191 kilometres thick and 70,000 km across can explain this, he says.
Harris has yet to account for perturbations to the satellites' orbits due to relativity, and the gravitational pull of the sun and moon. What's more, preliminary data from NASA's Juno probe, also presented at the AGU meeting, suggests its speed was as expected as it flew by Earth, casting doubt on the earlier anomalies.
But if Harris's explanation is correct, satellites could reveal properties of dark matter, such as whether its particles interact with each other.
This article appeared in print under the headline "GPS satellites hint at Earthly dark matter"
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