Failed GPS satellites to test Einstein’s theory

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Making lemonade from lemons: Scientists are going to repurpose two GPS satellites — launched into wrong orbits and thus useless for GPS — to conduct a test of Einstein’s theory of general relativity.

The satellites, operated by the European Space Agency (ESA), were mislaunched last year by a Russian Soyuz rocket that put them into elliptical, rather than circular, orbits. This left them unfit for their intended use as part of a European global-navigation system called Galileo.

But the two crafts still have atomic clocks on board. According to general relativity, the clocks’ ‘ticking’ should slow down as the satellites move closer to Earth in their wonky orbits, because the heavy planet’s gravity bends the fabric of space-time. The clocks should then speed up as the crafts recede.

On 9 November, ESA announced that teams at Germany’s Center of Applied Space Technology and Microgravity (ZARM) in Bremen and the department of Time–Space Reference Systems at the Paris Observatory will now track this rise and fall. By comparing the speed of the clocks’ ticking with the crafts’ known altitudes — pinpointed within a few centimetres by monitoring stations on the ground, which bounce lasers off the satellites — the teams can test the accuracy of Einstein’s theory.

There actually is little uncertainty here. No one expects this experiment to disprove Einstein’s theory, but the failed spacecraft provide a great opportunity to measure things at an accuracy never previously attempted, which in turn will help improve future GPS design.

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