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Lost Satellites Will Help Test Einstein's Theory of General Relativity

Two of the ESA's Galileo positioning satellites wound up in the wrong orbit, but it's not all for naught.

by Michael Byrne
Nov 19 2015, 12:30pm

Image: ESA

Putting a satellite into orbit doesn't always work out. In the summer of 2014, two spacecraft intended to support the European Space Agency's GPS-clone Galileo positioning system were delivered into the wrong orbit thanks to a wonky Soyuz rocket. They've remained fully functional, but still pretty much useless for their intended purpose.

All is not lost, however. On Nov. 9, the ESA announced that two teams would begin monitoring the atomic clocks aboard each spacecraft in the hopes of detecting the minute variations in time that should accompany a source of gravity such as the Earth. The Galileo craft are useless for positioning because they're on an elliptical rather than circular path, but this means that the satellites regularly change their distance from the planet, and, thus, experience gravity differently at different times.

Such a delicate probe of time as it relates to gravity should offer astrophysicists further verification (or not!) of Einstein's theory of general relativity. As an object finds itself closer and closer to a spacetime-warping gravitational force, time should slow down. As it drifts away, time should again speed up.

This has already been tested, of course, but with satellites whose altitudes above Earth can be measured down to the centimeter, general relativity can be demonstrated with a whole new level of precision.

Image: NASA

"While we don't know if and where relativity might break down, it is important to push the limits of our knowledge further and further, to eventually find hints of deviations," Gerald Gwinner, a physicist at the University of Manitoba unaffiliated with the ESA's project, tells Nature. "If this can be done as a money-saving opportunity, even better."

The Galileo experiments won't be the first time physicists have used orbiting atomic clocks to test general relativity. In 1976, a NASA launched a satellite known as Gravity Probe A just for this purpose. While it only lasted two hours in orbit (at about 10,000 kilometers above Earth's surface) Probe A was able to make some highly precise measurements. The forthcoming data, however, will trim any potential errors to within 0.004 percent. The catch is just that we have to wait a year for the results.

A further ESA experiment known as the Atomic Clock Ensemble in Space will start taking measurements aboard the International Space Station in 2017, offering precision down to .0002 percent. In other words, Einstein isn't getting off easy.

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