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Einstein's brain to be picked by satellites

Euro GPS misfires to test General Relativity

The European Space Agency is to make the most of two satellite misfires by using them to test Einstein's theory of relativity.

Last August, the fifth and sixth Galileo satellites were launched as part of a plan to create a Euro-run GPS system, but the launch didn't go according to plan.

A failure in the fourth stage of the rocket launcher left the two birds well out of their planned orbits. Since then, the ESA has been slowly nudging them back to where they can be used for geolocation. The final orbital tracks aren't perfect but they are workable.

"The satellites can now reliably operate their navigation payloads continuously, and the European Commission, with the support of ESA, is assessing their eventual operational use," explained ESA's senior satnav advisor Javier Ventura-Traveset.

One unexpected upside of the misfire however has been that it gives the agency a chance to do some unexpected science.

"The satellites have accidentally become extremely useful scientifically," said Ventura-Traveset, "as tools to test Einstein's Theory of General Relativity by measuring more accurately than ever before the way that gravity affects the passing of time."

Einstein predicted that time slows slightly close to a large gravitational object relative to something further away. This was proven in 1976, long after the great mathematician was dust, by NASA's Gravity Probe A mission, and now the Galileo satellites will refine that data with some experiments of their own.

The irregular orbits of the satellites mean that they rise and fall from the Earth's surface about 8,500 kilometers (5,282 miles) every day. By measuring the time distortion effects on the highly sensitive clocks that each satellite contains, scientists on Earth can check Einstein's calculations.

"While the Gravity Probe A experiment involved a single orbit of Earth, we will be able to monitor hundreds of orbits over the course of a year," said Ventura-Traveset.

"This opens up the prospect of gradually refining our measurements by identifying and removing systematic errors. Eliminating those errors is actually one of the big challenges. For that we count on the support of Europe's best experts, plus precise tracking from the International Global Navigation Satellite System Service, along with tracking to centimeter accuracy by laser."

Gravity Probe A found that a clock 10,000 kilometers up ran 140 parts in a million of a second faster than the same device on Earth, but it was a one-shot mission. The Galileo satellites' hundreds of passes should refine that figure by a factor of four. ®

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