When two Galileo global navigation satellites were launched into the wrong orbit last year, scientists decided to turn the multimillion dollar accident into the most rigorous test yet of Einstein’s theory of general relativity.
One of the theory’s predictions is that large objects like the Earth warp the fabric of spacetime, slowing time as smaller objects swoop in for a close encounter. The Galileo satellites, which were intended to follow circular orbits, were accidentally dropped into elliptical paths when the frozen fuel lines caused the control thrusters on the Soyuz rocket’s upper stage to fail. The satellites’ variable distance from the Earth made them ill-suited for navigation but perfect for the experiment.
Elizabeth Gibney, reporting for Nature News:
Launching space experiments takes enormous time and money, so using the off-course Galileo satellites is “a brilliant idea”, says Gerald Gwinner, a physicist at the University of Manitoba in Winnipeg, Canada, who is not involved in the work. “Even a mishap can be turned into something useful and fascinating,” he adds. “This is a classic case of ‘When life gives you lemons, make lemonade’.”
Each of the Galileo satellites contains an atomic clock, which is key to both their intended use and their new experimental mission. For global navigation, a receiver can determine its distance from the satellite by measuring the time it takes for a signal to travel from the satellite to the receiver. When a receiver is locked on to signals from three satellites, it can accurately determine its position on the globe. All global navigation satellites, including Galileo and the U.S.’s GPS, operate on this same principle, and the incredible temporal resolution of the atomic clocks aboard the satellites makes the whole endeavor possible.
Galileo’s atomic clocks—accurate to 0.45 nanoseconds over 12 hours—are also what will help test Einstein’s theory. As the two Galileo satellites swing closer and farther from the Earth, the time on their atomic clocks is expected to vary slightly. The clocks’ precision and the year-long duration of the experiment will help physicists determine how the nature of reality compares with the predictions from Einstein’s theory.
The last time this prediction of general relativity was measured was in 1976, and that experiment lasted just two hours. The impromptu Galileo experiment is expected to be four times more accurate than earlier results.