Pop quiz, hotshot. You've just launched a pair of GPS satellites into the wrong orbit, rendering them useless for navigation. What do you do? If you're the European Space Agency (ESA), you re-purpose them to precisely test Albert Einstein's theory that clocks slow down near heavy objects. Since the Galileo satellites were placed in elliptical, rather than circular orbits by Russian Soyuz rockets, they pass closer to Earth at certain points. Our planet bends the fabric of space-time, so the super-precise atomic clocks on-board the satnavs will theoretically slow during those times, then speed up again when the craft move away.