Having failed to achieve their intended orbit following a launch in August 2014, a pair of ESA Galileo satellites are being re-tasked to test Einstein's General Theory of Relativity. More precisely, the satellites will examine the effects of gravity on the passage of time, observing the phenomenon in a detail four times greater than any previous mission.
The original mission of the fifth and sixth Galileo probes was to, as part of a larger constellation of 30 identical satellites, provide a European global navigation system on a par with the Glonass and GPS systems used by the Russia and America. Unfortunately, a malfunction in the upper stage of the launch led to the twins being inserted into a low orbit, which temporarily rendered their primary mission inoperable.
Mission operators have since created a more favourable, yet still highly elongated orbit for the Galileo satellites, with the side effect of allowing each probe to observe minute fluctuations in time via its onboard atomic clock as it experiences a twice-daily 8,500-km (5,282-mile) rise and fall as it moves through its irregular orbit.
According to Einstein's General Theory of Relativity, time will pass slower for a person or object which is closer to a very massive body, such as a star or planet, than it would for someone who is further out. Around a supermassive body such as a black hole, time dilation would be extreme, but for a spacecraft orbiting Earth, the effects would be relatively minuscule.
Previous observations of the theory, carried out most notably by NASA's Gravity Probe A, had an underlying error which resulted in a navigational discrepancy of around 10 km (6.2 miles) per day, lowering the accuracy of the readings. Despite the difficulties, the spacecraft, which boasted a hydrogen mesa atomic clock, was able to make precise readings of time dilation as it passed along a single orbit. Readings from the mission supported the theory with an accuracy of 140 parts in a million.
The elongated, stable nature of the orbit of the twin Galileo satellites will allow scientists to take our understanding of the phenomenon one step further, whilst avoiding the errors that blighted Gravity Probe A. Furthermore, by carrying out observations of time dilation over hundreds of orbits instead of the single sub-orbital path travelled by Gravity Probe A, scientists will be able to undertake a far more comprehensive test of Einstein's theory
The impromptu experiment will last for one year, and collect readings around four times as accurate as any previous study of the phenomenon. It'll be studied in even greater depth upon by the agency's Atomic Clock Ensemble in Space mission (ACES), which is slated for launch in 2017.