Space

Galileo satellites to test Einstein's General Theory of Relativity

Galileo satellites to test Ein...
Artist's impression of a Galileo satellite orbiting Earth
Artist's impression of a Galileo satellite orbiting Earth
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Artist's impression of a Galileo satellite orbiting Earth
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Artist's impression of a Galileo satellite orbiting Earth
Graphic displaying the orbits of the Galileo satellites, pre- and post-correction
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Graphic displaying the orbits of the Galileo satellites, pre- and post-correction

Having failed toachieve their intended orbit following a launch in August 2014, apair of ESA Galileo satellites are being re-tasked to test Einstein'sGeneral Theory of Relativity. More precisely, the satellites willexamine the effects of gravity on the passage of time, observing thephenomenon in a detail four times greater than any previous mission.

The original mission ofthe fifth and sixth Galileo probes was to, as part of a largerconstellation of 30 identical satellites, provide a Europeanglobal navigation system on a par with the Glonass and GPS systemsused by the Russia and America. Unfortunately, a malfunction in theupper stage of the launch led to the twins being inserted into a loworbit, which temporarily rendered their primary mission inoperable.

Mission operators havesince created a more favourable, yet still highly elongated orbit forthe Galileo satellites, with the side effect of allowing each probe to observe minute fluctuations in time via its onboard atomic clockas it experiences a twice-daily 8,500-km (5,282-mile) rise and fall as it movesthrough its irregular orbit.

According to Einstein'sGeneral Theory of Relativity, time will pass slower for a person orobject which is closer to a very massive body, such as a star orplanet, than it would for someone who is further out. Around asupermassive body such as a black hole, time dilation would beextreme, but for a spacecraft orbiting Earth, the effects would berelatively minuscule.

Graphic displaying the orbits of the Galileo satellites, pre- and post-correction
Graphic displaying the orbits of the Galileo satellites, pre- and post-correction

Previous observationsof the theory, carried out most notably by NASA's Gravity Probe A,had an underlying error which resulted in a navigational discrepancyof around 10 km (6.2 miles) per day, lowering the accuracy of the readings.Despite the difficulties, the spacecraft, which boasted a hydrogenmesa atomic clock, was able to make precise readings of time dilationas it passed along a single orbit. Readings from the missionsupported the theory with an accuracy of 140 parts in a million.

The elongated, stablenature of the orbit of the twin Galileo satellites will allowscientists to take our understanding of the phenomenon one stepfurther, whilst avoiding the errors that blighted Gravity Probe A.Furthermore, by carrying out observations of time dilation overhundreds of orbits instead of the single sub-orbital path travelledby Gravity Probe A, scientists will be able to undertake a far morecomprehensive test of Einstein's theory

The impromptuexperiment will last for one year, and collect readings around fourtimes as accurate as any previous study of the phenomenon. It'll bestudied in even greater depth upon by the agency's Atomic ClockEnsemble in Space mission (ACES), which is slated for launch in 2017.

Source: ESA

2 comments
Lbrewer42
Since time is a concept - not an actual, physically measurable "thing," I often wonder if Einstein meant that gravity pulls on closer objects with more force, hence the perception of passing time, and the ability of mechanisms of any sort to measure time, are slowed.
Jacob Shepley
@Lbrewer42
no