Keeping time to go atomic on space station

Atomic Clock Ensemble in Space (ACES)

By David Shiga

THE International Space Station will soon host the most accurate clock ever sent into space. It will allow for better synchronisation of clocks on Earth and also probe exotic physics.

The experiment, called Atomic Clock Ensemble in Space (ACES), will be built by EADS Astrium and is scheduled to fly to the space station in 2014, the European Space Agency announced last week. It will keep track of time by measuring the frequency of microwaves absorbed by cooled caesium atoms.

On Earth, the accuracy of caesium clocks is limited by gravity. The atoms are cooled by using lasers to slow them down, then tossed upwards into a cavity where measurements are made to determine the precise frequency of microwave radiation that they absorb and emit. In microgravity, the atoms linger in the cavity, allowing for longer and more accurate measurements, explains John Prestage of NASA’s Jet Propulsion Laboratory in Pasadena, California, who is not involved in the project.

ACES should be at least 100 times as accurate as the clocks on GPS satellites, adds Prestage.

Using the space-station clock as a common point of reference, ground-based atomic clocks could be more accurately compared with one another. What’s more, variations between atomic clocks could reveal if a physical constant called alpha – which governs the electromagnetic force – is not constant after all.

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