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Riddle as Gaia's atomic clock runs slow

Paul Sutherland
04 February 2014, 0:00 UTC

Sen— Europe’s flagship star-mapping mission Gaia presented space scientists with a bit of a puzzle soon after its arrival in deep space to begin operations, Sen can reveal today.

The spacecraft’s onboard atomic clock appeared to be running slow by 1.3 seconds. And while that may sound like no time at all in ordinary life, it is a lot when critical time measurements are vital to Gaia’s work in making an accurate 3D model of the Milky Way galaxy.

Mission scientists were bemused because they knew the clock had been accurately set before the probe was launched from Kourou, French Guiana, by a Soyuz-STB/Fregat rocket on 19 December last year. And it was thought that proper account had been taken of relativistic effects - the fact that time slows down on a fast moving clock, compared to a stationary one, as predicted by Albert Einstein.

Gaia had to make a long and speedy journey following launch to reach a position 1.5 million km from the Earth called the L2 Lagrange point where it made a balanced stable orbit around the Sun in keeping with our own. Everything had gone to plan and Gaia arrived safely in what is called a Lissajous orbit around the L2 point. So why was the time out?

Fortunately the puzzle was swiftly solved and there is no real problem that would jeaopardise the spacecraft’s mission to survey around one bilion stars in the Milky Way, or one per cent of the galaxy’s 100 billion stellar population to high precision.

It turns out that while the time glitch was partly due to those Einsteinian effects of relativity, an error in synchronising signals from two widely separated radio dishes that listen for Gaia’s signals was also to blame.

Markus Bauer, ESA’s Science and Robotic Exploration Communication Officer, told Sen: “The first end-to-end check of the timing performance revealed a mismatch stemming from some kind of lack of synchronisation from one ground station to another and some relativistic effects not taken into account in some of the formulae. This mismatch has been fixed and the timing performances are now well within the specification.”

But wait, there is more! Early checks on Gaia revealed another unexpected result. It appeared to be a lot fainter through a telescope than had been expected. Keeping tabs on the spacecraft’s exact position is another essential part of the mission and it was expected to be able to do this with a moderate-sized professional telescope.

The five Lagrangian points for the Sun-Earth system. Gaia is at point L2. Image not to scale. Credit: NASA.

However, whereas the probe was expected to shine at a brightness of around magnitude 18, due to sunlight reflected from its sunshield, it was more than ten times fainter at about magnitude 21, according to an insider. Was this caused by over-polishing of the reflective shield, some wondered.

Markus told us that the spacecraft is fainter than expected but that ESA will get round this difficulty by using larger telescopes and taking longer exposures to image it. He said: “The expectation on the magnitude of Gaia was based on the optical tracking of Planck, which is a different spacecraft with different inclination to the Sun, and therefore brighter than Gaia. However it was the only spacecraft in L2 we could use.

“The current magnitude of Gaia will make it more difficult to measure it with one-metre-class telescopes. Therefore we either need to make longer observations or use larger telescopes. This will be solved without any doubt, and therefore this will not have any impact on the mission.”

So the good news is that Gaia will continue to carry out its amazing observations which, as well as mapping the stars, will discover new exoplanets, asteroids and icy bodies in the outer reaches of our Solar System.