By Jonathan Amos
The orbiting Gaia telescope will lose some performance because stray light is getting inside the observatory, the European Space Agency (Esa) says.
But the impacts are likely to be very small, scientists believe, and the expectation is that all the mission’s chief objectives will still be met.
Most of the unwanted light appears to be creeping around the giant shield Gaia uses to shade itself from the Sun.
The “pollution” makes it harder for the observatory to see the faintest stars.
“I must say this is not a major problem,” said Esa’s Gaia project manager, Giuseppe Sarri.
“The point is the spacecraft is doing very well in terms of everything is working, and now we’re focussing on the things we want to improve.
“We were expecting to get some stray light but the fact is, it is larger than we predicted,” he told BBC News.
Gaia was sent into orbit in December to do astrometry on a billion stars – to map their precise positions, distances and motions.
This huge sample should provide the first true picture of our Milky Way Galaxy’s structure.
As is normal after launch, the observatory was immediately put through a period of complex systems check-out and instrument calibration.
Engineers noticed early on that unexpected light was getting inside the big tent covering the satellite’s dual telescope mechanism.
Modelling indicates most of it is sunlight being diffracted around the observatory’s 10.5m-wide sunshield.
But further analysis suggests there is likely also some additional component – probably the general diffuse light on the sky itself.
The effect is certainly a nuisance because it makes it more difficult for Gaia to discern the least bright objects.
It was the mission’s aim to measure the positions of all stars down to magnitude 20 (about 400,000 times fainter than can be seen with the naked eye).
The stray light means about 40% of the accuracy of those measurements at this lowest magnitude will be lost.
Backwards and forwards
On the upside, it should be possible to get some of the performance back if Esa agrees to extend the mission and additional data can be taken.
And it is true to say that most of Gaia’s science will be done at magnitude 15 (4,000 times fainter than the naked eye limit) and brighter, which is unaffected.
Where the pollution issue may be felt more keenly is in determining the motions of stars towards or away (radial velocity) from the satellite.