ESA’s billion-star surveyor Gaia is slowly being brought into focus. This test image shows a dense cluster of stars in the Large Magellanic Cloud, a satellite galaxy of our Milky Way.
Once Gaia starts making routine measurements, it will generate truly enormous amounts of data. To maximise the key science of the mission, only small ‘cut-outs’ centred on each of the stars it detects will be sent back to Earth for analysis.
This test picture, taken as part of commissioning the mission to ‘fine tune’ the behaviour of the instruments, is one of the first proper ‘images’ to be seen from Gaia, but ironically, it will also be one of the last, as Gaia's main scientific operational mode does not involve sending full images back to Earth.
Gaia was launched on 19 December 2013, and is orbiting around a virtual point in space called L2, 1.5 million kilometres from Earth.
Gaia’s goal is to create the most accurate map yet of the Milky Way. It will make precise measurements of the positions and motions of about 1% of the total population of roughly 100 billion stars in our home Galaxy to help answer questions about its origin and evolution.
Repeatedly scanning the sky, Gaia will observe each of its billion stars an average of 70 times each over five years. In addition to positions and motions, Gaia will also measure key physical properties of each star, including its brightness, temperature and chemical composition.
To achieve its goal, Gaia will spin slowly, sweeping its two telescopes across the entire sky and focusing the light from their separate fields simultaneously onto a single digital camera – the largest ever flown in space, with nearly a billion pixels.
But first, the telescopes must be aligned and focused, along with precise calibration of the instruments, a painstaking procedure that will take several months before Gaia is ready to enter its five-year operational phase.
As part of that process, the Gaia team have been using a test mode to download sections of data from the camera, including this image of NGC1818, a young star cluster in the Large Magellanic Cloud. The image covers an area less than 1% of the full Gaia field of view.
The team is making good progress, but there is still work to be done to understand the full behaviour and performance of the instruments.
While all one billion of Gaia’s target stars will have been observed during the first six months of operations, repeated observations over five years will be needed to measure their tiny movements to allow astronomers to determine their distances and motions through space.
As a result, Gaia’s final catalogue will not be released until three years after the end of the nominal five-year mission. Intermediate data releases will be made, however, and if rapidly changing objects such as supernovae are detected, alerts will be released within hours of data processing.
Eventually, the Gaia data archive will exceed a million Gigabytes, equivalent to about 200 000 DVDs of data. The task of producing this colossal treasure trove of data for the scientific community lies with the Gaia Data Processing and Analysis Consortium, comprising more than 400 individuals at institutes across Europe.
VST-Schnappschuss von Gaia auf dem Weg zu einer Milliarde Sterne
Diese neuen Bilder vom Very Large Telescope Survey Telescope (VST) der ESO zeigen die ESA-Raumsonde Gaia etwa 1,5 Millionen Kilometer außerhalb der Erdumlaufbahn.
Am 19. Dezember 2013, einem Donnerstagmorgen, gestartet, hat der Satellit die Aufgabe in den nächsten fünf Jahren eine 3D-Karte unserer Galaxie zu erstellen. Den Himmel zu vermessen war seit Anbeginn der Zeit eine der zentralen Fragen der Menschheit. Gaia wird nun unser Verständnis von unserer stellaren Nachbarschaft auf ein ganz neues Niveau heben: Sie wird die Positionen und Bewegungen von etwa einer Milliarde Sternen in unserer Galaxie präzise ausmessen, um die Zusammensetzung der Milchstraße sowie ihre Entstehung und Entwicklung zu untersuchen.
Diese neuen Beobachtungen sind das Ergebnis einer engen Zusammenarbeit zwischen der ESA und der ESO, im Rahmen derer die Raumsonde von der Erde aus beobachtet wird. Gaia ist zwar das akkurateste astrometrische Gerät, das je gebaut wurde, um jedoch aus den Beobachtungen nützliche Informationen ziehen zu können, muss die Position der Sonde im Weltraum genau bekannt sein. Der einzige Weg, um die Geschwindigkeit und Position der Sonde mit sehr hoher Genauigkeit bestimmen zu können, besteht darin Gaia täglich von der Erde aus zu beobachten. Hierzu werden Teleskope, zu denen auch das VST der ESO zählt, in einer Kampagne, die als Ground-Based Optical Tracking (GBOT) bekannt ist, eingesetzt.
Das VST ist ein moderndes 2,6-Meter-Teleskop, das mit OmegaCAM ausgestattet ist, einer riesigen 268-Megapixel-CCD-Kamera mit einem Blickfeld, das viermal so groß ist wie die Fläche des Vollmonds. Das VST hat diese Bilder mit OmegaCAM am 23. Januar 2014 mit einem zeitlichen Abstand von 6,5 Minuten aufgenommen. Gaia ist deutlich als kleiner, rot markierter Punkt zu erkennen, der sich vor einem Hintergrund aus Sternen bewegt. In den Bildern ist die Raumsonde etwa eine Million mal schwächer als noch mit dem bloßen Auge erkennbar.
Gaia wurde bereits zuvor im Dezember 2013 mit dem VST beobachtet, sehr bald nach dem Start – und ist eines der nächsten Objekte, die jemals mit dem VST beobachtet wurden. Die Sonde erschien genau dort, wo sie vermutet wurde, was die erfolgreiche Zusammenarbeit zwischen der bodengebundenen und der weltraumbasierten Astronomie hervorhebt.
Europas Gaia Teleskop hat Probleme mit Streulicht
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, Guiseppe 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.
This information will have a number of applications but will be used to help make a 3D movie of the galaxy - to run forwards to see what happens millions of years into the future, and backwards to reveal how the galaxy was assembled in the deep past.
Gaia was hoping to get radial velocity data for about 150 million of the brightest stars.
It involves taking the light from a star and spreading it out into its component colours for analysis. For the faintest objects in the targeted sub-set of stars, this process again becomes much harder with stray light.
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There's no reason to believe Gaia won't be a triumph”
Prof Gerry Gilmore
The Gaia team thinks with some smart techniques it can recover some of the performance loss, but as it stands the mission may get radial velocity measurements now on only about 100 million stars.
"We say only about 100 million stars - that's still pretty spectacular," said Prof Gerry Gilmore, a Gaia scientist from the Institute of Astronomy at Cambridge University, UK.
"We don't actually know how many stars there are in the sky at the magnitudes needed to do the radial velocity measurements," he added.
"We had a guess that it was 150 million but that could be wrong by a factor of two; so it's quite possible there are a lot more stars out there [we can still measure].
"Gaia is the first ever high spatial resolution survey and so until Gaia has scanned the sky we won't know what's on the sky."
Engineers are also tracking an issue with what they call the "basic angle".
Part of Gaia's measurement strategy requires it to look at two parts of the sky at the same time to lock a frame of reference.
This is why it carries two telescopes held rigidly at an offset angle of 106.5 degrees. Great effort was put into making sure this basic angle was absolutely stable, with many components being constructed out of stiff silicon carbide as a consequence.
But a vanishingly small flexure is being detected - fractionally beyond what had been anticipated.
The team believes, though, it can nullify any impacts if the behaviour can be properly characterised.
"It's all nuisance stuff. Depending how you count it, there are about 500 critical components on Gaia and they're all working fine," said Prof Gilmore.
"Yes, it's complicated; yes, there are things we don't fully understand yet; but there's no reason to believe Gaia won't be a triumph."
Mr Sarri added that after the long commissioning process, the telescope was now ready to gather data continuously.
"In the last week, we had an informal handover and it's been working already in science mode on occasions. But now we will move into a period of 28 days of uninterrupted operation," he told BBC News.
Large sections of Gaia's telescope mechanism were constructed out of silicon carbide