Perhaps surprisingly, they did so as part of the preparations for ESA's Gaia mission, which scientifically is totally unrelated to WMAP.

The main goal of the Gaia mission is to make the largest, most precise three-dimensional map of our Galaxy. To this end Gaia will survey the entire sky to detect and very accurately measure the position and motion of each star down to mV~20 that passes its field of view.

The correct scientific evaluation of Gaia's position measurements makes it necessary that the absolute velocity of the spacecraft with respect to the Solar-System barycentre must be known to 2.5 mms-1, or to one part in 10 million, and the absolute position to 150 metres, or to one part in a thousand million.

This tremendous requirement cannot be satisfied by the usual satellite tracking techniques using their own radio signals, at least not for all times in Gaia's five-year science mission. It can be done, however, if sunlight reflected from the spacecraft is used for direct position measurements of the spacecraft on the sky. In orbit, Gaia will appear as a very faint speck of light, moving slowly among the distant background stars.

This ground-based optical tracking of Gaia was proposed by Ulrich Bastian (ARI, Heidelberg) a few years ago. Martin Altmann (also at ARI, Heidelberg) will be in charge of organizing and coordinating the ground-based optical tracking of Gaia in the years 2012 to 2017. He will need the support of quite a number of observers and observatories for this purpose.

Testing the Gaia tracking concept
What has all this to do with NASA's WMAP? Well, the ground-based optical tracking concept must of course be tested. Like WMAP, Gaia will be located at the second Earth-Sun Lagrange point L2, about 1.5 million kilometres from Earth. Like Gaia, WMAP has a deployable sunshield, partly covered with insulation material and partly with solar panels.

The Gaia shield is about 11 metres in diameter and inclined by 45 degrees to the Sun direction, that of WMAP is about 4.5 metres and inclined by 22.5 degrees. With all these parameters, WMAP is a reasonable (photo-)model for the brightness and observability of Gaia.

If the sunshield materials were strictly the same, and the proportion of insulation and solar panel areas similar, WMAP could be expected to be roughly 1.5-2 magnitudes fainter than Gaia. The actual brightness difference is still uncertain to some degree, however.

The above picture shows WMAP flying past the stellar background. Three images taken on 5 April 2008 at time intervals of a few minutes were added up to create this composite frame. Before superposition, the three images (actually black-and-white images) were artificially coloured red, green and blue.

For the stars, these three colours added up to neutral white. In contrast, the WMAP satellite shows up as the string of coloured points - since it is the only object having moved between the times the three images were taken. In addition to WMAP and a number of stars, a faint galaxy is visible as a slightly fuzzy blob at top centre of the picture.

The team acknowledges Dale Fink, Navigator of WMAP Spacecraft Control Team, for his specially supplied orbital ephemeris of WMAP.

Technical info: The exposures were 60 seconds each in the V band. Alexandre Andrei got a preliminary brightness of V=19.4 for WMAP, using the IRAF software, calibrating with 5 UCAC-2 stars, and applying a g-to-V magnitude correction. The WMAP ephemeris predicted an apparent magnitude for La Silla, at the time of observation, of V=18.7.

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