Radiation fears stalk stellar mission
Jun 30, 2010 1 comment
Europe's €450m GAIA satellite is suffering serious technical problems that could hinder its ability to create the largest and most precise 3D chart of our galaxy. Researchers are now in a race against time to ensure that the European Space Agency (ESA) mission will be able to map a billion stars within the Milky Way in extremely high resolution, which should provide a better understanding of the evolution and dynamics of our galaxy.
Due for launch in 2012, the five-year-long GAIA mission will orbit around the Sun from a point in space known as the second Lagrange point L2. It will have two cameras and a spectrometer, which will jointly use an array of 106 CCDs as photon detectors to determine the positions, distances, proper motions and brightness variations of stars. But to do their job, these CCDs need to avoid contamination from energetic particles mainly resulting from solar flares that could otherwise limit how well GAIA can detect faint light from stars.
The radiation issue is a top-level risk for GAIA and cannot be cured by changing the type of CCD or by shielding Giuseppe Sarri, GAIA's project manager
"When a CCD is subject to high-energy protons in space, its silicon structure is modified by the creation of holes in the material," says Giuseppe Sarri, GAIA's project manager. Sarri notes that when this happens, the electrons generated by incoming stellar photons impinging on the CCD "disappear" and then reappear a millisecond to a second later. While this effect is normally negligible for what he terms "normal" missions, the effect could be "dramatic" if GAIA is to measure stellar positions with accuracies down to just a few microarcseconds. ESA reports that this problem may well get worse by the expected solar maximum in 2012, when solar activity is greatest.
ESA, however, says it has been aware of the problem for at least a decade. Since 2005, researchers have been addressing the issue of how radiation damage could rearrange the charges on GAIA's CCDs as they are in the process of being electronically read. "The radiation issue is a top-level risk for GAIA and cannot be cured by changing the type of CCD or by shielding," says Sarri. "Calibration is the only way to understand exactly which parameters play a role in changing the CCDs' behaviour."
Concern over ESA's handling of the radiation issue caused Michael Perryman, former GAIA project scientist, to resign from the agency in 2008. But GAIA science-team member Lennart Lindegren, an astronomer at Lund Observatory in Sweden, is confident that GAIA's unprecedented accuracy will be feasible. GAIA researchers will continue to perform tests and calibrations until at least 2011, which will include irradiating the CCDs at space-like temperatures. Lindegren admits, however, that they can never be certain of success until the spacecraft is in orbit and starts sending back data.
About the author
Bruce Dorminey is a science journalist based in the US and author of Distant Wanderers: The Search for Planets beyond the Solar System (Springer, New York)