Riders on the storm
11 December 2000Whilst producing impressive science results, ESA's XMM-Newton and NASA's Chandra X-ray observatories have been weathering a truly harsh space environment. Radiation, which can hinder observations and even damage sensitive detectors aboard the two spacecraft has, at times, exceeded expected levels. During a three-day XMM-Newton workshop held at the mission's Science Operations Centre at VILSPA at the end of November, scientists have exchanged their findings and solutions to safeguard their missions.
Twice on each orbit, both X-ray observatories cross the Earth's radiation belts which extend out to a distance of some 40,000 kilometres. In this danger zone, highly energetic charged particles - essentially protons and electrons - are trapped by our planet's magnetic field. And even outside these belts and beyond the Earth's magnetosphere, the observatories are occasionally encountering large numbers of highly energetic charged particles.
Some ten years ago, it was realised that these energetic protons might damage XMM-Newton's CCD detectors and prevent them from collecting and measuring genuine X-rays. So the mission's science instruments were designed as robust as possible with heavy metal shielding around the detectors and shutters to occult them during passages through the belts and for occasions when solar flares send waves of highly charged particles out into space. A Radiation Monitor also acts as a sentinel for the whole spacecraft.
All appeared to have been catered for when, just as XMM-Newton was being shipped to Kourou for the launch campaign, news came that comparable instruments on the American Chandra satellite had suffered appreciable problems within their first few months of operation. With the help of NASA scientists, the XMM-Newton team was able to amend the operation procedures to improve protective measures. For instance, observation scheduling has been extremely prudent, delaying observations until XMM-Newton is well clear of the danger zone.
This protection strategy appears today to be working well and performance has not been significantly degraded. However the environment of space has brought a few surprises! On a number of observations, the XMM-Newton cameras have been subject to brief but intense doses of radiation. Some unknown particles have funnelled down the telescopes into the detectors and have masqueraded as a bright glow of X-rays, drowning out the targets of interest. Most astronomers can easily remove these short anomalies from their data, but there has been concern that these episodes might over the long term still harm the detectors.
" The most important conclusion reached at our meeting " - says the ESA workshop organiser Dave Lumb - " was that so far all our precautions are working well and that for both XMM-Newton and Chandra the small changes in performance with time of the CCD detectors is well within the range of pre-launch predictions, especially in the context of the large solar flares as the Sun reaches its maximum activity in its 11-year cycle. "
Whereas XMM-Newton is being watched over on a 24-hour basis, the Chandra Operations Control Center does not continuously receive the spacecraft telemetry. This has required the implementation of autonomous x-ray detector safing procedures, whereby Chandra automatically detects a danger and shuts down its science instruments. This has occurred four times this year because of high radiation events, such as the solar flare outbursts in July and November. Science observations were able to resume very quickly after the radiation subsided.
" No one realized that low energy protons would scatter off of Chandra's X-ray mirrors in the direction of the focal plane with such a high efficiency." explains Paul Plucinsky of the Chandra X-Ray Observatory Center, at the Harvard-Smithsonian Center for Astrophysics. " Our models were sufficient for the environment in the belts, we now have to improve our understanding of what is happening further out on the orbit. In particular we must continue to monitor closely the long-term cumulative effect of this radiation. But so far we are still on target to continue Chandra operations for at least ten years. "
The XMM-Newton team has found a smart way to ensure safe operations as ESA's Fabio Giannini explains: " We are now using the data from the science instruments themselves to warn us when the radiation environment is getting too 'hot'. We can measure levels before carrying out observations. The EPIC-pn camera, for example, acts like a scout, opening the window, sniffing the radiation weather and telling us whether it's safe to proceed. "
Never the less the experts on both missions are intrigued by the precise nature of certain mysterious waves of radiation. In one extreme case, a sheet or cloud of protons, possibly only 10km thick, produced some 300 particle impacts per square centimetre per second. And that onslaught lasted nearly 15 seconds! Who said it would be plain sailing for XMM-Newton?