Science data salvaged from Sun-gazing probe

New Scientist ^ | 18:03 04 July 03 | Will Knight

[demlosers]

Engineers have found a way to rescue valuable scientific data from a Sun-monitoring spacecraft, despite a serious mechanical problem with its main antenna. By rerouting the transmissions through a secondary antenna normally used to download system information, they hope to lose no more than two percent of data.

The Solar and Heliospheric Observatory (SOHO) uses a dozen different instruments to image the Sun's surface and coronal activity and to monitor particle flux in the solar wind. But as well as being of interest to solar scientists, SOHO monitors solar flares emitted by the Sun that cause magnetic storms when they reach the Earth's magnetosphere. That provides valuable early warning of space weather events that can seriously disrupt global satellite systems, aircraft and even power grids.

"This is very important," says Jeff Payne, a solar scientist at Rutherford Appleton Laboratory in the UK, who operates one of the spacecraft's instruments, the Coronal Diagnostic Spectrometer. "SOHO provides a lot of information not just to scientists but to space weather forecasters."

Losing contact

The mechanism that keeps SOHO's main signalling antenna pointed towards Earth was found to have developed a fault on 4 May. It was moved to a stable position to prevent damage. But this meant losing contact with probe's scientific instruments for around 19 days every three months. The probe's operators feared they would have to shut the spacecraft down during these intervals to keep it safe.

Now engineers have discovered that large amounts of scientific data can still be retrieved during these blackouts. They plan to reroute transmissions through SOHO's secondary antenna. This antenna isn't able to focus its signals towards Earth, instead it sends them in all directions, so the signals received on Earth are weaker. But the engineers are confident that by switching to larger, more sensitive listening posts within the Deep Space Network - a global network of ground stations - they will still be able to pick up SOHO's communications.

The spacecraft's main antenna drifted out of range on 27 June and will not be back in range until after 14 July when operators plan to rotate the craft by 180 degrees, to try to maximise the time during which the antenna points towards Earth. But Payne says communications so far indicate that it will be possible to retrieve around 98 percent of SOHO's scientific information.

The only potential problem with the new plan is that SOHO's operators must fight other scientists for access to the more powerful ground stations during periods when the antenna is out of range. But Payne adds: "The SOHO schedulers are already working with the Deep Space Network to get access for the next period in September."

The SOHO probe was launched in 1985 and is jointly operated by NASA and the European Space Agency (ESA). It is stationed about a 1.5 million kilometres from Earth at one of the LaGrange points, where the gravitational pull of the Earth and the Sun effectively cancel each other out. This position keeps the spacecraft stable but is not fixed in relation to the Earth.

Will Knight