The Solar and Heliospheric Observatory (Soho) is experiencing communication problems with the Earth.
By Dr David Whitehouse
BBC News Online science editor
The antenna it uses for high data-rate communication with the Earth is malfunctioning. Unless the problem is solved, its continuous transmission of scientific data to the Earth will suffer interruptions.
Constantly monitoring the Sun
Soho is an essential component among the array of satellites and ground-based observatories that monitor the Sun's surface and atmospheric activity.
Without Soho, our knowledge of potentially hazardous magnetic storms heading towards the Earth will be severely reduced.
Soho was launched in 1995 and is positioned at the L1 Lagrange point between the Earth and the Sun about 1.6 million km from the Earth. L1 is a region where the spacecraft can balance gravitational forces and remain in a stable position ideal for solar monitoring.
With its 11 instruments, Soho has become a crucial part in the scientific programme to study our star. Its website contains images that are updated daily.
Its position between the Sun and the Earth makes it particularly suitable for detecting the passage of magnetic storms en route to Earth.
Eruption warnings could be interupted
Such storms can occasionally disrupt satellite and radio communications. In extreme cases, astronauts on board the International Space Station can use a storm shelter area for protection from radiation.
Dr Bernhard Fleck, Soho's Project Scientist, told BBC News Online that Soho's High-Gain Antenna (Hga) can no longer move in a particular direction. A motor or gear assembly may be malfunctioning.
The Hga transmits data from Soho to the Earth at a high rate. It is vital to get the large amounts of data from Soho's many detectors.
The Hga has a beam about 14 degrees wide and is frequently moved so that it stays centred on Earth. As Soho and the Earth go around the Sun, the Hga beam's position needs such periodic adjustment.
Dr Fleck says the problem started on 4 May when Soho's monitoring computer detected that the Hga could not move properly.
Because of the Hga's wide beam, Soho is currently operating normally while the extent of the problem is evaluated.
Soho does have a Low-Gain Antenna (Lga) which does not need to be pointed in a specific direction. This will be used to monitor the spacecraft's health.
As Soho progresses in its orbit, the restriction of the Hga's movement will mean some data loss from Soho, says Dr Fleck. This is because the Earth will move out of the stationary Hga beam.
The Soho team estimates they will be unable to receive science data for about 18 days every three months. The first of these blackout periods will be in a few days' time.