Antenna Anomaly May Affect SOHO Scientific Data Transmission

Engineers are predicting this problem after detecting a malfunction in the pointing mechanism of the satellite's high-gain antenna (HGA), which is used to transmit the large amounts of data from SOHO's scientific observations to Earth.

The SOHO spacecraft is operating as safely as before the problem occurred. Its low gain antenna, which does not need to be pointed in a specific direction, will be used to control SOHO, monitoring spacecraft and instrument health and safety.

The anomaly in the HGA was recently discovered when engineers detected a discrepancy between the commanded and measured antenna position. In normal conditions, the antenna must be able to move along two axes, vertical and horizontal. The horizontal movement is no longer taking place properly. The problem is probably due to a malfunction in the motor or gear assembly that steers the antenna.

SOHO is located 1.5 million kilometers (one million miles) from Earth. It orbits around the First Lagrangian point, where the combined gravity of the Earth and the sun keep SOHO in an orbit locked to the sun-Earth line. To transmit data, the SOHO HGA must rotate to have Earth in its field of view.

If the problem is not solved, the Earth will be left outside the HGA beam on a periodic basis, with similar blackouts occurring every three months. European Space Agency (ESA) and NASA engineers are assessing several options to recover the situation, or minimize the scientific data loss.

SOHO is a project of international cooperation between ESA and NASA to study the sun, from its deep core to the outer corona, and the solar wind. It was launched in December 1995 on an Atlas IIAS/Centaur rocket. Besides watching the sun, SOHO has become the most prolific discoverer of comets in astronomical history. As of May 2003, more than 620 comets have been found by SOHO.

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