Bill Steigerwald<br />Goddard Space Flight Center, Greenbelt, Md<br />(Phone: 301/ 286-5017) June 24, 2003<br /><br /><br />ESA Media Relations Office<br />Paris, France<br />(Phone: 33/0/15369/7155)<br /><br /><br />Release: 03-214<br /><br /><br />ANTENNA ANOMALY MAY AFFECT SOHO SCIENTIFIC DATA TRANSMISSION<br /><br /><br /> The Solar and Heliospheric Observatory (SOHO) <br />spacecraft expects to experience a blackout in the <br />transmission of its scientific data this week. It is <br />estimated the blackout may last two to three weeks.<br /><br /><br />Engineers are predicting this problem after detecting a <br />malfunction in the pointing mechanism of the satellite's <br />high-gain antenna (HGA), which is used to transmit the large <br />amounts of data from SOHO's scientific observations to <br />Earth.<br /><br /><br />The SOHO spacecraft is operating as safely as before the <br />problem occurred. Its low gain antenna, which does not need <br />to be pointed in a specific direction, will be used to <br />control SOHO, monitoring spacecraft and instrument health <br />and safety.<br /><br /><br />The anomaly in the HGA was recently discovered when <br />engineers detected a discrepancy between the commanded and <br />measured antenna position. In normal conditions, the antenna <br />must be able to move along two axes, vertical and <br />horizontal. The horizontal movement is no longer taking <br />place properly. The problem is probably due to a malfunction <br />in the motor or gear assembly that steers the antenna.<br /><br /><br />SOHO is located 1.5 million kilometers (one million miles) <br />from Earth. It orbits around the First Lagrangian point, <br />where the combined gravity of the Earth and the sun keep <br />SOHO in an orbit locked to the sun-Earth line. To transmit <br />data, the SOHO HGA must rotate to have Earth in its field of <br />view.<br /><br /><br />If the problem is not solved, the Earth will be left outside <br />the HGA beam on a periodic basis, with similar blackouts <br />occurring every three months. European Space Agency (ESA) <br />and NASA engineers are assessing several options to recover <br />the situation, or minimize the scientific data loss.<br /><br /><br />SOHO is a project of international cooperation between ESA <br />and NASA to study the sun, from its deep core to the outer <br />corona, and the solar wind. It was launched in December 1995 <br />on an Atlas IIAS/Centaur rocket. Besides watching the sun, <br />SOHO has become the most prolific discoverer of comets in <br />astronomical history. As of May 2003, more than 620 comets <br />have been found by SOHO.<br /><br /><br />For information about NASA and Space Science on the <br />Internet, visit:<br /><br /><br />http://www.nasa.gov