The Solar and Heliospheric Observatory (SOHO) spacecraft expects to experience
a
blackout in the transmission of its scientific data during the week of 22 June
2003. This is estimated to last for about two and a half to three weeks.

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
(omni-directional), will be used to control the spacecraft and monitor both
spacecraft and instrument health and safety.

The anomaly in pointing the high-gain antenna 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 was 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, slowly
orbiting 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 high-gain antenna must rotate to have the Earth
constantly in its field of view as the spacecraft and the Earth progress in
their respective orbits.

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.

ESA and NASA engineers are currently assessing several options to recover the
situation, or minimize the scientific data loss.

Note to editors: More about SOHO

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.

SOHO moves around the Sun in step with the Earth, by slowly orbiting around the
First Lagrangian Point (L1), where the combined gravity of the Earth and Sun
keep SOHO in an orbit locked to the Earth-Sun line.

The L1 point is approximately 1.5 million kilometres away from Earth (about
four
times the distance of the Moon), in the direction of the Sun. There, SOHO
enjoys
an uninterrupted view of our daylight star.

Besides watching the sun, SOHO has become the most prolific discoverer of
comets
in astronomical history: as at May 2003, more than 620 comets had been found by
SOHO and now carry its name. SOHO’s easily accessible, spectacular data and
basic science results have captured the imagination of the space science
community and the general public alike.

For more information please contact:

ESA Communication Department
Media Relations Service
Paris, France
Tel: +33 (0)1.5369.7155
Fax: +33 (0)1.5369.7690

Dr Bernhard Fleck, ESA SOHO Project Scientist
c/o NASA/Goddard Space Flight Center
Greenbelt, USA
Tel: (1) 301.286.4098
e-mail: bfleck@nasa.gov
Further information:

ESA Media Relations Service
Tel: +33(0)1.53.69.7155
Fax: +33(0)1.53.69.7690

USEFUL LINKS FOR THIS STORY

* More about SOHO
http://sci.esa.int/soho/

IMAGE CAPTION:
[http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=32426&o
oid=12340]
SOHO depends on its antennae.