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From: Dan Dubrick
To: All
Date: 2003-06-28 01:54:00
Subject: 6\24 ESA - Antenna anomaly may affect SOHO science data trans

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Paris, 24 June 2003
Press Release
Nx 42-2003

Antenna anomaly may affect SOHO scientific data transmission

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{at}esa.nascom.nasa.gov

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