24-Jun-2003 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
SOHO is located 1.5 million kilometers 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
ESA and NASA engineers are currently assessing several options to
recover the situation, or minimise the scientific data loss.
For the purposes of my posts here, I'll still be able to provide the
majority of information. I'll be able to report geomagnetic activity,
solar wind data, and solar flare data. None of the current data
information will be affected. What will be affected is the forecast
information, and possibly information about whether or not a coronal
hole is there. Coronal holes show up in the extreme UV images SOHO
provides. Since UV light in this frequency range is blocked by the
ozone layer, these images can't obtained from the ground. The TRACE
(Transition Region and Coronal Explorer) satellite gathers data in the
UV range, but it doesn't provide the same kind of images that SOHO does.