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SOHO was launched into its
three-month journey to Lagrangian point No.1 on December 2nd,
1995, from Cape Canaveral by an Atlas-IIAS launcher. The whole
mass of the spacecraft when launched was 1610 kg plus 240
kg propellant, which is sufficient for at least six years
of operation.
When orbiting L1 between the
Earth and the Sun the spacecraft can enjoy an uninterrupted
view of the Sun, the heliosphere, and the solar wind particles.
One L1 orbit takes about 6 months.
The information from the SOHO
spacecraft is controlled in NASA's Goddard Space Flight Centre
near Washington DC, where the satellite's scientific data
is gathered via the three ground stations of NASA's Deep Space
Network (DSN) system.
The overall responsibility
for the spacecraft and its operations belongs to ESA. SOHO
has a robotic control system, which allows it to look after
itself up to 48 hours. An onboard computer controls the orientation
of the spacecraft relative to the Sun, using a fine-pointing
sun sensor and star-tracking system, and three reaction wheels,
which keep it sunpointing.
THE LOSS AND RECOVERY
After SOHO had completed its nominal two-year mission in April,
1998 and achieved spectacular results, its mission was extended
to 2003 to cover the upcoming period of maximum solar activity
expected to peak in 2000.
However, due to an unfortunate
sequence of operational mistakes, radio contact was interrupted
on June 25th, 1998. The spacecraft went into an uncontrolled
spin, causing the loss of power. SOHO was lost for a month
until the deeply frozen spacecraft was located by radar measurements
on July 23rd.
After some 'spacecraft emergency'
work the SOHO Recovery Team got the spacecraft to re-orient
towards the Sun in the middle of September.
SCIENTIFIC AIMS AND CAPABILITIES
OF SOHO
Soho aims to answer the following three
fundamental questions about the Sun: 1) What is the structure
and what are the dynamics of the solar interior? 2) Why does
the corona exist, and how it is heated? 3) Where and how is
the solar wind accelerated?
THE SOHO PAYLOAD
Can be divided into three segments: The helioseismology payload
consists of two velocity spectrometers (GOLF and MDI) and
several radiometers (VIRGO). They provide data for the study
of the structure and dynamics of the solar interior, from
the very deep core to the outmost layers. They can measure
the velocity and intensity of solar oscillations, investigate
non-periodic variations of the 'solar constant', and determine
its value. The coronal payload consists of several remote-sensing
instruments (CDS, EIT, LASCO, SUMER, UVCS) which are designed
to study the structure and dynamics of the outer solar atmosphere;
and one instrument (SWAN) to measure the ionised cavity which
the solar wind blows into the interstellar hydrogen breeze.
The solar wind and particle payload consists of mass spectrometers
(CELIAS) and particle analysers (COSTEP, ERNE) which analyse
the solar wind and energetic particles near the Earth's orbit.
Mission
Results:
- The first images of a star's convection
zone and the subsurface structure of sunspots .
- The most precise measurements of the temperature
structure and rotation profile in the solar interior, including
the discovery of a polar jet stream.
- The discovery of a magnetic carpet on the
solar surface.
- The first measurements of the acceleration
profile of the slow and fast solar wind .
- The identification of the source regions
of the fast solar wind in the magnetically "open" regions
at the Sun's poles.
- The most detailed view to date of the dynamics
in the outer solar atmosphere, including the discovery of
coronal "Moreton waves" and solar tornadoes.
- Most spectacular images and movies of coronal
mass ejections.
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