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Radio astronomers find a lost satellite
The detection of SOHO raises hopes for its recovery
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28 July 1998 Ground-based radio telescopes have been
able to detect the Solar and Heliospheric Observatory (SOHO)
spacecraft and have found it rotating slowly near its original
position in space, a potentially important step toward possible
recovery of direct communications with the spacecraft.
Radio contact with SOHO, a joint mission of the European Space
Agency (ESA) and NASA, was interrupted on June 24, an event under
review by a joint ESA/NASA investigation board.
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With the encouragement of Dr. Alan Kiplinger of the National
Oceanic and Atmospheric Administration's Space Environment Center
in Boulder, CO, researchers at the U.S. National Astronomy and
Ionosphere Center (NAIC) in Arecibo, Puerto Rico, used the facility's
305-meter (990-foot) diameter radio telescope to transmit a signal
toward SOHO on July 23. The 70-meter dish of NASA's Deep Space
Network in Goldstone, CA, acted as a receiver, locating the spacecraft's
echo and tracking it using radar techniques for more than an
hour. |
 The
70-meter (230-foot) diameter antenna in Goldstone California
is part of NASA's Deep Space Network (DSN). It is the largest
of the DSN antennas, and is capable of tracking a spacecraft
travelling more than 16 billion kilometers (10 billion miles)
from Earth. |
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How was SOHO Detected? |
The Arecibo Observatory, located in Puerto Rico, is operated
by the National Astronomy and Ionosphere Center. The giant 305
meter dish is the world's most powerful RADAR. It can transmit
pulses of radio energy with up to 500 mega-watts of power. The
Arecibo radar has been used to map the surfaces of Venus and
Mars, and to study the properties of near-Earth asteroids. |
Astronomers used a technique
called bistatic radar to detect SOHO. Bistatic
radar means that two radar dishes work together, in this case
the 70m Goldstone antenna and the 305m Arecibo dish. The Arecibo
dish is so large that its steering is limited. SOHO was only
in its field of view for about an hour. The Arecibo RADAR team
utilized the entire hour transmitting powerful pulses toward
the satellite. Meanwhile, NASA's Deep Space Network radio antenna
in Goldstone, CA, which is 60 degrees longitude to the west of
Arecibo, was used to pick up the reflections.  This is a technique that
has also been used to study the rings of Saturn and asteroids.
The image (right) shows the animated radar blip from the satellite
recorded at Goldstone. The image links to a larger animation
courtesy of Astronomy
Now. |
Artist's conception of a bistatic
RADAR experiment.
Preliminary analysis of the radar data, which is ongoing, indicates
that SOHO is still in its nominal halo orbit and is turning slowly
at a rate of roughly one revolution per minute. Staff members
of NAIC and the Deep Space Network, in close cooperation with
ESA and NASA, are continuing to analyze the radar data to extract
more precise information on SOHO's location and motion, which
in turn could help in future recovery efforts, as SOHO's solar
panels turn toward the Sun.
 The Solar and Heliospheric
Observatory is a joint project of NASA and the European Space
Agency. It was launched in 1995 and has been monitoring the Sun
with 11 different instruments from an orbit near the so-called
"L-1" Lagrangian point in space. The L-1 point is a
location in space about 1.5 million km from Earth where the gravitational
pulls of the earth and sun are balanced.
Until recently the mission has been a spectacular success. According
to Dr. David Hathaway of the NASA/MSFC Space Sciences Lab, SOHO
observations have greatly advanced our understanding of coronal
mass ejections ("CMEs"), solar magnetic fields, and
the internal structure of the sun. "One thing SOHO does
better than any earth-bound observatory is see the solar corona,"
explained Dr. Hathaway. "Coronagraphs here on Earth can
only detect the inner, brightest parts of the corona, but SOHO
can see 50 to 100 times further out, 24 hrs a day."
Solar physicists have been waiting expectantly for the year 2000
when the next solar maximum is expected to occur, but without
SOHO observers will be operating with a severe handicap. "We'll
be blind to many types of global activity on the sun," says
Dr. Hathaway. "Also, SOHO was expected to complement observations
by the ACE and TRACE spacecraft, already in orbit. We're really
hoping that SOHO will be recovered."
ESA and NASA engineers are continuing their efforts to re-establish
radio data communication with the spacecraft, encouraged by the
radar measurement of a slow spin rate, which suggests minimal
structural damage has occurred. The slow spin rate also raises
hopes that within the next two months SOHO's solar panels will
once again rotate into the correct position to power the spacecraft,
and when that happens ground controllers will be able to re-establish
communications.
The Arecibo radar team was led by Dr. Donald Campbell. The NAIC
is operated by Cornell University, Ithaca, NY, under a cooperative
agreement with the U.S. National Science Foundation, Washington,
DC. The Deep Space Network is managed by NASA's Jet Propulsion
Laboratory, Pasadena, CA. |
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