Radio Astronomers Find A Lost Satellite -- The Detection Of SOHO Raises Hopes For Its Recovery

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.

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.

How was SOHO Detected?

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. When SOHO passed overhead in Puerto Rico, an hour-long series of radar pulses were transmitted from Arecibo in the direction of the satellite. By the time the pulses reached SOHO, and the reflections travelled back to Earth the satellite was no longer visible at Arecibo. That's because the dish is so large that its steering is limited and the rotation of the earth had carried SOHO outside its field of view. So, the 70m 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 Earth-crossing asteroids.

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.

Editor's Note: The original news release, with images and related links, can be found at http://science.nasa.gov/newhome/headlines/ast28jul98_1.htm