Three interplanetary spacecraft will be tracked for three weeks in March and April in a NASA-European Space Agency experiment attempting to detect long-sought-after gravitational waves.
In the experiment, scientists will look for slight changes in the frequency of radio signals sent from Earth and echoed back by the Mars Observer, Galileo and Ulysses spacecraft.
Any such frequency shift, they say, could be caused by a passing gravitational wave emitted by a collapsing black hole or other distant celestial event.
The experiment will run from March 21 to April 11.
"Albert Einstein predicted the existence of gravitational waves in his theory of general relativity, and radio astronomy observations of pulsars have suggested they indeed exist -- but no one has ever detected a gravitational wave directly," said Dr. John W. Armstrong of JPL, who will work with the Mars Observer and Galileo spacecraft.
According to Einstein, waves of gravity are emitted by astrophysical bodies undergoing extreme acceleration. The waves themselves are ripples in the fabric of space-time moving at the speed of light.
Some cosmic events major enough to produce gravitational waves that could be detected near Earth include collapses of masses of stars in the hearts of galaxies and the spiraling together and collision of two black holes.
Detection of gravitational waves will give scientists new information on the interiors of these catastrophic events. Sensitive interferometer antennas are being built in the United States and Europe to search for gravitational waves with wavelengths of thousands of kilometers.
"In addition to searching for the shorter waves that can affect antennas here on Earth, we can use radio signals sent to spacecraft hundreds of millions of kilometers away to search for waves of much longer wavelength," said Dr. Frank B. Estabrook of JPL, who will work with the Galileo spacecraft.
If strong enough, a passing gravitational wave will warp the fabric of space between the spacecraft and Earth so that the frequency of the spacecraft's radio signal changes.
During the experiment, the antennas of NASA's Deep Space Network on Earth will beam radio signals to the three spacecraft at precisely known frequencies. Each spacecraft will be configured to send signals back to Earth at whatever frequency it receives. After these signals are finally received back at Earth, the total frequency change is measured.
The hydrogen maser clocks that control the DSN transmitters and receivers are so accurate that scientists will be able to detect a change in radio frequency of as little as a few parts in a quadrillion (1 followed by 15 zeroes).
"This should allow us to detect gravitational waves from objects such as massive pairs of black holes hidden in the hearts of other galaxies," said Hugo D. Wahlquist of JPL, who will work on the Ulysses spacecraft with Sami W. Asmar of JPL, Prof. Bruno Bertotti of the University of Pavia, Italy, and Prof. Luciano Iess of the University of Rome La Sapienza.
The experiment will be the first time observations have been made simultaneously with multiple spacecraft, which will greatly increase the reliability of any detection.
Scientists acknowledge, however, that snaring a gravitational wave during the experiment will depend on a good bit of luck -- whether or not a suitable astronomical event happens to occur during the three-week opportunity when data can be taken. The three spacecraft will all be in the night sky at that time, so interference with their radio signals due to charged particles in the solar wind will be at a minimum.
Mars Observer, launched in September 1992, will reach the red planet Aug. 24 of this year. Launched in 1989, NASA's Galileo spacecraft will arrive at Jupiter in 1995. The joint NASA-European Space Agency Ulysses spacecraft was launched in 1990 to fly over the sun's poles in 1994 and 1995.
Gravitational wave research is supported by the Astrophysics Division of NASA's Office of Space Science and by each of the three spacecraft projects, which scheduled the search during their interplanetary cruises.
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