Document From: Subject: To: Cc: Three interplanetary spacecraft, now headed quietly toward Mars, Jupiter and over the poles of the sun, soon may prove the existence of elusive waves in the universe's gravitational field by bobbing on ripples in space like corks bobbing on ripples in a pond. Such waves of gravity have never been directly detected, although their existence was predicted decades ago in Einstein's theory of relativity and there is indirect evidence that they exist. The waves are believed to be produced by supernova explosions, collapsing black holes and other catastrophic events. Past searches with ground-based equipment and single spacecraft have failed to discover them. Astrophysicists are hoping to make this major discovery by spending the next few weeks "listening" for passing gravitational waves with three "borrowed" spacecraft at the same time in the most sensitive detection system yet assembled to search for very low frequency gravitational waves. The spacecraft, now on their way to separate destinations in the solar system, are NASA's Mars Observer, Galileo and the European Space Agency (ESA) Ulysses spacecraft. The joint NASA-ESA experiment will run from March 21 to April 11, marking the first time three spacecraft will make observations simultaneously, greatly increasing the reliability of any detection. "If this experiment succeeds in detecting gravitational waves it may answer fundamental questions about the nature of gravity as well as give further support for Einstein's theory of general relativity," said Dr. Robert Stachnik, Gravitational Wave Program Scientist in NASA's Astrophysics Div., Office of Space Science, Washington, D.C. "We're also very excited about the possibility of making a major discovery with such a cost-effective experiment. We were able to take advantage of three spacecraft already in space which soon will be in the correct relative positions and distances we need to do this experiment. We can just borrow them for a few weeks, without any added cost for equipment and no change to their missions. It's big science on a small budget," Stachnik said. "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 NASA's Jet Propulsion Laboratory, Pasadena, Calif., who will work with the Mars Observer and Galileo spacecraft. The experiment is built around a simple concept. During the 3-week experiment, the antennas of NASA's Deep Space Network (DSN) on Earth will beam radio signals to the three spacecraft at precisely known frequencies. Each spacecraft will send signals back to Earth at the same frequency it receives. If no gravitational waves are passing through the Solar System, the signals returned to Earth should have exactly the same frequencies as the original signals sent from the DSN, shifted only by the Doppler effect of spacecraft motion. However, if a strong enough gravitational wave passes -- produced perhaps from collapsing masses of stars in the hearts of galaxies or from the spiraling together and collision of two black holes -- both the Earth and the spacecraft will experience a slight "bobbing" from the ripple-like passage of the gravitational wave. This interaction cannot be directly detected at either the Earth or the spacecraft alone, but would show up as a slight change in the frequency of the radio signal finally received back at Earth. 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 (a quadrillion is 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. Scientists emphasize, however, that snaring a gravitational wave during the 3- week experiment will depend on a good bit of luck -- whether a suitable astronomical event happens to occur during the relatively brief opportunity when data can be taken. All three spacecraft will be in the Earth's night sky at that time, so interference with their radio signals due to charged particles in the solar wind will be at a minimum. Successful detection of gravitational waves could open up an entirely new kind of astronomy. Because the gravitational waves do not readily interact with matter, detecting them may open a window to the interiors of powerful -- and sometimes catastrophic -- events such as supernova explosions and collapsing black holes. "Gravitational wave research is now in the hands of physicists. Once signals are detected, the astronomers will be beating down the doors," said Stachnik. Sensitive ground-based interferometer antennas now are being built in both 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 now will be using 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. Detection of the gravitational waves, even if they occur, will still take at least several months of patient data analysis. "The spacecraft systems can detect large enough gravitational waves, if they exist," said Dr. Bevan M. French, Program Scientist for the Mars Observer. "But it won't be one of those sudden 'Eureka!' situations. We'll be looking for a few small wiggles in a huge amount of radio data. It will take time." To identify the unique signals of gravitational waves, the scientists also will have to eliminate such mundane effects as planned changes in the orientation of the spacecraft, interference from charged particles (plasmas) in space and even atmospheric changes, rain and snow on Earth. 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 ESA Ulysses spacecraft was launched in 1990, and it will 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 radio searches during their interplanetary cruise periods. Source:NASA Spacelink Modem:205-895-0028 Internet:192.149.89.61 Luned“, 22 marzo 1993 20:14:00 NASA Press Releases Item From: Hank Mollenauer,IGUANA Subject: Gravitational Wave Search To: NASA Press Releases 3/17/93: 3 SPACECRAFT TO CONDUCT 3-WEEK GRAVITATIONAL WAVE SEARCH Donald L. Savage Headquarters, Washington, D.C. March 17, 1993 Franklin O'Donnell Jet Propulsion Laboratory, Pasadena, Calif. RELEASE: 93-48