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N° 13–1993: Searching for gravity waves in space

19 March 1993

Three interplanetary spacecraft, ESA's Ulysses and NASA's Mars Observer and Galileo, now quietly heading towards separate destinations (the poles of the Sun, Mars and Jupiter respectively), may soon prove the existence of waves in the universe's gravitational field by bobbing on ripples in space like corks bobbing on ripples in a pond.

Such gravity waves have never been directly detected, although their existence was predicted in Einstein's theory of relativity and there is indirect evidence that they exists. The waves are believed to be produced by supernova explosions, collapsing black holes and other events of this kind. Past searches with ground-based equipment and single spacecraft have failed to discover them.

This joint ESA/NASA experiment will run from the 21st of March to April the 11th and for the first time three spacecraft will make observations simultaneously, greatly increasing the reliability of any detection. Astrophysicists are hoping to make this major discovery by spending the next few weeks "listening" for passing gravitational waves with the three "borrowed" space probes at the same time in the most sensitive detection system yet assembled to search for very low frequency gravitational waves.

"For Ulysses it will be the second chance to search for these rare events. Last March, Ulysses "listened" for a period of about four weeks." said Dr. Richard G. Marsden, Deputy Ulysses Project Scientist at ESA's Research and Technology Centre, ESTEC, in Noordwijk, the Netherlands. "Although no gravitational waves were found on that occasion, the experiment set new upper limits to their intensity, thereby excluding a number of possible sources." said Prof. Bruno Bertotti, Principal Investigator of the Ulysses Gravitational Wave Experiment at the University of Pavia, Italy.
ESA's Ulysses spaceprobe was launched by Space Shuttle Discovery on October 6, 1990 to become the first probe ever to explore and circumnavigate the poles of the Sun. In February 1992 the spacecraft approached Jupiter and made use of the gravitational pull of the giant planet to "swing" itself out of the ecliptic plane, the imaginary "disc" in which all the planets of the Solar System orbit around the Sun.

"Ulysses is now 4,9 astronomical units -735 million km- from the Sun and 20 degrees South of the ecliptic plane on its way to fly over the South polar region of the Sun between May and September next year" said Dr. Peter Wenzel, ESA's Ulysses Project Manager.
A year later, in September 1995, Ulysses will have passed over the North pole of the Sun completing its almost 5 year long journey towards the unknown.


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