21 March
1993: On 21 March 1993, a trio of interplanetary spacecraft, ESA's Ulysses and NASA's Mars Observer and Galileo, began operating in a joint experiment to prove the existence of waves in the Universe's gravitational field.
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 would run from the 21 March to 11 April and for the first time three spacecraft would make observations simultaneously, greatly increasing the reliability of any detection.
