The term Opposition describes an alignment of the Earth, the Sun, and the Spacecraft with the Earth in the middle. Therefore the Sun-Probe-Earth angle (SPE) is close to 0º but the Sun-Earth-Probe angle (SEP) is close to 180º.

During an opposition, there is minimal Solar interference to the radio signals travelling between the Earth and the spacecraft. It is therefore a good opportunity to try to detect gravitational waves. The Ulysses Gravitational Wave Experiment (GWE) is a radio science experiment designed to attempt to detect low frequency gravitational waves (in the mHz band). Possible astrophysical sources in this frequency band are:

  • Collapse and collisions of black holes in the nuclei of galaxies and quasars
  • Periodic wave trains emanating from massive orbiting binary black holes
  • An isotropic background of cosmological origin

Second Opposition in February 1992 provided the best geometric conditions for the GWE. First Opposition was therefore used as a period to test the radio science techniques. No GWE observations have been planned for subsequent oppositions.

Alignment of Ulysses and the ACE, POLAR and SOHO spacecraft around the 5th Opposition of Ulysses in February 1998 also provided a unique scientific opportunity. Extra coverage was scheduled to support the Multi-project Investigation During the Alignment of Spacecraft (MIDAS) for the study of the evolution of Coronal Mass Ejections (CMEs), interstellar pickup ions and other phenomena.

  Date SPE Angle SEP Angle Scientific Activity
1st Opposition 30 Dec 1990 3.4º 174.6º Test Gravitational Wave Experiment
2nd Opposition 27 Feb 1992 0.1º 179.4º Prime Gravitational Wave Experiment
3rd Opposition 1 Mar 1993 4.6º 156.4º Routine
4th Opposition 24 Feb 1997 5.6º 151.6º Routine
5th Opposition 26 Feb 1998 0.8º 175.7º MIDAS
6th Opposition 1 Mar 1999 3.5º 161.5º Routine