Oppositions - Ulysses

Oppositions

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.