Conjunctions - Ulysses
The term Conjunction describes an alignment of the Earth, the Sun, and the Spacecraft with the Sun in the middle. Therefore both the Sun-Probe-Earth angle (SPE) and the Sun-Earth-Probe angle (SEP) are close to 0º.
During a conjunction, there is a great deal of interference when the radio signals to and from the spacecraft have to pass through the solar corona plasma. However, the signature of this interference contains information about the density and the dynamics of the solar corona. It is therefore a good opportunity to perform a Radio-Science Solar Corona sounding experiment (SCE). The SCE uses Doppler and ranging data to determine the density, velocity and turbulence spectrum of the Sun's atmosphere to distances well below 10 solar radii.
Radio-Science experiments have not been performed during all of Ulysses' Conjunctions since the spacecraft-Sun-Earth alignment has not always been good enough (particularly after Ulysses left the ecliptic plane). Also, during the Second Solar Orbit, power constraints have limited the use of the S-band transmitter.
|Date||SPE Angle||SEP Angle||Scientific Activity|
|1st Conjunction||21 Aug 1991||0.3º||1.2º||Prime Solar Corona Sounding Experiment|
|2nd Conjunction||2 Sep 1992||1.5º||7.8º||Secondary Solar Corona Sounding Experiment|
|3rd Conjunction||3 Sep 1993||5.8º||25.6º||Routine|
|4th Conjunction||4 Mar 1995||4.3º||5.8º||Routine|
|5th Conjunction||30 Aug 1997||2.0º||10.5º||Routine|
|6th Conjunction||1 Sep 1998||0.9º||4.8º||Routine|
|7th Conjunction||5 Sep 1999||4.6º||21.6º||Routine|
The conjunction geometry can impact spacecraft operations since it may compromise the capability to command or monitor the spacecraft. So for conjunctions where the alignment is good, all critical operations for a protracted period are loaded on-board the Ulysses spacecraft well in advance. These pre-loaded conjunction operations include:
- Essential bit rate changes
- Ranging operations for Radio-Science
- Special Sun-avoidance manoeuvres: Ulysses has an on-board program called CONJ which is designed to manoeuvre around the sun during the conjunction phase. CONJ was programmed to execute 15 manoeuvres (one per day) during the first Ulysses Conjunction.
In addition, some other special operations may be required:
- Switching off the tape recorders to use the power for the S-band transmitter
- S-Band transmitter switch on for Radio-Science, and a corresponding thermal reconfiguration
- Suspending the routine commands to the Ulysses scientific instruments