ESA Science & Technology - No. 58 - First Solar Conjunction

The spacecraft had been configured to sustain this low operations profile on 14 March and will remain in the same configuration until mid May. At the end of the reporting period the angular separation from the Sun was 5.5°.

In order to monitor accurately the actual up- and downlink signal degradation as the spacecraft angular distance to the Sun decreased, 20 New Norcia passes were taken during the reporting period with increased frequency, up to once per day around the angular distance minimum. Most of these passes have been used for tracking only, meaning no TM/TC links with the OCC were established.

A repetition of the commanding test performed on 7 April has been performed on 11 April to assess the degradation of the uplink signal. The results of this campaign will be used to plan the run-down of operations for all future Rosetta solar conjunctions.

At the end of the reporting period (DOY 118) Rosetta was at 378.8 million km from Earth (2.53 AU; one-way signal travel time was 20 minutes 14 seconds). The distance to the Sun was 229.1 million km (1.53 AU). The spacecraft reached its minimum separation angle from the Sun of about 0.55° on 13 April.

Spacecraft Status

Thermal
The thermal behaviour of the spacecraft is nominal and stable. Its configuration is, since DOY 230/2005, ACM1, with tank gauge heater 7A switched off.  The spacecraft is slowly warming up as the spacecraft to Sun distance decreases. In particular, the HGA Pointing Mechanism temperature is back to about 30°, and increasing. The hot thermal settings will be applied on-board mid-May to cope with the increasing Sun distance.

Since DOY 103/2006, the Sun is slowly moving below the +X spacecraft axis. At the end of the reporting period, the elevation of the Sun over the +X axis was -3.1°. The attitude guidance will be changed on DOY 130/2006 to bring the Sun back to the +X/+Z quadrant of the spacecraft. 

Payload
The only instrument operated in the reporting period is SREM, which remains active in the background for radiation monitoring. SREM's data collection rate is currently reduced to cope with the limited ground contacts during solar conjunction and the upcoming near Sun hibernation period.

The intense tracking campaign over the Solar Conjunction has allowed RSI to perform unplanned cruise science. The main objective of this RSI campaign was the sounding of the solar corona as the spacecraft was at small angular separations from the Sun.

Ground Facilities

A total of 20 New Norcia passes were taken over the reporting period, to support spacecraft monitoring and intensive RF signal tracking during the solar conjunction. In particular, AGC, tracking and other station data are collected to measure the RF signal quality while the spacecraft angular separation from the Sun is below 5°.

Dual ranging measurements were taken throughout the reporting period. In general, New Norcia performed well over the reporting period.

Future Milestones

The current solar conjunction phase will continue until mid May. All commanding activities over the entire phase have already been loaded on board the spacecraft Mission Timeline. After the Solar Conjunction the spacecraft will be configured into Near Sun Hibernation Mode for a period of about 2 months, from end May to end July 2006.

Operations for the Mars swing-by (February 2007) will start in August 2006, with another payload passive checkout (PC3), an intense tracking campaign around the Trajectory Correction Manoeuvre (DSM-2) in September, and the first payload active checkout (PC4) in November/December.