No. 57 - First Solar Conjunction Phase
On 15 March the spacecraft was configured for solar conjunction, including the activation of the S-band transmitter in parallel to the nominal X-band link. On 30 March the telemetry bit-rate was reduced as planned to 3.5 kbps, to cope with the increasing signal disturbance from the Sun. On 6 April a telecommand link test was successfully performed, all commands were successfully decoded on-board, although the uplink signal as received by the spacecraft was significantly disturbed by the Sun.
On 10 to 12 March the spacecraft was manoeuvred to point the OSIRIS camera towards asteroid Steins. The images collected over a period of about 24 hours were successfully downlinked to Earth on 15 and 16 March. In order to measure the link performance over the solar conjunction, and also to support radio science measurements of the solar corona, the frequency of ground station passes has increased to daily passes. Most of the new Norcia passes (a total of 14 in the reporting period) were of short duration (4 hours) and taken as pure tracking passes, without telemetry and telecommand connections.
|745||14.03.06||073||Spacecraft configuration for solar conjunction|
|746||15.03.06||074||Camera link test; Dump of OSIRIS Steins science|
|761||30.03.06||089||Monitor pass - TM bit rate to 3.5 kbps|
|762||31.03.06||090||Monitor pass - last WOL pre-conjunction|
|768||06.04.06||096||Monitor pass, TC test|
At the end of the reporting period (DOY 097) Rosetta was at 388.1 million km from Earth (2.59 AU; one-way signal travel time was 21 minutes 35 seconds). The distance to the Sun was 238.4 million km (1.59 AU).
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. During the OSIRIS observation of asteroid Steins, which brought the Sun about 23 degrees below +X for about 37 hours, a cooling of the +Z side was observed, as expected. All temperatures returned to the original values after the end of the slew back to GSEP. The thermal behaviour of the -Z thrusters was peculiar, as the temperature of all four thruster modules initially decreased once the Sun was moved to -23 degrees below +X, until their heaters were autonomously activated. The decrease in temperature is probably due to a shading of the -Z side of the spacecraft by the High Gain Antenna, which is in its +X-pointing position.
The only instruments operated in the reporting period were OSIRIS over two days at the beginning of March, and SREM, which remains active in the background for radiation monitoring. SREM's data collection rate was reduced in view of the upcoming solar conjunction and near sun hibernation period, in which limited ground contact will be available for data downlink.
OSIRIS Observations of Asteroid Steins
On a short notice request by the OSIRIS team, the instrument was pointed over a period of 37 hours from DOY 069 to 071 (10-12 March) to track asteroid Steins. These are the first observations taken by Rosetta of one of the scientific targets of the mission. This was also the last opportunity to observe the first target asteroid of the Rosetta mission before the fly-by, scheduled for September 2008.
The operation was carried out of coverage, and all data dumped to ground over the New Norcia passes of DOY 073 and 074. The instrument performance was nominal and all the commanded images were successfully collected and downlinked. The OSIRIS team has distributed pictures and is working on the reconstruction of the light curve.
A total of 14 New Norcia passes where taken over the reporting period, to support solar conjunction measurements. Both X-band and S-band signals were received and decoded in all passes. In general the performance of the S-band link was much better than expected, whilst the X-band link behaved according to the predictions in case of a quiet Sun.
The performance of the ground station was nominal. For the first time the dual-ranging set-up was used for Rosetta. In this mode the coherent ranging measurements from two IFMS (one tuned on X-band, the other on S-band) are collected and driven by the uplink in X-band of one of the two IFMS.
The spacecraft has now entered the final phase of the first solar conjunction. The spacecraft will reach its minimum separation angle of about 0.3° from the Sun (seen from Earth) on 13 April. Tracking passes will be taken daily in the next two weeks, but it is expected to lose the radio frequency signal soon. As from 23 April it is planned to resume normal TM/TC passes and to start dumping the housekeeping telemetry accumulated in the solar conjunction period.
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 2006.