No. 96 - Asteroid Flyby Test
During the reporting period the following major activities have been successfully executed:
- test of the asteroid flyby attitude dynamics on DoY 084
- switch to Near Sun Hibernation Mode (NSHM) on DoY 087
A total of 7 New Norcia (NNO) passes were taken during this period with nominal performance:
| DOY | Date | Pass | Main Activity |
| 079 | 19/03/08 | NNO 1482 | OSIRIS OBSM activity |
| 085 | 25/03/08 | NNO 1488 | Monitor past AFB test and guidance change |
| 086 | 26/03/08 | NNO 1489 | NSHM entry pre-conditions verifications |
| 087 | 27/03/08 | NNO 1490 | NSHM entry |
| 088 | 28/03/08 | NNO 1491 | NSHM entry verification |
| 093 | 02/04/08 | NNO 1496 | NSHM monitoring |
| 100 | 09/04/08 | NNO 1503 | NSHM monitoring |
At the end of the reporting period (DoY 102) Rosetta was at 73 million km from Earth (0.48 AU) and the one-way signal travel time was 244 seconds. The distance to the Sun was 222 million km (1.48 AU).
Asteroid Flyby Test
On 24 March a major test has been conducted to validate the spacecraft behaviour during the AFB phase. The AFB test has been executed to validate the spacecraft behaviour under the extreme dynamics conditions imposed by the selected flyby scenario for asteroid Steins in September 2008.
The test has also been taken as an opportunity to test the behaviour of the AOCS software in Asteroid Flyby Mode (AFM), the behaviour of the star trackers and the camera with straylight from the Sun, and that of the Lander.
The test consisted in commanding the required AOCS software attitude profile (open loop tracking in Fine Pointing Accuracy Phase and flip manoeuvre as for the real scenarios) and then the AFM mode, by-passing the fact that the camera could not track the asteroid. This has been achieved by setting parameters in the AOCS software that would allow the AOCS to stay in AFM and track the expected asteroid position without triggering any reconfiguration. The below schematic (from Flight Dynamics team at ESOC) is a graphical representation of the manoeuvre performed by the spacecraft.
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Representation of manoeuvres during the AFB test on 24 March, with time running from left to right and the spacecraft's attitude indicated by its +Z axis (toward fictive position of asteroid) and its +X axis (green) |
The test was executed on DoY 084 with the following timeline:
| Time (UT) | Activity |
| 08:39:59 | Wheel Off-Loading |
| 09:01:00 | Lander switch ON |
| 10:09:59 | Start slew to test attitude |
| 11:06:00 | NAVCAM A switch ON |
| 11:09:59 | End slew to test attitude |
| 11:20:00 | Start of attitude flip manoeuvre (rotation around +Z i.e. asteroid direction) |
| 11:36:30 | NAVCAM A commanded to asteroid tracking mode (with detection threshold modified in absence of asteroid) |
| 11:40:00 | End of flip manoeuvre (inverted pointing position reached) |
| 11:40:30 | Switch AOCS to Asteroid Flyby Mode |
| 12:00:00 | Simulated closest-approach time |
| 13:00:00 | End of Asteroid Flyby Mode and slew back to ephemeris pointing |
| 00:32:00 | Lander switch OFF (DoY 085) |
This flyby strategy, called inverted strategy, has been adopted in order to be able to track the asteroid at phase angle zero and beyond the closest approach point. Due to thermal constraints a major attitude manoeuvre had to be inserted between -40 and -20 minutes before closest approach (flip manoeuvre). For this reason the entry in AFM (closed loop guidance based on asteroid position measurement from the Navigation Camera NAVCAM) is as late as 20 minutes before closest approach, making the whole scenario very delicate and critical.
The AFM test was run successfully on the spacecraft and the attitude control error around closest approach was well within specification.
Spacecraft
ALICE
The instrument is OFF.
CONSERT
The instrument is OFF.
COSIMA
The instrument is OFF.
GIADA
The instrument is OFF.
MIDAS
The instrument is OFF.
MIRO
The instrument is OFF.
OSIRIS
The instrument was activated on DoY 079 as part of on-going on-board software maintenance (OBSM) activity. The test focussed on storing the shutter parameters with the best known configuration.
ROSINA
The instrument is OFF.
RPC
The instrument is OFF.
RSI
The instrument is muted.
VIRTIS
The instrument is OFF.
LANDER Philae
The instrument is OFF. It was activated on DoY 084 during the asteroid flyby (AFB) test to check the behaviour under Sun exposure. The preliminary analysis of TM data shows a nominal behaviour. A more detailed assessment of the test is being conducted by the PI team.
SREM
Since DoY 086/2008 the accumulation settings are configured for passive cruise mode.
Future Milestones
The cruise phase continues towards asteroid Steins that will be encountered early September 2008. The spacecraft will remain configured in Passive Cruise Mode (NSHM) till beginning of July when it will be reconfigured to Active Cruise Mode for the execution of the payload checkout 8 in preparation of the asteroid flyby.