No. 96 - Asteroid Flyby Test
18 April 2008 13:12
Report for Period 15 March to 11 April 2008
Rosetta is continuing its 4th orbit around the Sun as part of its interplanetary journey. The spacecraft distance from the Sun is increasing and will reach a maximum of about 2.26 AU in December 2008.
During the reporting period the following major activities have been successfully executed:
A total of 7 New Norcia (NNO) passes were taken during this period with nominal performance:
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.
The test was executed on DoY 084 with the following timeline:
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.
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.