Rosetta is preparing to make a close encounter with its comet tomorrow, on 14 February, passing just 6 km from the surface.
On 4 February, Rosetta began manoeuvring onto a series of new trajectories that will align the spacecraft for this week’s encounter. The series of thruster burns are happening (or have happened) as follows (distances are indicated from comet surface):
4 Feb – Depart from 26-km terminator orbit
7 Feb – Achieve 142 km from comet, then turn back
11 Feb – Arc back down to 101 km
14 Feb – Reach 50 km stand-off distance; turn and burn for the closest flyby arc
14 Feb – Conduct 6 km flyby at 12:40:50c GMT
The closest pass occurs over the comet’s larger lobe, above the Imhotep region (click on the image below to watch).
Note that, in the main science phase, Rosetta’s trajectory is being set by the Rosetta Science Ground Segment (RSGS) at ESAC, on a 16-week planning calendar (known as the LTP – the ‘long term planning’ process), and is fully optimised for the suite of instruments on board.
This means that 16 weeks before the start of each LTP cycle, RSGS proposes a trajectory for that LTP. This trajectory is then checked, against spacecraft and mission constraints, among other factors, at ESOC by the flight dynamics team.
While closest approach on 14 February is certainly a significant event for science observations, for the flight control team at ESOC, it’s a fairly routine operation and just one more activity during the main science phase.
Very short week in, very short week out
Since the intense manoeuvring and operations of 2014, the flight control team have adopted a regular weekly planning cycle. This covers two ‘VSTPs’ (very short term plans) covering Wednesdays to Saturdays (planned on Mondays) and Saturdays to Wednesdays (planned on Thursdays). Specifically:
- On planning days, last data inputs come in early in the morning – for optical navigation data, i.e. NavCam images, and radiometric data from the ground stations
- Production of all flight dynamics commands needed for the upcoming VSTP and covering (among others) trajectory and pointing strategy for that VSTP
- Merging of flight dynamics products with instrument commands and spacecraft platform commands
- Making sure the merge is conflict free (reconcile instrument, spacecraft and mission constraints)
- Generating, as output, not only commands to be uploaded to the spacecraft, but as well all data files needed on ground. These include, for example, the ground tracking station instructions for ESTRACK and NASA deep space stations for that VSTP, and the instructions for the automation tool in the ground control system.
Flybys – the new normal
For the rest of the current mission plan in 2015, in fact, Rosetta will always conduct flybys, and, based on predictions of increasing cometary activity, can no longer be manoeuvred so close to the comet as to be in a gravitationally bound orbit.
The net effect is that the workload for mission planning has settled into a more or less predictable, if still challenging, cycle. And, when not at work at ESOC during the week, the flight control team remain on call day/night/weekends.
Navigation: five times daily
The flight dynamics team at ESOC take optical images five times a day using Rosetta’s navigation camera, the NavCam. These images are then used in the optical navigation process to reconstruct the position and trajectory of Rosetta with respect to 67P.
“The NavCam is crucial for relative navigation and knowing where we are with respect to the comet,” says Rosetta Spacecraft Operations Manager Sylvain Lodiot.
He explains that a set of parallel trajectory plans are maintained; one set describing the ‘preferred’ case (making certain assumptions on comet activity), and the other describing the ‘high activity’ case (which is a fall back solution in case the preferred case can’t be flown, and is further from the comet). Also, the operations teams, with the approval of the mission manager, would command a transition to the high activity case if the optical navigation process were to fail for any reason.
Low bit-rate season
Beginning on 26 January, the comet and Rosetta entered conjunction season – a four-week period when both are orbiting more or less opposite Earth but on the other side of the Sun (see image below).
During ‘conjunction season’, the Sun interferes with the direct, line-of-sight radio signals transmitted between Earth and Rosetta.
“The practical effect is that data rates are reduced,” says Sylvain.
“Now, using ESA’s 35m Estrack stations, we get 14 kilobits per second, and this goes up to 45 kilobits per second when we use NASA’s 70m stations.”
This limitation regulates how much data can be downloaded in any one ground station pass.
By June, the orbital geometry will have improved with the spacecraft much closer to Earth, and data rates via ESA’s Estrack stations will then steadily recover to the maximum rate of 91 kbps.
The 2015 ‘routine’ challenge
The continuing challenge for operations in 2015 is to manage and reconcile all requirements, coordinate between the various specialists that support the mission, book tracking stations for the right times, keep an eye on spacecraft status and health and ensure that the science plan is implemented in the best possible way, while respecting all spacecraft constraints.
“For the flight control team and the supporting specialists at ESOC, like flight dynamics and Estrack stations, the operations workload has gone down significantly compared to the comet approach phase and the landing last year,” says Sylvain.
“Nonetheless, we can’t afford any less attention – it’s still a challenge and will remain so for the rest of the mission.”