Posted on 18/08/2014 by Daniel
What’s up with Rosetta?
Today: a quick recap of Rosetta orbital manoeuvres in the past fortnight since arrival at Comet 67P/C-G on 6 August. Today’s post is covers multiple manoeuvres, which means that the mission operations teams and flight dynamics experts at ESOC have been busy ensuring that everything is happening when it should!
First, before we go any further, a mandatory video! We say ‘mandatory’ because this animation explains in rather good detail what Rosetta has been doing and covers the current time frame up to the end of September. OK – lets watch:
Details, by date
6 August – Drifting slowly past the comet after a series of nine orbital manoeuvres (since May), Rosetta was commanded to conduct a 1-m/s thruster burn (which ran 7 min) to change its direction and enter onto the first arc (of three arcs) of two triangular (really, tetrahedral) orbits about the comet (00:12 time mark in the animation).
- It’s important to note Rosetta has not been captured by 67P/C-G gravity, and the continuing series of thruster burns are necessary to keep the spacecraft at the comet.
- The craft will execute two of these triangular orbits, referred to by the mission team at ESOC as ‘Close Approach Trajectory’ (CAT); there will be one large, at about 100km closest pass-by distance (‘Big CAT’) and the second will be done at about 50km (‘Little CAT’). This means that the thruster burns are not only changing Rosetta’s direction on each arc, but are also lowering the pass-by distance (i.e. altitude) as well.
10 August – CAT Change 1 burn – a 6min:25sec, 0.88-m/s burn that pushed Rosetta onto the next arc (00:17 time mark). We’re still at about 100km pass-by height.
13 August – CAT Change 2 burn – a 6min:22sec, 0.87-m/s burn that pushed Rosetta onto the next arc (00:21 time mark). Last arc at about 100km pass-by height.
17 August – CAT Change 3 burn – a 6min:19sec, 0.85-m/s burn that pushed Rosetta onto a transfer arc (00:26 time mark), down to about 80 km height to be achieved on 20 Aug (CAT 4).
To date, these have all been conducted as planned and Rosetta is now on the descent toward ‘Little CAT’ – steadily ‘falling’ lower to reach the approximately 50km distance for CAT 5 on 24 Aug.
The mission team are now planning the next CAT burns, CAT 4, 5 and 6, on 20, 24 and 27 August, respectively (see the animation time marks 00:30 to 00:42).
“The mission team are working intensively, and we’ve transitioned onto a new weekly planning cycle to cater for the CAT burns that happen every Wednesday and Sunday in August,” says Jose-Luis Pellon-Bailon, acting Spacecraft Operations Manager.
To give an idea of the incredible precision of the flight dynamics work being done to support these intricate manoeuvres, note that the orbit determination done after the 13 August burn found that Rosetta’s thrusters had over-performed by about 0.2% – a tiny amount in the order of an astonishing +2 mm/second!
Coming up: Global Mapping
On 31 August, Rosetta will begin the third and last arc of ‘Little CAT’ and the team will then pace Rosetta through a transition into the next set of two manoeuvres, referred to as ‘Transfer to Global Mapping’ (TGM) burns (00:48 in the animation).
The Global Mapping phase runs 10 September to 7 October, and will see Rosetta going down to just 29 km distance, a point when we expect the spacecraft to become actively captured by the comet’s gravity, and its orbit to become circular. The aim is to get down to 19km height, keeping Rosetta on the Sunlit side or orbiting on the terminator line.
Technology, systems, ground segment
On board the spacecraft, everything is operating as expected; no major issues have been seen. power & thermal, data handling, attitude & orbit control, thrusters, star trackers, NavCam and communication systems are all stable and nominal.
Tracking, telecommanding and science data download have been provided by ESA’s deep-space stations at Malargüe, Argentina, and New Norcia, Australia, as well as various NASA stations at Goldstone and Madrid.
On 17 August, Rosetta was 410 million km from Earth (2.74 AU); the one-way signal travel time was 22 min:49 sec (1369 sec).