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.

Getting CATty

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).

Comments

31 Comments

  • Donald Q says:

    Keep on truckin', lil buddy.

  • Eddie says:

    Hi all! You must be thrilled to bits! Bravo! A question please. With the huge delay between the sending, receiving and returning confirmation of instructions, how on earth are you able to so precisely manoeuvre the space vehicle?

  • Paul Edwards says:

    Fantastic post - thanks. I love the techy detail!

  • Roberto Nesci says:

    really a complex set of orbital operatios, and all preplanned (not real-time action) given the great distance of the spacecraft (23 light-minutes). This is really a big step for mankind...

  • Marco Masi says:

    The final elliptic orbit makes no sense to me. The nucleus is a highly irregular object and the gravitational potential must be too. The final part of the video is wrong.

    • Phil Stooke says:

      No, it's correct. The nucleus is small, and when you are 10 comet radii out the lumps in the gravity field even out - think of drawing concentric envelopes around the nucleus, each one 1 km further out than the one before, and by the tenth you have evened out most of the irregularities and are left with a smooth shape.

      • Marco Masi says:

        When the elliptic orbit is traced in the video I read "close observation at 10 km". Considering that the comet nucleus is about 4x3.5 km in size, you have not more than 2.5-3 comet radii. That won't be enough to approximate the gravity field with a spherical potential.

        • Rae says:

          Hi Marco, imagine 2 spheres side-by-side, each with radius 1km, and surface gravity of g. At a radius of 10km from the centre of this 2 sphere body, maximum gravity = (g/9^2)+(g/11^2) = 0.0206g and minimum gravity = (2g/101) (10/101^0.5) = 0.0197. A single sphere of same total mass would have a uniform gravity at r= 10km of 2g/10^2 = 0.02g. So gravity variation in this example is only around +/- 3%?

          • Marco Masi says:

            Ah... ok, this sounds more convincing... intuitively I expected more 'gravity rippling' (the comet diameter with respect to the elliptic major axis seems to me out of scale in the video). However, 3% is not so tiny after all, especially considering that further fluctuations rise due to the comet spin which is irrelevant in a spherical body, but might become significant here. I'm still not convinced that it won't diverge quickly from the elliptic trajectory after few orbits nevertheless. I don't think a quick approximate consideration might confirm or dismiss that, probably a numeric simulation is necessary to see if this is or not the case. Almost tempted to do that myself... 🙂

        • Steve Kasian says:

          OH... SNAP!!!

  • Clive Hartland says:

    That video explains a lot of the manouvres carried out by the probe.
    The fact that the gravity pull of the Comet is significant and requires orbit adjustment is interesting also.
    Looking at the stereo pair of last week it shows that gravity exists on the comet as the dust is settled in the valley between the 2 ends of the comet,
    I also feel that the comet is far more solid than envisaged as the rock wall in the view has rocky outcrops and makes me feel that it is a large chunk of rock thrown into space from some cataclysmic event?

    Clive

  • Heinrich Becker says:

    Very nice explanation.
    But what do you mean by "Night excursion"
    From the animation, it is not quite clear were Rosetta will be during this--will Rosetta actually be in the shadow of the comet?
    What will be the orbital period during the 30 km orbit_

    • Graham says:

      Heinrich

      You have to remember that Rosetta needs sunlight on its solar panel to work. Also the comet is incredibly dark - in reality about as dark as a black t shirt, so in order to get the best images Rosetta will be placed between the comet and the sun with its panels directly facing the sun as much as possible. The night excursion is a rare event but probably of little real value other than as a stepping stone to getting into the best possible orientation for the later stages of the mission. Given the small size of the comet and the speed of Rosetta relative to the comet I'd guess that it would be over very quickly

      • Steve Kasian says:

        Au contraire, being in the shadow of the comet will provide a one-of-a-kind opportunity to view the coma and to observe details which will undoubtedly help show it's surface origins and characteristics. The only way to do so otherwise involves overexposing the nucleus, which introduces it's own set of issues which make viewing it from within the shadow a uniquely important perspective.

  • idjles says:

    I calculate that 67P accelerated Rosetta by about a=GM/R²=2x10-8 m/s² at a distance of 100km.
    This gives a delta v of 0.007 m/s during the 3.5 days of each of CAT arc and a displacement of about 1m (s=at²/2) along each arc.
    Were you able to measure this acceleration/deltaV/displacement from 67P on Rosetta during Big CAT's 3 arcs?
    Were you then able to deduce anything about the mass distribution inside 67P?

    • Rainald Koch says:

      While your estimate of acc and delta v is correct (based on the 3.14E12 kg joke), the displacement is miscalculated. s = at²/2 = vt/2 is of the order of kilometers for the CAT arcs.
      Since the onboard atomic clock is accurate enough to measure the radial component of the displacement to tens of meters, ESA recently came up with first a fix of the mass, 1E13 kg, see
      http://blogs.esa.int/rosetta/2014/08/21/determining-the-mass-of-comet-67pc-g/

  • Gerald says:

    Thanks for the details and the comprehensive explanation! That is really excitigng!

  • Robert says:

    Thats an incredible mission !
    I will keep my fingers crossed for all that delicate maneuvers that will follow till the big landing day arrives !
    Further I suppose that it should be possible to image the lander from the 10km orbit easily with the narrow angle OSIRIS cam...
    Exciting times !

    • Andrew R Brown says:

      It is very exciting indeed.

      10 KM is still well above the surface, a little lower than the cruising altitude of most jetliners. Even with the OSIRIS NAC, Philae will not be easy to make out. Sunlight glinting off, yes but Philae will still look like a rock on the comet's surface from that distance.

    • Alte kalt says:

      The resolution of Osiris NFCam is about 20 micro-radians per pixel making 200mm at 10 km altitude. Im sure the lowest orbit will be under 5 km altitude and with sub pixel resolution and some other tricks like multi frame image processing it will be possible to recognize high contrast details the size of cherry, there is no atmospheric blur.

  • Stephan Andersohn says:

    What influence has the geometry of the comet to the CG? Is there the potential to find two center of gravity on each end of the "duck"? What is the infulence to the manoeuvres planed?

  • Bill says:

    As Phil says, it is correct. However, I think that the size of the nucleus as depicted in the video is misleading - ie, it appears much too large. The nucleus is, if I understand correctly, approx. 4km in length on it's major axis. The final orbital position of Rosetta should therefore be around two and a half times the diameter of the nucleus from its surface. This is not the case. This discrepancy can be seen at all stages of the animation, starting with the 100km legs (which should be forty times the diameter of the nucleus away from the comet).

  • Bill says:

    Apologies - I should have said 25 times the diameter of the comet toward the end of my last post!

  • Hermenator says:

    Hi Budnik,

    Congratulations on behalf of all of ISU SSP96 for making it to orbit!

    The Hermenator

  • Mike Halliwell says:

    Very interesting info and fascinating to watch but two questions all this manoeuvring must use lots of fuel how much spare have you for the rest of the mission?
    and what are the expectations on the life of the solar panels once the comet becomes more active?

  • Matt Taylor says:

    Regarding the nightside excursion, please remember that OSIRIS is only one of 11 instruments inboard the spacecraft. The excursion is done to make measurements with the MIRO and VIRTIS instruments of the un-sunlit nucleus. Indeed , the tail region would be interesting for a number of instruments but Rosetta's preponderance for the sunlit side is driven by navigational needs. It has to see some part of the sunlit part of the comet.

  • Jean Vallee says:

    Very interresting posts, keep it up Rosetta, you're my hero. ... My greeting to Philea.

  • hator.lethos says:

    Fantastic space exploration and human engineering keep it up ESA. Looking forward to the landing in November and hope all goes well.

  • Jacob says:

    Hi!
    Basically you have right, more precise gravity model was of coarse reconstructed from the comet shape to be accounted for the near comet manoeuvres. The demonstrated orbits is a simplification for a wide public. Much more important aspect is the rotational moment on the spacecraft originated from the extended comet mass. Rosetta behaves as a dipole in the inhomogeneous field, its attitude has to be permanently corrected in order to compensate the accumulated torque due to the comet, and this is an important technical requirement because of offloading of the reaction wheels and consumed fuel for thrusters.

    • Alte kalt says:

      A major concern is the drag in the not so perfect vacuum close to the comet and this gets worse as Rosetta goes against the wind. Its not a lot but it will consume an increasing amount of fuel, I'm guessing they will go further out again later on when the landing and mapping is done to save fuel. The atmospheric pressure will most likely never get above one or two milli Bar on the surface and a factor of 10 or less at the closest orbit but the sails are huge and the time scale is large it certainly must be compensated.

Comments are closed.