Down, down we go to 29 km – or lower?

Editor’s note: The Rosetta mission control team are transitioning Europe’s intrepid craft onto the next stage of cometary operations: the Global Mapping Phase (GMP). The update below is based on inputs provided by Flight Director Andrea Accomazzo at ESOC and Project Scientist Matt Taylor at ESTEC.

Before you read today’s post, take a quick look at our trajectory video to refresh your memory (we know you’ve already seen this – but it’s a very helpful aide memoire), which illustrates what Rosetta is doing now (skip through to the 00:58 – Global Mapping)

Note also that some round-figure references to “30, 20 or 10km” refer, specifically, to 29, 18.6 or 9.8 km, respectively.

We are now in the Transition to Global Mapping (TGM) phase, that is, the two hyperbolic segments of the trajectory that move us from the 50km pyramid of the Close Approach Phase (CAT) to the 30-km gravitationally bound orbit of the Global Mapping Phase (GMP).

“The aim of GMP is to get as close as we can and gather as high-res science data as we can to best characterise the potential landing sites – helping us to make the best decision on prime and backup and really start to understand what kind of thing we’re dealing with,” says Fred Jansen, ESA’s Rosetta Mission Manager.

“It’s a real synthesis of operations, science and analysis that is a first in many ways.”

On Wednesday, 10 September, 09:00 UTC (11:00 CEST) the spacecraft will be at the terminator plane (the plane passing through the centre of the comet and perpendicular to the Sun direction) and will perform a 19-cm/s manoeuvre (thruster burn) to insert the spacecraft onto the 30-km circular orbit.

“The orbital plane is 60 degrees away from the Sun’s direction and is such that we will orbit over areas of the comet in their ‘morning hours’,” says Andrea Accomazzo, Rosetta Flight Director.

“This results in orbits with periods of exactly 14 days. However, we will not fly all of each orbit!”

Rosetta GMP orbits Credit: ESA

Rosetta GMP orbits Credit: ESA


Andrea explains that, instead, after 7 days (i.e. when we are again on the terminator plane) we will conduct a manoeuvre (thruster burn)that  will change the orbital plane such that it will have the same characteristics as the previous orbit – but flying over ‘afternoon areas’ of the comet.

This will be the last time we fly bound orbits in front of the comet (see explanation below).

At the end of this second 30-km arc, we will let the spacecraft go onto the night side, just before dawn, to afford the instruments a look at the thermal characteristics of the comet.

Just before entering ‘night’ arc, we will do a small manoeuvre to lower the orbit such that when we come out again, that is crossing the terminator plane five days later, we will be at just 20km.

This manoeuvre, set to take place on 29 September, is actually subject to a GO/NOGO decision to be made on Thursday, 18 September, because we want to see how the spacecraft flies, and what conditions we encounter, at 30km for a week before we decide to go down any further.

Staying at 30 km

If conditions and performance dictate that we should not go any closer, we will continue until just before lander delivery in our 30-km orbits.

Two things to note:

  1. By ‘night’ we are not referring to eclipse, i.e. when the spacecraft is not illuminated by the Sun, but rather to when the ground track on the comet surface is on the night side*.
  2. The period of the orbit at 30x20km, that is the elliptic orbit we will use to go down to 20km, cannot be a multiple of 7 days and it is one of the few exceptions where the orbital plan cannot follow our ‘ideal’ weekly calendar (the other time when this happens will be, as you may guess, during landing); the mission control team have to be able to accommodate these exceptions.

“Once we reach the terminator plane at 20km, another small manoeuvre will set the orbital plane to coincide therewith,” says Andrea.

This is done in order to expose the minimum spacecraft surface (body plus leading edge of the arrays) to the gas flow emanating radially from the comet (if we were flying, for example in front of the comet, then the big 16-m long solar arrays would be exposed face-on to the flow).

Similarly, on 6 October (after having been for one week at 20km), we will again have a GO/NOGO decision as to whether we go down to 10km.

If so, on 8 October, another manoeuvre will change the orbit to 20x10km and once down to 10km on 15 October, we will circularise there.

This orbit would then be the one maintained until 26 October when we will start phasing the orbit for the lander delivery.

ESA’s most intense science operations ever

While the description so far of the spacecraft’s operational orbit evolution may sound relatively straightforward (we’ve left off many details on the intensive flight control and flight dynamics team work going on behind the scenes), it is especially important to understand that there is a related and massive level of science operations activity taking place.

The multiplicity of possible orbits (the aforementioned 30- 20- and 10-km orbital levels) make the planning for scientific observations immensely challenging since, given the lead time necessary for science operations planning, the Rosetta Science Operations Centre (RSOC) at ESAC, Madrid, Spain, will have to provide a separate plan, for all instruments, for each case.

“We are in the most intense science operations activity ever for ESA,” says Matt Taylor.

“The workload and the need to coordinate science planning with potential spacecraft orbits – all while venturing into the unknown of an entirely new comet – are challenging to say the least.”

Matt adds that the on-going science and flight operations planning processes don’t simply stop once Rosetta releases the lander.

“We are working on two parallel plans for post-landing also.”

Note*: For the technically inclined, this means that we will fly a ‘04:00 am arc’ (i.e. 30 degrees before the terminator plane, which is itself the 06:00/18:00 plane).



  • Errol Coder says:

    Unfortunately, imaging wise, when Rosetta enters the terminator orbit this will cause a half phase in illuminated surface. It will be interesting though to see how the lighting and surface features appear. It would be at this angle though that it would be a better chance to observe any activity caused by the sun as we will be at a 90 degree angle to the direction of the sun. Hoping for some observed jet activity 🙂

  • Kurt Muellauer says:

    I suppose the date should be corrected to Sept.24 in the following sentence:
    Just before entering ‘night’ arc, we will do a small manoeuvre to lower the orbit such that when we come out again, that is crossing the terminator plane five days later, we will be at just 20km.
    This manoeuvre, set to take place ===> on 29 September another small manoeuvre <=== will set the orbital plane to coincide therewith
    keep up the excellent work and best regards
    Kurt Muellauer

  • Kurt Muellauer says:

    The date for a manoeuvre on Sept.29 is correct for
    Once we reach the terminator plane at 20km, ===>another small manoeuvre <=== will set the orbital plane to coincide therewith
    keep up the excellent work and best regards
    Kurt Muellauer

  • Kamal Lodaya says:

    Earlier it was said that by around 10 September Rosetta would be in orbit around the comet. Does this mean that if no thrusters are fired it would from this point on orbit the comet forever, that is, it is “captured” by the comet’s gravity? How does one find out that it is captured?

    • Erka says:

      Hyperbolic trajectory means fly by good by,
      Parabolic is the limit to elliptic trajectory,
      Elliptic trajectory means i will stay here,
      Circular is a special case of elliptic with both axis the same length.
      To know this is to calculate, try it out and if it fails the calculation was wrong, therefore the double checking.

    • Daniel says:

      Hi Kamal:

      Rosetta Spacecraft Operations Manager Sylvain Lodiot, at ESOC, replies as follows:

      Not forever, as orbits are not perfect and we are only performing half orbits as you know. Bound orbits are OK until the comet is too active; from then onwards we’ll have to move away. But indeed we are on a bound orbit around the comet. Bound orbits around earth also need corrections from time to time! — Sylvain

    • Daniel says:

      Kamal: ESA’s Rosetta Flight Director Andrea Accomazzo provided this answer:

      Does this mean that if no thrusters are fired it would from this point on orbit the comet forever, that is, it is “captured” by the comet’s gravity?

      Yes, it is captured by the comet’s gravity.
      The orbit is however perturbed by the following factors:
      – a non perfectly spherical gravitational field (for obvious reasons)
      – the aerodynamic forces acting on the spacecraft due to the gas coming out of the comet
      – the solar radiation pressure
      Due to these effects the orbit is not stable on a long term so if one was to maintain it then correction manoeuvres would be required.
      We currently have a lost after 3.5 days (i.e. in the middle of the arc) to place a correction if needed.

      How does one find out that it is captured?
      Our current knowledge of the gravity field is enough to allow us to predict the type of orbit we would have by imparting a certain velocity to the spacecraft when at a specific position around the comet.
      I.e. we know that the spacecraft would not escape the comet’s gravity now (at least over a certain period of time).
      To verify the orbit we gather radiometric (Doppler and ranging) and optical (images) data that allow us to reconstruct the flight path of the spacecraft.

      By looking at it we see that it, more or less, follows our prediction i.e. in this case it stays around the comet at a distance of ca. 30km.

      • Kamal Lodaya says:

        Thank you for the detailed information.

        Am I right in thinking that this is absolutely the first time we have a spacecraft orbiting an asymmetrical object like this? I imagine we can learn something from this.

        Suppose you guys can find a way of keeping Rosetta’s orbit stable over a longer period of time, and suppose it survives perihelion. Is it hypothetically possible to take it all the way around and get a better qualitative idea of the kind of effect Jupiter has on the comet when it gets there? (In one of the posts someone said the comet was split, perhaps over several Roche passes, into these two lobes by the effect of Jupiter.)

        I am imagining Rosetta as a kind of probe through a large swathe of the solar system with the fuel requirements being taken care of by the comet.

      • Leo Vuyk says:

        Daniel, you wrote: about orbit perturbation origins: “a non perfectly spherical gravitaional field ( for obvious reasons)”
        It is obvious that both body parts of the duck have their own center of mass, which could lead to landing instability for location A ( in between these masses)
        Is this also an important reason for choosing landing site J?
        located more or less at the center line of both masses?

  • cosmo says:

    Could you please explain how exactly the rotation axis of the comet is orientated?

  • Erka says:

    Can the rosetta orbit be altered to a geostationary same as the comet spin direction orbit?

    This would minimize the influence of the comets almost vacuum atmosphere on the solar panels acting like sails.

    As for now rosetta is planing to fly against the wind.

    • Dertutenaix says:

      Well after a few turns in stable orbit it will clean its path as some moons does in the rings of Saturn.
      There must be a reason to fly in counter direction to the comet spin. I just don’t know what. My best guess is the extreme low orbital speed and if going counter-direction its over the same spot about every 6 hours or so and if going in the same direction it might take weeks. That is then a possible problem withe the communication to the lander. Such a problem i would solve with a few short range repeaters in orbit. Or even better a lot of landers all over the place. Communicating in a network.

  • Simon Frederik says:

    Dear Rosetta Team
    After a more than 10 years journey in the deep freeze of space , Rosetta´s and Philae´s hardware and other materials could have deteriorated a bit , becoming more brittle .
    Therefore , my question is if Philae can land as gently as possible to prevent any hardware-damage at the moment of landing with the inevitable shock .
    Just worried and hope for the very best .

  • Mark says:

    I would be neat if the Rosetta S/C could approach to within a few meters of the surface, drop the lander off at one site, then come back and grapple the lander and place it at another location, and so on. There’s probably a fuel constrain that prohibited this type of proximity operation. Better yet, the lander could obtain a sample, and the Rosetta S/C could then pick up the lander from the surface and return it to earth. Perhaps future spacecraft will have this capability. The extremely low g-field environment could make such operations feasible with minimal propellent usage.

  • Dave says:

    Fabulous and ambitious science to get to this stage and an excelent explanation of decreasing the the orbit and why the changes of orientation.
    I was only a boy when I watched the giant leap for mankind, this is on a par.
    I could of said pioneering, but of course Bruce willis had already figured out how to do it.
    Who would have believed from watching Brucies film that it was atually possible?
    An impossible dream, best of luck for the rest of the mission.

  • Alter schwede says:

    Obviously a lot of material was captured by this comet, that is how it became a comet, in general it is most likely growing, in short time with a mass of 100 kg Philae.
    The question is if the Rosetta will end up on it as well once the mission is finished. If so the comet will not mind at all.

    • Pete Williams says:

      I have also thought along these lines Alter. Can Rosetta itself land on the comet once its fuel is used up and then hitch a ride fore ever more. Most likely not, if it’s near a vent then it stands a good chance of being ejected back out into space.
      To be able to grapple with these thoughts says a lot for what’s been achieved so far.
      The rest of this year and beyond is going to be fascinating for those of us who look up eh!

    • Erich says:

      If some fuel is saved for a soft landing maneuver it is definitely possible to land the rosetta. But it will be lost signal vise as no more stable communication will be established. Once landed it will stay put. There are evidence of huge blocks standing on the surface tip toe. If left in orbit around the comet it will finally make a soft crash landing. The comet collects more then it releases. If i where the opposite it would be vanished a loooong time ago.

  • Nick tiller says:

    I am having an intense discussion with a friend as follows:

    Does the comet have a gravitational field to hold the Rosetta craft in orbit, it is my understanding that as it moves closer to the comet it reaches a phase where it is captured by the comets microgravity and hence starts to orbit in circular orbits. Also of coursea the Philae lander “lands” due to the gravitational force between it and the comet, the landing systems and harpoons merely being their to prevent it bouncing. Please add some links or explanations folks before there is a meltdown here in Manchester uk

    • Dertutenaix says:

      If you google for an online gravitation force calculator and enter the comets mass and the landers mass you can calculate the force that holds it down. Its not a lot but it is there and on images you can observe boulders that made this trick before Manchester uK was present on any map.

      • Nick tiller says:

        Thanks for the reply my difficult friend is now changing is argument to well it does not have sufficient gravity to hold the Philae probe in place or to enable it land, hence having harpoons to attach itself.

        Although if u watch the landing video it’s clear these are merely to stop it bouncing away, assistance would be appreciated I want to knock is argument into touch

        • Alter schwede says:

          The Philaes aids to stay put are to prevent it from bouncing and land again in som uncontrolled way and all over again until it comes to a final rest. It will stay put for sure without those aids as long as the velocities are under the escape velocity. With a worthless 100 kg lump that method is fine, the first impact velocity is well below the escape velocity and as some energy is absorbed the bounce has even less speed and so on until it stays put, but Philae is supposed to fulfil a mission and damages are no options. All the crates and boulders are indicating that things that land on the comet stay there for quite some time ranging from millions to billions of years. The place is scattered with derbies ranging from micro meter sized dust to the 3E12 kg sized head. Dust can get washed away from the surface again into orbit by some events, the growing tail of the comet indicates this process. The larger the object is the more prone is it to stay put. So far the biggest particles in orbit reported are the size of 0.1 mm and they might get collected again. Other forces than gravity are in play as well, the electrostatic force and the adhesion force play a large role to form colonies of the dust and condensing vapours lumping them up to larger structures as i read on an other comet flyby it was reported as ” 30 cm fluffy snowballs “. This comment came from a NASA scientist.

          Try to explain the creation of the clouds on earth, they are visible to most people but the majority do not know why they form these shapes and even do not care as life goes on anyway. A logical and simple model would suggest a uniform distribution of the vapours. Well simple logic don’t seem to fit.

  • Kopernik says:

    You said – “The orbit is however perturbed by the following factors:
    – a non perfectly spherical gravitational field (for obvious reasons)
    – the aerodynamic forces acting on the spacecraft due to the gas coming out of the comet
    – the solar radiation pressure.”
    Are there any calculations for the presence of dark matter?

    • Nick tiller says:

      any chance of being more specific of u were addressing my post?

    • Dertutenaix says:

      Dark matter is not present in a scale this size nor is dark energy. Not even in our solar systems planetary orbits anyone calculates with these parameters. Dark surface is obviously present. Dark matter and dark energy is only present in calculations in a cosmic scale meaning thousands of galaxies size. But even then only if this whole dark whatever is for real and not as usual an interpretation to make some theories fit on the blackboard.

  • Dertutenaix says:

    Just about dark matter, what we see is what we get and we cannot observe outside our event horizon. We can measure some strange things with our better and better instruments and to explain what ever anomaly there is we event something that fits the bill. What we do know is that there is a world outside our event horizon and it influences what we are observing. Say if it is a lot denser or maybe a lot more diluted it curves our space time in a way we can not verify. Our neighbors outside our event horizon have the opposite problem and just came up with the idea of white matter and white energy to make their bill fit.

  • Ben says:

    Earlier, the Rosetta science team provided a rough estimate of the comet’s mass as 1.0×10^13 kg, with an error of 10%. Surely they’ve fine-tuned this estimate by now. Is there a place to monitor updates for this type of information?

  • Alter schwede says:

    Trying all from dissertations to wikipedia and ESA FAQ the strange outcome is poor information quality. On top of that the ambition to update parameters are absent. The spin is said to be 12.7 hours and an accuracy like that in modern chronometer age is poor although easy to get accurate within 5 figures or better having an orbiter in situ for quite som time. The dimensions 3km to 5km are also poor accuracy as the way of measuring it with the onboard instruments with a lot higher resolution and precision is possible. The mass could be well defined by now, at least within 1% and not as in wiki +/- 10%. Maybe I’m wrong giving a picture that this is easy doing although the first estimate of the mass was a factor 1000% out of range. Or maybe its not important? I can give it a wild guess but am better of not doing that as the scientists of concern should have their chance first. The spin period might be closer to 12.4 hours rather then 12.7 hours per turn or not?

  • cosmo says:

    What about the GO/NOGO decision yesterday? Has Rosetta got permission to go down to 20 km?

  • cosmo says:

    Hello? Somebody there? 😉
    Will the trajectory correction maneuver on Wednesday bring Rosetta to within 20 km of comet 67P? What’s the decision?

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