New trajectories for Rosetta

Rosetta continues to recover well from the problems experienced during the close flyby over the weekend of 28 March that resulted in the spacecraft entering safe mode. Some of the science instruments are now switched back on again, and more will follow in the coming week.

As a result of the safe mode, Rosetta moved onto an ‘escape trajectory’ taking it approximately 400 km from Comet 67P/C-G. An orbital correction manoeuvre was executed on 1 April to start to bring the spacecraft back again, and with a second manoeuvre executed on 4 April, the target distance of 140 km was reached on 8 April.

But the previous difficulties in navigation mean that the operations team needs to be cautious while bringing the spacecraft even closer. In particular, they will need to assess the behaviour of the spacecraft’s star trackers in the environment of the increasingly active comet, since the previous navigation issues resulted from the star trackers becoming confused by comet particles.

An example of 'pyramid' trajectories close to the comet. (Note this is a screen shot taken from our 2014 animation "Rosetta's orbit around the comet")

An example of 'pyramid' trajectories close to the comet. (Note this is a screen shot taken from our 2014 animation "Rosetta's orbit around the comet")

“This has ultimately meant a complete replanning of the upcoming flyby trajectories,” says Rosetta spacecraft operations manager Sylvain Lodiot. “We’re first moving to a terminator orbit at a distance of 140 km and then we’re targeting 100 km. Then we will adopt a similar strategy to when we first approached the comet in August last year. That is, we will fly ‘pyramid’ trajectories, starting at about 100 km on 11 April, and we’ll monitor how the spacecraft reacts before moving closer.”

Three of these pyramid trajectories are currently planned up until the end of April. The team will assess the situation each week before deciding to move closer or, if necessary, to move further away again.

Visualistion of Rosetta's approach to the comet in August 2014. Click for animation. Credits: ESA.

Visualistion of Rosetta's approach to the comet in August 2014. Click for animation. Credits: ESA.

“We’re now assessing the impact of the new trajectory scheme on the planned science observations for the months ahead, including those which anticipated close flybys,” says Matt Taylor, Rosetta project scientist. “Our science operations team at ESAC is extremely busy working with the instrument teams to optimise science observations and associated spacecraft pointing for this new scheme. As we move forward, we will analyse what can be modified and improved in order to maximise science return within the capabilities of the spacecraft. We will be looking at examining all options to mitigate the issues we had and recover some of the science goals.”

We’ll provide details of future trajectory plans and the status of the science operations when this information is available.

 

Comments

18 Comments

  • leodp says:

    Hi,
    how much fuel is left?
    Can you please give an order of magnitude of how many correcting manoeuvres (for example how many pyramid "vertices" on orbits at around 100kmdistance ) could be done?

    Thanks

  • Bill Harris says:

    Ouch. I didn't know that the safe-mode from losing attitude during that close approach was a long-term situation-- I assumed that it cleared up in a couple of days.

    Although the appearance of the activity on 67P has been quite subtle thus far, my suspicion is that at or after perihelion the activity may become somewhat cataclysmic. There ar etoo many large boulders, aeolian dust flows and Maar-like craters scattered about to be from "subtle" processes. We don;t know much about the energy wrapped up in these cryo-clathrates with involved gases.

    --Bill

  • Dilip Shran says:

    I imagine that the issues faced by Rosetta provide a great learning opportunity to learn for Future Missions. So thought it has meant that all planned activities were not completed or delayed, the experience still has value.

  • logan says:

    Kudos to Darmstadt People 🙂

  • Mark Svenson says:

    I've looked briefly for this answer but haven't found it yet. How do you calculate orbits for such an irregular comet? Do you:
    > Assume it's a point mass or sphere and does this require occasional corrections
    >Actually have models for the exact shape of the comet
    > Something in between like assuming two spheres

    • Harvey says:

      The short answer; it's complicated 🙂
      Good discussion of it here, openly accessible.
      http://www.terrapub.co.jp/journals/EPS/pdf/5111/51111173.pdf
      To do it accurately, you need the internal density distribution of course.
      To a decent approximation, you won't be too far out if you use the centre of mass and are say ten times the radius of the object away for an object which is not too violently asymmetric like 67P.

  • Kasuha says:

    Furure cometary missions - if there will be any - should definitely consider switching to navigation using Sun and comet's surface features. Such things are much harder to miss in a cloud of dust. And available computer power should be sufficient to do that.

  • What was the key "unknown" re. C-G's coma that has led to the problems and the need for major mission replanning? As far as we know the general increase in gas and dust production isn't much different from previous apparitions, so something in the near-nucleus coma must have been quite unexpected to cause the navigation problems and safemode.

    • Marco says:

      I just think they were expecting "fog" rather than "large floating hailstones"

      Also the buildup has been quicker, which may be a reflection of a possible peak at "equinox" in May rather than at "perihelion" in August.

      • Kamal Lodaya says:

        Marco: I believe 67P has a record of higher post-perihelion activity. The call for amateur participation predicts a highest magnitude of 11 which seems considerably higher than seen earlier. Perhaps they are preparing for a breakup.

        • Marco says:

          No. All the predictions I see are slowdown due to conservation of AM while it stretches through Equinox. Then a speed up due to asymmetrical outgassing through and beyond perihelion. This is all based on previous observations of rotational period of 67P and other well studied comets.. . .

  • masanori says:

    I have no intention at all to make anyone in Rosetta team feel bad, but to be honest, this news made me get more excited. A little bit, at least. And made me naturally want to cheer and support Rosetta mission more. Just like Philae's First Science Sequence had to be replanned (if my understanding is correct), after the final touchdown.

    I have an impression that in general later missions tend to be done with having more informations in advance about target and tend to have less aspects of adventure. Not in terms of science perhaps. Like difference of impressions between Mars Exploration Rovers (Spirit & Oppy) and Curiosity.

    So, Rosetta & Philae and their teams, I'm sure you all have known since the beginning that such difficulty can happen!! I hope you get something you want, even facing that!! An adventure you are in!!

  • logan says:

    Maybe my geometry fails. Think that pyramid orbits offer a delta 8 in outbound particulate in between burnings.

    • logan says:

      Wrong, could be relevant only to 'triangular' orbits in the same plane of 67P. Not to 'pyramidal' ones.

    • logan says:

      'Triangular' orbits could be used for statistical samplings, and just extrapolate values toward the closer, dangerous distances.

    • logan says:

      Orbital distances selected on a 'logarithmic', integrating set of distances.

    • logan says:

      Several carefully selected 'triangular' orbits create an orbital 'cloud'. Just as electrons in atoms. Think it's ideal for statistics.

  • alex k says:

    Do you have a snapshot of what the orbital path of the comet 67p where it is now and where it will be in a year from now?

    The java program I have will not pull it up from:
    http://neo.jpl.nasa.gov/orbits/

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