Rosetta’s night-time excursion and a ‘Go’ for 20 km

For the last two weeks, Rosetta has been orbiting comet 67P/C-G at a distance of about 30 km on the “Global Mapping Phase” (GMP), and is now set to go even lower.

The aim of the GMP was to gather high-resolution science data to help characterise the potential landing sites for Philae, while also continuing to monitor how the spacecraft responds to the environment of an active comet, before getting closer still. See the table below for a recap of the manoeuvres conducted this month, starting with the transfer to the global mapping phase (TGM), and refer to the animation to help visualise these changes in trajectory.

Burn Date DV (m/s) Duration (min:sec)
TGM1 03/09 0.56 04:55
TGM2 07/09 0.45 04:18
GMP1 10/09 0.193 02:19
GMP slot 1 14/09 0.025 00:32
GMP2A 17/09 0.085 01:23
GMP2B 17/09 0.087 01:25
GMP slot 2 21/09 0.018 00:25

(DV = delta v; TGM = transfer to global mapping; GMP = global mapping phase; two more burns scheduled this month for 24/09 and 29/09)

In fact, as we discussed in an earlier blog post, rather than fly one complete orbit, the spacecraft conducted two seven-day-long half orbits at about 30 km, in different planes. That is, on 10 September, the spacecraft was at the terminator plane (the boundary between day and night, which is itself the 06:00/18:00 plane), and performed a thruster burn to insert onto the 30-km circular orbit. The orbital plane was then 60 degrees away from the Sun’s direction, such that the spacecraft orbited over areas of the comet in their ‘morning’ hours. Seven days later, when the spacecraft was again on the terminator plane, it conducted another thruster burn to change the orbital plane such that it had the same characteristics as the previous orbit – but instead was flying over ‘afternoon’ areas of the comet. Thus, from 18 September, the spacecraft was in a 28 km x 29 km orbit around the comet with an orbital period of 13 days 14 hours 59 minutes.


Rosetta GMP orbits Credit: ESA

Rosetta GMP orbits Credit: ESA

That brings us to today, when the spacecraft moves onto the night side of the comet to allow the instruments to look in more detail at the thermal characteristics of the comet. Note that by ‘night’ we are not actually referring to the spacecraft being in the dark, it remains illuminated by the Sun, but rather the ground track of the spacecraft’s instruments on the comet surface is on the night side. That is, the spacecraft flies along a 04:00 am arc, 30 degrees before the terminator plane.

Just before entering the ‘night’ arc, Rosetta will perform a very small manoeuvre to lower the orbit such that when it completes the arc, on 29 September, it will be at just 20 km from the comet. The command for this manoeuvre is already on board and executes today at 11:00 CEST (09:00 UTC) . At the same time Rosetta will begin the night excursion and start descending.

On Monday, another manoeuvre will circularise the orbit at 20 km along the terminator plane.After a week at 20 km, a decision will be made as to whether it is safe to proceed to just 10 km.




  • DavidM says:

    In order to release the lander, will Rosetta need to be in synchronous orbit of the nucleus? At what altitude does it need to be for release, and what kind of orbit will it be in then?

    • Erich says:

      The rosetta is, as can be seen in the animations, going against the orbit direction of the comet.
      From wich altidude the release is going to be is top secret information that is well guarded by proprietary politicians.
      In principle it could be released from an altidude of less then 1km but most likely it will be about 3 km. A geostationary orbit is about 3230 km radius but in the other direction.

    • emily says:

      Details of Rosetta’s trajectory and the separation altitude are being confirmed this week for both the primary and the backup site – we hope to report the outcome of these analyses later this week/early next week.

      • Sovereign Slave says:

        Hi Emily, I’ve been keeping up with Rosetta daily, and appreciate your blog and reports. I know it’s still early on, but it’s been somewhat frustrating to me that just about all the information coming out is still about mission details around what Rosetta’s doing, not about what is being discovered about the comet. Surely by now there’s got to be more data available from the science teams about 67P, which instruments are active, what they’re doing with them and finding out, etc. Would be great to have a weekly updates from each science team about what they’re working on and discovering, Maybe this information is elsewhere, if so let me know, thanks.

  • cosmo says:

    Oh, quite some action during the last two weeks. Thanks for the fortnightly update with some new bits of information too. Could you give me a hint why two distinct maneuvers were performed on last Wednesday?

    PS: I don’t understand why my last comment was censored!

    • cosmo says:

      Forget my PS! In the last five minutes or so, this comment from Monday was activated at last. 🙂

      • emily says:

        Apologies Cosmo, comments are moderated and so are not always activated immediately if the blog editors are working on other assignments.
        About the manoeuvres – as you can see they are very small, they are to maintain or slightly change Rosetta’s trajectory as described in the post/animation.

    • Erich says:

      All burns are made to correct the orbit to its proper path to be able to do some experiments or in other words gather information. The information of what experiments and why plus its outcome is delayed a few years.

    • cosmo says:

      Sorry, it seems my question was ambiguous. I didn’t ask why trajectory correction maneuvers (TCM) are generally necessary, well that’s pretty clear, but why was the scheduled TCM on September 17 split into two distinct maneuvers (GMP2A & GMP2B)?

      • Lynn says:

        It’s probably to stop going in one direction and start going in another

  • logan says:

    Congratulations to all Rosetta Teams.
    Especially to the ‘bridge’ people.

    P.D. Congratulations also to India in its MOM endeavor.

  • Dan Ioan says:

    Exciting!; Still, “For the last two weeks, Rosetta has been orbiting comet 67P/C-G at a distance of about 30 km ” and we got no OSIRIS image from this distance. What’s up with that?, maybe i missed it.. help me.

    • Jacint says:

      osiris crew are working so hard and have no time to show us images 😀

    • Jacob nielsen says:

      Maybe this helps: much (all?) of the data from Rosetta belong to someone. You? No, probably not. What we do get to know or see are: itinerary, instrumentation factsheets, occational “postcards”. Expect more and You will be frustrated. Meanwhile You can contribute by posting yuor own exotic theory based on, ok very little, or if You lack imagination: join a crowd. It can be lots of fun. Remember: sooner or later things will be published, and I think exiting news are ahead.

      • logan says:

        Hi Jacob. Not frustrated. Very sad. Even if S. Hawkins jump on and say something ‘clever’ before any ‘owners’, that is just collaboration. ESA/Rosetta university teams are the only ones ‘up’ there. No one is going come on an say that ‘carrot’ is mine.

    • Erich says:

      The osiris is so close that its resolution presents information that the public can not cope with.

      • THOMAS says:

        Is this fact or irony?

        It just seems to me that by now, between NAVCAM and OSIRIS and at the varying distances Rosetta has been orbiting the comet, they must have acquired images of ALL the most challenging surface features (especially those hot-spots on the neck) at ALL possible resolutions, including some which the public CAN cope with.

        There are apparently reasons for non-disclosure which have nothing to do with image-resolution….

        • Denis says:

          Do you mean there is an alien base on the comet ? That’s big news man!

        • logan says:

          If there is only one good thing about our spotless minds…

          World is sure that ESA was born to write his own chapter in History.

          Please be patient, THOMAS.

      • ethanol says:

        The NAVCAM and OSIRIS are run by different teams, which is why they have different policies. OSIRIS is more of a science instrument, while the NAVCAMs are more for, well, navigation. The higher-resolution OSIRIS images contain the most publishable information, so the team is likely withholding to make sure they don’t get scooped in the academic journals. Eventually the whole data set will be released publicly so others can take a crack at it, but for now we only get what they feel like releasing. If I had it my way, it would all be public right away like Cassini, but the policy for this mission is hardly unusual.

  • PaPe3814 says:

    It seems that weight is 9.89×10^12 Kg, which is close to previously announced 10^13 Kg,
    Thus, my personal density estimate is about 250 Kg/m^3, ie 1/4 of water.

    • THOMAS says:

      So it’s even lighter/less dense than a champagne cork, rather more like a piece of expanded polystyrene? Strange how looks can be deceptive in that case.

      Funny, but personally I still see a misshapen chunk of stratified rock which, brought back to Earth, would immediately sink like a stone to the bottom of any ocean you could try to float it on.

      • Denis says:

        You interpret what you see based on your intuition which is based on living on this planet.
        This is totally irrelevant for interpreting a picture taken from a place where everything (environment, gravity…) is different
        Maybe it “looks” like a dense rock based on your experience but this has zero correlation to the real density of this material.

  • Birgit Hofmann says:

    Well, where is the picture from osiris….?

    It is always an exciting….

    Go, Rosetta….

  • Kieran says:

    This has been really cool to watch over the past few months, and now I’m curious, what has Rosetta’s velocity been at all of these different altitudes? (relative to the comet)

  • Marc says:

    I am amazed about the sequence of mamoeuvres to get Rosetta with Philae in orbit. This is truly pure ‘rocket science’ to me ! Hats of to the whole Rosetta team.
    I hope the images and info will come back soon on a more regular basis. Together with the events happening at the red planet, it is very hard not to see we’re exploring our universe.
    Amazing, just incredibly amazing !

  • Erich says:

    By using the orbital period and radius presented in this page with Keppler’s third law i calculated the comet mass to be 9.88E12 kg, thats a lot better then Wikipedias figure.
    The volume of the comet will to my eyes fit in a box of 4000*3000*2500 meter sides. If someone could present a better figure of volume this would be nice.

    Has someone made a 3D model then please calculate the comets volume by assuming its maximal length to be 4350 meters.

    The density of the comet is about a third of water!

    • Robin Sherman says:

      Perhaps a cylinder would be a better approximation. A quick rough calculation of 4.5Km long cylinder with an average radius of 1Km would give a volume of about 14Km^3. This would give a density of around 70% of water.

    • Alexander Czernin-Morzin says:

      The German language monthly “Sterne und Weltraum” (Max-Planck, Heidelberg) has in its October issue another excellent article on Rosetta. Very impressive is the photo montage of Rosetta sitting on top of the city of Frankfurt. Rosetta is about the width of Frankfurt and the height of the Matterhorn.
      It is the best and most graphic image I have seen up to now.

  • Erich says:

    What is the volume of this comet in a 3 figures accuracy?

  • Erich says:

    The volume of the comet, as i tried to do my best estimation might be about 21 km^3, if so its density is almost the halve of water. Its maximum length could be 4.52 km. for sure is that the mass is 9.9E 9 ton +/- 2%

    With so many persons at esa carrying an academic title, why is it so hard to get accurate figures of essential but definitely not proprietary protected information in this blog?
    I have a suspicion that i have misinterpreted the meaning of modern academics. If this is confirmed i will put my title into the junkbox because i have no longer an urge to join that club.

  • Robin Sherman says:

    Hears my take on the landing zone using the Navcam image taken on Sep 14th. The scale is pretty approximate, basically calculated from the size of the cross and the size of the image of the landing zone.,(each arm of the cross is about 25m), so please don’t take the figures as fact.

  • Michael Wiggins says:

    Are you guys detecting gravitational perturbations due to the “dumbbell” shape of the comet?

  • Donald Q says:

    Fantastic. Keep up the exemplary job of steering this mission around such a fascinating object.
    Thanks, Team Rosetta!

  • David W says:

    Right Emily!! You have a small but dedicated band of followers on the Rossetta Blog. Now, you know we are ALL hungry fo some info from Osiris, some fantastic photos we can purr over. How about just one, if we all promise not to show it to anyone else.
    I think I can speak for all of us- Mum’s the word- One peak and we won’t put it out there to the public 🙂 Will we Guys x

  • Peter K says:

    Dear Rosetta Team, I understand that publishing scientific Information takes some time and the priority rights of authors also have to be considered. But I think, taking into acount all the data you have already published here on the blogg, it should be possible to at least show us 1 or 2 new OSIRIS Images in high res of for instanve the landing site or some other interesting region, tell us approximately the density, mass and size of the nucleus and give a general overview of its composition, without compromising your scientific publications. I think everyone here would grately appreciate that!
    Thanks and best wishes for this great Mission!

  • A mass of 10.5 giga ton is a better fit.
    A volume about 21km^3 is what i get.
    A density of 0.5 ton/m^3 is the halve of water.

    If my figures are right or wrong is not influencing my career.

    • The escape velosity at 2000 m radius is about 0.75 m/s
      The netto force holding Philae to the surface is about 17.5 mN for the g pull minus 4.5 due to the rotation makes it 13 milli Newton. Lucky number 😉

  • Jason Rowberg says:

    There’s been a lot of discussion about the changing rotation period of the comet. Is that possibly in relation to Rosetta’s orbit? If Rosetta is orbiting opposite 67P at 14 days, and 67P rotates at 12.7 hours, if my algebra is right the apparent rotation period would be about 12.2 hours.

  • Jason Rowberg says:

    In regard to the propriety of the data, I can wait for a year to see the results (heck, we’ve waited 10 already), but the only thing that really concerns me is that NOW is the hour when decisions must be made. At the end of the mission, we can’t go back and dig into something that the ESA team may have overlooked given the huge amount of data they have to review.

    If the crowd were able to evaluate the data, every possible interesting feature and question would come to light and the only limitation would be what Rosetta is able to make time to explore.

  • Llynisa says:

    From orbital semi-axis 29 km and orbital period of 13 days 14 hours 59 minutes, I calculate the mass as 1.0411×10^13 kg

    • I used 28.5 km as an average, the differency is not a lot and the main issue is that we finally have a proper accuracy of its mass. Compared to the wikipedias +/- 10 % the figure improved >>10 times.
      The hard part is the volume 21km ^3 is the best i get and im not happy with that.

  • Tom says:

    Rosetta is doing very exciting low orbit work.!

    Please give us pictures! Osiris high res of very small areas gives you an idea what is relay goin on down there.

    No pictures for almost a week: very sorry to see that “propriety of the data” arrogance is still in place. It will lead to ESA loosing it funds!

  • simon says:

    I have the “presentiment” that , because 67P is a very irregular and quite unstable piece of dirty rocky ice ,
    maybe , if we are lucky and Rosetta´s eyes will be open long enough , we can witness in our lifetime the breaking up of Comet 67P i.e. separation of head and body !!

    • Kamal Lodaya says:

      My guess is the narrowest portion of the neck is around a third of a kilometre.

      • Kamal Lodaya says:

        No, I made a mistake. From some of the other images the neck looks almost a kilometre thick. There is a curved depression when seeing the neck from the front but that cannot make too much of a difference. That is a lot of material to break up considering that at its perihelion 67P/CG will be well outside the Earth’s orbit. But it would be interesting to estimate an approximate rate of sublimation and see how much material can be lost during one visit to perihelion.

        • logan says:

          Don’t know how profound are ‘those’ ‘scars?’ on the neck.

        • logan says:

          Alter have made a very good point at gravitational tide being damaging there.

    • THOMAS says:

      Rocky and dirty, OK. But where’s the ice? None has yet been detected AFAIK.

      And what is”rocky ice” supposed to look like, exactly?

      • Kamal Lodaya says:

        Interesting point: if 67P is to break up, will it get softened up due to (1) sublimation and escape of internal material leading to subsidence and consequent separation, or (2) escape of dust from the surface, or (3) other forces acting on it like gravitational tides (what is the estimate of its strength at perihelion?), or (4) some internal strains (where is all this energy now?) There might be more possibilities too.

        The best data we have is the Rosetta images taken from various angles and distances. Calibrating them against each other to detect perceptible differences over a few months is not easy, but that looks a possible way forward. We may need a quick informal way of naming features for which we don’t have to go through a complicated approval process. A rough-and-ready map with some named features should be handy, so that we can make conversation like: “hey, check out Dimple number 2 east of landing site C.”

  • laurence says:

    I certainly don’t know of how the ‘politics’ work on this type of mission. I am definitely sad that we cannot see some of the close-up images, but I’m stretching all the patient that my mind can muster up.

    A WONDERFUL part of this is that you guys in the ‘general public’ are providing estimates of orbital data, density, gravitational effects, etc. I salute your smart minds and it gives me ‘food’ to marvel at. Exciting is an understatement, and you guys (including Emily) really provide unprecedented types of information. Even ‘close guesses’ are so fun, and it will be neat to go back and compare the estimates against real data when it is published. This whole thing is just so awesome (to coin a much-used superlative).

  • DavidM says:

    I haven’t got why ESA behave as if better science will result if detailed and various images are withheld from the public. The science will depend upon a lot more data than the imagery, and much of this can be held back for the teams’ own use.

    What deals have been done to preclude early publication of OSIRIS images from the public who are paying around 1bn Euro for this mission?

  • Dertutenix says:

    I think its dependent of the ciricumface area and not its shape of orbit that is to take in account. If orbiting a dual system its no circular or eliptic but rather wiggly orbit, then the radius makes no sense if not aproximated to a Circular with the same area as the complex orbit area has.

  • Chris Gartner says:

    Not sure if this topic has come up already but have any natural satellites been discovered orbiting around the comet? Also what “g” value does the comet produce, I’m guessing about 0.0001 g that of planet Earth. I know the force of gravity is low for this object but how low?

  • Yahia Muflehi says:

    Quoting from above i.e.
    ” On Monday, another manoeuvre will circularise the orbit at 20 km along the terminator plane. After a week at 20 km, a decision will be made as to whether it is safe to proceed to just 10 km.”

    Now that an amendment landing plan was announced whereby Philae will be released either at 22 km to ” J ” landing site OR at 12 km to ” C ” landing site , it is clear from the above quote that it was considered unsafe for Rosetta to proceed to a lower orbit. The question what were risks considered?

  • Cesare Guaita Milano Plantarium says:

    to Emily. Thanks for a reply.
    I have read carefully all EPSC abstracts but they are quite disappointing because they contain only generic and qualitative information . For example, in the VIRTIS report is reported this sentence:
    …they see some strong hints of carbon-bearing compounds, with some spectral features that are compatible with the complex macromolecular carbonaceous materials found in the most primitive carbonaceous meteorites. These materials are often referred to as “organics”, even if their origin is unrelated to life.
    This information should be strongly interesting if accompanied by some kind of spectra. Otherwise it is
    only dissappoint.
    I hope these (quantitative) informations have to be presented at APSC, so becoming of public knowledge. There is some site reporting the streamed APSC comunications ? Is so, all problems should be resolved.

  • Dan Owen says:

    Ten years ago I was able to get NASA cylindrical maps of Mars for both the full color visual appearance and an elevation map. With these was able to create the planet with all its features like Olympus Mons and Valles Marineris. This allowed me to create a flyby / fly through animation that was kind of amazing due to the quality of those image maps.

    I am hoping that ESA will eventually provide a high quality 3D model of the comet along with visual and elevation maps.

  • Kamal Lodaya says:

    Chris: Two reasons why this is unlikely, one is the low g as you mentioned; the other is that when the comet is active its jets will tend to drive away anything orbiting around it. The latter is of course the main danger for Rosetta later on. The lander Philae can also be destroyed by an outburst somewhere near it, or its instruments damaged by an ancillary dust sprayed onto it by the outburst. But assuming this does not happen it should be sitting pretty for some time so long as its batteries keep getting recharged regularly by the Sun’s visibility. We know that Rosetta has cameras which can spot Philae, I wonder whether Philae’s cameras can make out Rosetta as a star-like image.

  • THOMAS says:

    Sorry, but unless we are required by high scientific authority to accept the idea that no image acquired of objects in outer space has any intrinsic value and that they are in reality the exact opposite of what they look like (which I absolutely refuse), we need to turn the logic around: This object being visibly solid rock (just like the other four comets which we have images of), it is necessarily the alleged density which has only relative value in this “place where everything (environment, gravity…) is different”, as you yourself put it. Back on Earth, its density would be calculated quite differently, as a function of Earth’s gravity, and it would indeed “immediately sink like a stone to the bottom of any ocean you could try to float it on”, as I said above. No-one will make me believe that this comet is less dense than a champagne cork.

  • THOMAS says:

    Sorry, I should have specified that my comment above was a response to Denis’s comment of 28/09/2014 at 01:53 which ended “Maybe it “looks” like a dense rock based on your experience but this has zero correlation to the real density of this material.”

Comments are closed.