Landing site selection: the race is on

Rosetta has been at comet 67P/Churyumov-Gerasimenko for two weeks, collecting critical data that are now being used to identify candidate landing sites for Philae. This coming weekend up to five candidate landing sites will be proposed for further analysis (they will be announced on Monday), but what exactly goes into choosing a landing site, and who gets to decide?

Let’s start with the ‘who’: to summarise, responsibility for the selection of the candidate sites lies with the Landing Site Selection Group (LSSG), which comprises engineers and scientists from Philae's Science, Operations and Navigation Centre (SONC) at CNES, the Lander Control Centre (LCC) at DLR, scientists representing the Philae Lander instruments, and supported by the ESA Rosetta team, which includes representatives from science, operations and flight dynamics. The final Go/No Go decision for the primary landing site is given after a formal review by ESA, including the lander team, in mid-October.

Artist impression of Philae on the surface of 67P/C-G. Credit: ESA/ATG medialab

Artist impression of Philae on the surface of 67P/C-G. Credit: ESA/ATG medialab

Now for the ‘what’: in general terms, the landing site selection process will ultimately identify a safe and scientifically interesting landing site for Rosetta's lander, Philae. Very well, but what defines “safe” and what is “scientifically interesting”?

Initially, the primary decider is whether or not landing on a given site is technically feasible, independent of scientific desirability. In this case, 'feasibility' relates to the factors concerning the orbiter’s trajectory to deploy Philae and the speed at which Philae is released, details of the descent (duration, the illumination conditions along the way, and radio communications with Rosetta as the orbiter moves away from the comet after separation), as well as conditions at touch-down.

Surface conditions include the illumination of the site at the landing time (it is preferred to land in day time), the slope of the surface and the orientation of the lander with respect to the illumination conditions and local topography. For example, too steep a surface may cause Philae to topple over – although its two harpoons, and ice screws in its three feet will help secure it onto the comet surface. Similarly, it goes without saying that hazards such as large boulders or deep crevasses should be avoided.

There must also be a balance between day and night to cater for the scientific needs of the instruments, as well as ensuring that the solar cells can recharge the battery to power the instruments, while not overheating the lander. Taking into account the comet’s rotation period (12.4 hours) and axis of rotation (see animation below), the LSSG will likely consider some sites close to the equator, offering roughly six hours daylight and six hours of ‘night’. Conversely, they will inevitably exclude sites that are either always in shadow or always in sunlight.

The sites should also allow for operations of the CONSERT experiment, which requires radio signals to be transmitted between the orbiter and lander through the body of the comet, in order to learn about the internal structure of the comet.

To help home in on the landing site and to determine the scientific potential of candidate sites, essential observations have been provided by Rosetta's scientific instruments, in particular OSIRIS, MIRO, VIRTIS, ALICE, GIADA and ROSINA, and from the navigation camera, NAVCAM. As well as providing images, the instruments are helping to refine parameters such as the comet’s shape, rotation parameters and gravity field, along with measurements of surface temperature, outgassing, and the density and velocity distribution of particles in the coma, all of which play a key role in determining the feasibility of operations at specific sites.

Some possible landing regions were already tentatively discussed during the Rosetta arrival event held at ESOC on 6 August, based on very preliminary analysis of illumination conditions and initial flight dynamics inputs (see animation above, and watch a replay of the full science session here).

This week, up to ten possible sites are being laid on the table for a first round of dedicated discussions and for the LCC and the SONC to carry out a technical analysis on each site, ready to be presented at the weekend meeting. Participants of the LSSG will then review the results from the technical analysis and discuss the scientific merits of the candidate sites. By the end of the weekend meeting, as many as five sites could be selected for further detailed investigation.

The candidate landing sites will be announced on the ESA Portal on Monday.

The content of this blog post is based on a longer article published on ESA’s Science & Technology pages: Selecting a landing site for Rosetta’s lander Philae.



  • Roger Kline says:

    Using the rubber duck term for the comet, would landing in the neck area allow you to sample material from the core as the other areas? Any spot on the outer surface would be melted or disturbed by space where the "neck" area would be more protected?

  • John Broughton says:

    The comet is almost certainly a contact binary and therefore the neck is not its interior. Most likely it represents a deep layer of accumulated dust (devoid of ices) around the contact point. Other smooth surfaces are dust covered well. Since the orbiter itself can investigate dust, we should expect a landing on rough ground, where pristine comet material will be found.

  • Chris says: 'are' being collected. Data is the plural form of datum, like bacteria and bacterium. No offense meant, but if you're going to write about scientific topics, please use the correct terminology. :)

    • emily says:

      Good spot on that typo - thanks!

    • Mickey says:

      Hum, hum ... from a strict grammarian and translator -sub-categories deriving from the linguistic science- point of view, "data" refers to a "bunch" of information. Including some latin errors in a stupid language just makes it even more stupid and meaningless, though I like latin ;).

  • Brian says:

    Well yes, data ARE being collected, in the plural, not a datum point IS being collected, lots of data. Probably better to say nothing!

  • Clive Hartland says:

    Quantify it by saying, 'All Data possible is being collected,' as a layman will have very little idea what the data is and to an experienced Comet-phile he will be satisfied that everything is being done to satisfy scientific requirements.
    As said earlier a landing site chosen has to fullfil a myriad of criteria to suit all the needs of all departments.


  • Dave says:

    One of the most exciting events in the history of exploring space, and all you can talk about is grammar?

  • Colin Wood says:

    Is it just me,or does that comet look like a chunk of rock? I would of thought it would have been smooth. If it is rock will the harpoon s still be able to penetrate?

  • Daniele says:

    I hope it'll be all good beside the landing site. I think is one of the greatest mission in space of the lasts years! If you only think about how much time it tooks to be accomplished..!

  • Greg says:

    Yes, better to not quibble over nuances of figures of speech in the midst of having a on-the-spot view and great, open access to the biggest space shot since Mars landing. Science today; grammar tomorrow!
    Kudos and hip, hip, hooray! to the ESA for this marvelous job of including the public to the show. I hope NASA takes the hint and invites us along in its next space mission, wherever and whenever.

  • John Rogers says:

    I haven’t been following discussions in detail but is it really likely that this is a contact binary? From the images, it looks to me like the remnant of an initially layered and cratered single body from which the central band has been eroded. The outer surfaces are largely covered with circular features which look like ancient impact craters that were covered over with more ice and dust while the comet was accreting, and are now being exhumed. Conversely the edge of the central chasm, shown so spectacularly in the OSIRIS stereo pair on Aug.7, which is free of dust because in the gravity field it is the underside of an overhanging cliff, has no crater-like features but shows possible layering parallel to the end surface. I would guess that the central chasm has been created by venting, which was amplified there as erosion proceeded because (i) the subsurface layers were more volatile, and (ii) once the sides became overhanging they were unprotected by dust. Does that seem plausible?

  • Asish says:

    Old habits die hard....look at that animation, how our old spherical coordinate system look weird & battered on that out-of-this world rock!

  • Asish says:

    We can do all the guess work...and that is absolutely okay, but will we ever come to know if this is indeed a contact binary or a single body since origin which morphed to this present shape, and who knows, might even dissociate in 2 pieces !

  • Wesley says:

    Contact binaries and the evolution, and physics of masses in space is of interest to me. Looking at the pre landing material distribution scans, I would almost think that this was one mass that may have been broken in two by impact shock waves. If this is the case they may have traveled together as a binary pair. This might be more common than previously thought. I think the Hartley asteroid also went through this process. The alignment of magnetic grains, and elements that make up the neck might be a big part of what 67P/Churyumov-Gerasimenko can teach us. Does the solar, and interstellar radiation charge particles that travel between the two bodies? Is enough mass transferred in this way to help decay the orbit of a moonlet? Does the Casimir effect play a part over thousands or millions of years this evolution may take? I would think solving this answer on the larger scale might give us insight into the evolution of "solid rock", and its' chemical ratios of which we have thousands of samples in the form of meteorites.

  • Colin Wood says:

    If they are impact crarers I am very surprised that it still in one piece. According to Nasa this body may well be charged , might cause problems for the lander.

  • S0 says:

    Let's hope it goes better than the space tether experiment… or rather, that they learn from it when picking a landing site. BEST WISHES, TEAM. Go get 'em.

  • Asish says:

    hmmm.......thought provoking insight indeed.....

  • Damir says:

    All comet flybys preceding this mission contradicted “dirty snowball” theory, supporting the idea that there is no real difference between comets and asteroids. After reading the article:
    my grave fear for the lander is that, even if its instruments somehow survive a massive electrocution just before the touch-down, the lander will simply bounce-off the comet’s rock-hard, bone-dry surface.

  • Haring says:

    "Safety" of the landing site should be the top priority. Just using common sense - and very few data ;) - I would avoid the neck area where dust could be channelized once the comet wlll be closer to the sun, and produce unwilled forced that could hit laterally the lander. I would prefer "external" landing sites, like the bottom of the body of the duck or the top of the head, but anyway far away from areas where particles could be flowing.
    I also wonder if there is any chance the comet will break up in two parts. In this case again an "external" site would be safer.

  • Alte Kalt says:

    I made some calculations to the latest accessible data.
    About 3.5*4.0 km shape
    The mass is 1E13 +/-1E12kg
    The density is 102 +/-9kg/ m3
    The rotation period is12.7h
    The rotation axis not really worst case but still a challenge

    This given the two lumps are barely holding together and a geostationary orbit is 1+/- 1km outside the outermost rotating "hilltop". Its a bit hard to get it more accurate due to the poor definition of data. Every close encounter to the Jupiter certainly made structural changes to the comet,
    If it once was a pair of comets orbiting each other and finally colliding this was a very gentle collision .
    It almost looks like they are barely filling each-others Roche spheres meeting@the neck.

    On newer pictures i noticed a pair of huge stretched longitudinal marks, cal them craters if you like, one on each body and i speculate than those are the stamp-marks of their first encounter prior to come to a rest.

  • Will the team consider adding locations from the southern hemisphere after the global mapping is complete?

  • Kit says:

    I'm curious about where the comet will vent or evaporate when it gets warmed by the sun. Is it known or assumed that most of the surface will do this? Or will there be one or a few "ports"? Is there any observable evidence of this now? Are there any suggestions of the answers from what has been seen so far? Is any of this important to the landing location? Thanks! (I think this is a very exciting and interesting mission. I'm amazed and very pleased that so much information is given and interaction is possible!)

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