Rosetta’s Comet Target is a Rotating Two-body Comet

What do You Do When Your Target Looks like a Rubber Ducky in Space?

A sequence of 36 interpolated images of comet 67P/Churyumov-Gerasimenko each separated by approximately 20 minutes. The images were obtained by OSIRIS on July 14th, 2014 from a distance of approximately 12,000 kilometers (Courtesy: SA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Imagine designing a mission to land a probe on a comet. You have to make some assumptions about the comet nucleus, such as its shape, rotation rate, velocity, and what kind of ice it’s made of.  That’s what the planners for the European Space Agency’s Rosetta mission had to do, and this week, they’re getting the first up-close images of the target, Comet 67P/Churyumov-Gerasimenko. When the first views came from the spacecraft, I’m sure the scientists were probably incredibly excited to find out that this comet is not just your run-of-the-mill dented-in icy nucleus with a few jets. No way.  Instead, they’ve drawn the cosmic equivalent of a winning lottery ticket: a sort of double-lobed shaped nucleus that someone described as a rotating “rubber ducky” in space.

Check it out for yourself.

Yes, this is not like any other comet astronomers have seen.  And, it’s spurring a LOT of speculation. I used to study comets in grad school, and several questions came to mind immediately. For starters, how did it get to be this shape? Was this nucleus once two chunks of ice that somehow slammed together in the past and are now orbiting wildly in orbit? That would make it the first “contact binary” comet discovered.  Or, was it one huge chunk of ice that somehow got eroded or broken apart, leaving behind this rotating ducky-shaped object?

To answer the question the mission scientists will use the Rosetta spacecraft’s instruments and cameras to study the surface characteristics of the comet. The data they gather will tell them the ice and mineral makeup. If the nucleus came from one body, then the whole thing should show the same mineralogical makeup. If it came from two different bodies, then the studies will show slight (or perhaps not-so-slight) differences in the ices and dust grains on the surface.

When Rosetta gets to the comet (and the scientists decide to deploy it), it will send a small lander called Philae to settle down to the surface to give us some views from the comet, and also give some first-hand information about the surface materials it will be sitting on. Of course, with a two-body comet, now the big question is, WHERE do you land it?  In particular, if this comet turns out to be made of two different chunks of ice and dust, which side do you pick to study?

Stay tuned because Rosetta is supposed to be at its closest approach to the comet on the morning of August 6th, 2014. It will be an exciting morning for another solar system “first”!


  1. Adolf Schaller

    This may indeed prove a tricky target for the Philae lander. Speculation all week that declare ‘obvious’ regions for it to alight and harpoon itself – near the object’s axis of rotation or center of mass where the two lobes join – are simplistic and naïve. At the junction between the lobes of contact binary bodies there is often a thick mantle or meniscus of fine dust that accumulates after hundreds of millions of years of ‘bump-and-grind’ erosion between the two lobes. For example, whenever one lobe is struck by a sufficiently energetic impactor, the loosely connected objects are temporarily knocked off their previous equilibrium configuration of repose and the two objects can wobble or rock back and forth with respect to each other for some period of time before friction brings them back to rest. (Another potential source for disturbance is tidal disruption by passing close to a planet). Whenever a disturbance places the two pieces temporarily out of kilter before doming to rest in their new equilibrium configuration, a significant amount of debris – rubble from fine dust particles to large cobbles – can get knocked loose or generated and will accumulate at the junction between the two main bodies, like a mutual talus debris apron. The problem is that while the Philae lander may find the rotational axis suitably stationary for a landing attempt, the surface at those spots may consist of a deep layer of loosely-bound dust particles, preventing it from anchoring itself securely to the surface by means of its ‘harpoon’ system. This will become a very perplexing issue that the controllers will be examining very carefully before committing Philae to a final candidate landing site. Attempting to land on a single body would have been difficult enough. If they are lucky, they may be able to find local ‘outcrops’ that bulge up from beneath the debris blanket near the poles; hopefully, such clean exposures might belong to one or the other main lobe, or to a giant boulder that happens to be lodged at the junction. This complex contact-binary shape with the added surface composition issue at the junction places much more severe constraints on the problem of landing. Its going to be a gripping and nerve-wracking thing to watch.

  2. Adolf Schaller

    I’ve wondered that too until the other day when I came across this; it seems many people have been wondering about it:

    It remains to be seen how it pans out, but there has been a reluctance by the Rosetta mission handlers (if not ESA itself) to provide open access to raw imagery and data, as for example, JPL and other US-based facilities provide for missions (like Cassini and New Horizons, etc.). Perhaps continued pressure might help them to recognize the potential value in such a rare opportunity to enhance public awareness. The power of sheer emotional excitement that can be generated by a real-time adventure in interplanetary space can’t be ignored – especially when it comes time for reviewing ESA’s next interplanetary exploration budget…

  3. C.C. Petersen

    Adolph, I remember a time not all that long ago when ESA’s outreach was pretty much nil and the institution didn’t seem to feel that people were interested or that they needed to know what was happening with an ESA mission. The HST/ESA members actually do a good job, and ESA is slowing improving on other fronts. This whole issue that has arisen with the Rosetta data could have been handled better, but I’m afraid there are still those autocratic viewpoints among some of the hierarchy that see the public as “undeserving” or not worthy of catering to. I’m not saying to release all the data — that would be counterproductive. But, there’s a way to release exciting images without hurting a scientist’s work.

  4. Adolf Schaller

    Carolyn, agreed, just so. There are many ways to provide real or near-real-time images without compromising anybody’s work. And though ESA seems more inclined to pre-package its press releases with produced videos, I’m sure there are plenty of news agencies amongst EU member states who would gladly take up providing live-coverage feed directly from the control room with cameras trained on the monitors and the mission team members for anything as momentous as the Philae landing. How difficult could it possibly be to provide a live podcast of the event?*

    It frankly amazes me that the author of the Rosetta blog entry I linked to reminded readers that real-time updates on twitter or similar minimalist social media avenues was sufficient to satisfy the public appetite for coverage. Frankly, I consider that overture to be condescending, and judging from most of the commentary surrounding this issue, many share that view. I hold out hope for coverage at least as complete as that associated with the Huygens probe landing on Titan might be arranged; but then, that was assisted by JPL..

    *Though not based in Europe, I would not hesitate to nominate Loch Ness Productions to carry it out overseas to an international audience with aplomb! (Given a reasonable contract with a sufficient budget, naturally). 8)

  5. C.C. Petersen

    That’s very kind of you. We’d be more likely to do the fulldome video show explaining all the cool science in context, of course. 😉

    I am a bit flabbergasted that some in the hierarchy think that twitter updates would suffice to keep people informed. You really need a multi-pronged approach with social media, liveblogging, streaming video (a la NASA and the Ustream ‘casts) so that people in all the different time zones can stay in tune with the latest updates.

    I remember the amazing feats of reporting that we all did from JPL for the Voyager and Mars missions; images were distributed, presscons were held, people were available to answer questions — it was the gold standard for the LEAST a mission could do to help get the word out.

    A live feed from a control room shouldn’t be that hard to do…

  6. Adolf Schaller

    Update: This seems encouraging…

    Apparently press briefings will be live-streamed at:


    We’ll need to see how invited media will do.

    Also, on the landing: a quick calculation shows that the rotation may not pose as much of a problem as the animation of the goofy-shaped tumbling object might first suggest. assuming the outermost extensions are 1 km from the center of gravity (CG), from earlier estimates of a 2-km-diameter average for an irregularly shaped body, and its approximately 12.4-hour rotation period, the outermost extensions will only be moving at about 0.5 km/hour, which translates to about 0.14 meter per second. Relative velocities with respect to the CG of surfaces closer to the axis of rotation will be below even that. So the combination of its motion and shape doesn’t represent that much of a problem, and any urgency to alight near the CG/axis near the contact junction evaporates. However, another potential but relatively minor problem might emerge: depending on the landing site, the geometry of the object could periodically break line-of-sight communication with the orbiter. Right now, the axis of the nucleus rotation is aimed roughly toward the Sun, but that will shift significantly as the nucleus nears perihelion over the course of the mission, which can mean that the lobes may cast shadows over large areas that would otherwise have been exposed to sunlight for most of every 12.4-hour day on this sun-facing ‘hemisphere’. That will be a factor in determining an optimum landing site too.

  7. Adolf Schaller

    BTW: We may expect the possibility of some truly awesome images from the lander showing an immense overhanging ceiling at a crazy angle to the surface on which the lander rests. (I hope they don’t decide to land on the ends of either lobe from which the other cannot be seen).

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  9. Adolf Schaller

    Correction on the July 19 note: the outer radius appears to be twice that which I originally suggested. At those extremities that suggests a transverse velocity with respect to the center of gravity of the object to be about a quarter of a meter per second.

    Still, not a hard target.

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