Rosetta's 10-billion-tonne comet

67P Rosetta is now moving within 80km of Comet 67P/Churyumov-Gerasimenko

The comet being followed by Europe's Rosetta spacecraft has a mass of roughly 10 billion tonnes.

The number has been calculated by monitoring the gravitational tug the 4km-wide "ice mountain" exerts on the probe.

Ten billion tonnes sounds a lot, but it means Comet 67P/Churyumov-Gerasimenko has quite a low bulk density, something in the region of 300kg per cubic metre.

If you could put the object in an ocean, it would float.

The calculation would seem to confirm suspicions that the comet is highly porous, and may even hide voids inside its body - but this is all to be determined.

"The mass is in the realms of what was expected," says European Space Agency (Esa) project scientist Matt Taylor.

"At this stage, it simply constrains what we believe it is made from, and as we get better measurements (closer up), there will be a lot of work to interpret whether the comet is heterogeneous or more 'bitty'. But not yet; this is the first measurement."

Rosetta arrived at 67P on 6 August after a 10-year chase through the Solar System.

It immediately began a complex fly-around manoeuvre, and it is from the way the comet's close proximity bent the satellite's path that scientists could gauge a mass.

Having such a number - which is equivalent to about 150,000 aircraft carriers, to continue the ocean analogy - is critical for mission planners as they design a safe orbit for Rosetta when it truly starts circling 67P in the coming weeks.

"Operationally, this can mean that planned orbits may be modified, as they assumed a different mass and hence gravity field," explains Taylor.

"Later, we will get more accurate information, in particular focusing on the centre of mass location, which will certainly come into play for landing site selection."

Rosetta is due to put down its contact robot Philae in November. A long list of five possible locations could be announced on Monday.

Philae Where precisely the centre of mass is located is important for planning the landing

Sunday saw the probe drop another step in altitude, with pictures now being acquired from inside 80km.

Esa has been issuing daily images taken by Rosetta's navigation cameras.

Sadly, we have not had a new picture this week from the Osiris science cameras, which see much more detail.

The Max Planck Institute for Solar System Research, who own the Osiris views, are keeping them under wraps for the moment.

Nonetheless, everyone you speak to continues to be enthralled by this object.

The surface features are so diverse - and go from the very sharp to the very smooth.

Some of the craters are truly intriguing. One, located on the comet's largest lobe, has tall, thin rims.

It resembles the crown-like splash-back you see when a liquid drop hits water in slow-motion.

But the form that is stirring perhaps the most debate is the streaks that are apparent in 67P's narrow "neck" region.

Are they a consequence of erosion, and possibly connected in some way to the boulders that clutter the neck terrain?

Or are they the result of some layering process, maybe when the body first formed and was accumulating material?

"Every time we get a picture back, it just blows me away," says Stephen Lowry, a Kent University researcher working on Osiris.

"We're going to be really busy trying to get our heads around all the features - to the point where the task is actually quite daunting."

67P How do you form craters with tall, thin rims?

Even scientists not formally connected with the mission are taken aback by what they are seeing.

Gareth Collins at Imperial College London is a specialist on impacts and cratering, and is "following the imagery with awe".

He is intrigued by what happens to the surface of 67P when it gets hit by another object in space.

Rosetta will establish precisely the nature of the materials that make up the comet and just how porous it might be.

Such properties will determine the styles of craters that can be sculpted, explains Collins.

"If the impactor is dense (rocky/metallic) then it likely produces a very deep, carrot-shaped cavity, largely by compacting pore space inside the comet, which then collapses to form a shallow depression on the surface.

"But if the impactor were to be another low-density comet or cometary fragment then it is possible the cratering process may be more similar to rock-on-rock impacts.

"Another factor is cohesion. At such low-surface gravities, even very small amounts of cohesive strength (think blancmange) can support towering cliff-faces."

This may help explain the crown-like rim of that fabulous crater.


One of the graphics we have used in the BBC coverage to try to convey the size of 67P shows a comparison with central London.

Which got me thinking: what would happen if 67P came down on the UK capital?

I should emphasise that this cannot happen because the comet does not come far enough into the inner Solar System to reach Earth, but let us unhook our imaginations for a moment.

Gareth Collins has an impact calculator that allows you to work out the kind of damage we could expect.

He has two versions, but the one I've been playing with allows you to vary more easily an impactor's parameters.

And for a 10-billion-tonne, 4km-wide, low-density object, travelling at some 50km per second - you can dig out an impressive bowl almost 40km across.

That is pretty much everything obliterated inside London's M25 orbital motorway system. Give it a go.

There's a serious side to this. Objects very similar to 67P, and bigger, have hit the Earth in the past; and they will likely do so again in the future.

What we learn from Rosetta and 67P may inform future generations on how best to deal with this threat.

Graphic of 67P hitting the Earth What if? Earth has been hit by much bigger objects in the past
Jonathan Amos Article written by Jonathan Amos Jonathan Amos Science correspondent

Rosetta: Date fixed for historic comet landing attempt

Europe's daring attempt to put a small robot on the surface of Comet 67P will take place on Wednesday 12 November.

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  • rate this

    Comment number 22.

    Instead of speculating what would happen if this thing were to hit London, why not imagine it clobbering the border between Syria and Iraq? That might shake up the gangs of murderous savages operating there.

  • rate this

    Comment number 21.

    @Major Mustachio Smythe We wouldn't come barging into one of your religious shindigs and start disproving god exists, not that I'd bother.. so please have the common courtesy and don't post comments espousing your beliefs on something that IS happening and can be proven to be happening, the world isn't flat and the Sun doesn't revolve around the Earth, and we're decended from apes .....get over it

  • rate this

    Comment number 20.

    @14. 'Faith' is belief without evidence, and even against evidence. Why would you even be interested in this amazing mission? It is collecting evidence and knowledge which you and all the others stuck in the 12th Century will disregard. Unless, of course, science were to discover evidence for god (calm down, this mission won't) - I wonder if you'd still ignore it? Take your mumbo jumbo elsewhere.

  • rate this

    Comment number 19.

    A most interesting article, which makes me feel that my £12.18 per month licence fee money is actually being well spent. These articles, as well as BBC radio and TV programmes which feature topics of scientific interest are part of what is best about having a BBC and a licence fee.
    If only there were more of these articles and programmes......

  • rate this

    Comment number 18.

    "..., can the lander take off again and hop from one to the other? ..."

    Sadly not. I think Philae is a pretty minimal machine, able to self orient and with a basic retro braking engine. Its basically a kinetic trajectory machine and landing a strictly one shot event.. it basically drifts in.

  • Comment number 17.

    This comment was removed because the moderators found it broke the house rules. Explain.

  • rate this

    Comment number 16.

    14.Major Mustachio Smythe

    . As it says in 1 Timothy 6:20 "Avoid the profane and idle babblings

    And how are we to avoid these idle babblings whilst you are still posting messages here ?

  • rate this

    Comment number 15.

    @ Major Moustache Smythe.
    Fortunately some people are more able than you. Don't worry, you don't need to understand the technicalities.

  • Comment number 14.

    This comment was removed because the moderators found it broke the house rules. Explain.

  • rate this

    Comment number 13.


    A smaller chance true, but a large comet hitting Earth would be a catastrophic event that could kill tens of millions - especially if it hit the ocean and caused a mega-tsunami that would wipe out all the coastal cities on the surrounding continents. Even a smaller impact such as the Tunguska event is like a nuclear explosion, and these could happen far more regularly...!

  • rate this

    Comment number 12.

    A worthy and interesting project.

    No sign of the god squad yet? Yippee!, the chance of a rational conversation...

  • rate this

    Comment number 11.

    Absolutely wonderful science! The world would be so much better if we spent more on projects like this and (much, much) less on weapons, wars, banks, foreign aid, subsidised drinks for MPs, funding terrorists, etc, etc.

  • rate this

    Comment number 10.

    "I should emphasise that this cannot happen because the comet does not come far enough into the inner Solar System to reach Earth, but let us unhook our imaginations for a moment."

    Well, there are plenty of celestial bodies out there that have, can and will hit earth as this article points out, learning about these objects is a critical step in potentially avoiding a collision in the future.

  • rate this

    Comment number 9.

    For the team to appear to actually land Rosetta on the comet, they need to do what the Americans did for their so-called “manned moon landings” between 1969 and 1972 -- fake the whole thing on a film set.

    Otherwise it will be: 5…4…3…2…splat!

  • rate this

    Comment number 8.

    Looking at the picture (which looked more like a Yorkshire terrier than a rubber duck) in an earlier story on 67P, it bears a remarkable resemblance to one of those Belgian chocolate truffles with flakes stuck to the surface. Now wouldn't that be a discovery?

  • rate this

    Comment number 7.

    Given that it looks like it's actually an amalgamation of two bodies, can the lander take off again and hop from on to the other? It would be interesting to see if their composition was the same or different and in what ways.

  • rate this

    Comment number 6.

    When we're talking about astronomical threats causing trillions of £ of damage to the global economy, it annoys me that asteroids/comets are always hitting the headlines. A ~0.5% chance every decade. Solar super-storms are little known in the public eye yet the likelihood of an event causing equivalent damage is on the order of 10% every decade...

  • rate this

    Comment number 5.

    Fantastic resolution on these photographs now, and a good article. Well worth a read.

  • rate this

    Comment number 4.

    Breaking News!!! A giant ice cube can float in water!... ugh BBC

  • rate this

    Comment number 3.

    Wow, 10 billion tonnes. I can't even comprehend that sort of weight! This is what I love about our universe; it is so unfathomably colossal, my unscientific imagination alone simply isn't enough to begin to understand or contextualise what is out there.


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