Swell space pictures: A space probe catches up with a comet

August 13, 2014 • 10:12 am

Landing on a comet! That’s the goal of the European Space Agency’s Rosetta probe, which for ten years (three of them spent dormant) has been speeding toward the Comet 67P, which comes around the sun once every 6.5 years.  Once it’s near the comet, and in orbit around it, the probe will launch a lander which, for the very first time, will enable humans to sample the surface of a comet. To show the magnitude of this achievement, the BBC reports that the chase has involved 6 billion km, and the comet that the probe has now rendezvoused with is travelling 55,000 km per hour (34,175 mph). That a species can do this defies the imagination.

Here’s what the comet looks like:


How big is it? Not too large:


A YouTube video (notes below) show that the probe’s approach to the comet was via a series of triangular motions, and the Economist excerpt below that shows why they did it this way:

(YouTube): After a ten year journey through space, ESA’s Rosetta spacecraft will reach comet 67P/Churyumov-Gerasimenko in August 2014 [JAC: it did that on Aug. 6]. After catching up with the comet Rosetta will slightly overtake and enter orbit from the ‘front’ of the comet as both the spacecraft and 67P/CG move along their orbits around the Sun. Rosetta will carry out a complex series of manoeuvres to reduce the separation between the spacecraft and comet from around 100 km to 25-30 km. From this close orbit, detailed mapping will allow scientists to determine the landing site for the mission’s Philae lander. Immediately prior to the deployment of Philae in November, Rosetta will come to within just 2.5 km of the comet’s nucleus.

This animation is not to scale; Rosetta’s solar arrays span 32 m, and the comet is approximately 4 km wide.

From The Economist:

So how does Rosetta move in a triangle? Essentially, by cheating. Every few days it fires its thrusters to execute the turn at each corner of the triangle. Rosetta will remain 100km from the comet for a couple of weeks, before closing to 70km. The long sides of the triangle, and the amount of fuel burn required to execute the turns at its corners, will allow the probe’s controllers to observe the effect of the comet’s gravity, and thus determine its mass. To complicate matters, the comet is oddly shaped, which makes its gravitational field irregular. Next month, once the comet’s mass has been established and its gravity field is understood, Rosetta will go into a circular orbit at a distance of 30km. After making further observations, it will then shift to an elliptical orbit in which it passes 10km from the comet at its closest point.

In November, Rosetta will eject the robotic lander Philae, which is small (total mass 100 kg, payload 27 kg). Here it is:


According to Wikipedia, the small sampling probe contains the following instruments.

  • APXS (Alpha Proton X-ray Spectrometer) APXS analyzes the chemical element composition of the surface below the lander. The instrument is an improved version of the APXS of the Mars Pathfinder.
  • COSAC (COmetary SAmpling and Composition) The combined gas chromatograph and time-of-flight mass spectrometer perform analysis of soil samples and determine the content of volatile components.
  • Ptolemy an instrument measuring stable isotopic ratios of key volatiles on the comet’s nucleus
  • ÇIVA (Comet Nucleus Infrared and Visible Analyzer)
  • ROLIS (Rosetta Lander Imaging System)
  • CONSERT (COmet Nucleus Sounding Experiment by Radiowave Transmission). The CONSERT radar will perform the tomography of the nucleus by measuring electromagnetic wave propagation from Philae andRosetta throughout the comet nucleus in order to determine its internal structures and to deduce information on its composition.
  • MUPUS (MUlti-PUrpose Sensors for Surface and Sub-Surface Science)
  • ROMAP (Rosetta Lander Magnetometer and Plasma Monitor)
  • SESAME (Surface Electric Sounding and Acoustic Monitoring Experiment)
  • SD2 (Drill, Sample, and Distribution subsystem) Obtains soil samples from the comet at depths of 0 to 230 millimetres (0.0 to 9.1 in) and distributes them to the Ptolmy, COSAC, and Civa subsystems for analysis. The system contains four types of subsystem: drill, carousel, ovens, and volume checker.[15] There are a total of 26 platinum ovens to heat samples—10 medium temperature 180 °C (356 °F) and 16 high temperature 800 °C (1,470 °F)—and one oven to clear the drill bit for reuse

Finally, although I thought comets were made of ice, this one apparently isn’t. While still releasing water from its surface, it’s rocks, or rather two rocks that may have formed when two comets collided.

I had no idea this was going on, though you space buffs surely did, but the idea of landing a probe on a comet only a few km across, and travelling 55,000 km per hour, is stunning. They’re almost there, and I have little doubt that the probe landing will be successful. Stay tuned.

h/t: Steve

29 thoughts on “Swell space pictures: A space probe catches up with a comet

    1. Not only does it work, it works incrdibly precisely. At 70 km from the comet, the gravitational force on the spacecraft should be ~10,000,000 times weaker than the spacecraft would “feel” at the surface of the earth … and they are not just measuring the force, they’re measuring the assymetry of that force.

      I was amazed at how NASA’s GRACE project measures changes in gravitational forces to learn whare and how fast land ice is melting all over the Earth.

  1. Yes, stunning. And to build suitable equipment that is able to collect such intricate data from the comet – absolutely stunning. I think we can be proud of our species.

  2. It is a little jolting to see that the comet isn’t at all like the depictions in my grade school textbooks. I almost understand how the pope felt about Galileo’s observations.

  3. It simultaneously thrills me and makes me sad. It thrills me that we can and are doing things like this, but it also makes me kind of sad to think that so few people care.

  4. No, comets isn’t made of ice. The Langmuir probe lead (investigates ion and electron densities and temperatures) is a local guy, and he describes them as “a communal snow heap” after the winters scraping. There seems to be a continuity from asteroids (10 % ices and tens of percent porosity) to comets (50+ % ices and very porous).

    My wish list is that a sample D/H ratio will help pin down the amount of Earth volatiles that impactors contributed after its formation.

    A note of caution: Hayabusa’s asteroid landing mission had the worst time. It got back samples by an Apollo-13 save mission on steroids.

    1. I’m pressed on time, so take my percentages et cetera as not researched, taken from memory, likely erroneous.

  5. I really love the Rosetta mission and have been following it for a long time.

    What really blows me away is to what precision those trajectories need to be calculated. This is not unique to the Rosetta mission. Here is an animation of Rosetta’s trajectory (space characters added to prevent embedding):

    https:// http://www.youtube. com/watch?v=ktrtvCvZb28

  6. I too am looking forward to the landing and sampling missions over the coming year.

    The shape of the comet could be due to differences in the rate of outgassing. That is, the skinnier part of the nucleus could have more volatile stuff in it, and had outgassed more.

    1. That is, the skinnier part of the nucleus could have more volatile stuff in it, and had outgassed more.

      … and so the surface has sunk down below the apparent “surface” (depending on how smooth you want to make your progenitor asteroid/ comet).
      But, since the un-sunken parts contain fewer volatiles, one would expect them to be denser. So, they should experience forces sufficient to crush the weak “central” material, which you posit has collapsed under the weight of it’s surface crust.
      Doesn’t work, as far as I can see.

  7. I actually exclaimed “Awesome!” and felt childhood-levels of excitment at seeing that comet photo! I saw the photo they released a week or so ago, but that was mainly just indiscernible blobs. I didn’t realise they’d got so close and had taken such an epic image. It’s an exciting time. Fingers crossed that the landing goes well. How spectacular! 😀

  8. In the diagram of the comet superimposed on London and viewed from above, the scale shows
    2000 ft > 1 km.

    1.609344 km = 5280 ft.

    1.609344 km/5280 ft = x/2000 ft

    x = 0.6096 km = 2000 ft

    0.6096 km < 1 km

    2000 ft < 1 km, eh?

    1. Yeah, you’re right. Looking at the Google map of London indicates that neither of the scales are right either – and they’re inconsistent with the dimensions of the comet drawn on the map too.

  9. Peeve alert!

    Science journalists love to express spacecraft speeds in miles or km per hour, and it always makes me cringe. Sure, it makes for some big numbers, but those numbers end up being meaningless to the reader. Nobody can visualize what it means to travel 55,000 km/hour.

    But in the units that astronomers use, that comes out to about 15 km/s. Now that’s something you can visualize. Lots of people drive 15 km to work or to the grocery store, and know exactly what that route looks like. Now picture doing your daily commute in a second.

    To me there’s a huge missed opportunity here for journalists to bring the science to life in a visceral way. But instead they’d rather club us over the head with big numbers, because km/s sounds too sciency, or something.

    As an aside, 15 km/s is not all that fast as such things go. After all, it took ten years to get there. The Earth’s orbital speed around the sun is about twice that fast, and just getting off the Earth into independent solar orbit requires about 11 km/s.

    But yes, it’s amazing that a species can get vehicles off its home planet in order to do this sort of thing.

    1. “The Earth’s orbital speed around the sun is about twice that fast…”

      What is the quoted speed relative to, if not the sun? The only relevant speed here is the relative velocity of the comet and the probe, and I doubt the quoted speed is that, since that velocity varies over the probe’s journey. It would be meaningless to say that this relative velocity is 55,000 km/h, without saying when it has that magnitude. So I think the quoted speed is irrelevant.

      In any case, matching velocities in a vacuum is probably only a minor achievement by the standards of this mission. When we think of a high-speed rendezvouz, something like mid-air refuelling of aircraft probably comes to mind. But that takes place in an atmosphere, and is a completely different kettle of fish.

      1. Speed relative to the sun isn’t wholly irrelevant, since it gives a rough order of magnitude of the kind of speed required for interplanetary travel. If your spacecraft can’t execute velocity changes of tens of km/s, you’re not going to get beyond Earth orbit, much less catch up with a comet. Using planetary flybys for gravitational boost yields delta-vees of about the same order of magnitude, because that’s how fast planets move.

  10. Yesterday I forgot in my rush that in this and recent article context Google Doodles celebrated (also in US i hear) Anders Ångström, one of the founders of spectroscopy.

    And an obvious name drop, he came from my alma mater Uppsala University. =D

    1. I’m glad to know for and after whom 10^-8 m was named.

      When I was taking high school science neither teacher nor text offered to say where this and other units (Joule, erg, Newton, dyne, ohm, hertz, etc.) came from. Of course, if the prevailing attitude of the Powers That Be toward science is purely instrumental and not in the least bit historical or reflective or numinous, one shouldn’t be surprised.

    2. But did Angstrom name drop that his alma mater used to host Carolos Linneaus, or that it would in future host Torbjörn Larsson, OM ?

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