The shape of rocks to come

By Phil Plait | March 14, 2007 7:57 pm

This news came out a few days ago, but I’ve been too busy to comment. And it’s funny, when I see news releases like this, my take on them is always a little different…

The news itself is very cool. Literally! Take one asteroid. Set it spinning. Now let it get near the Sun, maybe as far from the Sun as the Earth is. What happens?

Well, one side is heated by the Sun, and the other is not. But remember, the asteroid spins. So the warm side rotates away from the Sun. It immediately starts to cool by radiating away infrared light.

The thing is, light particles — photons — have momentum. It’s a weird outcome of quantum mechanics, but even though they have no mass they can still carry momentum. So a photon hitting an asteroid "impacts" it, and the asteroid gains a little bit of momentum. When a photon is radiated away, the asteroid loses momentum. It’s a little more complicated than this (what isn’t?) but that’s the gist.

If the asteroid were a perfect sphere then this would have no real impact (har har) — the gain and loss balance out. But they’re not symmetric: they have lumps, and craters, and crevices. This means that photons can be re-reradiated away at some angle, and the overall effect is to add momentum to the asteroid. In general, this means that the spin of the asteroid is affected, since the photon momentum is not lost directly away from the center of mass of the asteroid (in other words, it’s an off-center force, a torque).

Long story short– over long periods of time, a spinning asteroid can have its spin rate affected by this effect. It’s called YORP, for the Yarkovsky-O’Keefe-Radzievskii-Paddack effect.

It’s incredibly weak, so it’s never been seen until now. The folks at the European Southern Observatory have found an asteroid — (54509) 2000 PH5 — that spins once every 12 minutes, which is pretty fast (the ESO guys even made a movie of it as it brightens and dims due to its rotation). After four years of observations, they have found that the spin of the asteroid is increasing, and just by the amount predicted by YORP. This graph is amazing:

The dots show the rotation period change, and the line is the predicted amount. I call that a match.

The thing is, they couldn’t have made this prediction without knowing the actual shape of an asteroid, since it’s the shape that affects the YORP effect. So they used the Arecibo radio telescope in Puerto Rico to bounce radar blips off the asteroid, and time how long it takes them get back. By doing that, they can build up a 3D model of the rock — a pulse that takes fractionally longer to get to the asteroid and back means there is a crater there (it had to travel farther), and one that gets there and back faster means it hit a bump (it had to travel less). We know this method works, because it’s been done on asteroids that have been visited by spacecraft. We know what shape those asteroids are, and the 3D Arecibo maps have matched pretty well.

Now, sit back and just take that in for a moment. We can map asteroids from Earth!

Man, that’s cool.

So here’s what (54509) 2000 PH5 looks like as it rotates.

Click it to see the whole set of images. The left hand column in both images is the actual radar data. The middle columns are the models, and the right columns are the 3D renderings. It looks like a weird tooth, or a nugget. Maybe it’s Fiddle Faddle!

And that’s my skewed take on the press release. Sure, we have finally seen this teeny tiny YORP effect, and it’s very cool, but man, WE CAN MAP ASTEROID SURFACES! This isn’t new, but it still kills me. When I was a kid… heck, just a few years ago, asteroids weren’t anything but blips of light. Even the biggest were just barely resolvable into blurry lumps. Now we send probes to them! And we can even sit back here at home and ping them (literally) with radar, and create very cool maps of their surface topology.

This truly rocks.


Comments (42)

  1. Cameron

    One word:

  2. elgarak

    Wow. Just wow.

  3. Cassini just took pictures of large liquid bodies on Titan, I wonder how big this is.

  4. The 3D renderings look different from the other 2 columns to my eyes.

    Maybe one of those kids are doing their own things.

  5. Dan Gerhards

    The renderings do look different from the other columns, but that makes sense to me. If a part of the asteroid wasn’t pointed anywhere near us, the signal wouldn’t come back at all, and the “photo” would be dark. The renderings would use data from all the pictures combined, so they wouldn’t be affected by this effect.

  6. icemith

    Nah, I think you will find that the center column needs to be somewhat interpolated from earlier images due to the ‘shadowing’ effect. Or maybe if the contrast can be stretched a little, the info may be there in the ‘dark’ anyway.

    I would like to see a ‘movie’ made of those images, (anybody?), as that would be fascinating. Now, what color could we choose? Yeah, let’s go Technicolor.

  7. Might this observation have some sort of practical use? I can’t really think of anything, but Phil’s first paragraph got me thinking about what Sagan talks a lot about in Pale Blue Dot, which is colonies on asteroids and the moving of asteroids as either weapons or sources of ore.

    Very cool in its own right, though. Another example of how different branches of science can serve to reinforce/confirm other branches’ theories.

  8. Ahruman

    It’s clearly an alien starship.

  9. bswift

    Great description of the Yarkovsky effect!

  10. PK

    What are the error bars on that graph?

  11. bswift

    If you take a look at their article, they show error bars. Getting a dP/P of a few times 10^-7 comes from lots of observations, coordination, and of course, picking the right rock to look at.

  12. bswift

    damn I screwed that link up. this should work.

  13. Jason!

    So, why does it only _add_ momentum to the spin?

    I mean, I can see if the effect is counteracting the spin, eventually the asteroid will stop and start spinning in the direction of the effect, and then momentum will be added, but the statement seemed to imply that the effect only adds momentum.

  14. Cindy


    The NY Times had a great article about this in it’s science section on Tuesday. The article did say that depending on the torques, the spin would either speed up or slow down.

    Perfect timing, I’m going to be teaching torques to my students soon! Of course, they’ll probably focus on the fact that light has momentum (high school students so I can’t go too much into quantum mechanics).

  15. Gary Ansorge

    Any data on the surface composition contained in the radar scattering?

    Looks kinda like a big, chunky snowball in the frames,,,or a smores,,,

    GAry 7

  16. Allen Thomson

    >The thing is, light particles — photons — have momentum. It’s a weird outcome of quantum mechanics…

    It’s a lot more intuitive to think of light pressure in terms of absorption and emission of photons, but the effect also exists in the classical theory of electromagnetic radiation (as in Maxwell’s equations).

  17. John Oliver

    Just a note. Some of us have worked to “map” asteroids long before the radar observations by observing and timing asteroidal occultations of stars. A shadow of the asteroid, cast by the starlight, sweeps across the surface of the earth. Observers located perpendicular to the path observe chords of the projected cross-section of the asteroid. Such observations are still carried out, primarily by observers with relatively small portable telescopes, on dozens of occassions every year.

  18. I was going to say “what asteroids studied by radar have been visited by spacecraft?” but I prayed to Ghugle instead. Eros, 1975. Itokawa, 2004.

  19. L Ron Hubbub

    What’s even cooler is that the radar scan must have also changed the spin of (54509) 2000 PH5 – however slightly.

  20. Jason- it can add or subtract momentum, but it’s a vector, so you can add negative momentum. :-)

  21. PK

    Thanks for the link, bswift.

    Re photon momentum: The fact that light has momentum (in addition to energy) is a relativistic effect (remember that Maxwell’s theory is a relativistic theory). The fact that light is built up from particles (if you look at it the right way) is quantum mechanical.

  22. Jef Spalding

    Thanks, Phil. Nice to have YORP and the technology explained so interestingly. Humans are only limited by their imagination, so your great example proves again how the most obvious technology can often be useful in other ways if someone just thinks about it. I’m sure its future applications may go beyond asteriods, but unfortunately limited in resolution by wavelength of the radiowaves. Still, fascinating!

    And as for Fiddle Faddle, do they still make that stuff?

  23. Just Al

    Icesmith: I would like to see a ‘movie’ made of those images, (anybody?), as that would be fascinating. Now, what color could we choose? Yeah, let’s go Technicolor.

    Well, let’s not, but as for the movie, I animated the jpeg provided on the linked site, which can now be seen at Asteroid Animation

    It’s big (about a Mb) so give it time to load. Sorry about the jiggling, I was trying to correct for it, failed.

    And while I’m at it, three of my own (with help) animated gifs from the eclipse the other evening can be seen at the links below. Exposures had to be varied as it went along, since at some point you choose between catching the shaded portion in detail, and catching the sunlit portion – ya ain’t getting both! 😉

    Eclipse Landscape (3.8 Mb)

    Eclipse Further (1.4 Mb)

    Eclipse Sequence (1.2 Mb), not up to APOD’s standards, but hey, not bad for an amateur using a 500mm lens 😉

    Anyone else having problems with the movie BA linked to? I seem to get just a fixed image.

  24. I am sooo excited that they found seas on Titan! WOHOO! yea! But I only wish that they could go to europa and snoop around there.. I just love outer space!

  25. Duane

    What are the dots that appear to surround the image of the asteroid (from the “actual radar data”)? Are they tiny pieces of rock that orbit the asteroid, or inevitable image artifacts?

  26. Gary Mcleod

    It might sound far-fetched now, but I’ve long supported the idea of learning to herd near-earth asteroids into safe and convenient orbits and mine them for materials to build space habitats. We could turn potential hazards into a vital resource to ensure the survival of the human race.

    Mineral-rich asteroids could be gradually nudged into earth’s orbit then pushed into the L4 and L5 positions. The materials mined can be used to build large space environments like those proposed by Gerard O’Neil in the seventies, supporting thousands or millions of people. Stephen Hawking recently remarked that humanity need to colonise space to survive, and I believe asteroids offer our best opportunity to achieve this.

    We could build immense islands as torus, sphere or cylinder structures with incredible curved vistas and no problems with earthquakes, volcanoes or destructive weather. Turning deadly asteroids into space islands is a fanciful notion, but it’s an option that should be worth serious consideration I’d have thought?

  27. Stark

    Just Al – Just a brief hijack of the thread regarding photography of high contrast subjects (like an eclipse).

    Actually, with a little bit of work you CAN get both the shadows and the sun… if you have access to Photoshop CS2 it doesnt even take work. Google HDR (High Dynamic Range) photography for details. The basics are to take several photos in rapid succesion and widley varying exposures (the one that gets the bright sun and the one that gets the shadows at a minimum) and combine them into one spectacular image.

    I plan on doing this the next time I get a decent view of an eclipse! I do it often for lanscape photos – especially sunset shots and it can produce amazing results.

    I now return you to your regularly scheduled thread…

  28. yagwara

    Wasn’t Arecibo at risk of being shut down? Is it still? Obviously there is still beautiful work done there.

  29. DennyMo

    So if we can take such detailed images of such small objects from such great distances, how much longer until we can use similar technology take similar images of the lunar landing sights?

  30. We know this method works, because it’s been done on asteroids that have been visited by spacecraft.
    Sure, we have finally seen this teeny tiny YORP effect, and it’s very cool, but man, WE CAN MAP ASTEROID SURFACES! This isn’t new, but it still kills me.

    Agreed – the coolest part of this story to me is the HOLY COW – we’ve visited frickin’ asteroids! When you have to do something that cool to test your newest remote-sensing method, that’s pretty amazing, in my opinion.

    Why is the predicted YORP-acceleration line on the graph wobbly? Shouldn’t it be a straight line?

    Stephen Hawking recently remarked that humanity need to colonise space to survive, and I believe asteroids offer our best opportunity to achieve this.

    I agree with Gary Mcleod – orbital habitats are at least a good idea worth serious consideration. But I’m pretty sure Arthur C. Clark has been talking about eggs and baskets n=1 for a while, now. Perhaps somebody is finally listening to him?

  31. Irishman

    DennyMo, the technology is not relevant. Taking radar images of asteroids to 3D map them is a very different thing that visually imaging the terrain of the Moon to see tiny artifacts.

    Also note the resolution scales are not in the same range. The asteroid has a diameter of 114 m diameter. The LM would show up as maybe 1 pixel in the image.

    Just Al, great animation!

  32. Bill Nettles

    If the wobble really is part of the prediction, my guess is that it is because the asteroid isn’t symmetrical, so as it rotates it presents a larger cross section at some time and a smaller cross section at others. That means that the applied torque is not constant, causing a variation in the change, kind of like when your car has a vibration that comes and goes while cruising down the road at 75 m/s (wheee!).

    Question: Is this asteroid in a circular orbit, or did they account for the eccentricity (and hence varying photon flux)?

    I sure wish they had more than 4 points. I looks like the data has some negative curvature compared to the prediction. Also wish the prediction bisected every other data point. It’s really hard to trust 4 data points, but, hey, psychologists trust 2 and build educational paradigms on a sample of size 1. 😉

  33. If representing a geoid like Earth, Mars or Moon in a plane (see Google Maps, Google Mars or Google Moon) it’s very difficult, certainly the representation of asteroid surfaces are pretty complicated. But if rotation is highly influenced by the irregular shape of the asteroid, I suppose that translation (i.e. movement around the Sun and, perhaps, crossing the Earth orbit) can also be influenced. So, the asteroid police must use this mappings in tracking the most dangerous guys of the Solar System

  34. Does it really require quantum mechanics to conclude that light carries momentum? As far as I can tell, that conclusion follows from special relativity (as long as you accept that light carries energy and has no mass): E^2 = p^2 c^2 + m^2 c^4, so if m=0 and E>0 then p>0. Quantum mechanics is required to relate momentum to frequency, but that level of detail isn’t necessary to understand this effect.

  35. Just Al

    Irishman said: Just Al, great animation!

    Thanks! It was actually corrected a little while the post was still in moderation limbo (as I began to learn how to use Adobe ImageReady), so it’s better than the original.

    Stark, regarding your hijack: Good advice, and something that I have used in the past. I have a certain “purist” approach to most subjects though, especially ones that are hard to photograph, and try to keep things close to how they were taken. Sometimes it is useful to do things like flatten the contrast between highlights and shadows, but this can be misleading for those who later want to try the same subject themselves. And some people love to cry “foul.”

    Okay, the comment about “amateur” was a little misleading (since I’m about to give a seminar on nature photography), but I’m truly not equipped or experienced in astrophotography and do not possess a decent telescope, so it applies to these images anyway 😉

  36. PK

    Steuard, see my comment above. You also don’t need QM for the connection between momentum and frequency of light. You get that for free in the momentum four-vector of a (plane) wave: (k,0,0,k). The energy of a wave is a function of it’s frequency, also in classical EM. Therefore, the momentum is a function of the frequency as well. No quantum required.

  37. The curve compared to the data on the graph is the radar data, so the agreement is between the radar determined spin rate change and the optically determined spin rate change. Not too surprising. The actual comparison shows that the computed YORP effect is a factor of 2 to 7 times higher than the observed effect. Not too bad, but not the spectacular agreement shown in the Figure.


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