≡ Menu

Sun Boosts Asteroid’s Spin

OK, we sometimes encounter scientific terms with large numbers of syllables, but how about this one, perhaps the prize winner: the Yarkovsky-O’Keefe-Radzievskii-Paddack Effect. This multicultural monicker — drawn on the names of a Russian engineer, an American scientist, a Russian astronomer and a NASA aerospace engineer — has something interesting to say about sunlight. As solar radiation heats and cools an asteroid, released energy can change its rotation. In the case of the asteroid 2000 PH5, the effect increases the spin rate to the point where the asteroid may eventually come to spin faster than any asteroid known.

This complicated study used a variety of telescopes to make the case that the asteroid’s rotation period decreases by 1 millisecond every year. That’s a long and slow effect, but the results build over time, and they’re more readily observable because 2000 PH5 approaches Earth every year. The cause is the heating of the asteroid’s surface by the Sun. Stephen Lowry (Queens University Belfast) puts it this way:

“The warming caused by sunlight hitting the surfaces of asteroids and meteoroids leads to a gentle recoil effect as the heat is released. By analogy, if one were to shine light on a propeller over a long enough period, it would start spinning.”

Tracking an asteroid's motion

Is this how some binary asteroids have formed, spinning so fast that they eventually break apart? Nobody has ever observed the YORP effect in action before, but it’s thought to play a role in changing asteroid orbits between Mars and Jupiter, perhaps leading some into planet-crossing orbits. With observations from a bevy of telescopes around the world and radar observations from Arecibo and the Goldstone facility in California, the researchers were able to create a 3D model of the asteroid’s shape.

The spin effect thus noted was seen to increase according to theory throughout a four year observing period. Projecting forward with these data, the team sees 2000 PH5 stable for another 35 million years, with its existing 12 minute rotation being reduced to just 20 seconds in that time. No currently known asteroid is rotating so fast. Says Lowry: “This exceptionally fast spin-rate could force the asteroid to reshape itself or even split apart, leading to the birth of a new double system.”

Image: Radar images obtained at the Arecibo facility in Puerto Rico on July 28, 2004, covering one full rotation of asteroid 2000 PH5 (columns 1 and 4). Corresponding shape-model fits to the images are shown in columns 2 and 5. Columns 3 and 6 are detailed 3-D renderings of the shape model itself. Credit: European Southern Observatory.

Another asteroid under study is 1862 Apollo, observed by a different team that finds the asteroid is making one extra rotation per orbit around the Sun every forty years. That’s a gain of more than four minutes a year, making it clear these effects can play a serious role in the way such objects move. No orbital changes from this source will be quick, but over the long haul YORP may help us make better predictions about what near-Earth asteroids are going to do as they make their close approach.

One of two papers slated to appear in Science Express is Lowry, “Direct Detection of the Asteroidal YORP Effect,” which should be online soon. The paper on 1862 Apollo is Kaasalainen et al., “Acceleration of the rotation of asteroid 1862 Apollo by radiation torques,” posted online in advance of print publication by Nature and available here.

Comments on this entry are closed.

  • andy March 8, 2007, 15:35

    I just noticed that the image there has pretty much exactly the same spacing as the distance scrolled when moving the mouse wheel on my screen, which gives a nice animation as I scroll past it.

  • Douglas Blane March 29, 2007, 7:57

    I ended a short note I wrote about the Yarkovsky-O’Keefe-Radzievskii-Paddack Effect (YORP) effect with the following words:

    “I still want to know more about Yarkovsky, O’Keefe, Radzievskii and Paddack though.”

    A few days later I got a reply from ‘Mary’, which I’d like to share with you:

    “O’Keefe was my father. He was also responsible for discovering that the earth is (slightly) pear shaped, for which reason, we received many lugs of pears for Christmas for several years. He figured out how to map China when going from flat to round was unusual (during WWII) and he figured out that a satellite would help us to map the earth. He said tektites were from the Moon, and when nobody would listen, we put it on his funeral program so he’d have one more chance.

    He liked Wyeth and always talked about his appreciation of how light looks red when it comes through smoke or fog, but looks blue if it bounces off. I found his earliest comments about this in a letter to my mother before they married; he had read Goethe’s Farbenlehre.

    He believed very strongly that, as a civil servant, he was in the employ of the citizens of the US, who had a claim on his attention. Therefore those who wrote to him always received polite answers, even the little old ladies who wanted him to repent of his belief that the earth is round, or that it circuits the sun.

    He always wanted to be an astronomer; now he has an asteroid and an astral effect named after him. Good.”

  • Administrator March 29, 2007, 8:15

    Douglas, thanks so much for sharing this information about O’Keefe with us! What a wonderful note. Much appreciated.

  • ljk March 29, 2007, 9:02

    Today is the 200th anniversary of the discovery of 4 Vesta.

    On March 29, 1807, German astronomer Wilhelm Olbers
    discovered the minor planet Vesta, the brightest planetoid in
    the sky and the second largest in the Main Belt after 1 Ceres.

    A dark feature seen on Vesta by HST was named after Olbers.

    The NASA probe Dawn will orbit Vesta for nine months in 2010-2011.

    More information here:


  • ljk April 19, 2007, 7:46

    Thermal inertia of near-Earth asteroids and implications for the magnitude of the Yarkovsky effect

    Authors: Marco Delbo (INAF-Osservatorio Astronomico Di Torino, Oca), Aldo Dell'oro (INAF-Osservatorio Astronomico Di Torino), Alan W. Harris (DLR Institute of Planetary Research), Stefano Mottola (DLR Institute of Planetary Research), Michael Mueller (DLR Institute of Planetary Research)

    (Submitted on 15 Apr 2007)

    Abstract: Thermal inertia determines the temperature distribution over the surface of an asteroid and therefore governs the magnitude the Yarkovsky effect. The latter causes gradual drifting of the orbits of km-sized asteroids and plays an important role in the delivery of near-Earth asteroids (NEAs) from the main belt and in the dynamical spreading of asteroid families. At present, very little is known about the thermal inertia of asteroids in the km size range. Here we show that the average thermal inertia of a sample of NEAs in the km-size range is 200 $\pm$ 40 J m−2 s−0.5 K−1. Furthermore, we identify a trend of increasing thermal inertia with decreasing asteroid diameter, D. This indicates that the dependence of the drift rate of the orbital semimajor axis on the size of asteroids due to the Yarkovsky effect is a more complex function than the generally adopted D^(−1) dependence, and that the size distribution of objects injected by Yarkovsky-driven orbital mobility into the NEA source regions is less skewed to smaller sizes than generally assumed. We discuss how this fact may help to explain the small difference in the slope of the size distribution of km-sized NEAs and main-belt asteroids.


    Icarus (30/03/2007) in press


    Astrophysics (astro-ph)



    Cite as:

    arXiv:0704.1915v1 [astro-ph]

    Submission history

    From: Marco Delbo [view email]

    [v1] Sun, 15 Apr 2007 19:20:24 GMT (964kb)


  • ljk February 22, 2008, 10:58

    Redetermination of the space weathering rate using spectra of Iannini asteroid family members

    Authors: Mark Willman, Robert Jedicke, David Nesvorný, Nicholas Moskovitz, Željko Ivezić, Ronald Fevig

    (Submitted on 21 Feb 2008)

    Abstract: We obtained moderate S/N ($\sim85$) spectra at a realized resolution of R $\sim100$ for 11 members of the Iannini family, until recently the youngest known family at under 5 million years of age \citep{bib.nes03}. The spectra were acquired using the Echellette Spectrograph and Imager in its low-resolution prism mode on the Keck II telescope. The family members belong to the S-complex of asteroids with perhaps some K class members. The Iannini family members’s average spectral slope, defined as the slope of the best-fit line constrained to pivot about 1 at 550 nm, is $(0.30\pm0.04)/\mu m$, matching the $(0.26\pm0.03)/\mu m$ reported by \citet{bib.jed04} using SDSS \citep{bib.ive02} color photometry. Using our spectra for this family as well as new observations of Karin family members \citep{bib.ver06} and new classifications of some older families we revised the space weathering rate of S-complex asteroids originally determined by \citet{bib.jed04}. Following \citet{bib.jed04} we parameterize the space weathering rate of the principal component color of the spectrum ($PC_{1}$), which is correlated with the spectral slope, as $PC_{1}(t) = PC_{1}(0) + \Delta PC_{1}[1 – \exp^{-(t/\tau)^{\alpha}}]$.

    Our revised rate suggests that the characteristic time scale for space weathering is $\tau = 570\pm220$ Myr and that new S-complex clusters will have an initial color of $PC_{1}(0) = 0.31\pm0.04$. The revised time scale is in better agreement with lab measurements and our measurements support the use of space weathering as a dating method. Assuming all the spectra should be identical, since members derived from the same parent body are presumably covered with similar regolith, we combined them into a high-S/N composite family spectrum which is within the S-complex.

    Comments: 27 pages, 6 figures, 3 tables

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.2977v1 [astro-ph]

    Submission history

    From: Mark Willman [view email]

    [v1] Thu, 21 Feb 2008 04:32:57 GMT (76kb)


  • ljk May 16, 2008, 12:23

    Effect of Density Inhomogeneity on YORP: The case of Itokawa

    Authors: D.J. Scheeres

    (Submitted on 14 May 2008)

    Abstract: The effect of density inhomogeneity on the YORP effect for a given shape model is investigated. A density inhomogeneity will cause an offset between the center of figure and the center of mass and a re-orientation of the principal axes away from those associated with the shape alone. Both of these effects can alter the predicted YORP rate of change in angular velocity and obliquity.

    We apply these corrections to the Itokawa shape model and find that its YORP angular velocity rate is sensitive to offsets between its center of mass and center of figure, with a shift on the order of 10 meters being able to change the sign of the YORP effect for that asteroid. Given the non-detection of YORP for Itokawa as of 2008, this can shed light on the density distribution within that body.

    The theory supports a shift of the asteroid center of mass towards Itokawa’s neck region, where there is an accumulation of finer gravels. Detection of the YORP effect for Itokawa should provide some strong constraints on its density distribution. This theory could also be applied to asteroids visited by future spacecraft to constrain density inhomogeneities.

    Comments: 23 pages, 3 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.2168v1 [astro-ph]

    Submission history

    From: Daniel Scheeres [view email]

    [v1] Wed, 14 May 2008 21:02:22 GMT (110kb,D)


  • ljk July 10, 2008, 12:25

    The Sunny Side of Asteroids

    Written by Nancy Atkinson

    Asteroids with moons, called binary asteroids, are fairly common in the solar system. But scientists haven’t been able to figure out the dynamics of these asteroids, especially how the moons form.

    But a group of astronomers studying binary asteroids say the surprising answer is sunlight, which can increase or decrease the spin rate of an asteroid. The researchers also say that since there are a number of “double craters” on Earth – side-by side craters that appear to have formed at about the same time — these binary asteroids may have hit our planet in the past.

    Derek Richardson, of the University of Maryland, and Kevin Walsh and Patrick Michel at the Cote d’Azur Observatory, France outline a model showing that when solar energy “spins up” a “rubble pile” asteroid to a sufficiently fast rate, material is slung off from around the asteroid’s equator. This process also exposes fresh material at the poles of the asteroid.

    If the spun off bits of asteroid rubble shed sufficient excess motion through collisions with each other, then the material coalesces into a satellite that continues to orbit its parent.

    Full article here: