ScienceWeek

2. ASTRONOMY: ASTEROID SPIN

The following points are made by Richard P. Binzel (Nature 2003 425:131):

1) What controls the spin of the mountain-sized rocks in space that we call asteroids? These leftover building blocks from the era of planetary formation have undergone random and relentless collisions that have sculpted their shapes over the past 4.5 billion years. These collisions should result in random orientations of asteroid spin axes, yet observations have revealed unexpected spin alignments(1), and Vokrouhlický et al(2) have demonstrated how the slow but steady recoil force of thermal re-radiation can win out over the heavy hand of collisions in controlling the direction of asteroid spins.

2) Planetary bodies such as asteroids maintain an equilibrium temperature by re-radiating (at thermal infrared wavelengths) the same amount of energy that they absorb from sunlight. Approximately in 1900, I. O. Yarkovsky (1844-1902)(3,4), a Russian engineer, wrote about the effect that this recoil could have on the motion of a planetary body. The "Yarkovsky effect" relies on the same principle that drives more swimmers to the beach in the afternoon than in the morning -- the afternoon side of a rotating planet is hotter as a consequence of having absorbed a full day's worth of sunshine. Being hottest, the afternoon side therefore produces the greatest amount of thermal re-radiation. Yarkovsky reasoned that the recoil from this one-sided thermal re-radiation could preferentially slow down or speed up the orbital velocity of an asteroid, depending on whether the tilt of its spin axis meant that its afternoon side faced forwards or backwards. The resulting "Yarkovsky drift" is most effective for small asteroids and might be a key component in the delivery of meteorite samples from the asteroid belt to the Earth(5).

3) Thermal re-radiation has also been recognized to have some effect on the spin rate and orientation of an asteroid, dubbed the "YORP effect" after the names of researchers who detailed the mechanics involved (Yarkovsky, O'Keefe, Radzievskii and Paddick). The YORP effect, acting over millions of years, might have asteroids dancing -- spinning them up, spinning them down and turning their spin axes around -- but there was no reason to think that any particular rotation state would be preferred. Besides, random collisions would shake them all about and start the dance all over again.

References (abridged):

1. Slivan, S. M. Nature 419, 49-51 (2002)

2. Vokrouhlický, D., Nesvorný, D. & Bottke, W. F. Nature 425, 147-151 (2003)

3. Öpik, E. J. Proc. R. Irish Acad. 54, 165-199 (1951)

4. Hartmann, W. J. et al. Meteoritics Planet. Sci. 34, 161-167 (1999)

5. Bottke, W. F., Vokrouhlický, D., Rubincam, D. P. & Broz, M. in Asteroids III (eds Bottke, W. F. et al.) 395-408 (Univ. Arizona Press, Tucson, 2002)

Nature http://www.nature.com/nature

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ON THE ASTEROID EROS

The following points are made by Erik Asphaug (Nature 2001 413:369):

1) Planetary formation apparently began as delicate sedimentation, with granules clinging to other granules in the solar nebula. The process climaxed 10 million to 100 million years later with planets smashing together at velocities of tens of kilometers per second. The Moon, for example, was apparently created when a body the size of Mars hit the early Earth approximately 4.5 billion years ago. In between was a fast-paced epoch, as poorly comprehended as it was brief, which we try to understand by studying asteroids and comets, the remnants that wander the Solar System like ghosts from a bygone time.

2) One such ghost is 433 Eros, recently contacted by the NASA NEAR-Shoemaker spacecraft (NEAR = Near Earth Asteroid Rendezvous). Eros originated in the main asteroid belt, beyond the orbit of Mars, where Jupiter's gravitational stirring stunted planet growth. The belt is crowded, and the long-term fate of asteroids is to batter each other to bits. A variety of dynamical process cause objects to leak from the main belt into the inner Solar System, where they typically crash into the Sun or into a planet or are ejected from the Solar System, after approximately 10 million years. Planet-crossing orbits tend to be chaotic, so the fates of asteroids are expressed as probabilities. Eros has a chance of approximately 5 percent of striking Earth, but not anytime soon. Eros is approximately 34 x 13 x 13 kilometers in size, and is the second largest of the known near-Earth asteroids; the largest is 1036 Ganymed, which is approximately 40 kilometers in diameter. Eros bears enormous impact scars from its 4 billion years in the main asteroid belt, and it may have lost much of its original mass and shape to cratering.

Nature http://www.nature.com/nature

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