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ScienceWeek
ASTROPHYSICS: ON THE YARKOVSKY EFFECT
The following points are made by S.R. Chesley et al (Science 2003 302:1739):
1) The Yarkovsky effect is a weak non-gravitational acceleration believed to act on asteroids and meteoroids. According to theory (1-5), absorbed solar radiation is re-emitted in the infrared with some delay, which is related to the thermal inertia of the surface. This delay, in concert with the object's orbital and rotational motion, offsets the direction of the thermal emission and its associated recoil force from the Sun's direction, resulting in a slow but steady drift in the semimajor axis of the object's orbit. Over millions of years, this drift can move main-belt asteroids and meteoroids until they reach a resonance, at which point gravitational perturbations take over and reroute them into the inner solar system (3).
2) The Yarkovsky effect also explains meteorite cosmic-ray exposure ages that are too long for the classical delivery scenarios (3) and the large dispersion of asteroid family members that would otherwise have required unrealistically large collisional ejection velocities. It can also limit the long-term predictability of possibly hazardous close-Earth approaches.
3) The Yarkovsky effect has been detected in the motion of artificial Earth satellites but not for any natural bodies. Vokrouhlick et al (2000) explored the possibility of direct detection by means of the precise determination of near-Earth asteroid (NEA) orbits and concluded that such a detection would be feasible for NEAs up to a few kilometers in size, given precise radar astrometry spanning a decade or more. In particular, they predicted that radar ranging in May 2003 to the asteroid 6489 Golevka (which has a 530-m diameter) would reveal direct evidence for Yarkovsky accelerations.
4) The authors report the outcome of that radar experiment, which confirms Yarkovsky-induced modification of asteroid orbits. Measurements of the distribution of radar echo power in time delay (range) and Doppler frequency (radial velocity) constitute two-dimensional images that can spatially resolve asteroids. The fine fractional precision of radar time-delay measurements and their orthogonality to optical plane-of-sky angular astrometry make them powerful for refining orbits. Radar observations of Golevka were conducted during its close-Earth approaches in 1991, 1995, and 1999. Delay-Doppler measurements were made at Arecibo, PR, and Goldstone, CA, in 1991 and extensively at Goldstone in 1995 (18).
5) In summary: Radar ranging from Arecibo, Puerto Rico, to the 0.5-kilometer near-Earth asteroid 6489 Golevka unambiguously reveals a small nongravitational acceleration caused by the anisotropic thermal emission of absorbed sunlight. The magnitude of this perturbation, known as the Yarkovsky effect, is a function of the asteroid's mass and surface thermal characteristics. Direct detection of the Yarkovsky effect on asteroids will help constrain their physical properties, such as bulk density, and refine their orbital paths. Based on the strength of the detected perturbation, the authors estimate the bulk density of Golevka to be 2.7 grams per cubic centimeter.
References (abridged):
1. D. P. Rubincam, J. Geophys. Res. 100, 1585 (1995)
2. D. P. Rubincam, J. Geophys. Res. 103, 1725 (1998)
3. P. Farinella, D. Vokrouhlick, W. K. Hartmann, Icarus 132, 378 (1998)
4. D. Vokrouhlick, Astron. Astrophys. 335, 1093 (1998)
5. D. Vokrouhlick, Astron. Astrophys. 344, 362 (1999)
Science http://www.sciencemag.org
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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 Vokrouhlick 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. Vokrouhlick, D., Nesvorn, 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., Vokrouhlick, 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
ScienceWeek http://scienceweek.com
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