YORP effect

The Yarkovsky–O'Keefe–Radzievskii–Paddack effect, or YORP effect for short, is a second-order variation on the Yarkovsky effect that changes the rotation period of a small body (such as an asteroid). The term was coined by David P. Rubincam in 2000.

In the 19th century, Ivan Yarkovsky realised that the infrared radiation escaping from a body warmed by the Sun carries off momentum as well as heat. Translated into modern physics, each photon escaping carries away a momentum p = E/c where E (= hν) is its energy and c is the speed of light. Vladimir Radzievskii applied the idea to rotation based on changes in albedo and Stephen Paddack and John A. O'Keefe realised that shape was a much more effective means of altering a body's spin rate. It was also suggested that the YORP effect may be the cause of rotational bursting and that it eventually eliminates small asymmetric bodies from the Solar System.

In 2007 there was direct observational confirmation of the YORP effect on the small asteroids 54509 YORP (then designated 2000 PH5) and 1862 Apollo. The spin rate of 54509 YORP will double in just 600,000 years, and the YORP effect can also alter the axial tilt and precession rate, so that the entire suite of YORP phenomena can send asteroids into interesting resonant spin states, and helps explain the existence of binary asteroids.

Observations show that asteroids larger than 125 km in diameter have rotation rates that follow a Maxwellian frequency distribution, while smaller asteroids (in the 50 to 125 km size range) show a small excess of fast rotators. The smallest asteroids (size less than 50 km) show a clear excess of very fast and slow rotators, and this becomes even more pronounced as smaller populations are measured. These results suggest that one or more size-dependent mechanisms are depopulating the centre of the spin rate distribution in favour of the extremes. The YORP effect is a prime candidate. It is not capable of significantly modifying the spin rates of large asteroids by itself, so a different explanation must be sought for objects such as 253 Mathilde.

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