Yarkovsky-O'Keefe-Radzievskii-Paddack effect
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The Yarkovsky-O'Keefe-Radzievskii-Paddack effect, or YORP effect for short, is a second-order variation on the Yarkovsky effect which causes a small body (such as an asteroid) to spin up or down. The term was coined by Dr. David P. Rubincam in 2000.
Imagine a rotating spherical asteroid with two wedges attached to its equator. The reaction force from photons departing from any given surface element of the sphere will be normal to the surface, such that no torque is produced. Energy reradiated from the wedges, however, can produce a torque because the wedge faces are not parallel to the sphere's surface. An object with some "windmill" asymmetry can therefore be subjected to minuscule torque forces that will tend to spin it up or down as well as make its axis of rotation precess. Note that the YORP effect is zero for a rotating ellipsoid.
In the long term, the object's changing obliquity and rotation rate may wander randomly, chaotically or regularly, depending on a slew of factors. For example, assuming the Sun remains on its equator, asteroid 951 Gaspra, with a radius of 6 km and a semi-major axis of 2.21 AU, would in 240 Ma (240 million years) go from a rotation period of 12 h to 6 h and vice versa. If we gave 243 Ida the same radius and orbit values as Gaspra, it would spin up or down twice as fast, while a body with Phobos' shape would take several billion years to change its spin by the same amount. Shape makes a big difference.
So does size —smaller objects will spin up or down much more quickly. Shrink Gaspra by a factor of 10 (to a radius of 500 m) and its spin will halve or double in just a few million years. Likewise, the YORP effect is more effective as you move closer to the Sun. Moving our small Gaspra to 1 AU allows it double/halve its spin rate in a mere 100,000 years. Give it a million years and its period may shrink to ~2 h, at which point it could start to break apart.
This is one mechanism through which binary asteroids may form, and it may be even more common than collisions and planetary near-encounter tidal disruption as the primary means of binary formation.
Observations show that asteroids larger than 125 km 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, however, so a different explanation must be sought for large asteroids like 253 Mathilde.
[edit] References
- O'Keefe, John A., Tektites and Their Origin, Elsevier, Amsterdam. 254 pp. (1976)
- Paddack, Stephen J., Rotational bursting of small celestial bodies: Effects of radiation pressure, J. Geophys. Res., 74, 4379–4381 (1969)
- Radzievskii, V. V., A mechanism for the disintegration of asteroids and meteorites, Dokl. Akad. Nauk SSSR, 97, 49–52 (1954)
- Rubincam, David P., Radiative spin-up and spin-down of small asteroids, Icarus, 148, 2–11 (2000)
