Photons streaming outward from the Sun can impart momentum, which is how a solar sail works. But even more subtle effects produced by the warming of irregular objects may have visible results. A new study of asteroid moons and how they form invokes the tongue-twister known as the Yarkovsky, O’Keefe, Radzievskii, Paddack effect, mercifully shortened to ‘YORP effect’ by those who study it. A body warmed by the Sun gives off infrared radiation, which carries momentum as well as heat. An asteroid’s spin can thus be speeded or slowed by sunlight.
Add plenty of time and things get interesting. Start with the kind of asteroid that is little more than a pile of rocky rubble held together by gravity, then spin that rubble pile up slowly over a period of millions of years and eventually material will be slung off from the asteroid’s equator. Colliding materials of this nature can eventually coalesce into the satellite we see orbiting its parent, says Patrick Michel (Cote d’Azur Observatory, France), who goes on to note the implications for defending Earth against an incomng asteroid:
“Based on our findings, the YORP effect appears to be the key to the origin of a large fraction of observed binaries. The implications are that binary asteroids are preferentially formed from aggregate objects [rubble piles], which agrees with the idea that such asteroids are quite porous. The porous nature of these asteroids has strong implications for defensive strategies if faced with an impact risk to Earth from such objects, because the energy required to deflect an asteroid depends sensitively on its internal structure.”
Image: Three views of the binary asteroid KW4, a ‘rubble pile’ that may have spawned its own moon. Credit: NASA/JPL.
A binary impact is quite a thought, but the authors of the study point out that doublet craters formed by the nearly simultaneous impact of similar objects can be found on Earth as well as other planets. Learning how to counter such asteroids is going to push our technology to the limit, involving as it will missions to different types of asteroids to assess their makeup and figure out which methods for trajectory change are most likely to succeed.
If the YORP theory proves out, it will have solved an earlier conundrum. Small binary asteroids are not thought to have formed early in the history of the Solar System, so what brought them about later? You could create a model of collisions and planetary encounters to account for the binaries, but their sheer numbers make that model dubious. Fully fifteen percent of near-Earth and main belt asteroids (two dynamically different environments indeed) with diameters of less than ten kilometers are now believed to have satellites. The YORP model, which fits the test case of binary asteroid KW4, may well hold the key.
The paper is Walsh et al., “Rotational breakup as the origin of small binary asteroids,” Nature 454, (10 July 2008), pp. 188-191 (abstract)