An international research team led by Academy Research Fellow Mikko Kaasalainen has found an asteroid whose rotation receives an extra kick from solar radiation.
The asteroid 1862 Apollo’s diameter is about 1.5 km, it has a small moonlet, and its orbit crosses that of the Earth.
The team reconstructed Apollo’s shape and determined its rotational state using brightness measurements from several years. They found that Apollo’s rotation speed steadily increases, and showed that this is due to the re-radiation of solar energy from its surface. The study was published in Nature online on 7 March.
Apollo’s rotation period is slightly over three hours, and it decreases only by four thousandths of a second per year, so the analysis required accurate mathematical methods. Because of the acceleration, Apollo is likely to break apart or radically change its figure in the future. It may already have done so earlier, and its present moonlet may be a remnant of such a breakup.
The study confirms that non-gravitational forces are important in the dynamical evolution of asteroids. Re-radiation of solar energy acts as a propulsion engine on the asteroid’s surface. There are two coupled manifestations of this phenomenon: the one changing the orbit (the Yarkovsky effect), and the one changing the spin state (the Yarkovsky-Radzievskii-O’Keefe-Paddack or YORP effect).
The study confirmed the latter, and the former was detected by radar in 2003. Non-gravitational orbital and spin changes can be significant or even critical over long time intervals. They affect the motion of asteroids that may collide with the Earth. The phenomenon can also be used to estimate the masses of asteroids. Apollo is now the first object larger than one kilometre across for which the propulsion effect has been detected.