16 Mar, 2007 12:04 pm
Ask any researcher about their mental picture of the dynamics of our solar system, and the answer you are most likely to get is that planets and asteroids just move in their orbits and spin around with constant periods. The Sun coordinates everything with its gravity -- add some mutual perturbations and a few close encounters or collisions, and that's about it. But the Sun also lights up the whole show, and now we know that its light has an important part to play as well.
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. The fact that the data could not be fitted with a constant period was the key: only the addition of an acceleration parameter could bring all the modelled rotational phases in agreement with the observational ones. 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) (see, e.g., Bottke et al. 2006). The study confirmed the latter, and the former was detected by radar by Chesley et al. (2003). YORP theory predicts precisely the sort of acceleration term that was needed in the analysis, and simulations verified that radiation effects on the surface of an asteroid similar to the reconstructed model would indeed cause the observed acceleration. Thus the observation was completely consistent with the YORP effect.
Non-gravitational orbital and spin changes can be significant or even critical over long time intervals, and now we are certain that they must be taken into account in calculations. 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. Detailed knowledge about this phenomenon via observations of YORP effect helps us to understand it better and even to plan deflection strategies. This mechanism also explains why there are several asteroid moonlets and very fast or very slowly spinning asteroids (YORP can also decelerate the spin).
The spin-up detection also turns out to have an important meaning to observers in general.
One of the co-authors of the study (Warner) is an amateur astronomer, and the data (asteroid brightness measurements, so-called lighturves) taken with his telescope and CCD camera were crucial for the analysis. This shows that science can still be very democratic and inexpensive: almost anyone interested can take part in frontline observation projects. In fact, such observer networks are absolutely necessary in modern astronomy.
Apollo's YORP detection was part of a large project aiming at mapping thousands of asteroids within the next 10 years or so using the same methods that were employed with Apollo. The goal of the large international network of observers, with telescopes varying from 20-cm to 10-m ones, is to get the first comprehensive picture of the asteroid population. We have already analyzed and imaged over 100 asteroids in this manner, and upcoming large-scale sky surveys (such as Pan-STARRS) will increase this number very fast.
Figures: Views of the reconstructed shape model of 1862 Apollo, shown at equatorial level (ApolloBroad, ApolloEnd) and from above (ApolloPole).
Bottke, W.F., Vokrouhlicky, D., Rubincam, D.P. and Nesvorny, D. 2006:
The Yarkovsky and YORP effects: Implications for asteroid dynamics.
Annu. Rev. Earth Planet. Sci. 34, 157.
Chesley, S.R. 2003: Direct detection of the Yarkovsky effect via radar ranging to the near-Earth asteroid 6489 Golevka. Science 302, 1739.
Kaasalainen, M., Durech, J., Warner, B, Krugly, Y. and Gaftonyuk, N. 2007: Acceleration of the rotation of asteroid 1862 Apollo by radiation torques. Nature online DOI:10.1038/nature05614