NEAR EARTH ASTEROIDS: YARKOVSKY and
In recent memory, asteroid collisions with the prehistoric Earth are cited as one of the reasons for the dinosaurs demise†. The mechanisms by which asteroids are said to collide with the Earth seem, to most in the “public,” be based upon gravitational perturbations. But, not much is mentioned as why or how these perturbations may occur. In the late 19th century, a Russian mathematician, I. O. Yarkovsky suggested that sunlight may affect the manner in which asteroids “orbited;” and since that initial hypothesis, the theory has evolved into an active research area. So, what is the Yarkovsky effect? It may be defined as an effect of the “uneven heating of an asteroid by the Sun resulting in the alteration of an asteroid’s orbital path.” Needless to say, it effects are cumulative and may prove to be important in future considerations in the Torino scale‡.
The Yarkovsky effect might be understood in the following terms: (1) radiation from the Sun impinges upon the asteroid, (2) effects of radiation cause the asteroid to heat, (3) heating of the asteroid causes it to slowly drift away from its “orbit about the Sun ” (4) the drifting asteroid is tugged by the gravity of a larger body (i.e. Jupiter, Saturn, or other larger body), (5) upon leaving the asteroid belt—the “free” asteroid is subject to further gravitational attraction to other bodies in the Solar System (i.e. Earth), and another effect is YORP (modified Yarkovsky effect) which serves to modify the torque of the rotating asteroid and its eventual drift from the asteroid belt.
Another way to think of the Yarkovsky/YORP effects is to attempt to envision a “recoil” of the asteroid by the effects of radiation. Fairly recent studies have measured these cumulative effects and have shed considerable light on a handful of asteroids. In a pre-publication print (by Farnocchia et al) submitted to the journal Icarus—the authors indicate that the uncertainty with which Near Earth Asteroids are tracked may be due (in part) to the Yarkovsky effect.
The authors cite the asteroid known as Apophis (a popularly named asteroid known for its close approach due in the years 2029 and 2036)–the authors claim that the effects of gravity perturbations (along with Yarkovsky effects) will make it an interesting candidate for more consideration. As computational and observational techniques improve in the coming years, it will become easier for astronomers and scientists to help keep an informed public knowledgeable of the skies around us.
For very detailed (and technical) information on the YORP/Yarkovsky effects—see:
David Vokrouhlicky and William F. Bottke (2011) Yarkovsky and YORP effects. Scholarpedia, 7(5):10599., revision #122352
†Drs. Luis and Walter Alvarez (father and son) posited that an asteroid impact led to dinosaur demise—See: Alvarez Theory on Dinosaur Die-Out Upheld: Experts Find Asteroid Guilty of Killing the Dinosaurs « Berkeley Lab News Center