For more than a century, astronomers have predicted that the energy released as the sun heats and cools an asteroid could change the rock's rotation or even speed up its orbital velocity. The effect is so weak that it ought to take centuries to really nudge an asteroid. In a tour de force of precision measurements, however, researchers have now confirmed the phenomenon. With this proof in hand, astronomers should be able to calculate the threats of these rogue bodies more precisely, although it's unlikely they'll be able harness the effect to save us from Armageddon.
The various models for the phenomenon eventually were lumped together under the name YORP effect--short for Yarkovsky, O'Keefe, Radzievskii and Paddack, the four principal researchers in the field. The problem has been that the process can take millions of years to impact an asteroid's motion significantly, so it's very difficult to observe. Undaunted, two international teams of researchers decided to employ an array of the most sensitive radar instruments available, backed up by optical telescopes, to track asteroids and attempt to detect how the YORP effect was changing their motion.
The teams chose two small asteroids, one called 54509 (2000 PH5), or PH5 for short, and the other called 1862 Apollo. PH5's rapid period of rotation (about 12 minutes) and close approaches to Earth every summer made it a particularly good observing target. Reporting in two papers published online tomorrow in Science, the PH5 team says they have compiled high-resolution radar and optical images over 4 years to track the rock's orbit and measure its spin rate with great precision. Each year, PH5 shaves about 1 millisecond off the duration of its rotation, says astrophysicist Patrick Taylor of Cornell University, lead author of one of the Science Express papers. "Without any other plausible causes for the change in spin rate, this is the best evidence for the YORP effect acting on an asteroid," he says.
The 1862 Apollo asteroid provided much more dramatic results. Reporting online today in Nature, the other team of researchers said the asteroid, which has a diameter of about 1400 meters, managed one extra rotation per orbit around the sun within a timeframe of only 40 years--a gain of more than 4 minutes per year.
Now that the effect has been confirmed, it could be used to determine future asteroid paths with much greater accuracy, says astrophysicist William Bottke of the Southwest Research Institute in Boulder, Colorado. But don't expect Bruce Willis and a team of crack scientists to pull an Armageddon-style rescue by harnessing the YORP effect. Bottke, who wrote an accompanying commentary in Nature, says these orbital changes occur so slowly "that they only have a marginal effect on near-Earth asteroid orbits over a timescale of 100-1000 years," not nearly enough to change the path of any Earth-threatening asteroids.