Sunlight Inflicts Faster Spin in Asteroids

Researchers have measured for the first time a slight force from sunlight that affects the spin rate of the asteroids, a characteristic useful in forecasting fine-tune long-term asteroid orbits and solves the mystery of the rear moonlet displayed by some asteroids. "Sunlight can make asteroids spin so fast they break up, spontaneously shedding a moonlet," said Stephen Lowry, a planetary astronomer from Queen's University in Belfast, UK. "They can do it on their own, essentially, which is really fascinating", he said.

The sun inflicts a light push on space rocks as they absorb the light on their day sides and reflect it as they spin, producing a slight recoil effect, like a set of mini-propulsion engines on the asteroid surface.

As asteroids have an irregular shape, the mini-engines act more powerfully on one part of the asteroid more than on another, generating a twisting force named YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect. YORP effect could not be assessed directly, but now two teams have been able to do it on two asteroids that periodically cross the Earth's orbit.

Lowry's team followed the spin speed of the near-Earth asteroid named 2000 PH5 from 2001 to 2005. This space body rotates once every 12 minutes and is just 120 m (360 ft) long. "It's like a very odd-looking potato," said Lowry.

PH5 was found to spin more rapidly over time, confirming YORP predictions of knocking 1 ms off its rotation period every year during the study.

The other team led by mathematician Mikko Kaasalainen from the University of Helsinki in Finland investigated a much bigger Earth-crossing asteroid, 1862 Apollo, 1.4 km wide and rotating every three hours from 1980 to 2005. The Finnish researchers also confirmed the tiny YORP effect: 1862 Apollo's spin also quickened, cutting off from its rotation time roughly 4 ms yearly, again in line with predictions.

The YORP effect would require about 2.6 million years to double the spinning rate of 1862 Apollo. But as YORP changes slowly the orbits of asteroids over time, it has its importance for fine-tuning predictions of asteroid orbits, especially those too close to the Earth. "If you see an asteroid coming fairly close to the Earth and you want to predict exactly how its orbit evolves over a few thousand years or so, these effects would be important," said Kaasalainen. "Tiny changes in an orbit can even make a significant difference in 30 years or so." he added.

"The results are important for understanding binary asteroids, space rocks with little moonlets circling around them", said Lowry.

Previously, it was thought they appeared in the wreckage of asteroid collisions or when one rock's gravity captured another, but now it is known that a single asteroid could split into a binary by itself due to the YORP effect.