Making light of asteroid spin

Scientists in Helsinki have spotted an asteroid behaving badly, and it's all down to light. Mikko Kaasalainen, from the University of Helsinki in Finland, has been watching an asteroid called Apollo, which is about 1 mile across and spinning. But careful measurements have revealed that the asteroid is very slowly altering its rotational period, by about one 4000th of a second every year. It's the result of a phenomenon called the YORP effect, short for Yarkovsky-O'Keefe-Radzievskii-Paddack effect. This occurs when light hits a surface and gives it a push. But because the asteroid is an irregular shape some parts of it receive a bigger push when sunlight hits it than other parts. The result is that the object's rotational period begins to change. So could this push an asteroid into the path of the Earth, provoking a collision? "Theoretically yes," says Kaasalainen, "but the reverse is also true and scientists have suggested making part of the surface of an Earth-bound asteroid more reflective to take advantage of this effect and avoid a collision". The observations are published in this week's Nature.