The basic idea behind the Yarkovsky effect is that the surface of an asteroid is heated by the Sun during the day, and then cools off during the night. This means that the asteroid tends to emit more heat from its “afternoon side”, and less from the opposite side. The recoil from the afternoon side is therefore larger as well, and the overall effect is that a tiny, non-gravitational force acts on the asteroid.

The acceleration caused by this force is also tiny, but over millions of years its effect could become large enough to displace an asteroid from its natural orbit around the Sun. This might be enough to push the asteroid inwards from the “main belt” - which lies between Mars and Jupiter – and towards the Earth.

Steven Chesley of the Jet Propulsion Lab (JPL) in California and colleagues at JPL, Charles University in Prague, the Arecibo Observatory in Puerto Rico and the University of California at Los Angeles have now studied how the orbit of asteroid 6489 “Golevka” has changed over a period of 12 years. They analysed radar data taken using telescopes at Arecibo and the Goldstone Observatory in California, and found that the Yarkovsky effect has changed Golevka’s orbit by about 15 kilometres since 1991. Chesley and co-workers also calculated that the asteroid is just 530 metres in diameter, weighs about 210 billion kilograms and has a bulk density of 2.7 grams per cubic centimetre.

“Never before has the mass of a small solitary asteroid been measured,” Chesley told PhysicsWeb. “This allows us to determine the object’s density, which provides clues to its internal structure. In this case it suggests that the asteroid has a heavily fractured interior.”

The US-Czech team now plans to study other asteroids and hopes that its new method will eventually become routine when tracking these objects. The astronomers also believe that measuring the strength of the Yarkovsky acceleration is the only way to determine the mass and density of small (sub-kilometre) asteroids from Earth.