How Asteroids Split Up

Mystery of asteroid pairs solved

26 August 2010

Asteroids are often thought of simply as big rocks orbiting the Sun, but they can have quite exciting lives. Small irregularly-shaped asteroids can be “spun up” to fast rotation rates by sunlight falling on them — much as the asymmetric profile of a propeller blade helps it to spin up in the wind. New results show that when asteroids spin fast enough, they can split into two pieces which then begin orbiting each other. Scientists call this process “rotational fission”.

A new study released this week, led by Petr Pravec of the Astronomical Institute in Czechia and involving many other institutions around the world, shows that many of these binary asteroids do not remain bound to each other but escape, forming two asteroids in very similar, but independent, orbits about the Sun where previously there was just one. Many such asteroid pairs have been discovered in recent years and the new work shows that their properties match perfectly with what is expected from the rotational fission model. It was predicted that in such asteroid pairs the larger one would always be at least five times more massive than the smaller object, and this is exactly what is found. More equally-sized binary asteroids remain bound to each other.

The work is based on very precise measurements of the brightness of 35 asteroid pairs using the Danish 1.5-metre national telescope at ESO’s La Silla Observatory in Chile, among other telescopes around the world.


The asteroids that populate the Solar System are primarily concentrated in the main asteroid belt between Mars and Jupiter about 300 million kilometres from the Sun, but some have orbits that extend into the inner Solar System, and are known as the near-Earth asteroids. There are likely about a million asteroids larger than one kilometre in diameter orbiting the Sun.

Astronomers believe most asteroids are not solid chunks of rock, but rather are rubble piles of debris that come in shapes ranging from snowmen and dog bones to potatoes and bananas, with each asteroid held loosely together by gravitational forces.

More Information

This research will appear in a Letter to Nature on 26 August 2010.

The team is led by led by Petr Pravec of the Astronomical Institute in Czechia. Other co-authors are from institutions in France, Spain, Chile, Israel, the United States, Slovakia and Ukraine.



Dr. Petr Pravec
Astronomical Institute AS CR
Fricova 298
CZ-25165 Ondrejov
Tel: +420-323-620352
Fax: +420-323-620263

Richard Hook
ESO, La Silla, Paranal and E-ELT Press Officer
Garching bei München, Germany
Tel: +49 89 3200 6655

About the Announcement



An asteroid that has split due to rotational fission (artist’s impression)
An asteroid that has split due to rotational fission (artist’s impression)