The rubble pile
Itokawa
Just 535 metres long, Itokawa is not a block of rock but a heap of boulders and gravel held together by gravity alone. Japan's Hayabusa spacecraft reached it in 2005, touched its surface twice and in 2010 brought back to Earth the first grains ever collected from an asteroid.
- Size
- 535 × 294 × 209 m
- Distance from Sun
- 198 M km
- Orbital period
- 557 days
- Rotation
- 12.1 hours
- Discovered
- 1998
- Samples
- On Earth since 2010
Physical characteristics
Itokawa measures 535 × 294 × 209 metres and is shaped like a peanut, or a sea otter: a larger "body" and a smaller "head" joined by a neck. Its mean density, 1.9 grams per cubic centimetre, is too low for solid rock: about 40% of its volume is empty space. It is what astronomers call a rubble pile, born from the fragments of a larger asteroid that was destroyed.
The surface surprised everyone: almost no craters, and in their place fields of boulders up to 50 metres across alternating with smooth areas of fine gravel, such as the "Muses Sea" where the spacecraft touched down. Vibrations from impacts make the smaller stones slide towards low-lying areas, rather as in a shaken box. The head and the body also have different densities: they may once have been two separate objects that gently merged.
Orbit and rotation
Itokawa takes 557 days to travel an orbit lying between 0.95 and 1.70 AU from the Sun, 198 million km on average: it crosses the orbits of both Earth and Mars. It belongs to the Apollo group. Its orbit lies almost in the same plane as Earth's, tilted just 1.6°, and that is what made it an easy target to reach.
It spins once every 12.1 hours. Its rotation is very slowly speeding up under the effect of sunlight, which heats the irregular surface and, re-emitted as heat, pushes it like an extremely feeble engine: this is the YORP effect, measured here with precision. There is no known risk of impact with Earth.
Discovery and exploration
It was discovered on 26 September 1998 by the LINEAR programme in New Mexico. In 2003, once it had been chosen as the mission's target, it was named after Hideo Itokawa, the engineer regarded as the father of the Japanese space programme.
The Hayabusa spacecraft, launched from Japan in May 2003, reached it in September 2005 and touched its surface twice the following November. The sampling device did not work as planned and contact with the spacecraft was lost for weeks, but the capsule came down in Australia on 13 June 2010 with about 1,500 grains of dust, almost all smaller than a tenth of a millimetre. They were enough to show that rocky asteroids are the source of the most common meteorites.
Frequently asked questions
What is a rubble pile?
It is an asteroid made not of solid rock but of fragments, from boulders to dust, held together only by their weak mutual gravity, with many voids inside. It forms when a larger asteroid is destroyed by a collision and part of the debris reassembles. Itokawa, Bennu and Ryugu are all bodies of this kind.
How much material did Hayabusa bring back?
About 1,500 microscopic grains, weighing less than a milligram in total. That sounds like very little, but with today's laboratory instruments it is enough to measure their composition and age. They were the first samples of an asteroid ever brought to Earth.
Why is Itokawa shaped like a peanut?
Probably because it formed from the merger of two bodies. The "head" is denser than the "body", a sign that they may be two separate fragments of the same parent asteroid that slowly drifted together until they touched. Two-lobed shapes like this are common among small bodies: Arrokoth and comet 67P have them too.
Watch the whole Solar System in motion
Real orbits, moons and speeds rebuilt from NASA/JPL data, right in your browser. Free, nothing to install.
Launch the 3D simulation