An asteroid that could one day hit the Earth is a flying heap of rubble, experts have discovered.

The peanut-shaped space missile called Itokawa is like a flying gravel bank made up of loose-packed rocks, pebbles and dust. The rubble is held together by Itokawa’s weak gravity, according to photos and other data from a space probe that has spent months shadowing the mysterious mini-world.

Itokawa, which is 585 yards long (535m), is one of the group of Near Earth Asteroids, so-called because its orbit around the Sun crosses that of our own planet. Astronomers say it will collide with us within the next million years unless it is first destroyed or has its orbit changed.

Japan’s Hyabusa probe – the name means Falcon – pulled up alongside Itokawa in September last year and briefly perched on its surface in November to collect samples. Japan’s space agency has had trouble controlling the probe but still hopes to bring the asteroid bits home, parachuting them into the Australian Outback by June 2010.

Results from the daring space mission are revealed in the latest issue of the journal Science this week.

The experts says the presence of large boulders in the rubble pile suggest that Itokawa was formed after a collision broke up a parent asteroid. The resulting fragments, including material from which the planets formed 4.5 billion years ago, fell together to produce the asteroid circling the Sun today.

The journal says: “Understanding how they are built may not only someday help us deflect or destroy one on a collision course with Earth, but tell us how Earth was formed.”

The accompanying image is courtesy of the Japanese space agency JAXA.