Itokawa, the “sea otter” rubble pile

Why a 535-meter asteroid looks like two pieces stuck together, and what its boulders and missing craters reveal.

Published 2026-10-23

When Hayabusa reached Itokawa in September 2005, it revealed an object unlike any asteroid seen before: small, lumpy and strangely smooth in places.^[5]

A sea otter in space

Itokawa measures about 535 × 294 × 209 m.^[3] Writing in the journal Science, Hayabusa's team described its shape as resembling a sea otter, with a distinct “head” and a larger “body.”^[1] Others call it peanut-shaped.^[4]

Two piles stuck together

The favored explanation is that after Itokawa's parent body was shattered by a collision, the fragments' weak mutual gravity pulled them into two or more rubble piles — and two of those piles gently bumped together and stuck. Another possibility is that a large impact broke off a chunk that later fell back and reattached.^[1] Either way, Itokawa is likely a contact binary.^[4]

Boulders, gravel — and few craters

Itokawa's low density (about 1.9 g/cm³) and high porosity mean it is a rubble pile, not a solid rock.^[3][4] Hayabusa's images showed a rough surface studded with boulders and a surprising lack of impact craters.^[4] NASA notes that any impact energetic enough to make a crater probably shakes rocks loose that fill the hole back in.^[1]

The largest boulder is about 50 m across — too big to have been thrown out by the impact that made Itokawa's largest crater, so it is probably a leftover fragment from the collision that built the asteroid.^[1]

Why it matters

Many small near-Earth asteroids may be rubble piles like Itokawa. Knowing how loosely they are held together matters for planning sample-return missions — and for any future effort to deflect a hazardous asteroid.

Numbered citations link to the sources list on the Itokawa overview page.