A GALACTIC sea otter, or merely an orbiting heap of rubble? The Itokawa asteroid, examined by Japan鈥檚 Hayabusa spacecraft, has been called both. The latest analysis has thrown up a more vexing question: how on earth does the rubble stay together?
Named after late rocket scientist Hideo Itokawa, the asteroid is basically a 535-metre-long pile of rubble packed even more loosely than a handful of sand. Some astronomers 鈥� who clearly don鈥檛 get out enough 鈥� claim it looks like a sea otter. Bafflingly, Itokawa appears to be 40 per cent porous, while a handful of sand is only 20 per cent porous. Its gravity seems to be just strong enough to hold it together (Science, vol 312, p 1328).
鈥淚t鈥檚 very hard to get porosities greater than that,鈥� says Erik Asphaug, a planetary scientist at the University of California in Santa Cruz, who is not involved with the mission.
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