Itokawa "impossible" says New Scientist

New Scientist has a new “impossible” &mdash the Asteroid Itokawa, recently (Sep 2005) visited by Japan’s Hayabusa spacecraft.

Itokawa has a porosity of about 40%, versus a mere 20% for ordinary sand. It also appears to have suffered much pounding, which should have collapsed said porosity quite handily.

Hayabusa scientists used the data — combined with measurements of the space rock’s size — to estimate its density. It appears to be 40% porous, or filled with empty space.

“That is astonishing,” says [Erik] Asphaug, adding that a handful of sand has a porosity of 20%. “It’s very hard to get porosities greater than that. You've got to start balancing things delicately, like you were building a house of cards,” he says. “The only way to do it is to gently pack the stuff together.” Tamping down

But that raises another mystery, he says, since repeated impacts with other space rocks over millions of years should have made Itokawa denser. “Every time you have an impact, you're going to tamp it down,” he says.

...and so on.

Impossible from end to end — and, as with much else — if only one starts solely with standard assumptions about how such bodies formed in the first place.

Memorable line: Everything we suspected about it turned out to be wrong.