25143 Itokawa
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
On tiny worlds with little gravity, all the rules are different. Asteroid 25143 Itokawa measures 500 x 300 x 200 meters, making it the size of a large hill. Its surface gravity is about 1/100,000 that of the earth, meaning a 70 kilogram person would weigh only 0.7 grams on Itokawa. Its escape velocity is estimated to be only 0.2 meters per second, meaning a human could easily jump off it altogether. Its mass is about 35 million tons. We have such accurate information because of the way the asteroid's gravity affected the path of the spacecraft. Itokawa is so tiny the spacecraft didn't orbit it. Rather it simply "parked" nearby in space and used its maneuvering thrusters to maintain its position.
The asteroid was only discovered in 1998 and was photographed by the Japanese probe Hayabusa in 2005. The asteroid is not round because its gravity is far too feeble to pull it into a round shape. Its peanut shape suggests it may be a contact binary, that is, two small asteroids held in direct contact by their gravity. Contact binaries are completely impossible for large planets and moons because their gravity would pull them together into a single sphere. But tiny asteroids have such feeble gravity they can be pulled together enough to touch but not be completely merged.
Itokawa appears to be a loosely compacted pile of rocky rubble. It may have been shattered by an impact and some of the fragments were gathered up by the remnant asteroid.
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Created 17 December 2007, Last Update 15 January 2020