Most moons are either round and smooth, or lumpy pieces of space rock.

大多数卫星要么是圆形的，要么是光滑的，要么是块状的太空岩石。

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It is a space rock captured and now held in orbit by the gravity force of Mars.

它只是星际空间的一块岩体，受火星引力作用而被吸引住，并沿着轨道环绕火星运动。

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Of course, a lot can happen to an errant space rock in 172 years, hence the odds of one-in-a-thousand.

的确，在今后172年中，这种在太空中漫游的巨石的去向有各种可能，其撞击地球的概率为千分之一。

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But scientists think it was either a comet or rock from space.

VOA: special.2009.08.25

As the space rock burned up over the city of Chelyabinsk, the shockwave blew out windows and rocked buildings.

BBC: Russia meteor's origin tracked down

Leaving this too long would make it impossible to build a spacecraft powerful enough to move the space rock out of harm's way.

BBC: Foresight mission spacecraft Image

Ever spend an afternoon blasting space rock with your wedge of death?

FORBES: Magazine Article