The comet nucleus itself is too small to hold on to these gases through gravitation.

慧核本身太小，它的引力不足以束缚住这些气体。

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For all the fiery display it makes in the sky, the nucleus of a comet is relatively small.

尽管彗星在天空中显得光彩夺目，但彗核相对来说是较小的。

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The fan-shaped region that stretches upward from Johnson's nucleus (yellow ball in the middle) represents the dust "tail" of the comet.

从Johnson彗星的彗核（照片中部的黄色球体）向上延展的扇状区域描绘出了这颗彗星的尘埃“尾”。

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In addition to taking normal images of the impact in the visible spectrum, the HRI provides an infrared "fingerprint" of the material from inside the comet's nucleus.

BBC: Q&A: Deep Impact comet mission

It's never happened before that we've fired a very large bullet that's made largely of copper and aluminum, the size of a washing machine, at a comet that's about--the comet's nucleus is about four or five miles across--to make a big crater.

NPR: NASA 'Deep Impact' Probe Targets Comet

The first is that the asteroid is really a ball of ice, like the nucleus of a comet, and that the minerals which the spectroscope detected are only a coating.

ECONOMIST: Asteroids