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have higher proportions of deuterium relative to normal hydrogen.
So, ice that formed in the outer reaches of the solar system can be
predicted to be more deuterium-rich than that (like the Earth's water)
which condensed closer in.
In recent years, humans have moved in closer to comets with space-
craft to collect particles of comet-tail, or to analyse the light that
shines from them. Memorably, the NASA Deep Impact mission in 2005
sent a probe to crash directly into the 9P/Tempel comet, a camera on
the main craft catching the sudden glare of light upon impact (Fig. 2).
The first half-dozen comets to be analysed in this way turned out to
have the high deuterium levels predicted—and so could not represent
a major source of the Earth's water.
Recently, this simple picture has changed. In 2006, the Stardust
spacecraft came back to Earth, having flown through the coma of the
Wild 2 comet and captured up to a million tiny particles of comet
dust on collectors made of aerogel, a highly porous but tough silica-
based material. Analysis of the particles brought a large surprise, for
there were melt-rock droplets there, and minerals that had condensed
fig. 2. Watery comet 9P/Tempel at its point of collision with the impact probe
of the spacecraft Deep Impact .
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