Hubble has snapped some snazzy images of Comet Tempel-1, the target of NASA's Deep Impact probe, just days before the two bodies are set to collide.

The pictures show a jet of dust streaming towards the sun from the icy core of the comet, in what astronomers hope is a warm up for the display many are expecting when Deep Impact hits the comet on 4 July.

Comets are prone to outbursts of activity like this, but the exact reasons for the activity are not well understood, according to the European Space Agency. One theory is that the heat from the sun opened a crack on the surface of the comet's nucleus, allowing trapped dust and gas to escape.

Another explanation is that a chunk of the surface itself was lifted off the nucleus as gas pressure built up below it. As the gas explodes, it might break the crust into tiny pieces, resulting in the fan shaped expulsion seen in the pictures.

When Deep Impact arrives at the comet on Monday next week, astronomers hope the impact will cause a similar display as the probe crashes in to the comet at many thousands of miles an hour. ®

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