AFTER a 5.1 billion-kilometre journey, a spacecraft is about to encounter a comet discovered by an Australian astronomer.

Travelling at 12.5 kilometres a second, the Deep Impact spacecraft will make its closest approach to the comet Hartley 2 early on Friday, passing within 700 kilometres of the 1.2-kilometre wide icy world.

Deep Impact

It will be the first time a single spacecraft has had encounters with two different comets.

On July 4, 2005, Deep Impact flew by comet Tempel 1, dropping a probe on to its surface, creating unprecedented celestial fireworks to coincide with Independence Day in the US.