A Nasa probe is about to sweep past Comet Hartley 2, acquiring a swathe of pictures to send back to Earth.
The Deep Impact spacecraft will get as close as 700km (430 miles) to the colossal block of ice and dust.
Its two visible-light and one infrared imager will endeavour to pick out features not seen on the four previous cometary encounters made by spacecraft.
This should give scientists further insight into the diverse properties and behaviours of these remarkable objects.
"The nuclei of the four comets that we have seen up close are very different from one another - both the overall shape and the kinds of features seen on the surface," says principal investigator Mike A'Hearn from the University of Maryland, College Park.
"What we want to understand is why these differences occur when we don't see obvious differences in the processes that should shape what we see".
The closest approach to Hartley 2 - a roughly 2km-long, gherkin-shaped object - should occur at 1402 GMT. The probe will whiz by at a relative speed of 12.5km/s. The event is occurring just over 20 million km from Earth.
Deep Impact is on an extended mission, having been re-tasked to visit Hartley following its successful flyby of Comet Tempel 1 in 2005.
On that primary mission, the spacecraft released an impactor that crashed into Tempel's nucleus kicking up thousands of tonnes of icy debris.
BBC News
