The Cassini probe, in orbit around the gas giant, is going to take a picture of Earth, and the imaging team wants everyone to wave and smile.

Because of the great distance to Saturn, our home will appear as a mere pixel in the final photo.

But Cassini scientist Carolyn Porco says it is a moment to "celebrate life on the Pale Blue Dot".

"Waving time" - the period when the spacecraft's cameras will be operating - starts at 21:27 GMT and ends at 21:42 GMT (22:27-22:42 BST).

These timings include the 80 minutes it will take reflected light from the surface of the Earth to travel the nearly 1.5 billion km (900 million miles) to reach the outer Solar System.

Dr Porco's Ciclops imaging team will be producing a large mosaic of Saturn and its ring system on Friday.

Earth will appear as small speck in the lower-right of the final picture.

It is likely to be several days before the first images are processed and released.

The probe snapped a similar mosaic in 2006. On that occasion, Earth was positioned in the upper-left of the frame.

But Dr Porco says the set-up six years ago was not ideal. For the re-shoot, she plans to use Cassini's highest resolution camera, and the most appropriate filters to capture Earth in natural colour.

More than that, however, she says, people on Earth will know this time they are on camera, and that offers everyone the opportunity to participate.

Dr Porco hopes the picture will be reminiscent of the famous "Pale Blue Dot" image captured by the Voyager-1 probe in 1990.

That was a picture she helped organise with the astronomer and popular science writer Carl Sagan.

He memorably described the Earth as looking like a "mote of dust suspended in a sunbeam", such was its apparent insignificance in the vastness of space.

Inspired in part by the Cassini team's plans, scientists working on the Messenger probe at Mercury will also be picturing Earth on Friday and Saturday.

Messenger will see Earth as it scans the skies for any previously unrecognised objects that might be circling the innermost world.

Timings for these pictures are 11:49, 12:38, and 13:41 (all GMT) on both Friday and Saturday.

Parts of the Earth not illuminated in the Cassini images, including all of Europe, the Middle East and Central Asia, will appear illuminated in the Messenger pictures.

More details on the Saturn project can be found at The Day The Earth Smiled website.