But don't fret if your hair is out of place for the planet-wide portrait: the space camera will point not at us but those lucky Americans, with smiles as wide as the Grand Canyon.

“North America and part of the Atlantic Ocean will be in sunlight – leaving Australia, unfortunately, on the wrong side of the world,” says Swinburne University astrophysicist Sarah Maddison. “Nevertheless, it's the idea that counts. So smile anyway,” she recommends.

The interplanetary portrait, the first to be taken without ultraviolet or infrared filters, will be helped by a unique version of an outer solar system eclipse in which the sun's glowing dial tucks away safely behind Saturn.

This photo-artwork will be short on detail: Earth, after all, will be a trifling 1.5 pixels wide – with the illuminated part less than a pixel across – from Cassini's remote vantage point, 1.44 billion kilometres off.

The Earth image will form part of a mosaic, or multi-image gallery, of the Saturnian system being composed by Cassini, named after the Italian astronomer who made discoveries about the ringed planet and its multiple moons.

Be warned, though, this is no instant Earth snap: it will be some time before any images are released. “A narrow-angle picture will take a few days,” says the initiator of the Earth-image project and leader of Cassini's imaging team, Carolyn Porco of the Space Science Institute in Boulder, Colorado.

“A small version of the mosaic will take about three weeks,” Dr Porco explains. “And, the big, full mosaic of all the rings, with Earth, will be ready in roughly two months.”

Expected result

Like the old Apollo-era shots of Earth rising, Saturday's picture will show our home orb as little more than a pale blue speck perched precariously in deep space between Saturn's delicate rings.

“And yet here, in our solar system's habitable zone, our planet can sustain life,” associate professor Maddison notes.

“The day the Earth smiled”, as the event has been dubbed, offers a sense of the amazing scope and achievements of space science, she points out. “To be able to send a human-built spacecraft nearly 1.5 billion kilometres away, have it orbit another planet and do amazing science – for 15 years in the case of Cassini – is a remarkable feat.”

Ring research

Although the Earth picture itself is of little scientific significance, the spacecraft's position, lurking in Saturn's shadow, will provide a backlit view of the rings – enabling researchers to scrutinise variations in their shape, colour and brightness. This will shed light on the ring composition.

“Cassini has a broad range of instruments for investigating the origin, evolution, dynamics and composition of Saturn's atmosphere, rings and moons,” says another Swinburne astrophysicist, Francesco Pignatale.

As the sun will be blocked out by Saturn, more light-sensitive equipment can be used, CSIRO astrophysicist Kurt Liffman explains: “In 2006, a new ring was discovered under similar circumstances and more detailed images were obtained of Saturn's E-ring.”

Throughout much of human history, the planets have been regarded as tiny, bright, wandering points of light in the sky, says Swinburne University astronomer Chris Fluke. “Now it is the Earth's turn.”

Few opportunities have arisen for photographing Earth from the solar system's outer suburbs, he notes: “So, it's great that the spacecraft can take time out from its scientific schedule tomorrow to take a rare portrait of our home planet.”

From a scientific standpoint, the picture will demonstrate the progress made by space science, Mr Pignatale adds: “As well as sending humans into space and robotic probes to the outer solar system, we are now able to take a 'self-portrait' of ourselves from afar.”

Rationale

The new image will not be the most distant snap taken of Earth. That was achieved on 14 February 1990, when NASA's Voyager 1 probe, now leaving the solar system, imaged our planet from beyond the orbit of Neptune, roughly 6 billion kilometres away.

Scientists also observed Earth among Saturn's rings in September 2006, in a mosaic that has become one of Cassini's most popular images. “Since then, I wanted to do it all over again – only better,” says Dr Porco, Cassini's imaging team leader.

To realise this, she and members of the Cassini Imaging Central Laboratory for Operations in Boulder, Colorado, studied the flight plan to find a time when Earth would not be obstructed by Saturn or its rings. Tomorrow offered the best chance for Cassini to fly in Saturn's shadow, and collect different types of images of the planet and its ring system.

“What also makes this so special is that we're letting everyone know ahead of time,” Dr Porco explains. “This will be the first time people of the world will know in advance that their picture is being taken from a billion miles away.”

She adds: “I wanted this to happen so it would serve as a day to celebrate life on planet Earth and our accomplishments in exploring the solar system.”

Links:

Try your hand at this contest.

Enjoy a Cassini Imaging Team gallery here.

Take part in a Flash-based game that lets you play golf on Saturn's moons here.

Discover more about the Earth snap here.

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