From the remote vantage point of Saturn, planet Earth seems mighty small and insignificant. Yet the first interplanetary portrait to be taken in natural colour is a stark reminder of how alone and potentially vulnerable we are in the vastness of space.
The series of snaps was taken early last Saturday by NASA's Saturn-orbiting space probe, Cassini, from a distance of 1.44 billion kilometres.
Space shot: This image taken on July 19, 2013, the wide-angle camera on NASA's Cassini spacecraft has captured Saturn's rings and our planet Earth and its moon in the same frame. Photo: NASA/JPL-Caltech/Space Science Institute
The images – facilitated by a unique version of an outer solar system eclipse in which the sun's glowing dial hid safely behind Saturn – are short on detail. Earth, after all, shows up as a trifling 1.5 pixels wide, with the illuminated part less than a pixel across.
The Earth images, perhaps more art than science, will eventually 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.
"A small version of the mosaic will take about three weeks," 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. "The big, full mosaic of the all the rings, with Earth, will be ready in roughly two months."
She is pleased with the results so far. "What made this special was that we let everyone know ahead of time," Dr Porco explains. "This was the first time people of the world knew in advance that their picture was being taken from a billion miles away."
Dr Porco 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."
Like the old Apollo-era shots of Earth rising, the pictures show our home orb as little more than a pale blue speck perched precariously in deep space somewhere beneath Saturn's delicate rings.
"And yet here, in our solar system's habitable zone, our planet can sustain life," says Swinburne University astrophysicist Sarah Maddison.
"The day the Earth smiled", as Saturday's event was dubbed, offered a sense of the amazing scope and achievements of space science, Associate Professor Maddison 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 – was a remarkable feat."
Although the Earth pictures themselves are of little scientific significance, the spacecraft's position, lurking in Saturn's shadow, provided backlit views of the rings – enabling researchers to scrutinise variations in their shape, colour and brightness. This shed light on the ring composition.
Cassini has a range of instruments for investigating the origin, evolution, dynamics and composition of Saturn's atmosphere, rings and moons. As the sun was blocked out by Saturn, more light-sensitive equipment could 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. "This was 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 could take time out from its scientific schedule to take a rare portrait of our home planet."
From a scientific standpoint, the pictures demonstrate the progress made by space science, says another Swinburne astrophysicist, Francesco Pignatale:
"As well as sending humans into space and robotic probes to the outer solar system, we could now take a 'self-portrait' of ourselves from afar."
The new images are not the most distant snaps 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," beams Dr Porco, Cassini's imaging team leader. "And we did it!"
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Take part in a Flash-based game that lets you play golf on Saturn's moons at www.ciclops.org/sector6/index.php
Discover more about the Earth snap at: saturn.jpl.nasa.gov/waveatsaturn
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