Humans are getting the rare opportunity Friday to have their photo taken from space as Earth 'photobombs' a picture of Saturn that is being taken by the Cassini spacecraft.
Cassini will be in the far reaches of the solar system when it snaps the picture -- roughly 1.44 billion kilometres from Earth, or 10 times the distance between the Earth and the Sun.
Cassini is shooting a mosaic, or composite image of Saturn as it is backlit by the sun, giving astronomers a unique angle on the planet and its rings.
Coincidentally, Earth will be in the bottom right-hand corner of the frame when the picture is taken Friday between 5:27 and 5:42 p.m. ET, NASA said on its Jet Propulsion Laboratory (JPL) . Earth will represent just one tiny pixel of the massive image.
"There have been only two images of Earth from the outer solar system in all the time humankind has been venturing out into space," NASA said in a statement.
"The first and most distant one was taken 23 years ago by NASA's Voyager 1 spacecraft from 6 billion kilometres away, showing Earth as a pale blue dot. The other opportunity was Cassini's image in 2006 from 1.49 billion kilometres."
Opportunities for such a photo are few and far between, largely due to the fact NASA's sensitive cameras typically have to avoid pointing in the direction of the sun, due to the risk of damage.
That means it's usually unsafe to photograph Earth, except in rare cases where the sun is blocked by another planet -- in this case Saturn.
North America and part of the Atlantic Ocean are expected to be visible when the Cassini photo is taken.
The picture of Earth is expected to take a few days to process, while the image of the entire Saturn system will take several weeks.
NASA's JPL said the positioning of the sun behind Saturn gives astronomers the chance to learn more about the planet's rings, as "the viewing geometry highlights the tiniest of ring particles and will allow scientists to see patterns within Saturn's dusty rings."
A previous mosaic was made of the Saturn system by Cassini in 2006. It revealed that one of the rings had unexpectedly large colour and brightness variations.
"We'll want to see how that looks seven Earth years and a Saturnian season later, giving us clues to the forces at work in the Saturn system," said NASA's Linda Spilker, Cassini project scientist, in a blog about the event.
Once NASA scientists realized Earth would be in the frame when the Saturn picture is taken, members of the team working with the MESSENGER spacecraft, which is currently orbiting Mercury, began looking for a similar opportunity to capture Earth from a great distance.
As it turns out, MESSENGER will also find Earth within its lens Friday, and again on Saturday, as it searches for natural satellites orbiting Mercury and records images of the planet.
"Parts of Earth not illuminated in the Cassini images, including all of Europe, the Middle East and Central Asia, will appear illuminated in the MESSENGER images," NASA said in a statement.
