Nasa’s Cassini spacecraft will take an image of Earth from 1.44bn kilometres away. From there, our planet will look like the expected images of alien earths around other stars
Nasa takes pictures of Earth all the time – but not like this. The Cassini spacecraft is in orbit around Saturn, and so is currently 1.44bn kilometres away. From that distance, Earth is only going to appear as a pixel or two across.
The image, which will be taken on 19 July at around 10:30pm BST, will show our whole, living breathing world of six billion inhabitants, as a single point of light.
It is expected that the Earth will look like a “pale, blue dot”. This was the phrase used to describe an image of Earth taken in 1990 from even further away. The Voyager 1 spacecraft was 6bn kilometres away, roughly the distance to Pluto’s orbit, when it took pictures of most of the planets in the solar system.
Because of Earth’s predominant oceans, the planet appeared as a pale blue dot but was difficult to pick out on the final image.
This attempt should provide something more photogenic. The great ringed world of Saturn will be eclipsing the Sun so its rings will be visible but the great bulk of the planet itself will be in silhouette. Earth will appear to the bottom right of the ring system.
A pale blue dot is what we should expect the first picture of a habitable planet around another star to look like. Although all surface details will be lost in such a confined image area, it will retain a lot of information that can be extracted.
Since 2005, the European Space Agency’s Venus Express spacecraft has been turning its instruments back on Earth while in orbit around Venus. Across the gulf of space, the VIRTIS instrument registers enough light to tell the chemical composition of Earth’s atmosphere. Such studies will be vitally important to tell whether a newly discovered planet world has a breathable atmosphere.
An earlier attempt to do this was made by Carl Sagan using Nasa’s Jupiter-bound spacecraft Galileo during its flyby of Earth in 1990. The subsequent results were published in a Nature paper, cheekily titled, “A search for life on Earth from the Galileo spacecraft”.
Cassini has imaged Earth from Saturn twice before, in 2006 and in 2012. This time, however, is different because it will be the first time with a visible light camera.
This will show us what our eye would see if we were in a spacecraft at Saturn, looking back towards home. North America and part of the Atlantic Ocean will be in view at the time, although impossible to discern.
In a press release announcing the attempt, Nasa invited the public to acknowledge the event by waving at the planet when the photo is being taken. Hmmm. Did somebody just say cheese?
Stuart Clark is the author of Voyager: 101 Wonders Between Earth and the Edge of the Cosmos (Atlantic).
[Ligeia Mare, shown here in a false-color image from NASA's Cassini mission, is the second largest known body of liquid on Saturn's moon Titan.]