On July 19, 2013, NASA spacecraft got not one but two rare and unique views of Earth from opposite ends of the solar system. While exploring Saturn, NASA’s Cassini spacecraft took the top image of Earth from a distance of about 1.45 billion kilometers (898 million miles) away. NASA’s MESSENGER spacecraft was 98 million kilometers (61 million miles) from Earth, in orbit around Mercury, when it acquired the lower image.
The Cassini view is the third-ever image of Earth from the outer solar system. Views of Earth from distant planets are rare because our planet is so close to the Sun. Sunlight would damage the spacecraft's sensitive imagers, so they are rarely pointed homeward. On July 19, however, Cassini was positioned so that Saturn blocked the Sun’s light while Earth was within the spacecraft’s field of view. Sunlight glimmers around the giant planet’s limb and lights its icy, dusty rings. The sunlit Earth is light blue. The Moon is a faint white dot to the side, but is more clearly visible in the narrow-angle camera view.
Cassini was launched in 1997 to study Saturn and its moons and rings. The July 19 image that includes Earth was part of a wider mosaic of the Saturn system as it was backlit by the Sun. This view will allow scientists to see particles and patterns in Saturn’s rings that are not often visible. The mosaic will take weeks to compile.
On the other side of the solar system, MESSENGER (Mercury Surface, Space, Environment, Geochemistry, and Ranging) was looking for potentially dim, small moons around Mercury when it acquired the lower, black and white image. The sensor required a long exposure time to capture these dim objects, which means Earth and Moon are overexposed. They appear exceptionally bright and large when, in reality, both are less than a pixel in size in this image.
“Cassini’s picture reminds us how tiny our home planet is in the vastness of space, and also testifies to the ingenuity of the citizens of this tiny planet to send a robotic spacecraft so far away from home to study Saturn and take a look-back photo of Earth,” said Cassini project scientist Linda Spilker. That observation holds true for the view from Mercury, as well.
Cassini image courtesy NASA/JPL-Caltech/Space Science Institute. MESSENGER image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Caption by Holli Riebeek.