On September 22 NASA’s OSIRIS-REx spacecraft made a “slingshot” gravity-assist pass by Earth in order to adjust the angle of its flight toward Bennu. Mission scientists took the opportunity to test out the spacecraft’s cameras with planned observations of Earth and the Moon, and I’m happy to report that everything worked out perfectly! Some of the first images shared with the public were of Earth from a distance of 106,000 miles; this one shows the Moon imaged from 746,000 miles away three days later on Sept. 25. It’s literally a view of the Moon we can’t ever get from Earth!
Why can’t we see the Moon like this? Simple: because it was taken from beyond the Moon’s orbit, it shows terrain on the lunar surface that never faces Earth. The Moon is tidally locked with Earth, meaning its rotation is at a 1:1 rate to its orbit and it always keeps the same side facing Earthward, with only a slight bit of variation over the course of a year (libration). So almost half of the Moon is never visible from Earth—this is called the farside (and sometimes wrongly as the “dark side,” although it receives sunlight regularly just like the half of the Moon that we do see. In terms of radio communication from Earth though the farside is indeed dark, as Apollo astronauts experienced radio blackouts every time their CSMs passed behind the Moon during lunar orbit!)
The right half of the OSIRIS-REx image shows parts of the Moon that never face Earth. East of the irregularly-shaped Mare Marginis just north of Mare Smythii was literally “luna incognita” until the first images of the farside were captured by the Soviet Luna 3 spacecraft in October 1959.
Here’s an annotated version of the same image:
Various probes and robotic landers followed and less than a decade later humans would walk on the Moon, but to this day no spacecraft has landed on the Moon’s farside…but occasionally we get images like this from outbound robotic spacecraft on their way to explore the Solar System.
To produce the image the OSIRIS-REx team registered and combined nine one-megapixel PolyCam images taken in quick succession using a technique called super-resolution imaging. (I enlarged the original image by a factor of two and sharpened the image for extra detail.)
In addition, OSIRIS-REx also captured this view of Earth and the Moon together in color from even farther away—almost 3.2 million miles (5.12 million km) on Oct. 2, 2017. The Moon has been brightened (because it really is quite dark compared to Earth) but the color is as they might appear to our eyes. (Source)
OSIRIS-REx launched from Cape Canaveral on Sept. 8, 2016 aboard an Atlas V rocket. It will arrive at asteroid Bennu in August 2018 and will spend three years surveying and sampling the surface of Bennu before returning to Earth in September 2023 to drop off its sample. Learn more about the mission here.