It’s often said that a photo is worth a thousand words, but this one’s worth 127 million miles (205 million kilometers):
This portrait of the Earth and Moon was taken on November 20, 2016 by NASA’s Mars Reconnaissance Orbiter. Its primary camera is the High Resolution Imaging Science Experiment (HiRISE), and like any instrument, it occasionally requires calibration. It’s analogous to photographers metering their cameras off an 18% gray card. Only problem is that there aren’t any such “cards” in orbit around Mars. Fortunately, Earth’s Moon has a very well-known spectral reflectance so it will do very nicely as HiRISE’s “gray card.”
The result is a grainy image that reveals a surprising amount of detail; Australia marks the dark-reddish blob in the middle of Earth’s disk, with southeast Asia at the top and Antarctica at the lower left. Even the Moon has some decent detail, with darker maria clearly visible.
This isn’t the first time HiRISE took a look back home; it snapped a pic of us back in 2007. They’re taken for the purpose of calibrating the camera so that it can continue to image Mars’ surface. But every picture tells a story, and this one is no exception.
It’s the story of a small blue world and its moon, floating in the darkness. It’s the story of us.