(Every day until Christmas, I’ll be posting a science-related image.)

Day 10

Humans have only set foot on one other world: the Moon. Apollo 11 astronauts Neil Armstrong and Buzz Aldrin were the first to walk on the Moon, on July 20, 1969. They collected rock and dust samples to bring back to Earth, but they also left two significant scientific instruments on the surface. The first was a seismograph to measure moonquake activity, and the second (dear to my own heart) was the Lunar Ranging Retro Reflector (LRRR). The LRRR is a mirror designed to reflect light back directly the way it originated (unlike ordinary mirrors, which bounce light off at an angle equal to the incoming angle); when a powerful laser from Earth is directed at it, the timing of the light that comes back to Earth can be used as a precision measurement of the Earth-Moon distance. This in turn can be used as a way to test Einstein’s general theory of relativity.

The image above is of the Apollo 11 landing site, taken by the Lunar Reconnaissance Orbiter (LRO), in the process of mapping the Moon’s surface in exquisite detail. This particular photo was taken from a height of 24 kilometers (15 miles), and the detail is amazing. I recommend that you view this image at full resolution: you can literally see the footprints of the astronauts, the LRRR, the camera they left behind, and other minute details of the first foray onto the surface of another world.