From 9:45 p.m. tonight until 4:40 a.m. Tuesday, Greek time, astronauts on NASA's Artemis II mission are expected to pass through the dark side of the Moon.
At Mission Control at Johnson Space Center in Houston, all eyes are on this historic flyby. Lunar scientists are particularly interested, as the far side of the Moon is noticeably different from the side facing Earth: it has a thicker crust, many more impact craters and few of the extensive plains of frozen lava that characterize the visible side.
Based on the current trajectory of the Orion capsule, Artemis II scientists in Houston are constantly updating their predictions of what the crew will be able to observe.
The lighting conditions, unfortunately, are not ideal. The Moon and Sun are in a geometry such that only 20% of the dark side will be illuminated when astronauts pass by.
That's a bit disappointing for scientists, who had hoped for a wider area to be visible. However, they remain excited that astronauts will see geological features on the illuminated portion of the far side of the Moon, and they gave Nature a first glimpse of them.
A mass of craters
At the top of many researchers' lists is the Orientale basin, a giant impact basin 930 kilometers across in the Moon's southern hemisphere. It is the largest and youngest of the basins formed during the so-called Late Heavy Bombardment, a prolonged asteroid storm that began about 4 billion years ago. Its three concentric rings are thought to have been created when a massive asteroid struck the Moon, spewing out material that was ejected like a tidal wave and causing the Moon's crust to collapse around the impact site.
Orientale “is of great importance to understanding impact crater formation throughout the solar system,” says Kelsey Young, Artemis II’s lead scientist for the Moon.
Because it is so large and detailed, scientists use it as a model to understand how impact basins form on other planets. Although it has been imaged by robotic spacecraft, its full view has never been seen by human eyes.
Image of Ohm Crater taken by Lunar Orbiter 5 in 1967
Other craters that have not yet been observed optically in sunlight will also be illuminated during the flyby, such as the 64-kilometer-diameter Ohm crater, with a central peak rising above lava flows on its floor, and the 9-kilometer-diameter Pierazzo crater.
Astronauts will try to detect subtle changes in the color and brightness of the surface, as well as observe how the changing angle of sunlight alters the perception of lunar topography. According to mission scientists, this direct human observation can add new information to the detailed photographs already collected by the orbiters.
Cameras on standby
The Artemis II crew will also see the Earth set and rise above the lunar horizon, in a repeat of the historic "Earthrise" image first captured by Apollo 8 astronauts in 1968.
Artemis II astronauts Jeremy Hansen (with camera) and Reed Wiseman practice photographing the Moon at the Johnson Space Center in Houston.
For nearly an hour, as Orion passes by the far side, astronauts will also be able to watch a solar eclipse, when the Moon covers the Sun. They will then observe and photograph the solar corona, while also looking for any bright flashes on the Moon's surface from meteor impacts.
All of these images will be captured with three Nikon cameras, one of which has a telephoto zoom lens up to 400mm. The astronauts also have iPhones with them for more informal shots.
At closest approach, Orion will be about 6.600 kilometers above the lunar surface, with the Moon appearing about the size of a basketball when held at arm's length.
