China now has two history-making robots sending back images from an area of the Moon where humankind has never been before. This side of the Moon is distant and mysterious, but, despite pop culture references to the contrary, it isn’t always dark. In fact, after touching down on the lunar surface, the probe sent back a snapshot of its new home that shows a rocky, cratered, and distinctly lit landscape.
China’s probe — which includes a lander and a rover — landed at 10:26AM Thursday, Beijing time, as part of China’s Chang’e-4 mission to scout out the side of the Moon we can’t see from Earth. Since it takes the Moon roughly the same amount of time to spin around its axis as it does to orbit the Earth, we only see one half of the Moon: its near side. China’s landing on the Moon’s far side was a world first, in part because of the technical difficulties posed by that distance. It’s really difficult to get radio signals from Earth to robots on the far side — or vice versa — when the entire bulk of the Moon is planted in between.
— LunarOrbiter (@LunarOrbiter) January 3, 2019
China bridged the signal gap by sending up a satellite called Queqiao, which communicates with the probe and relays information, including photos, back to Earth. There’s light in the photos because there’s light on the far side of the Moon: in fact, there is no permanently dark side of the Moon. “Half the moon is always lit by the Sun — just like the Earth,” Frederick Walter, a professor of physics and astronomy at Stony Brook University, says in an email to The Verge.
Our planet experiences daylight and night because Earth spins on its axis as it orbits the Sun. The side pointing toward the Sun is bright, the side pointing away is night. Over the course of 24 hours, the slow spin of the world cycles through both. (Things get weird at the poles, but even they experience both light and darkness.) The Moon goes through a similar cycle, but on a slower schedule: a full lunar day is roughly 29 Earth days long. Walter calculates that when the Chang’e-4 probe touched down on the Moon’s far side, it was roughly 9AM local lunar time.
“Where the Chinese lander came down, it’s daylight,” Walter says. Shots taken by a camera on the Chang’e-4 lander show the rover, called Yutu-2, casting a shadow on the Moon’s surface, the Planetary Society reports. That’s perfect for the mission, which relies on solar power, according to The New York Times.
— China Xinhua News (@XHNews) January 3, 2019
Even though you won’t be able to see their landing site, you’ll be able to tell when the Chang’e-4 lander and Yutu-2 are bathed in light or cloaked in darkness just by looking up at the sky on a cloudless night. When the Sun is shining fully on the side of the Moon facing Earth, we see a bright, full Moon — but the far side of the Moon is dark. And when the Sun lights up the far side of the Moon, the near side is dark and we see a new Moon. In between, the waxing and waning Moon will mark whether the robots are entering dusk or dawn.
In the meantime, astronomers and space enthusiasts will await any news from the mission. We’ve seen distant glimpses of the far side of the Moon before. The first time was in 1959, when the Soviet Luna 3 spacecraft took a grainy photo of the far side’s crater-covered landscape. Since then, NASA’s Lunar Reconnaissance Orbiter has captured photos and mapped the surface, so we can see the far side in crisp, composite images.
But those images were all still taken from a distance. Now that China’s probe has touched down, we’re getting a closer look than ever before. The spacecraft landed on target in the Von Kármán crater in the South Pole-Aitken basin, according to China’s state media. The basin is a massive, 1,550-mile-wide impact site that’s miles deep and billions of years old. The Yutu-2 rover is equipped with ground-penetrating radar to investigate beneath the Moon’s surface. And while we wait for it to collect its scientific data, we can enjoy the views it sends back of the far side of the Moon — which, right now, is sunlit.