The Chinese rover vehicle Yutu-2 crawls cautiously across the rugged terrain of the dark side of the Moon. It takes photos as it moves, transmitting them back to mission control in Beijing. So far, things seem routine. But then Yutu-2 sends back an image of a mysterious gel-like, colorful substance lying in a little crater. This gets Beijing’s attention. What is this baffling substance and what’s it doing in a Moon crater?
Perplexed personnel at the Beijing Aerospace Command and Control Center have no idea what this material is. But they they want to find out more, so Yutu-2’s movement schedule is quickly modified. Instead of roving further afield as originally planned, it will now spend more time concentrating on this puzzling find.
The Yutu-2 rover is part of the China National Space Administration’s Lunar Exploration Program. It was transported to the Moon aboard the Chang’e-4 spacecraft, which itself was sent to space atop a rocket from south-west China’s Xichang Satellite Launch Center in December 2018. The spaceship entered lunar orbit after just over a 4-day flight.
Chang’e-4 left its orbit and made a successful soft landing on the Moon’s surface on January 3, 2019. It came down on the Van Kármán crater, which is some 110 miles across. That crater itself is in a much larger impact crater, the South Pole-Aitken basin. That is just over eight miles deep and has a diameter of some 1,600 miles.
This soft landing was extraordinary, because it took place on the dark side of the Moon. No other spacecraft had ever achieved this before. After all, the terrain there presents particular challenges. While the side of the Moon visible to Earth has a comparatively flat topography, the dark side is much craggier, with mountains around double the size of Mount Everest.
We’ll get back to that mysterious substance that the Yulu-2 rover stumbled upon. But let’s first learn a little more about the Chang’e-4 mission and the dark side of the Moon. In fact “dark side” is something of a misnomer. Each side of the Moon has two-week-long periods of light, with another fortnight of darkness coming next.
So given that the so-called “dark side” of the Moon does receive two weeks of light, it’s actually more appropriate to call it the “far side.” And from our perspective here on Earth, that’s exactly what it is. Generally speaking, we can’t see it – which gives it something of an air of mystery.
The first time we earthlings ever saw the far side of the Moon, though, wasn’t until 1959. That was when the Soviet Luna 3 spacecraft orbited the Moon and photographed its far side. That mission managed to send back 17 rather poor-quality pictures of the Moon’s far side. And though the grainy photos might not have been great, they nonetheless caused a sensation on Earth.
Almost another decade passed until human eyes actually viewed the far side of the Moon in real-time. Luna 3 was an unmanned mission, but America’s 1968 Apollo 8 mission had astronauts aboard. These three people were the first men to fly to the Moon, as well as being the first to personally witness its dark side.
All of the spacecraft that have landed on the Moon since 1959 have done so on its visible side. That is, until Chang’e-4 landed on the far side in January 2019, as part of the Chinese Lunar Exploration Program. This project got underway in 2007 with the launch of Chang’e 1.
Chang’e, by the way, is the goddess of the Moon in Chinese mythology. But the Chang’e 1 spacecraft which borrowed the goddess’ name was an altogether more practical proposition. After just under a year and a half, the spacecraft had completed its mission to fly around and map the Moon.
The success of Chang’e 1 opened the way for a second mission, Chang’e 2, in 2010. This was also a lunar orbiting mission, basically collecting data needed for Chang’e-3. This one was to be more ambitious and would involve a Moon landing, as well as the deployment of a rover vehicle. Chang’e-3 set off from Earth in December 2013.
Chang’e-3 made its soft landing on the Moon on December 14, 2013, eight days after its launch. It was the first craft to make a controlled landing on the Moon’s surface since the Soviet Luna 24 in 1976. Unlike Chang’e-3, that Soviet mission had lacked a rover, although it did take soil samples with a robotic probe.
These samples were flown back to Earth aboard Luna 24’s return vehicle. They offered strong evidence that there was water on the Moon. But the existence of lunar water was later confirmed by the Indian probe Chandrayaan-1. That mission carried NASA’s Moon Mineralogy Mapper which detected ice at both of the Moon’s poles.
Once it had landed on the near side of the Moon, Chang’3 deployed its rover vehicle. This was Yutu, which had been named by participants of an online ballot. Like the goddess Chang’e, Yutu is a figure from traditional Chinese mythology. In fact, Yutu – which translates as ‘jade rabbit’ – is Chang’e’s pet and resides on the Moon, appropriately enough.
Yutu was the first Moon rover that the Chinese space program had ever used. The vehicle had six wheels and weighed some 310 pounds. It was capable of carrying a payload of around 44 pounds. Its equipment included a camera that could broadcast real-time video footage back down to Earth.
Yutu could also carry out basic analysis of the soil samples it took. It navigated the Moon’s surface using automatic sensors which stopped it bumping into obstacles. It was capable of climbing up modest slopes, and it had the capacity to travel a little over six miles. It ultimately proved able to survive a two-week lunar night when it awoke from sleep mode in January 2014.
However, Yutu did experience some technical glitches. Reports in January 2014 from the Chinese government’s press agency Xinhua said that a “mechanical control abnormality” had been triggered by the “complicated lunar surface environment.” In plain English, it seems the rover was having difficulty crossing the Moon’s rough terrain. Nevertheless, the vehicle continued to transmit data until March 2015.
So although they weren’t complete successes, Chang’e-3 and Yutu were, at the very least, proofs of concept. Chang’e-3, in particular, had succeeded in making the first soft landing on the near side of the Moon in nearly four decades. A landing on the far side of the Moon for Chang’e-4 and Yutu-2 were the next items on the agenda.
But Chang’e-4 developers had to address a problem with communications. Because Chang’e-4 would land on the far side of the Moon, it wouldn’t be able to make direct contact with mission control back on Earth. So, the answer was to launch a dedicated communications satellite which could then relay signals.
The satellite was intended to pick up signals from Chang’e-4 on the Moon’s surface and forward them to Earth. But in order to do this, it needed to be in a position between both locations. In May 2018 a relay satellite called Queqiao was sent into space on a rocket from the Xichang Satellite Launch Centre.
Like the Chang’e spaceships and the Yutu rovers, the relay satellite had a name taken from Chinese mythology. Queqiao, as it happens, means ‘magpie bridge.’ In a story from Chinese folklore, magpies flock together to make a bridge right across the Milky Way. This allows two lovers – the Weaver Girl and the Cowherd – to meet each year. Fittingly, Queqiao the satellite allowed Chinese mission control to speak to Chang’e-4.
After 24 days in space, Queqiao was in the right position to do its job – around 37,000 miles from the Moon and 280,000 miles from Earth. Meanwhile, engineers and technicians had been preparing the Chang’e-4 spacecraft. In fact, many of its components had already been manufactured as back-up parts for the Chang’e-3 mission.
Although certain adaptations were made to the design of Chang’e 4, it was ultimately very similar to its predecessor Chang’e-3. After all, the Chang’e-4’s mission was very much like that of Chang’e-3. It was to travel to the Moon and launch a rover – Yutu-2 this time around – onto the lunar surface.
But as we know, the two lunar missions had one very crucial difference. Where Chang’e-3 had landed on the near side of the Moon, Chang’e-4 was to land of the far side. And that’s what made the Chinese mission unique. No other spacecraft had ever made a soft landing on the dark side of the Moon.
Six months after the launch of the Queqiao relay satellite, Chang’e-4 and Yutu-2 were launched on December 7, 2018. At the start of January 2019, Chang’e-4 left its lunar orbit and headed for the the Moon’s surface. Mission control couldn’t directly control this operation, leaving Chang’e 4’s computers to complete the task alone.
Once it was a little over 9 miles from the Moon’s surface, Chang’e-4 fired rockets to decrease its speed. At an altitude of around 330 feet, the landing craft paused in its descent. Its sensors – including a camera and lasers – scouted the ground for a suitable landing spot. And then slowly, the craft completed its descent and landed gently.
Speaking to British newspaper The Guardian, Professor Andrew Coates of the University College London’s Mullard Space Science Laboratory recognized the outstanding achievement this landing represented. He said, “This is a great technological accomplishment, as it was out of sight of Earth, so signals are relayed back by their orbiter. And most of the landing was actually done autonomously in difficult terrain.”
As we saw before, Chang’e-4 landed in the South Pole-Aitken basin. This eight-mile-deep depression on the Moon’s surface is 1,600 miles across and is reckoned to be among the largest impact craters in our Solar System. It’s also thought to be the Moon’s oldest crater. Although most of this basin is invisible from Earth, the mountains on its outer edge – known as the Leibnitz range – can, in fact, be seen.
Within the South Pole-Aitken basin, the Chang’e lander had specifically come down in the Von Kármán crater. This is named for Theodore von Kármán, an American physicist and aerospace engineer of Hungarian descent. Fittingly, Von Kármán was PhD mentor to a Chinese scientist called Qian Xuesen. This man is widely credited as having been a key figure within China’s space scheme.
Once Chang’e-4 was on the Moon, personnel at Beijing mission control were again able to assume control of the spacecraft. They transmitted commands, which meant the lander’s antenna and solar panels were now extended. The next step was to launch Yutu-2 on its exploration and sampling mission.
Like the first Yutu which traveled to the Moon with Chang’e-3, Yutu-2 is a six-wheeled lunar buggy. The Chinese technicians had learned a great deal from the experience of the original Yutu. With improvements based on that knowledge, they were optimistic that this latest model would be operational for years.
Yutu-2 has a heater which is powered by decaying radioactive material. This allows it to maintain its temperature through the frigid lunar nights. Solar power provides the electricity to run the vehicle and its instruments. Yutu-2 is about five feet long, and its height and width are both just a little more than three feet. It weighs around 310 pounds.
The lunar rover carries a formidable array of scientific equipment. On-board instruments include a panoramic camera that can rotate all the way around. A piece of equipment called a visible and near-infrared imaging spectrometer can identify substances and gases. There’s also radar equipment that can delve to a depth of about 100 feet.
Once Chang’e 4 deployed its Yutu-2 launching ramp, the lunar rover was ready to start exploring and analyzing the far side of the Moon. In fact, its first sortie across the Von Kármán crater lasted only a few days. Then, the two-week lunar night started and the rover automatically powered down into its hibernation mode, where it stayed until the start of the next Moon day.
It was on lunar day eight of Yutu-2’s mission – on July 28, 2019 – that something weird was stumbled upon. This phenomenon was noticed by a researcher analyzing images from Yutu-2’s main camera. The researcher had seen an odd substance sitting at the bottom of a small crater just a little more than six feet across.
This discovery was enough to convince Beijing mission control to modify Yutu-2’s schedule. Instead of immediately traveling further across the Van Kármán crater, it would stick around this new find for a while to investigate further. So, just what had Yutu-2 stumbled across? The Chinese-language science publication Our Space gave a description on social media in October.
Our Space reported that the unknown substance that Yutu-2 had come across was “gel-like.” It apparently included bright spots, as well as coloring that was different to the more familiar Moon rubble scattered around it. One photograph showed that the substance had a green and a red area. However, some scientists suggested that these colors might be a photographic effect, rather than what appears in reality.
So, what was this substance? Clive Neal, a lunar specialist from the University of Notre Dame in Indiana, told Space.com that he had a pretty good idea what it was. He said that the pictures he’d studied showed a substance very similar to something that the Apollo 17 mission had found in 1972.
Neal said that material was most probably impact glass, a sample of which Apollo 17 had collected. This substance consists of broken pieces of various minerals that have been jammed together by the actions of a high-velocity meteor impact on the surface of the Moon. Neal told Space.com that finding such material in a crater like the one found by Yutu-2 “is to be expected.”