This weekend, China embarks on a historic mission to land on the far side of the Moon - The Verge

Early Saturday morning in China, a rocket will launch, carrying a lander and a rover bound for the Moon. It will mark the beginning of China’s ambitious lunar mission known as Chang’e-4, which will attempt to land spacecraft on the Moon’s far side — the region that always faces away from Earth. No other nation has ever attempted such a feat — which means the mission could catapult China into spaceflight history.

So far, China is among an elite group of three countries that have landed a spacecraft softly on the surface of the Moon. Apart from America’s notable Apollo missions, the former Soviet Union also landed robotic spacecraft on the lunar surface, with the last mission occurring in 1976. In 2013, China entered the fray, putting a lander and a rover on the Moon. That mission, known as Chang’e-3, was part of a decades-long campaign that China devised to study the Moon with robotic spacecraft. Prior to Chang’e-3, the country had put a spacecraft in lunar orbit and had also crashed a vehicle into the lunar dirt. Now, the next step is to visit a part of the Moon that’s never been fully explored.

It’s a significant step because landing on the far side of the Moon is an incredibly challenging task. The Moon is tidally locked with Earth, meaning it rotates around its axis at about the same time it takes to complete one full orbit around our planet. The result: we only see one half of the Moon at all times. This near side of the Moon is the only region that we’ve landed on gently, because there’s a direct line of sight with Earth, enabling easier communication with ground control. To land on the far side of the Moon, you must have multiple spacecraft working in tandem. In addition to the lander itself, you need some kind of probe near the Moon that can relay communications from your lander to Earth.

And that’s exactly what China has. In May, the China National Space Administration launched a satellite called Queqiao, specifically for the purpose of aiding with communications for the upcoming Chang’e-4 mission. After about a month in space, Queqiao settled into a spot facing the far side of the Moon, more than 37,000 miles away from the lunar surface. The satellite is doing circles around a point in space known as the second Earth-Moon Lagrange point. It’s a place akin to a parking spot for spacecraft. At a Lagrange point, the gravitational forces of two bodies (stars, planets, etc) equal out in such a way that a spacecraft stays put in relation to the two entities. At this particular Lagrange point, Queqiao will stay facing the far side of the Moon, allowing communication between the spacecraft and Earth using a large curved antenna.

“Demonstrating that you can communicate and perform roving on the lunar far side using a relay satellite is going to be quite a technological feat, and it’s going to bring a lot of prestige,” Andrew Jones, a freelance journalist covering China’s spaceflight program, tells The Verge.

If it all works, China will be getting an up-close view of one of the most tantalizing areas of the lunar surface: the South Pole-Aitken basin. It’s believed that the Chang’e-4 lander and rover will touch down in the Von Kármán crater inside this region, according to Jones, though the exact landing site hasn’t been confirmed. The South Pole-Aitken basin is a large impact crater on the far side of the Moon that’s roughly 1,550 miles in diameter and 7.5 miles deep. It’s thought to be one of the oldest impact sites on the lunar surface, but we don’t know exactly how old it is — and its true age could tell scientists a lot about the early Solar System.

Most of the craters on the Moon are thought to have formed around 3.9 billion years ago, based on analysis of the lunar rocks collected during NASA’s Apollo missions. Many scientists think these holes occurred during a period of the Solar System known as the Late Heavy Bombardment — a period when a huge number of asteroids smacked into the inner planets. It’s thought this time occurred after most of the planets in our cosmic neighborhood had formed, which is why it’s considered “late” in our Solar System’s development. If the South Pole-Aitken basin is also 3.9 billion years old, it supports the idea that this bombardment happened. If it’s much older than that, it puts a dent in that theory. “This actually helps us understand not just about the Moon, but the whole Solar System,” Clive Neal, an engineering professor at the University of Notre Dame and emeritus chair of the Lunar Exploration Analysis Group, or LEAG, tells The Verge. “That’s why it’s important; it’s bigger than the Moon.”

Because of its potential to tell us about our history, the South Pole-Aitken basin has long been a priority target of study. Scientists have proposed sending spacecraft to this region in order to collect samples and return them to Earth for in-depth analysis. “The South Pole-Aitken basin is exceedingly important, and we still haven’t done it because it’s too difficult,” says Neal.

Unfortunately, Chang’e-4 won’t be returning anything to Earth, so it probably won’t be able to tell us the exact age of the basin. But it should learn a few interesting tidbits. The Chang’e-4 rover will be carrying ground-penetrating radar to figure out what the structure of the Moon is like underneath the surface of the basin, which could tell us more about how this area formed. It will also have an instrument designed to figure out what the surface is made of in this region. And it’s carrying a Swedish instrument designed to figure out how particles streaming from the Sun interact with the lunar rocks.

Meanwhile, the lander, which is tasked with carrying the rover to the Moon’s surface, will also be doing science from its landing spot, taking advantage of its location on the Moon. Since these vehicles will be on the Moon’s far side, they’ll be shielded from much of the electromagnetic interference from Earth and don’t have to deal with our planet’s atmosphere. The lander will be studying the space environment and the Universe in low frequencies — something we can’t do from our planet.

And of course, both the lander and the rover will carry cameras to take detailed images of the lunar surface, just as Chang’e-4’s predecessor, Chang’e-3, did. Much of the Chang’e-4 design is modeled after Chang’e-3, which landed on the nearside of the Moon and told scientists a great deal about an area known as the Imbrium basin. Hopefully, Chang’e-4’s rover will move farther than the rover on Chang’e-3, called Yutu, which stopped being able to travel after about a month.

While it’s definitely unique, Chang’e-4 is just one step in the ladder of China’s decade-long Chang’e mission plan (Chang’e is a goddess of the Moon in Chinese mythology). Following this mission, China plans to launch another robotic mission to the Moon next year called Chang’e-5, which is designed to return samples from the nearside of the Moon. If successful, it’ll be the first time lunar material has been brought back to Earth since 1976. Beyond that, Neal thinks that a sample return from the far side of the Moon is on the horizon. “Chang’e-4 is a first step, and I’m sure it will raise more questions than it answers,” says Neal. “But showing the capability is there to land on the far side and rove, that tells us what’s the next step, and, as I say, robotic sample return would be the logical next step.”

In the more distant future, it’s possible that China hopes to put people on the Moon, though it hasn’t been open about those plans. Jones says that it looks like China is working toward crewed flight, by developing a new huge launch vehicle and concepts for a rocket that can carry people. “There’s no officially government-approved plan to put Chinese astronauts on the moon, however you can see that they are working on the various components that you need,” he says.

Any human missions are still years away, and for now China is focused on Chang’e-4. But as is the case with many of China’s missions, the details surrounding this flight have been hard to come by. We know that the mission is set to launch on top of one of China’s Long March 3B rockets from the country’s Xichang Satellite Launch Center. And thanks to air closure notices, takeoff time is estimated to occur around 1:30PM ET on Friday, December 7th. China may only announce that the mission was a success after the spacecraft is on its way to the Moon, though Jones says we might hear earlier than that from other sources.

“It might be that the first indication we have of launch is that some poor soul near Xichang launch center is woken up thinking there’s an earthquake and complaining about it on social media.” Jones says.

If Chang’e-4 does make it to space, it will spend less than a month traveling to the Moon, likely touching down sometime in the first week of January. If that happens, China will have officially moved into its own elite group, as the only country to visit the side of the Moon we cannot see from Earth.

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https://www.theverge.com/2018/12/6/18127180/china-national-space-administration-change-4-mission-lander-rover-far-side

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