For centuries, humans have written books, songs, and poems about the so-called “dark side of the Moon”, but only today did a man-made object make physical contact with the rugged, unexplored area. The news, revealed in the early hours of Thursday, that China’s space agency successfully landed the unmanned Chang’e-4 probe on the far side of the Moon prompted the appropriate levels of excitement, with photographs of its arid landscape beamed over to Earth within minutes. Announcing the momentous step, Chinese state media said the landing “lifted the mysterious veil” from the far side of the Moon and “opened a new chapter in human lunar exploration”.
But for those of us without any expertise in space travel, the news raised as many questions as it answered. Haven’t we already landed on the Moon? What will this new mission teach us? And what exactly is the ‘far side of the Moon’, anyway?
Andrew Coates, professor of physics at UCL’s Space Science Laboratory, answers some common questions.
What is the ‘far side of the Moon’?
The ‘far side of the Moon’ (or “dark side of the Moon” as it is more commonly known, thanks in no small part to the 1973 Pink Floyd album) refers to the previously unexplored area of the Moon’s surface invisible from Earth. It’s all explained by ‘tidal locking’: the Moon moves in gravitational lockstep with the Earth, rotating at the same rate that it orbits our planet, which means only one section of the Moon is ever visible from Earth (the ‘Earth-facing side’). Over time, we can see slightly more than half of the Moon’s surface from Earth, says Prof Coates, due to small variations in the Moon’s orbit speed (a process known as ‘libration’).
What is new about the Chinese landing?
Since the Chinese probe touched down at 02:26 GMT, photographs of the area’s rugged landscape have been beamed across the world. But Thursday’s Chang’e-4 landing is not the first time we have seen the far side of the Moon. That happened on 7th October 1959 when, at the height of the Cold War’s Space Race, the Soviet probe Luna 3 broadcast a number of photographs of the previously unseen area, mapping one-third of its surface area. Subsequent probes were able to make more detailed maps of the area’s terrain. But Chang’e-4 is the first time a man-made device has successfully landed on the Moon’s far side. “This is the first time we’ve actually been able to touch down,” says Prof Coates. “That’s the distinguishing feature which which makes it quite exciting.”
What’s the point?
Politics aside, researchers hope that Chang’e-4 will make a number of important scientific findings. They are most interested in the composition of the South Pole-Aitken Basin, which at 2,500 kilometres wide and 13 kilometres deep is the oldest and deepest basin area on the Moon, and probably a remnant of a collision which occurred early in the Solar System’s history. “The hope is that looking at the composition of rocks … will give interesting information about the formation of the Moon and also the early history of the Solar System,” says Prof Coates. This will build on scientific strides made in the 1960s, when American probes found plains on the Moon’s Earth-facing side suggesting a history of volcanic eruptions, along with the famous ‘Sea of Tranquility’, which served as the landing site for the Apollo 11 mission in 1969. Chang’e-4 could also conduct “space weather experiments”, he says, attempting to track weather events that are heading towards Earth.
How will scientists take advantage of the far side’s radio silence?
Radio astronomy (using radio waves to explore the Solar System) is made famously difficult on Earth due to signal interference; something as simple as a microwave in next door’s kitchen can throw a spanner into the works, says Prof Coates. But this problem does not apply on the Moon’s far side, which is unaffected by the Earth’s radio signals. Now that Chang’e-4 has landed, he thinks, the far side may become a prime location for radio astronomy. “It’s relatively radio quiet, all you have to listen to is the rest of the cosmos, so it makes it an exciting target for that,” he says. But this does, of course, raise another question ...
How are scientists keeping in contact?
Out of reach of radio signals, Chang’e-4 can only communicate with scientists back on Earth by ‘bouncing’ signals off a separate satellite in orbit before beaming them back to Earth.
Is the American flag still there?
Painted with the Stars and Stripes, it was planted by Neil Armstrong and his comrades in 1969 at the iconic moment that man first walked on the Moon. And the flag is almost certainly still there, says Prof Coates. “As far as we know it will still be there. There’s no wind on the Moon … so it won’t have been blowing, but it will have been affected because there’s still a wind from the sun. It’s been sitting in that solar wind environment for almost 50 years. I imagine it will be almost as good as new.”
Is it all about politics?
Thursday’s Chang’e-4 landing excited scientists across the world, many of whom are optimistic about the progress it could bring in mankind's understanding of space. But as with all space exploration, politics is never far behind. Indeed, CNSA - the Chinese space agency - released very little news about Chang’e-4’s mission before Thursday’s official announcement that it had successfully touched down, in what looks like an attempt to play down the mission in case it failed. Fred Watson, the Australian government’s ‘astronomer-at-large’, has compared this secrecy to that shown by the Soviet Union in the early years of its competition with Nasa. He told the BBC: “The Chinese space agency is a young organisation, but perhaps in years to come it will catch up.”