By Shannon Hall
In a spaceflight first, China’s Chang’e-4 has landed where no spacecraft has touched down in one piece before: the far side of the moon. “This is a historic step in international scientific exploration of the moon, opening up the ‘Luna Incognita’ of the lunar far side to surface exploration for the first time,” said James Head, a planetary scientist at Brown University.
If successful, the mission could answer fundamental questions about Earth’s only natural satellite. There are still mysteries, for example, about the moon’s formation and early evolution, which, in turn, hold clues to the history of the solar system.
Additionally, the mission will conduct the first radio astronomy experiments from the moon’s far side and the first investigations to see whether plants can grow on the moon — a crucial step toward long-term human missions beyond Earth.
The moon has two sides
If you were to watch the moon over the course of a month, the “man in the moon” would never truly disappear. Despite the moon’s phases, he’s always there, keeping a watchful eye over you from his heavenly perch. That’s because the moon is tidally locked with Earth. It rotates exactly once every time it circles our planet, thus keeping the same hemisphere pointing toward Earth at all times.
Astronomers refer to the side we always see from Earth as the “near side” and the side we can never see as the “far side.”
It’s not really the “dark side”
While the far side can never be seen from Earth, it is still illuminated by the sun and has the same phases as the near side. There is no permanently “dark side” of the moon, although it has been described this way in popular culture to refer to the moon’s unknown side.
A flood that never came
When the Soviet spacecraft, Luna 3, sent back the first images of the far side of the moon in 1959, it revealed a world that looked vastly different from the one we see.
The face of the “man in the moon” on the near side is so noticeable because it’s composed of dark areas, which stand out against the light lunar soil. Those dark areas formed when ancient asteroids struck the moon’s surface, unleashing lava that darkened the facade and smoothed it, erasing the records from previous impacts.
But when ancient asteroids struck the far side of the moon, there were no floods of lava. The impacts simply left a surface pockmarked with craters. That makes the far side much lighter, much older and much more heavily cratered than the near side.
Some astronomers suspect the dichotomy arises because the crust on the near side is much thinner than that of the far side. That would make it easier for magma to emerge on the near side, explained Briony Horgan, a planetary scientist at Purdue University. But why the crust’s thickness would vary so dramatically from one hemisphere to the next remains a mystery. And Horgan is hopeful that Chang’e-4 will provide hints to the answer.
A treasure trove that’s always out of view
The early history of the solar system was violent. That’s when large objects — asteroids or comets — pounded the rocky planets and left craters, some that are more than 600 miles across.
But most evidence of this tumultuous past has disappeared, erased by time. On Earth and other rocky worlds, volcanoes have washed away these craters over billions of years.
The far side of the moon has retained a pristine record of its youth, particularly the number of times that ancient objects pummeled its surface.
“The history of the very early solar system is locked up in the rocks of the far side,” Horgan said.
Land on the far side, see the universe anew
Earth is loud. Any radio antenna will reveal a symphony of strange noises that emanate from cellphones, TV stations, power lines, electrical fences, distant lightning, GPS satellites, cars, Wi-Fi and, well, radios, to name a few. But the moon shields the far side from much of that chatter, thus creating the ideal location to study the radio universe.
That’s why astronomers have long dreamed of building a radio telescope on the far side of the moon.
“We use radio wavelengths to probe everything from black holes in the local neighborhood to distant galaxies, so a radio observatory on the far side could be a huge boon to astronomy,” Horgan said. “The Chang’e-4 mission will be the first mission to test out this theory and to see just how much better the lunar far side is for radio observations than our observatories back on Earth.”
A future lunar observatory — if it works as intended — could improve the study of phenomena like the primordial clouds of hydrogen gas that coalesced into the universe’s first stars.
“We would be able to listen to a distant echo of the Big Bang and see the universe in a state before the first-ever stars formed,” said Heino Falcke, a radio astronomer at Radboud University Nijmegen in the Netherlands.
Such a signal would allow astronomers to probe the early development of the universe. Falcke pointed to the famous cosmic heat map of the universe as it appeared only 370,000 years after the Big Bang, also known as the cosmic microwave background. “That was a baby picture,” he said, but from the far side of the moon it might be possible to “see a movie of the evolution of the young universe from toddler to puberty.”
Seeing what else might grow on the far side
Chang’e-4 is also carrying an onboard experiment to test how well plants — specifically potatoes and small flowering Arabidopsis plants — grow on the moon, which could contribute to China’s human spaceflight goals. It is the first mini-greenhouse to land on another world in the solar system.
Anna-Lisa Paul, a horticultural scientist at the University of Florida in Gainesville, said that the tiny experiment “is a step forward in preparing people to return to the moon for longer than a brief visit.”