THE MOON is familiar. Amateur star-gazers can examine it with a telescope, and humans have stood on it several times since the 1960s. But later this year China will carry out a mission that could dramatically increase human understanding of our astronomical neighbour. Scientists will learn vastly more about the Moon’s formation, and perhaps about the way in which the Earth and even other planetary masses came together billions of years ago. This mission, Chang’e-4, has such potential because it intends to drop a lander and a rover for the first time ever on the Moon’s far side. Why can so much be learned here? The Moon’s orbit is tidally locked to the Earth, which means that it rotates once as it circles the planet. As a result, Earth-bound observers always see the Moon’s same face, and observers on the Moon watch the Earth spin in a fixed position above them. While slight eccentricities in the Moon’s position relative to the Earth mean that slightly more than half its surface can be seen during the course of a year, most of the far side is never visible. In 1959 the Soviet Union’s Luna 3 probe made the first circumnavigation of the Moon and beamed back the first pictures of its far side. This is how Earth learned of one of the largest impact craters on any body in the solar system. The South Pole–Aitken basin is 2,500km across and 13km deep. Dozens of spacecraft have observed the near side of the Moon and six have landed people on it. The far side has been mapped, and multiple Apollo crews have seen the surface on fly-bys. In 2011 NASA’s Lunar Reconnaissance Orbiter sent back the most detailed images to date. But all observations have been from a distance. No craft has landed. What is known of the far side is intriguing. The near side is covered with maria, or ancient lava flows; the far side has almost none. The concentrations of chemical compounds on the two sides are noticeably different. The near side’s lava flows cover much of the oldest ground, whereas the far side’s relatively untouched face has experienced billions of years of solar wind and solar exposure. Brad Tucker, an astrophysicist and cosmologist at the Australian National University, says this means missions can uncover “unique material that would be a direct snapshot of the beginnings of the Moon.” One of the most popular theories about the Moon’s formation is that it is debris from a collision between the Earth and a proto-planet billions of years ago; another suggests that multiple orbiting moons or a debris ring around Earth coalesced. More data from the far side will help refine or refute these theories. Dr Tucker also notes that the far side has more helium-3, a substance that is both relatively rare on Earth and excellent for rocket fuel. The difficulty in exploring the far side stems from its position: the Moon’s bulk blocks all radio signals. China earlier this year launched a satellite that now sits at the L2 Lagrange point, a locus of comparative gravitational stability where the satellite can, with some maintenance, keep a nearly fixed position relative to the Earth and the Moon. This relay will allow live remote control and data reception from the lander and rover. Better understanding of the far side will affect governments’ and private plans for putting people back on the Moon, for establishing stations permanently orbiting the Moon and, one day perhaps, for developing the Moon into a refuelling station for missions to Mars and beyond. “The Moon has been shown to be a giant gas station for the Solar System,” says Dr Tucker. Far-side missions might reveal where the pumps are.
