Where Did Chang'e 4 Go?
2019 is off to a strong start in the space community. NASA flew by the farthest object ever visited, and now the China National Space Administration (CNSA) has successfully landed Chang’e 4 on the far side of the Moon. This landing is the first time a spacecraft has ever soft-landed on the far side of the Moon. But what exactly is the far side of the Moon and what do they expect to find there? In short, unique terrain that can answer fundamental questions about lunar and Solar System evolution.
The far side of the Moon covers the half of the Moon that never faces Earth. While the Moon does rotate around its own axis, it is tidally locked to Earth. This means that the same side of the Moon always faces Earth (and the same side always faces away from Earth). The Moon has a rotational period and orbital period around the Earth of about 27.3 days (the sidereal month). If the rotation period and orbit period are the same it is called synchronous rotation. Having a synchronous rotation around Earth causes the Moon to have a day/night cycle.
Since the far side of the Moon always faces away from Earth, it has a environment that is different from the near side. Visually, the far side has a more cratered appearance. This is due to Earth protecting the near side from most asteroids, while exposing the far side to many more impacts. Without a thick enough atmosphere, asteroids of all sizes impact the Moon's surface. This creates a fractal like effect, where no matter how close you get to the lunar surface, you continue to find smaller craters and impacts.
No radio transmissions from Earth’s surface can reach the far side due to the Moon blocking them. This means the radio environment on the far side is pristine and only contains radio signals from non-human sources. Such an environment is ideal for radio astronomy where radio noise reduces the sensitivity of measurements made. Measuring the "Cosmic Dawn" of the early universe and magnetic structures of exoplanets both need very high sensitivity observations.
The far side also receives no reflected light from Earth (planetshine), yet constant radiation from the universe. Similarly, there is also more direct exposure to the sun on the far side, increasing the amount of Helium-3 present in the surface. The difference in photons and radiation that hit the far side's surface will make the surface composition differ from the near side. The Apollo missions brought back lunar surface samples from the near side, but scientist have not had direct access to the far side's surface. Chang'e 4 is our first chance to study this area in more detail.
Chang'e 4 landed in Von Kármán crater, a 180 km (112 mi) wide crater that is within the South Pole-Aitken (SPA) Basin. SPA is one of the largest impact feature in the Solar System, stretching about 2500 km (1550 mi) across and 13 km (8 mi) deep. It is located between the South Pole and Aitken crater on the lunar far side. The SPA region is a high priority location for study because of the diversity of features it contains, including excavated pre-impact material, material melted in the impact, and basalt flows [Jawin et al., 2018]. The actual SPA impact event likely exposed material from the lower lunar crust and mantle [Petro et al., 2011]. Studying this material in-situ and via a sample return would answer fundamental questions about Solar System evolution.
Von Kármán crater is roughly circular in shape, with a central peak indicating where the crater initially formed. The northern third of the crater is rough and cratered, with the southern portion filled with mare basalt flows. Chang'e 4 landed on mare basalt in the southern portion of the crater. Although some images of this area look smooth, there are still significant meter wide craters and pits.
Key science goals for Von Kármán crater include [Snape et al., 2010]: (1) constraining the lunar bombardment history, (2) studying the lunar interior, (3) studying the distribution of surface rocks (including thorium and iron-rich rocks), (4) quantifying the far side mare basalt, and (5) studying potential differentiation in the material. Chang'e 4's spectrometers, camera, and ground penetrating radars will explore some of these goals.
The science that comes out of the Chang'e 4 mission promises to change our understanding of the Moon and Solar System. The far side of the Moon is an ideal location for revolutionary science and resource extraction. The CNSA Chang'e 4 mission is spearheading human kind's first foray into this unknown environment.
February 2, 2019 - We initially stated that the Moon does not rotate about its own axis, however, this is incorrect. The Moon does rotate about its axis during its orbit around Earth. We sincerely apologize for the mistake.
Petro, N. E., S. C. Mest and Y. Teich (2011), Geomorphic terrains and evidence for ancient volcanism within northeastern South Pole-Aitken basin, in Ambrose, W.A., and Williams, D.A., eds., Recent Advances and Current Research Issues in Lunar Stratigraphy: Geological Society of America Special Paper 477, 129-140.
Snape, J. F., et al. "Science-rich Mission Sites Within South Pole-Aitken Basin, Part 2: Von Kármán Crater." Lunar and Planetary Science Conference. Vol. 41. 2010.