CHANG’E 4 MISSION TO THE DARK SIDE
The far side of the Moon is the hemisphere of the Moon that always faces away from Earth. The far side’s terrain is rugged with a multitude of impact craters and relatively few flat lunar maria.
It has one of the largest craters in the Solar System, the South Pole–Aitken basin. Both sides of the Moon experience two weeks of sunlight followed by two weeks of night; the far side is sometimes called the “dark side of the Moon,” meaning unseen rather than lacking light.
Until the late 1950s, little was known about the far side of the Moon. Librations of the Moon periodically allowed limited glimpses of features near the lunar limb on the far side. These features, however, were seen from a low angle, hindering useful observation. (It proved difficult to distinguish a crater from a mountain range.) The remaining 82% of the surface on the far side remained unknown, and its properties were subject to much speculation.
Before space exploration began, astronomers did not expect that the far side would be different from the side visible to Earth. On October 7, 1959, the Soviet probe Luna 3 took the first photographs of the lunar far side, covering one-third of the surface invisible from the Earth. The images were analysed, and the first atlas of the far side of the Moon was published by the USSR Academy of Sciences on November 6, 1960.
On April 26, 1962, NASA’s Ranger 4 space probe became the first spacecraft to impact the far side of the Moon, although it failed to return any scientific data before impact.
The first truly comprehensive and detailed mapping survey of the far side was undertaken by the American unmanned Lunar Orbiter program launched by NASA from 1966 to 1967. Most of the coverage of the far side was provided by the final probe in the series, Lunar Orbiter 5.
The far side was first seen directly by human eyes during the Apollo 8 mission in 1968.
The China National Space Administration’s Chang’e 4 made the first soft landing on the lunar far side on 3 January 2019. The craft includes a lander equipped with a low-frequency radio spectrograph and geological research tools.
CHINESE LUNAR EXPLORATION PROGRAM
The Chinese Lunar Exploration Program is designed to be conducted in three phases of incremental technological advancement:
- The first is to reach lunar orbit, a task completed by Chang’e 1 in 2007 and Chang’e 2 in 2010;
- The second is to land and rove on the Moon, as Chang’e 3 did in 2013 and Chang’e 4 did in January 2019;
- The third is to collect lunar samples from the near-side and send them to Earth, a task for the future Chang’e 5 and Chang’e 6 missions.
The program aims to facilitate a crewed lunar landing in the 2030s and possibly build an outpost near the south pole.
An ancient collision event on the Moon left behind a very large crater, called the Aitken Basin, that is now about 13 km (8.1 mi) deep, and it is thought that the massive impactor likely exposed the deep lunar crust, and probably the mantle materials. If Chang’e 4 can find and study some of this material, it would get an unprecedented view into the Moon’s internal structure and origins.
The specific scientific objectives are:
- To measure lunar surface temperature over the duration of the mission
- Measure the chemical compositions of lunar rocks and soils
- Carry out low-frequency radio astronomical observation and research using a radio telescope
- Study of cosmic rays
- Observe the solar corona, investigate its radiation characteristics and mechanism, and to explore the evolution and transport of coronal mass ejections (CME) between the Sun and Earth.
Direct communications with Earth are impossible on the far side of the Moon, since transmissions are blocked by the Moon itself. Communications must go through a communications relay satellite, which is placed at a location that has a clear view of both the landing site and the Earth.
On 20 May 2018, the China National Space Administration (CNSA) launched the Queqiao relay satellite to a halo orbit around the Earth–Moon L2 point.
The robotic probe Chang’e 4 landed in the unexplored South Pole-Aitken basin, the biggest known impact structure in the solar system.
A significant challenge faced by the Chinese team was the inability to communicate directly with the spacecraft. Signals to and from the rover are being relayed through a satellite called Queqiao (Magpie Bridge). Queqiao is in a “halo orbit” on the other side of the moon, from where it can communicate with both Chang’e and the Earth.
At 100 metres above the lunar surface, the probe briefly hovered to identify obstacles and measured the slopes on the surface. Avoiding any boulders or ditches, it selected a relatively flat area and resumed a slow, vertical descent, touching down softly in the Von Karman Crater within the South Pole-Aitken Basin, according to a report by the Xinhua news agency.
During the final phases of the approach, however, Chang’e 4 was on its own and could not be operated remotely. Starting from an altitude of 15km, the craft used a rocket booster to decelerate and a high-tech camera and laser measurements to avoid boulders and ditches.
Chang’e 4 targeted the Von Kármán crater, which was predicted to have a smooth volcanic floor and which sits within the great Aitken basin.
Instruments onboard the Chang’e lander and rover will aim to study the local lunar geology, probe the moon’s interior, and analyse the solar wind – a stream of high-energy particles that flow from the sun. Onboard experiments will also test how well plants grow in the weak lunar gravity.
The rover – named Yutu 2, or Jade Rabbit 2 – left the spacecraft, drove off a ramp and began making tracks on the moon’s surface at 10.22pm on Thursday, about 12 hours after Chang’e 4 landed.
Lunar project chief designer Wu Weiren called the separation of the rover “a small step for the rover, but one giant leap for the Chinese nation.”
China’s Chang’e 4 mission could use soil tests and temperature measurements to reveal new clues to the cataclysmic collision that created the moon and uncover the origins of the water that is unexpectedly abundant in lunar soil.