The mission is the follow-up to Chang'e 3, the first Chinese landing on the Moon. The spacecraft was originally built as a backup for Chang'e 3 and became available after it landed successfully. The configuration of Chang'e 4 was adjusted to meet new scientific objectives. Like its predecessors, the spacecraft is named after Chang'e, the Chinese Moon goddess.
The Chinese Lunar Exploration Program is designed to be conducted in three phases of incremental technological advancement: The first is simply reaching lunar orbit, a task completed by Chang'e 1 in 2007 and Chang'e 2 in 2010. The second is landing and roving on the Moon, as Chang'e 3 did in 2013 and Chang'e 4 is planned to achieve. The third is collecting lunar samples from the near-side and sending 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. The Chinese Lunar Exploration Program has started to incorporate private investment from individuals and enterprises for the first time, a move aimed at accelerating aerospace innovation, cutting production costs, and promoting military-civilian relationships.
The Chang'e 4 mission was first scheduled for launch in 2015 as part of the second phase of the Chinese Lunar Exploration Program. But the adjusted objectives and design of the mission imposed delays, and finally launched on 7 December 2018, 18:23 UTC. The spacecraft entered lunar orbit on 12 December 2018, 08:45 UTC. Landing is scheduled for 3 January 2019.
This mission will attempt to determine the age and composition of an unexplored region of the Moon, as well as develop technologies required for the later stages of the program.
An ancient collision event on the Moon left behind a very large crater called Aitken Basin that is now about 13 km (8.1 mi) deep, and it is thought that the massive impactor punched through the Moon's crust, spewing some of its mantle to the surface nearby. 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 science 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
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.
Lagrangian points in a two-body system. A satellite in a halo orbit around L2, which is behind the Moon, will have a view of both the Earth and the far side of the Moon.
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 side and the Earth. On 20 May 2018, CNSA launched the Queqiao (Chinese: 鹊桥; pinyin: Quèqiáo, meaning "Magpie Bridge") relay satellite to a halo orbit around the Earth–Moon L2 point. The relay satellite has a mass of 425 kg, and it uses a 4.2-meter antenna to receive X band signals from the lander and rover, and relay them to Earth control on the S band.
The spacecraft took 24 days to reach L2, using a lunar swing-by to save fuel. On 14 June 2018, Queqiao finished its final adjustment burn and entered the L2 halo mission orbit, which is about 65,000 kilometers from the Moon. This is the first lunar relay satellite at this location.
As part of the Chang'e 4 mission, two microsatellites (45 kg each) named Longjiang-1 and Longjiang-2 (Chinese for "Dragon River"), were launched along with Queqiao in May 2018. However, Longjiang-1 failed to enter lunar orbit, while Longjiang-2 succeeded and is currently operational in lunar orbit. These microsatellites were tasked to observe the sky at very low frequencies (1 MHz-30 MHz), corresponding to wavelengths of 300 m to 10 m, with the aim of studying energetic phenomena from celestial sources. Due to the Earth's ionosphere, no observations in this frequency range has been done in Earth orbit, offering potential breakthrough science.
Lander and rover
As is the case with many of China's space missions, the details of the spacecraft and the mission in general have been limited. What is known is that much of the Chang'e 4 lander and rover design is modeled after Chang'e-3 and its Yutu rover. In fact, Chang'e 4 was built as a backup to Chang'e 3, but has been adapted to the specifics of the new mission. The lander and rover were launched six months after the launch of the Queqiao relay satellite, on 7 December 2018, 18:23 UTC.
The total landing mass is 1,200 kg. Being a backup copy of Chang'e 3, the stationary lander is also equipped with a radioisotope heater unit (RHU) in order to heat its subsystems during the long lunar nights and power its operations, along with some solar panels, during its planned one-year mission.
After landing, the lander will deploy a ramp to deploy the rover to the lunar surface. The rover measures 1.5 × 1.0 × 1.0 m and has a mass of 140 kg. The rover was fabricated at Dongguan, Guangdong province, is solar powered, and it is propelled by six wheels. The rover's nominal operating time is three months.
The communications relay satellite, orbiting microsatellites, lander and rover, each carry scientific payloads. The relay satellite will perform radio astronomy, whereas the lander and rover will study the geophysics of the landing zone. These payloads are, in part, supplied by international partners in Sweden, Germany, the Netherlands and Saudi Arabia.
The primary function of the Queqiao relay satellite, which is already deployed in a halo orbit around the Earth-Moon L2 point, is to provide continuous relay communications between Earth and the lander on the far side of the Moon. Additionally, this satellite hosts the Netherlands-China Low-Frequency Explorer (NCLE), an instrument which performs astrophysical studies in the unexplored radio regime of 80 kHz to 80 MHz.
The lander and rover carry scientific payloads to study the geophysics of the landing zone, with a very limited chemical analysis capability.
The lander is equipped with the following payloads:
Silkworm larvae will be grown in a container on the Moon
Landing Camera (LCAM)
Terrain Camera (TCAM)
Low Frequency Spectrometer (LFS)  to research solar bursts.
Lunar Lander Neutrons and Dosimetry (LND), a neutron dosimeter developed by Kiel University in Germany. It will gather radiation dosimetry for future human exploration of the Moon, and will contribute to solar wind studies.
The lander also carries a 3 kg sealed container with seeds and insect eggs to test whether plants and insects could hatch and grow together in synergy. The experiment includes seeds of potatoes and Arabidopsis thaliana, as well as silkworm eggs. If the eggs hatch, the larvae would produce carbon dioxide, while the germinated plants would release oxygen through photosynthesis. It is hoped that together, the plants and silkworms can establish a simple synergy within the container. A miniature camera will image any growth. In 1982, the crew of the SovietSalyut 7 space station grew some Arabidopsis, thus becoming the first plants to flower and produce seeds in space. They had a life span of 40 days. (See: Plants in space)
Regarding its composition, it contains about 10% by weight iron oxide (FeO) and 4-5 parts per million of thorium, which has been used as a replacement for uranium as nuclear fuel on several thorium reactors. The landing date is in "early 2019", likely to happen in January 2019.
Launches are separated by dashes ( – ), payloads by dots ( · ), multiple names for the same satellite by slashes ( / ). CubeSats are smaller. Manned flights are bolded. Launch failures are in italics. Payloads deployed from other spacecraft are (enclosed in brackets).