|Mission type||Lander, lunar rover|
|Mission duration||Lander: 12 months|
Rover: 3 months
|Launch mass||Lander: 1,200 kg|
Rover: 140 kg
|Landing mass||Total: ~1,200 kg; rover: 140 kg|
|Dimensions||Rover: 1.5 × 1.0 × 1.0 m|
|Start of mission|
|Launch date||Queqiao relay satellite: 20 May 2018|
Lander and rover: 7 December 2018, 18:23 UTC
|Rocket||Long March 3B|
|Launch site||Xichang Satellite Launch Center|
|Landing date||Lander and rover: 3 January 2019, 2:26 UTC|
|Landing site||Von Kármán crater in the South Pole-Aitken Basin|
Chang'e 4 (//; Chinese: 嫦娥四号; pinyin: Cháng'é Sìhào; literally: 'Chang'e No. 4') is a Chinese lunar exploration mission that achieved the first soft landing on the far side of the Moon, on 3 January 2019. A communication relay satellite, Queqiao, was first launched to a halo orbit near the Earth-Moon L2 point in May 2018. The robotic lander and Yutu 2 (Chinese: 玉兔二号; literally :"Jade Rabbit No. 2") rover were launched on 7 December 2018 and entered orbit around the Moon on 12 December 2018.
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 Chang'e 3 landed successfully in 2013. The configuration of Chang'e 4 was adjusted to meet new scientific objectives. Like its predecessors, the mission 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 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. 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. The orbit's perilune was lowered to 15 km (9.3 mi) on 30 December 2018, 00:55 UTC. Landing took place on 3 January 2019 at 02:26 UTC, shortly after lunar sunrise over the crater Von Kármán.
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 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:
Direct communication with Earth is impossible on the far side of the Moon, since transmissions are blocked by the Moon. 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 (Chinese: 鹊桥; pinyin: Quèqiáo; literally: 'Magpie Bridge') relay satellite to a halo orbit around the Earth–Moon L2 point. The relay satellite is based on the Chang'e 2 design, has a mass of 425 kg (937 lb), and it uses a 4.2 m (14 ft) 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 kilometres (40,000 mi) 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 or 99 lb each) named Longjiang-1 and Longjiang-2 (Chinese: 龙江; pinyin: Lóng Jiāng; literally: 'Dragon River'), were launched along with Queqiao in May 2018. Longjiang-1 failed to enter lunar orbit, but Longjiang-2 succeeded and is currently operational in lunar orbit. These microsatellites were tasked to observe the sky at very low frequencies (1–30 MegaHertz), corresponding to wavelengths of 300 to 10 metres (984 to 33 ft), with the aim of studying energetic phenomena from celestial sources. Due to the Earth's ionosphere, no observations in this frequency range have been done in Earth orbit, offering potential breakthrough science.
As is the case with many of China's space missions, the details of the spacecraft and the mission 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, and based on the experience and results from that mission, Chang'e 4 was adapted to the specifics of the new mission. The lander and rover were launched on 7 December 2018, 18:23 UTC, six months after the launch of the Queqiao relay satellite.
The total landing mass is 1,200 kg (2,600 lb). Both the stationary lander and Yutu-2 rover are equipped with a radioisotope heater unit (RHU) in order to heat their subsystems during the long lunar nights, while electrical power is generated by solar panels. After landing, the lander extended a ramp to deploy the Yutu-2 rover (literally: "Jade Rabbit") to the lunar surface. The rover measures 1.5 × 1.0 × 1.0 m (4.9 × 3.3 × 3.3 ft) and has a mass of 140 kg (310 lb). Yutu-2 rover was fabricated at Dongguan, Guangdong province; it is solar-powered, RHU-heated, and it is propelled by six wheels. The rover's nominal operating time is three months, but after the experience with Yutu rover in 2013, the rover design was improved and Chinese engineers are hopeful it will operate for "a few years."
A few days after landing, Yutu-2 went into hibernation for its first lunar night and it resumed activities on January 29, 2019 with all instruments operating nominally. During its first full lunar day, the rover travelled 120 m (390 ft), and on 11 February 2019 it powered down for its second lunar night. The Yutu-2 rover and lander will resume operations on February 28 and March 1, respectively.
The communications relay satellite, orbiting microsatellite, lander and rover each carry scientific payloads. The relay satellite is performing radio astronomy, whereas the lander and Yutu-2 rover will study the geophysics of the landing zone. The science payloads are, in part, supplied by international partners in Sweden, Germany, the Netherlands, and Saudi Arabia.
The primary function of the Queqiao relay satellite that is 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 performing astrophysical studies in the unexplored radio regime of 80 kilohertz to 80 megahertz. It was developed by the Radboud University in Netherlands and the Chinese Academy of Sciences. The NCLE on the orbiter and the LFS on the lander will work in synergy performing low-frequency (0.1 MHz–80 MHz) radio astronomical observations.
The lander and rover carry scientific payloads to study the geophysics of the landing zone, with a modest chemical analysis capability. The lander is equipped with the following payloads:
The landing site is within a crater called Von Kármán (180 km or 110 mi diameter) in the South Pole-Aitken Basin on the far side of the Moon that was still unexplored by landers. The site has symbolic as well as scientific value. Theodore von Kármán was the PhD advisor of Qian Xuesen, the founder of the Chinese space program.
Chang'e 4 marks the first major US-China collaboration in space exploration since 2011 Congressional ban. Scientists from both countries had regular contact prior to the landing. This included talks about observing plumes and particles lofted from the lunar surface by the probe's rocket exhaust during the landing to compare the results with theoretical predictions, but NASA's Lunar Reconnaissance Orbiter (LRO) was not in the right position for this during the landing. The US also informed Chinese scientists about its satellites in orbit around the moon, while China shared with the US scientists the longitude, latitude, and timing of Chang'e 4's landing.
China has agreed to a request from NASA to use the Chang'e 4 probe and Queqiao relay satellite in future US moon missions.
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