''Queqiao'' relay satellite (), also known as the Chang'e 4 Relay, is a communications relay and radio astronomy satellite for the Chang'e 4 lunar farside mission. As part of the Chinese Lunar Exploration Program, the

China National Space Administration China National Space Administration (CNSA; ) is the government agency of the People's Republic of China that is responsible for civil space administration and international space cooperation, including organizing or leading foreign exchanges ...

(CNSA) launched the Queqiao relay satellite on 20 May 2018 to a

halo orbit A halo orbit is a periodic, three-dimensional orbit near one of the L1, L2 or L3 Lagrange points in the three-body problem of orbital mechanics. Although a Lagrange point is just a point in empty space, its peculiar characteristic is that it ca ...

around the Earth–Moon L₂ Lagrangian point Queqiao is the first ever communication relay and

radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming f ...

satellite at this location. The name ''Queqiao'' ("Magpie Bridge") was inspired by and came from the Chinese tale '' The Cowherd and the Weaver Girl''.

Design and development

Queqiao was designed to function as a communication relay for the Chang'e 4 mission to the far side of the Moon, as well as a deep space radio astronomy observatory for the Chinese space program. 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. A circular orbit, while easy to achieve, would periodically carry the satellite out of sight of either the lander or the Earth. A constellation of multiple satellites can solve this problem at the cost of greater expense and risk. With this in mind, placing a satellite in orbit not around the Moon itself, but around an equilibrium point of the Earth-Moon system on the far side of the Moon () becomes an attractive option. The types of orbits near the equilibrium points, include Lyapunov orbits,

Lissajous orbit In orbital mechanics, a Lissajous orbit (), named after Jules Antoine Lissajous, is a quasi-periodic orbital trajectory that an object can follow around a Lagrangian point of a three-body system without requiring any propulsion. Lyapunov orbits ...

s, and quasi-halo orbits. Lyapunov orbits pass behind the Moon, restricting communication opportunities with Earth for long periods of time, and as such were not considered. Lissajous orbits require less stationkeeping than halo orbits, but suffer from occasionally passing behind the Moon as well. Their non-periodicity–a trait shared with quasi-halo orbits–further complicates maintaining the pointing of antennas and solar arrays. Thus a halo orbit was chosen, at the cost of greater stationkeeping expense. An halo orbit as a communications relay for an

Apollo Apollo, grc, Ἀπόλλωνος, Apóllōnos, label=genitive , ; , grc-dor, Ἀπέλλων, Apéllōn, ; grc, Ἀπείλων, Apeílōn, label=Arcadocypriot Greek, ; grc-aeo, Ἄπλουν, Áploun, la, Apollō, la, Apollinis, label= ...

mission to the far side of the Moon was first suggested in 1966 by

Robert W. Farquhar Robert Willard Farquhar (September 12, 1932 – October 18, 2015) was an American mission design specialist who worked for NASA. He designed halo orbits and was involved in a number of spaceflight missions. Farquhar was elected as a member into t ...

. In the end, no relay satellite was launched for Apollo. Although a number of spacecraft have operated in halo orbits in the Earth-Sun system since then, China was the first to realize Farquhar's original idea of a communications relay satellite in a halo orbit around the Earth-Moon point. The satellite is based on the Chang'e 2 design. It utilizes the CAST100 small satellite bus with an aluminum honeycomb sandwich plate structure, and several 3D-printed parts. Communication with the lunar surface is accomplished in the

X band The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is rather indefinitely set at approxim ...

, using a high-gain deployable parabolic antenna, the largest antenna used for a

deep space exploration Deep space exploration (or deep-space exploration) is the branch of astronomy, astronautics and space technology that is involved with exploring the distant regions of outer space. However, there is little consensus on the meaning of "distant" regi ...

satellite. The lunar link uses PCM/PSK/PM modulation in the forward link and BPSK in the backward link. The forward link data rate of the lander and the rover is 125 bit/s. The return link data rate is up to 555 kbit/s for the lander and up to 285 kbit/s for the rover. Data transmission to the Earth operates in the

S band The S band is a designation by the Institute of Electrical and Electronics Engineers (IEEE) for a part of the microwave band of the electromagnetic spectrum covering frequencies from 2 to 4 gigahertz (GHz). Thus it crosses the convention ...

in BPSK modulation mode, using a single mid-gain helix antenna at a data rate of up to 10 Mbit/s.Chang'e 4 relay satellite, Queqiao: A bridge between Earth and the mysterious lunar farside
. Xu, Luyan, ''The Planetary Society''. 19 May 2018. Retrieved on 20 May 2018

Mission

20180912 6258TPS-TPR-2018Q3-18-09-04-p14legacy

On 20 May 2018, several months before the Chang'e 4 mission, the Queqiao was launched from Xichang Satellite Launch Center in China, on a Long March 4C rocket. The spacecraft took 24 days to reach L₂, using a gravity assist at the Moon to save propellant. On 14 June 2018, Queqiao finished its final adjustment burn and entered the mission orbit, about from the Moon. This is the first lunar relay satellite ever placed in this location. In addition to its communication relay equipment, Queqiao carries the Netherlands-China Low Frequency Explorer (NCLE), a radio-astronomy experiment to detect faint radio signals from the early universe. The instrument is intended to perform a wide range of observations in the low-frequency radio regime, such as studying space weather and characterizing the radio background environment at L₂. The far side of the Moon is an ideal environment for radio astronomy, because the Moon can shield instruments from man-made radio frequency interference coming from the Earth. While Queqiao's primary mission will keep the instrument constantly in line of sight of the Earth, and expose it to radio interference from the primary communication relay hardware, the accumulated experience and data from NLCE will serve as a pathfinder for future deep space radio astronomy instruments. NLCE successfully deployed its antennas on 27 November 2019. Queqiao is additionally fitted with a laser reflector developed by Sun Yat-sen University as a pilot study for the TianQin gravitational-wave observatory project. A pair of scientific microsatellites, Longjiang-1 and Longjiang-2, has been launched with the Queqiao as a secondary payload. The microsatellites weigh 45 kg each and measure 50x50x40 centimeters. Developed at the

Harbin Institute of Technology Harbin Institute of Technology (; abbreviation: HIT or ) is a public research university and a member of China's elite C9 League and a member of the University Alliance of the Silk Road. HIT is a Chinese Ministry of Education Class A Doubl ...

, the microsatellites were to fly in formation in a 300x3000 km orbit to perform ultra-long-wavelength astronomical interferometry. Contact was lost with Longjiang-1 shortly after trans-lunar injection, but Longjiang-2 successfully entered a 350x13700 km altitude lunar orbit on 25 May. Longjiang-2 was equipped with a micro-optical camera provided by

King Abdulaziz City for Science and Technology King Abdulaziz City for Science and Technology (KACST; ar, مدينة الملك عبدالعزيز للعلوم والتقنية) in Riyadh, Saudi Arabia is an organization established in 1977 as the Saudi Arabian National Center for Science & ...

, returning color images of the Earth and the lunar surface. On 24 January 2019, Longjiang-2 performed an end-of-mission maneuver, lowering its periapsis to 500 km. The orbit gradually decayed due to gravitational perturbations with the microsatellite impacting the far side of the lunar surface at 14:20 UTC, 31 July 2019.

International collaboration

China and Radboud University of Netherland collaborated on the Netherlands-China Low Frequency Explorer (NCLE), a radio-astronomy experiment. China has also agreed to a request from

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

to use the Chang'e 4 probe and Queqiao relay satellite in future U.S. Moon missions.

References

{{Portal bar, Astronomy, Stars, Spaceflight, Solar System, Science 2018 in China Chinese Lunar Exploration Program Chinese space probes Space probes launched in 2018 Spacecraft launched by Long March rockets Spacecraft using halo orbits Satellites orbiting Earth