Queqiao-1

File:Lagrangianpointsanimated.gif around {{L2|nolink=yes}}, which is behind the Moon, will have a view of both the Earth and the Moon's far side]]

File:Queqiao antenna.jpg

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.{{cite news |last1=Wall |first1=Mike |title=China Launching Relay Satellite Toward Moon's Far Side Sunday |url=https://www.space.com/40626-china-launching-moon-mission-sunday-change-4.html |work=Space.com |date=18 May 2018 |archive-url=https://web.archive.org/web/20180518183823/https://www.space.com/40626-china-launching-moon-mission-sunday-change-4.html |archive-date=18 May 2018}}{{cite web |url=http://www.planetary.org/blogs/emily-lakdawalla/2016/01141307-updates-on-change-program.html |title=Updates on China's lunar missions |date=14 January 2016 |access-date=24 April 2016 |publisher=The Planetary Society |author=Emily Lakdawalla |archive-url= https://web.archive.org/web/20160417061245/http://www.planetary.org/blogs/emily-lakdawalla/2016/01141307-updates-on-change-program.html |archive-date= 17 April 2016 |url-status= live }}{{cite web |last=Jones |first=Andrew |url = https://gbtimes.com/change-4-lunar-far-side-satellite-named-magpie-bridge-from-folklore-tale-of-lovers-crossing-the-milky-way |title=Chang'e-4 lunar far side satellite named 'magpie bridge' from folklore tale of lovers crossing the Milky Way |work=GBTimes |date=24 April 2018 |access-date=28 April 2018 |archive-url=https://web.archive.org/web/20180424104558/https://gbtimes.com/change-4-lunar-far-side-satellite-named-magpie-bridge-from-folklore-tale-of-lovers-crossing-the-milky-way |archive-date=24 April 2018 |url-status=live }}

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 ({{L2|nolink=yes}}) becomes an attractive option.{{Cite journal|last1=Wu|first1=Weiren|last2=Tang|first2=Yuhua|last3=Zhang|first3=Lihua|last4=Qiao|first4=Dong|date=2017-12-12|title=Design of communication relay mission for supporting lunar-farside soft landing|url=https://doi.org/10.1007/s11432-017-9202-1|journal=Science China Information Sciences|language=en|volume=61|issue=4|pages=040305|doi=10.1007/s11432-017-9202-1|s2cid=22442636|issn=1869-1919|url-access=subscription}}

The types of orbits near the equilibrium points, include Lyapunov orbits, halo orbits, Lissajous orbits, 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 {{L2|nolink=yes}} halo orbit as a communications relay for an Apollo mission to the far side of the Moon was first suggested in 1966 by Robert W. Farquhar.{{cite journal |last1=Robert Farquhar |title=Station-Keeping in the Vicinity of Collinear Libration Points with an Application to a Lunar Communications Problem |journal=AAS Science and Technology Series: Space Flight Mechanics Specialist Symposium |date=1966 |volume=11 |pages=519–535}}, see Farquhar, R. W.: [https://ntrs.nasa.gov/citations/19710000821 "The Control and Use of Libration-Point Satellites"], Ph.D. Dissertation, Dept. of Aeronautics and Astronautics, Stanford University, Stanford, California, 1968, pp. 103, 107–108. In the end, no relay satellite was launched for Apollo.{{cite web | url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680015886_1968015886.pdf | title=Lunar Far-Side Communication Satellites | last1=Schmid | first1=P. E. | date=June 1968 | publisher=NASA | access-date=2008-07-16 }} Although a number of spacecraft have operated in halo orbits in the Earth-Sun system since then,Dunham, D.W. and Farquhar, R. W.: "Libration-Point Missions 1978-2000," Libration Point Orbits and Applications, Parador d'Aiguablava, Girona, Spain, June 2002 China was the first to realize Farquhar's original idea of a communications relay satellite in a halo orbit around the Earth-Moon {{L2|nolink=yes}} point.{{cite news |last1=Xu |first1=Luyuan |title=How China's lunar relay satellite arrived in its final orbit |url=http://www.planetary.org/blogs/guest-blogs/2018/20180615-queqiao-orbit-explainer.html |publisher=The Planetary Society |date=2018-06-15 |language=en |quote=This is the first-ever lunar relay satellite at this location.}}

The satellite is based on the Chang'e 2 design.[https://nssdc.gsfc.nasa.gov/planetary/lunar/cnsa_moon_future.html Future Chinese Lunar Missions: Chang'e 4 - Farside Lander and Rover]. David R. Williams, NASA Goddard Space Flight Center. 7 December 2018. It utilizes the CAST100 small satellite bus with an aluminum honeycomb sandwich plate structure, and several 3D-printed parts.{{Cite journal|last1=Zhang|first1=LiHua|last2=Xiong|first2=Liang|last3=Sun|first3=Ji|last4=Gao|first4=Shan|last5=Wang|first5=XiaoLei|last6=Zhang|first6=AiBing|date=2019-02-14|title=Technical characteristics of the relay communication satellite "Queqiao" for Chang'e-4 lunar farside exploration mission|journal=Scientia Sinica Technologica|language=Chinese|volume=49|issue=2|pages=138–146|doi=10.1360/N092018-00375|s2cid=88483165|issn=2095-946X|doi-access=free}}

Communication with the lunar surface is accomplished in the X band, using a high-gain {{convert|4.2|m|ft}} deployable parabolic antenna, the largest antenna used for a deep space exploration satellite.{{Cite web |url=http://www.chinanews.com/gn/2018/05-21/8518942.shtml |title=鹊桥号发射成功 将成为世界首颗连通地月中继卫星 |accessdate=2018-05-26 |date=2018-05-21 |archive-url=https://web.archive.org/web/20180527070317/http://www.chinanews.com/gn/2018/05-21/8518942.shtml |archive-date=2018-05-27 }} 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 in BPSK modulation mode, using a single mid-gain helix antenna at a data rate of up to 10 Mbit/s.[http://www.planetary.org/blogs/guest-blogs/2018/0519-change-4-relay-satellite.html Chang'e 4 relay satellite, Queqiao: A bridge between Earth and the mysterious lunar farside] {{Webarchive|url=https://web.archive.org/web/20180521001050/http://www.planetary.org/blogs/guest-blogs/2018/0519-change-4-relay-satellite.html |date=21 May 2018 }}. Xu, Luyan, The Planetary Society. 19 May 2018. Retrieved on 20 May 2018

File:20180912 6258TPS-TPR-2018Q3-18-09-04-p14legacy.png

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.{{Cite web|last1=Barbosa|first1=Rui|last2=Bergin|first2=Chris|date=2018-05-20|title=Queqiao relay satellite launched ahead of Chang'e-4 lunar mission|url=https://www.nasaspaceflight.com/2018/05/queqiao-relay-satellite-launched-change-4-lunar-mission/|url-status=live|archive-url=https://web.archive.org/web/20201109011332/https://www.nasaspaceflight.com/2018/05/queqiao-relay-satellite-launched-change-4-lunar-mission/|archive-date=2020-11-09|access-date=2021-10-17|website=NASASpaceFlight.com|language=en-US}} The spacecraft took 24 days to reach L₂, using a gravity assist at the Moon to save propellant.{{cite news |last1=Xu |first1=Luyuan |title=How China's lunar relay satellite arrived in its final orbit |url=http://www.planetary.org/blogs/guest-blogs/2018/20180615-queqiao-orbit-explainer.html |publisher=The Planetary Society |date=15 June 2018 |archive-url=https://web.archive.org/web/20181017123833/http://www.planetary.org/blogs/guest-blogs/2018/20180615-queqiao-orbit-explainer.html |archive-date=17 October 2018}} On 14 June 2018, Queqiao finished its final adjustment burn and entered the mission orbit, about {{Convert|65000|km||abbr=}} 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.{{Cite journal|last1=Vecchio|first1=Antonio|last2=Bentum|first2=Mark|last3=Falcke|first3=Heino|last4=Boonstra|first4=Albert-Jan|last5=Ping|first5=Jinsong|last6=Chen|first6=Linjie|last7=Klein-Wolt|first7=Marc|last8=Brinkerink|first8=Christiaan|last9=Rotteveel|first9=Jeroen|last10=Pourshaghaghi|first10=Hamid|last11=Karapakula|first11=Sukanth|date=2021-01-01|title=The Netherlands-China Low-frequency explorer (NCLE)|journal=43rd COSPAR Scientific Assembly. Held 28 January - 4 February|url=https://ui.adsabs.harvard.edu/abs/2021cosp...43E1525V|volume=43|pages=1525|bibcode=2021cosp...43E1525V}} 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.{{cite web|url=https://www.space.com/radio-telescope-beyond-moon-far-side-antennas-deploy.html |title=Radio Telescope Unfurls 3 Antennas Beyond the Far Side of the Moon |website=Space.com |date=2 December 2019 |first=Meghan |last=Bartels

}}

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.{{cite web |title=鹊桥号启程，为嫦娥四号登陆月球背面架设通信桥梁 |url=https://www.guokr.com/article/442961/ |author=Lsquirrel |website=果壳网 |date=2018-05-20 |accessdate=2019-01-04 |archive-date=2019-01-04 |archive-url=https://web.archive.org/web/20190104021738/https://www.guokr.com/article/442961/}}

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.{{Cite web|title=Chinese satellite snags new views of Earth from lunar orbit|url=https://www.planetary.org/articles/20180614-longjiang-2-earth-pics|access-date=2021-10-17|publisher=The Planetary Society|language=en}} Developed at the Harbin Institute of Technology, the microsatellites were to fly in formation in a 300x3000 km orbit to perform ultra-long-wavelength astronomical interferometry.{{Cite web|title=Lunar Orbiter Longjiang-2 Smashes into Moon|url=https://www.planetary.org/articles/longjiang-2-impacts-moon|access-date=2021-10-17|publisher=The Planetary Society|language=en}} 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, 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.

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 to use the Chang'e 4 probe and Queqiao relay satellite in future U.S. Moon missions.{{cite web|last1=Needham|first1=Kirsty|title=Red moon rising: China's mission to the far side|url=https://www.smh.com.au/world/asia/red-moon-rising-china-s-mission-to-the-far-side-20190117-p50s0y.html|newspaper=The Sydney Morning Herald|date=19 January 2019}}

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