''Queqiao-2'' relay satellite (), is the second of the communications relay and radio astronomy satellites designed to support the fourth phase the

Chinese Lunar Exploration Program The Chinese Lunar Exploration Program (CLEP; ), also known as the Chang'e Project () after the Chinese Moon goddess Chang'e, is an ongoing series of robotic Moon missions by the China National Space Administration (CNSA). Engineering Program ...

, after Queqiao-1 launched in 2018. The

China National Space Administration The China National Space Administration (CNSA) is a government agency of the People's Republic of China headquartered in Haidian District, Haidian, Beijing, responsible for civil space administration and international space cooperation. These ...

(CNSA) launched the Queqiao-2 relay satellite on 20 March 2024 to an elliptical

frozen orbit In orbital mechanics, a frozen orbit is an orbit for an artificial satellite in which perturbations have been minimized by careful selection of the orbital parameters. Perturbations can result from natural drifting due to the central body's shap ...

around the Moon to support communications from the

far side of the Moon The far side of the Moon is the hemisphere of the Moon that is facing away from Earth, the opposite hemisphere is the near side. It always has the same surface oriented away from Earth because of synchronous rotation in the Moon's orbit. C ...

and the

Lunar south pole The lunar south pole is the southernmost point on the Moon. It is of interest to scientists because of the lunar water, occurrence of water ice in Crater of eternal darkness, permanently shadowed areas around it. The lunar south pole region fea ...

. The name ''Queqiao'' (, "Magpie Bridge") was inspired by and came from the Chinese tale ''

The Cowherd and the Weaver Girl ''The Cowherd and the Weaver Girl'' are characters found in Chinese mythology and appear eponymously in a romantic Chinese folk tale. The story tells of the romance between Zhinü (; the weaver girl, symbolized by the star Vega) and Niulang ...

''.

Background and mission planning

The initial phase of the International Lunar Research Station (ILRS), consisting of the Chang'e 7 and Chang'e 8 probes, was scheduled to be built in 2026 and 2028 on the southern edge of the

South Pole–Aitken basin The South Pole–Aitken basin (SPA Basin, ) is an immense impact crater on the far side of the Moon. At roughly in diameter and between deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest, and dee ...

located on the

. While the Queqiao so far only had to connect with two probes on the far side of the Moon (

Chang'e 4 Chang'e 4 (; ) is a robotic spacecraft mission in the Chinese Lunar Exploration Program of the CNSA. It made a soft landing on the far side of the Moon, the first spacecraft to do so, on 3 January 2019. A communication relay satellite, , w ...

lander and ''

Yutu-2 ''Yutu-2'' () is the robotic lunar rover component of China National Space Administration, CNSA's Chang'e 4 mission to the Moon, launched on 7 December 2018 18:23 UTC, it entered lunar orbit on 12 December 2018 before making the first soft land ...

'' rover), future mission would include more workload, with up to ten robots being active on the moon for the ILRS project, which requires a complex and sophisticated communication network. The Queqiao relay satellite was inserted in a

halo orbit A halo orbit is a periodic, non-planar orbit associated with 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 th ...

around the Earth-Moon L₂ since 2018. China planned another relay satellite, called ''Queqiao 2'', to support and supplement Queqiao-1. Originally, the idea was to design the relay satellite as an improved version of the Queqiao and launch it together with the Chang'e 7 probe. After a project revision, the Center for Lunar Exploration and Space Projects at the

CNSA The China National Space Administration (CNSA) is a government agency of the People's Republic of China headquartered in Haidian, Beijing, responsible for civil space administration and international space cooperation. These responsibilities ...

decided to launch it separately. This allowed the building of a larger variant of the relay satellite that could be launched earlier and used in the

Chang'e 6 Chang'e 6 () was the sixth robotic lunar exploration mission by the China National Space Administration (CNSA) and the second CNSA lunar sample-return mission. Like its predecessors in the Chinese Lunar Exploration Program, the spacecraft is ...

sample return mission A sample-return mission is a spacecraft mission to collect and return samples from an extraterrestrial location to Earth for analysis. Sample-return missions may bring back merely atoms and molecules or a deposit of complex compounds such as lo ...

that was also launched in 2024 to the Apollo crater on the

. Although the first Queqiao can provide the unique function of relaying constant communications to and from the far side of the Moon, aided by Chinese Deep Space Network, its halo orbits around the Earth-Moon L₁ and L₂ were inherently

unstable In dynamical systems instability means that some of the outputs or internal state (controls), states increase with time, without bounds. Not all systems that are not Stability theory, stable are unstable; systems can also be marginal stability ...

and requires the satellite to consumes of fuel for a small orbit correction maneuver approximately every 9 days. Therefore, a frozen

elliptic orbit In astrodynamics or celestial mechanics, an elliptical orbit or eccentric orbit is an orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. Some orbits have been referre ...

around the Moon itself was chosen for Queqiao 2 due to its more stable nature. The frozen elliptic orbit can provide visual contact with the Moon for eight hours, i.e., two-thirds of its 12-hour orbit, since the point of its periselene lies above the side of the southern polar region facing away from the Earth. When Queqiao-2 reaches a position about 200 km from the lunar surface, it will perform capture braking and enter a lunar

parking orbit A parking orbit is a temporary orbit used during the launch of a spacecraft. A launch vehicle follows a trajectory to the parking orbit, then coasts for a while, then engines fire again to enter the final desired trajectory. An alternative trajec ...

of 200 × 100,000 km with a period of about 10 days. Eventually, Queqiao-2 will enter a large elliptical

of 200 × 16,000 km with a period of 24 hours, which is inclined at 62.4° to the equator. No further orbit correction maneuvers are necessary for a period of 10 years, the assumed lifespan of the satellite. However, it did not enter that orbit and instead entered a 119.25° 1992 × 18679 km retrograde orbit.

Design

Queqiao 2 relay satellite and radio observatory is based on the CAST 2000 bus from DFH Satellite, a subsidiary of the Chinese Academy of Space Technology. It carries a total of of hydrazine and oxidizer in tanks with a total capacity of , giving it a take-off weight of around . The three-axis stabilized satellite has eight engines with a thrust of 20 N each for orbit correction maneuvers as well as eight engines with a thrust of 5 N each and four engines with a thrust of 1 N each for attitude control; it can be aligned with an accuracy of 0.03° (three times as good as the standard version of the

satellite bus A satellite bus (or spacecraft bus) is the main body and structural component of a satellite or spacecraft, in which the payload and all scientific instruments are held. Bus-derived satellites are less customized than specially-produced satelli ...

). Two rotatable

solar cell A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.

wings, each with two

solar array A photovoltaic system, also called a PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to abs ...

s, deliver a total output of 1350 W, the operating

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

is 30.5 V. During blackoutor eclipse period, it has accumulators with a charge storage capacity of 135 Ah. The manufacturing company assumes that Queqiao 2 will work properly for at least 8 to 10 years. Adopted from the first Queqiao, a

parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or p ...

with a diameter of 4.2 m and an

antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...

of 44 dBi is permanently mounted on the top of the bus- the alignment is carried out via the satellite's

attitude control Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle or satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, ...

- and is used for radio communication with the lunar surface. In order to be able to accommodate the satellite in the

payload fairing A payload fairing or nose fairing is a nose cone used to protect a launch vehicle, spacecraft payload (air and space craft), payload against the impact of dynamic pressure and aerodynamic heating during launch through an atmosphere. An additiona ...

of the

launch vehicle A launch vehicle is typically a rocket-powered vehicle designed to carry a payload (a crewed spacecraft or satellites) from Earth's surface or lower atmosphere to outer space. The most common form is the ballistic missile-shaped multistage ...

, the segments of the reflector are folded together during launch. After separating from the

upper stage A multistage rocket or step rocket is a launch vehicle that uses two or more rocket ''stages'', each of which contains its own Rocket engine, engines and Rocket propellant, propellant. A ''tandem'' or ''serial'' stage is mounted on top of anoth ...

of the rocket and unfolding the solar modules, the antenna is also unfolded at the beginning of the

transfer orbit Orbits Astrodynamics In orbital mechanics, a transfer orbit is an intermediate elliptical orbit that is used to move a spacecraft in an orbital maneuver from one circular, or largely circular, orbit to another. There are several types of tran ...

to the Moon. 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 set at approximately 7.0–11.2&nbs ...

, using a high-gain deployable parabolic antenna, the largest antenna used for a deep space exploration satellite. The large

provides 10 simultaneously usable

X-band The X band is the designation for a band of frequency, 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 set at approximately 7.0� ...

channels for radio traffic down to the Moon and 10 channels for traffic up to the satellite, as well as the possibility of communicating in the decimeter wave range. In the opposite direction, telemetry and payload data from the robots can be transmitted upwards at a speed of 50

kbit/s In telecommunications, data transfer rate is the average number of bits ( bitrate), characters or symbols ( baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are mu ...

when using an omnidirectional antenna, and at 5

Mbit/s In telecommunications, data transfer rate is the average number of bits (bitrate), characters or symbols (baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are multi ...

when using a parabolic antenna. The signals are then demodulated and decoded in the satellite. The K_a band is used to transmit payload data to the

ground station A ground station, Earth station, or Earth terminal is a terrestrial radio station designed for extraplanetary telecommunication with spacecraft (constituting part of the ground segment of the spacecraft system), or reception of radio waves fr ...

s of the

Chinese Academy of Sciences The Chinese Academy of Sciences (CAS; ) is the national academy for natural sciences and the highest consultancy for science and technology of the People's Republic of China. It is the world's largest research organization, with 106 research i ...

, both from the surface probes on the Moon and from the satellite itself. With quadrature phase shift keying,

encryption In Cryptography law, cryptography, encryption (more specifically, Code, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the inf ...

with low-density parity check code and a traveling wave

tube amplifier A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by so ...

with 55 W output power, the

data transfer Data communication, including data transmission and data reception, is the transfer of data, transmitted and received over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical ...

rate is on average 100 Mbit/s. The antenna used is a small parabolic antenna with a diameter of 0.6 m in a gimbal suspension, which is mounted on the

nadir The nadir is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the zenith. Et ...

side of the satellite bus on a fold-out arm that allows it to protrude above the large parabolic antenna.

Telemetry Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', 'far off', an ...

and control of the satellite is usually carried out on 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 ...

, for which there is an S-band

omnidirectional antenna In radio communication, an omnidirectional antenna is a class of antenna (electronics), antenna which radiates equal radio power in all directions perpendicular to an Cartesian coordinate system, axis (azimuthal directions), with power varying wi ...

at the

focal point Focal point may refer to: * Focus (optics) * Focus (geometry) * Conjugate points, also called focal points * Focal point (game theory) * Unicom Focal Point, a portfolio management software tool * Focal point review, a human resources process for e ...

of the small parabolic antenna in addition to the K_a band

transceiver In radio communication, a transceiver is an electronic device which is a combination of a radio ''trans''mitter and a re''ceiver'', hence the name. It can both transmit and receive radio waves using an antenna, for communication purposes. The ...

. The

data transmission Data communication, including data transmission and data reception, is the transfer of data, signal transmission, transmitted and received over a Point-to-point (telecommunications), point-to-point or point-to-multipoint communication chann ...

rate for commands from the Earth to the satellite is 2000 bit/s, the telemetry data is transmitted from the satellite to the Earth at a speed of 4096 bit/s. This is twice as fast as the first Queqiao. The position is determined using a combination of the so-called Unified S-Band Technology (USB), where the distance and speed of the satellite are calculated from the

Doppler shift The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...

of the

carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or freq ...

for the telemetry signals, and long-base

interferometry Interferometry is a technique which uses the ''interference (wave propagation), interference'' of Superposition principle, superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important inves ...

, where connected

radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...

s are using the Chinese

VLBI Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. T ...

network to determine the exact

angular position In geometry, the orientation, attitude, bearing, direction, or angular position of an object – such as a line, plane or rigid body – is part of the description of how it is placed in the space it occupies. More specifically, it refers to t ...

. The systems are alternately redundant. In the event of a failure of the S-band system, the telemetry and control signals can also be transmitted via the K_a band, and if the K_a band signals are subject to strong

attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a Transmission medium, medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and ...

by the

water droplets Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known living organisms ( ...

in the Earth's

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

during the hot and wet season, the payload data can also be transmitted via the S-band, but only with a data transfer rate of a maximum of 6 Mbit/s. Similar to a

satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are ope ...

system, the

time of arrival Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequence events, to compa ...

, i.e., a transit time measurement of the signals between the partners involved in communication, is used to determine their position in orbit or on the surface of the Moon with high accuracy.

Scientific payloads

There are three scientific payloads on the spacecraft: *Grid-based Energetic Neutral Atom imager (GENA):

Images An image or picture is a visual representation. An image can be two-dimensional, such as a drawing, painting, or photograph, or three-dimensional, such as a carving or sculpture. Images may be displayed through other media, including a project ...

particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing elementary particle, particles, such as t ...

for neutral atoms for observing the

terrestrial Terrestrial refers to things related to land or the planet Earth, as opposed to extraterrestrial. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on o ...

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...

, especially the

magnetotail In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo. ...

. *Extreme

Ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

Camera (EUC). *Lunar Orbit

EXperiment: The intention is to use the 4.2 m antenna as a

during the four hours the satellite spends over the Moon's

north pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...

during each orbit. The satellite will be used in conjunction with terrestrial telescopes for long-base

with a baseline of 400,000 km. The aim is not only to determine the position and composition of radio sources outside the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

, but also as part of the Chinese deep space network, i.e, the position of spacecrafts such as the asteroid probe

Tianwen-2 ''Tianwen-2'' () is a Chinese asteroid sample return and comet exploration mission that launched on 28 May 2025. China National Space Agency (CNSA) plans for the probe to return samples from asteroid 469219 Kamoʻoalewa in 2027. After the ...

. For use as a

, a cooled

receiver for the

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

range 8–9 GHz with a

noise temperature In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. The power spectral density of the noise is expressed in terms of the temperature (in kelvins) that would produce ...

of less than 50 K and four selectable bandwidths (64, 128, 256 and 512

MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base u ...

) is mounted on the antenna. In order to be able to accurately determine the transit time difference between the satellite and the terrestrial radio telescope for a given signal and thus calculate the position of the radio source or the spacecraft (the position of the satellite itself can be determined with an accuracy of 30 m), the satellite has an

atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

with a maximum deviation of 10 −12 per second or 10 −14 per day. The receiver and clock together have a mass of and have an average power consumption of 220 W.

Mission

Queqiao-2 was launched on 20 March 2024 at 00:31

UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...

by a Long March 8 rocket from the

Wenchang Space Launch Site The Wenchang Space Launch Site ( zh, 文昌航天发射场, links=no) is a rocket launch site located in Wenchang on the island of Hainan, in China. Formally a suborbital test center, it currently serves as China's southernmost spaceport. T ...

, supporting China's

in 2024 and future 7 and 8 lunar missions scheduled for 2026 and 2028 respectively. The upgraded Queqiao-2 entered lunar orbit on 24 March 2024 at 16:46

, where it is expected to operate for 8–10 years and by using a elliptical

of 200 km × 16,000 km with an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

of 62.4°, instead of the halo orbit. The initial mission of Queqiao-2 is to provide relay communication support for Chang'e 6. After Chang'e 6 completed its mission, it adjusted its orbit to provide services for Chang'e-7, Chang'e-8 and subsequent

lunar exploration The physical exploration of the Moon began when ''Luna 2'', a space probe launched by the Soviet Union, made a deliberate impact on the surface of the Moon on 14 September, 1959. Prior to that the only available means of lunar exploration had ...

missions. In the future, Queqiao-2 will also work with Chang'e 7 and Chang'e 8 to build the International Lunar Research Station. Queqiao-2 also carries two smaller Deep Space Exploration Laboratory communication satellites, Tiandu-1 and Tiandu-2, to verify the technicality of the lunar

communication Communication is commonly defined as the transmission of information. Its precise definition is disputed and there are disagreements about whether Intention, unintentional or failed transmissions are included and whether communication not onl ...

and

navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

based on the Queqiao technology. After launch, the two satellites underwent lunar orbit insertion on 24 March 2024 at 17:43

and entered a large

elliptical orbit In astrodynamics or celestial mechanics, an elliptical orbit or eccentric orbit is an orbit with an orbital eccentricity, eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. Some or ...

around the Moon (Both were attached to each other and separated in lunar orbit on 3 April 2024). Both are equipped with a communications payload and first one has a laser passive retroreflector and an in-space router, with another has navigational devices. In a large elliptical orbit around the moon, satellite-to-ground

laser ranging A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

are inter-satellite microwave ranging are to be carried out by these satellites via high-precision

lunar orbit In astronomy and spaceflight, a lunar orbit (also known as a selenocentric orbit) is an orbit by an object around Earth's Moon. In general these orbits are not circular. When farthest from the Moon (at apoapsis) a spacecraft is said to be at apo ...

determination technology. On 12 April 2024, CNSA announced that Queqiao-2 had successfully completed in-orbit communication tests with

on the far side of the moon and the Chang'e 6 probe while still on the ground. The satellite entered its targeted elliptical orbit on 2 April after a correction midway, near-moon braking and orbital manoeuvre around the moon. It facilitates communication between Earth and lunar probes signaling China's commitment to space exploration and international cooperation. On 23 September 2024, it was discovered by independent astronomer Scott Tilley that the satellite was instead in a 119.25° 1992 × 18679 km retrograde orbit.

Comparison of relay satellites

Here is a comparison of some of the key differences of the two lunar relay satellites:

References