Queqiao-2 relay satellite

Queqiao 2

Rendering of Queqiao 2 satellite
Mission type	Communication relay Radio astronomy
Operator	CNSA
COSPAR ID	2024-051A (QUEQIAO-2)
SATCAT no.	59274
Mission duration	Planned: 8-10 years 11 days (in progress)
Spacecraft properties
Bus	CAST-2000[1]
Manufacturer	DFH Satellite Company LTD
Dry mass	1,200 kilograms (2,600 lb)
Dimensions	Antenna: 4.2 metres (14 ft) in diameter[1]
Power	1350W[1]
Start of mission
Launch date	20 March 2024, 00:31:28 UTC[2]
Rocket	Long March 8[3]
Launch site	Wenchang Space Launch Site LC-2[4]
Orbital parameters
Reference system	Selenocentric frozen orbit
Periselene altitude	16,000 km (9,900 mi)[5]
Aposelene altitude	200 km (120 mi)[5]
Inclination	62.4°[6]
Period	24 hours[5]
Lunar orbiter
Orbital insertion	24 March 2024, 17:05 UTC[7]
Instruments
Grid-based Energetic Neutral Atom imager (GENA) Extreme Ultraviolet Camera (EUC) Lunar Orbit VLBI EXperiment (LOVEX)
Chinese Lunar Exploration Program ← Chang'e 5 Chang'e 6 →   Queqiao satellites ← Queqiao

Queqiao-2 relay satellite (Chinese: 鹊桥二号中继卫星; pinyin: Quèqiáo èr hào zhōngjì wèixīng; lit. 'Magpie Bridge 2 relay satellite'), is a second of the two communications relay and radio astronomy satellites designed to support the fourth phase the Chinese Lunar Exploration Program.^[8][9][10] The China National Space Administration (CNSA) launched the Queqiao-2 relay satellite on 20 March 2024 to a elliptical frozen orbit around the Moon to support communications from the far side of the Moon and the Lunar south pole.^{[11][12][13][14]}

The name Queqiao ("Magpie Bridge") was inspired by and came from the Chinese tale The Cowherd and the Weaver Girl.^[11][10][15]

Background and mission planning

The initial phase of the International Lunar Research Station (ILRS), consists of the Chang'e 7 and Chang'e 8 probes, i.e., is to be built from 2026 on the southern edge of the South Pole–Aitken basin located on the far side of the Moon.^[16] While the Queqiao so far only had to look after two probes on the far side of the Moon (Chang'e 4 lander and Yutu-2 rover), it is assumed that up to ten robots will be active there at the time of ILRS, which requires a complex and sophisticated communication network.^[17]

So, in March 2024, the Queqiao relay satellite, which has been orbiting in a halo orbit around the Earth-Moon L₂ since 2018, will be supplemented by another relay satellite, called Queqiao 2.^[14][17] 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,^[18] the Center for Lunar Exploration and Space Projects at the CNSA decided to launch it separately.^[19] This allowed the buildibng of a larger variant of the relay satellite that could be launched earlier and used in the Chang'e 6 sample return mission planned for 2024 to the Apollo crater on the far side of the Moon.^[17]

Although the first Queqiao has the advantage of relaying constant communications to and fro the far side of the Moon aided by Chinese Deep Space Network, it is disadvantageous, as halo orbits around the Earth-Moon L₁ and L₂ are inherently unstable^[20] and the satellite therefore consumes 80 g (2.8 oz) of fuel for a small orbit correction maneuver approximately every 9 days. Therefore, a frozen elliptic orbit around the Moon itself was chosen for Queqiao 2, in which it has 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.^[21]

When Queqiao-2 reaches a position about 200 km from the lunar surface, it will perform capture braking and enter a lunar parking orbit of 200 × 100,000 km with a period of about 10 days. Eventually, Queqiao-2 will enter a large elliptical frozen orbit 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 a good 10 years, i.e., in principle the assumed lifespan of the satellite.^[5]

Design

Queqiao 2 relay satellite and radio observatory is based on the CAST 2000 bus from YTO Group, a subsidiary of the Chinese Academy of Space Technology.^[22] It carries a total of 488 kg (1,076 lb) of hydrazine and oxidizer in tanks with a total capacity of 606 L (133 imp gal; 160 US gal), giving it a take-off weight of around 1,200 kg (2,600 lb). 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). Two rotatable solar cell wings, each with two solar arrays, deliver a total output of 1350 W, the operating voltage 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.^[23][24]

Adopted from the first Queqiao, a parabolic antenna with a diameter of 4.2 m and an antenna gain of 44 dBi is permanently mounted on the top of the bus- the alignment is carried out via the satellite's attitude control - and is used for radio communication with the lunar surface.^[25] In order to be able to accommodate the satellite in the payload fairing of the launch vehicle, the segments of the reflector are folded together during launch. After separating from the upper stage of the rocket and unfolding the solar modules, the antenna is also unfolded at the beginning of the transfer orbit to the Moon.^{[11][26][27][23][28][1]}

Communication with the lunar surface is accomplished in the X band, using a high-gain 4.2 metres (14 ft) deployable parabolic antenna, the largest antenna used for a deep space exploration satellite.^[29]

The large parabolic antenna provides 10 simultaneously usable X-band 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 when using an omnidirectional antenna, and at 5 Mbit/s when using a parabolic antenna. The signals are then demodulated and decoded in the satellite.^[25]

The K_a band is used to transmit payload data to the ground stations of the Chinese Academy of Sciences, both from the surface probes on the Moon and from the satellite itself. With quadrature phase shift keying, encryption with low-density parity check code and a traveling wave tube amplifier with 55 W output power, the data transfer 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 side of the satellite bus on a fold-out arm that allows it to protrude above the large parabolic antenna.^[11][24]

Telemetry and control of the satellite is usually carried out on the S-band, for which there is an S-band omnidirectional antenna at the focal point of the small parabolic antenna in addition to the K_a band transceiver. The data transmission 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 of the carrier wave for the telemetry signals, and long-base interferometry, where connected radio telescopes are using the Chinese VLBI network to determine the exact angular position.^[24]

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 by the water droplets in the Earth's atmosphere 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 system, the time of arrival, 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.^[10]

Scientific payloads

There are three scientific payloads on the spacecraft:^[30][31]

• Grid-based Energetic Neutral Atom imager (GENA): Images particle detector for neutral atoms for observing the terrestrial magnetosphere, especially the magnetotail.^[30][31]
• Extreme Ultraviolet Camera (EUC).^[30][31]
• Lunar Orbit VLBI EXperiment: The intention is to use the 4.2 m antenna as a radio telescope during the four hours the satellite spends over the Moon's north pole during each orbit. The satellite will be used in conjunction with terrestrial telescopes for long-base interferometry 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 , but also as part of the Chinese deep space network, i.e, the position of spacecrafts such as the asteroid probe Tianwen-2. For use as a radio telescope, a cooled X-band receiver for the frequency range 8–9 GHz with a noise temperature of less than 50 K and four selectable bandwidths (64, 128, 256 and 512 MHz) 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 with a maximum deviation of 10 −12 per second or 10 −14 per day. The receiver and clock together have a mass of 45 kg (99 lb) and have an average power consumption of 220 W.^[30][31]

Mission

Queqiao-2 was launched on 20 March 2024 at 00:31 UTC by a Long March 8 rocket from the Wenchang Space Launch Site,^[32][33] supporting China's upcoming Chang'e 6 and future 7 and 8 lunar missions scheduled for 2026 and 2028 respectively.^[34][35] The upgraded Queqiao-2 entered lunar orbit on 24 March 2024 at 16:46 UTC,^[36] where it is expected to operate for 8-10 years and by using a elliptical frozen orbit of 200 km × 16,000 km with an inclination of 62.4°,^[5] instead of the L₂ halo orbit.^[37][38]

The initial mission of Queqiao-2 is to provide relay communication support for Chang'e 6. After Chang'e 6 completes its mission, it will adjust its orbit to provide services for Chang'e-7, Chang'e-8 and subsequent lunar exploration missions. In the future, Queqiao-2 will also work with Chang'e 7 and Chang'e 8 to build the International Lunar Research Station.^[10]

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 and navigation constellation based on the Queqiao technology. After launch, the two satellites underwent lunar orbit insertion on 24 March 2024 at 17:43 UTC and entered a large elliptical orbit around the Moon (Tiandu-2 was attached to Tiandu-1 and separated in lunar orbit).^[36] Both are equipped with a communications payload and first one has a laser passive retroreflector and a in-space router, with another has navigational devices.^[39] In a large elliptical orbit around the moon, satellite-to-ground laser ranging are inter-satellite microwave ranging are to be carried out by these satellites via high-precision lunar orbit determination technology.^[40][10][41]

Comparison of relay satellites

Here is a comparison of some of the key differences of the two lunar relay satellites:^{[1][11][42][43][44][5][6]}

References