Chang’e-4 relay satellite enters halo orbit around Earth-Moon L2, microsatellite in lunar orbit

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HELSINKI, Finland — The relay satellite which will facilitate China’s Chang’e-4 lunar far side landing mission late in 2018 has entered its intended halo orbit around Earth-Moon Lagrange point 2.

The Beijing Aerospace Command and Control Center (BACC) sent commands for the spacecraft to fire its engines at 11:00 p.m. EDT June 13, with the burn complete at 11:06 p.m.

The satellite will now undergo on-orbit testing of its communications functions, while maintaining a complex Lissajous orbit, which is a three-dimensional irregular curve, rather than a two-dimensional halo.

The relay satellite was launched from Xichang, southwest China, on May 20, and performed a braking maneuver May 25 when passing within 100 kilometers of the lunar surface, sending it on a trajectory towards the second Earth-Moon Lagrange point, some 65,000-80,000 kilometers beyond the moon.

The relay satellite is a precursor mission to the launch of the Chang’e-4 lander and rover, which will attempt to land in the vicinity of the Von Kármán crater within the South Pole-Aitken Basin on the lunar far side around November or December. No such mission has been attempted before.

As the lunar far side at no time faces the Earth, due to the moon’s orbital period matching its rotational period, a relay satellite is required to facilitate communications with the Earth.

“The satellite is the world’s first communication satellite operating in that orbit, and will lay the foundation for the Chang’e-4, which is expected to become world’s first probe soft-landing and roving on the far side of the Moon,” Zhang Hongtai, president of the China Academy of Space Technology (CAST), told Xinhua news agency.

Named Queqiao (‘Magpie Bridge’), a name taken from a Chinese folklore tale that sees two lovers reunited once a year when a flock of magpies form a bridge across the Milky Way, the satellite will orbit around the Lagrange point, with an orbital radius allowing it line-of-sight with both the far side of the most moon and tracking stations on Earth at all times.

It will use a 4.2-meter parabolic antenna to relay telecommands from the ground to the Chang’e-4 lunar spacecraft and transmit data and telemetry back to Earth via S-band, while using X-band to communicate with the lander and rover.
The relay satellite is also equipped with the Netherlands-China Low-Frequency Explorer (NCLE), a low-frequency, space-based astronomy pathfinder experiment that will attempt to detect radio signals from the cosmic dark ages, before emission of light by the first stars in the universe.

The three 5-meter-long antennas will be deployed after the lander and rover section of the mission, which is designed to last a minimum of three months.

Longjiang microsatellite operating in lunar orbit
The State Administration for Science, Technology and Industry for National Defense (SASTIND), which oversees China’s space activities, on June 14 also provided an update on the pair of microsatellites launched along with Queqiao.

An image captured by the Saudi camera aboard the  Longjiang-2 microsatellite shows a portion of the northern hemisphere of the far side of the moon near Petropavlovsky crater, and the distant Earth. Credit: China Lunar Exploration Project (CLEP)
An image captured by the Saudi camera aboard the Longjiang-2 microsatellite shows a portion of the northern hemisphere of the far side of the moon near Petropavlovsky crater, and the distant Earth. Credit: China Lunar Exploration Project (CLEP)

Unlike the relay satellite, the two 47-kilogram satellites developed by the Harbin Institute of Technology were intended for lunar orbit.

The report states that Longjiang-2, also known as DSLWP-B, successfully entered an elliptical lunar orbit using its own propulsion May 25, while Queqiao headed towards Earth-Moon L2.

SASTIND confirmed however that Longjiang-1 (DSLWP-A) had suffered an anomaly during Earth-Moon transfer and did not enter lunar orbit. This was the first official acknowledgement that the spacecraft had been lost.
Longjiang-2 carries a low-frequency antenna and has been facilitating out amateur radio experiments while in lunar orbit. It was also to test space-based interferometry with Longjiang-1.

The China Lunar Exploration Project (CLEP) today also released images from the small camera payload aboard Longjiang-2, developed by the King Abdulaziz City for Science and Technology (KACST) of Saudi Arabia, capturing the surface of the moon with the Earth in the background.

Despite the loss of one of the microsatellites, Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, says that the mission is a harbinger of things to come.

McDowell told SpaceNews that the Longjiang/DSLWP probes mark the beginning of ambitious plans for small deep space probes, with the even smaller NASA MarCO probes currently on their way to Mars.

“I think in the decade to come, we’ll continue to see ambitious, large planetary probes, like the European-Japanese Bepi-Colombo and China’s Chang’e-4 which are both preparing for launch, but we’ll also see the flourishing of these small, simple and highly focussed probes,” McDowell said.

“Fifteen years after the first cubesat we’re seeing the first large constellations now. Perhaps in 10 years we’ll see big smallsat constellations in the inner solar system for interplanetary comms, GPS, space weather and asteroid searches.”

Professor Ping Jinsong of the National Astronomical Observatories of China and principal investigator for the Low Frequency Spectrometer payload on the Chang’e-4 lander, told SpaceNews that future plans are being considered.

“To fly a low-cost constellation in lunar space or at the Sun-Earth L1/L2 points area, with EMC quiet platform for each satellite unit, and with space Very-long-baseline interferometry (VLBI) working mode…should be a reasonable choice in the near future,” Ping said.