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Chang’e-4 lowers orbit ready for first-ever landing on far side of the Moon

The Chang’e-4 lunar spacecraft lowered its orbit early on Sunday in preparation to attempt the first-ever landing on the far side of the Moon in the first days of the New Year.

An official release by the China Lunar Exploration Project (CLEP) under the China National Space Administration (CNSA) on Sunday stated that the Beijing Aerospace Command and Control Centre (BACC) issued instructions to Chang’e-4, which then fired its at 00:54 UTC (08:54 Beijing time).

Telemetry received by ground stations monitoring the mission confirmed by 00:56 UTC that Chang’e-4 had lowered its orbit from a circular 100 x 100 kilometre orbit to an elliptical 15 x 100 km orbit.

CNSA stated said the control centre will choose a proper time to land the probe on the far side of the moon, with the landing expected to target a section of the Von Kármán crater.

Chang’e-4, though carrying a small radioisotope thermoelectric generator (RTG) for limited lunar nighttime activity, will rely mainly on solar energy to power its operations. This means the landing will take place only after the Sun is deemed high enough in the sky over Von Kármán crater.

Since reaching the vicinity of the Moon on December 12, Chang’e-4 has refined its orbit a number of times, tested communications with the Queqiao(‘Magpie bridge’) relay satellite, and checked the imaging instruments and ranging detectors on the spacecraft in preparation for the landing.

China’s Chang’e-4 Probe Changes Orbit to Prepare for Moon Landing.

When will Chang’e-4 land?

China has been vague on the landing date, though early reports following launch suggested a date some 27 days later, equating to January 4 Beijing time.

Scott Tilley, an amateur astronomer, has together with fellow tracking enthusiasts @DF2MZ and @uhf_satcom and with data assistance from Fabian P. Schmidt, been following the progress of Chang’e-4, providing clues to the activity and status of the mission in the absence of regular official updates from China.

“We’ve been tracking CE4 since it arrived in lunar orbit on X-band [radio frequency],” Tilley says, noting that a burn was performed on December 26 to circularise the orbit of Chang’e-4.

“Propagating estimated orbital parameters gives a first landing opportunity centred on January 2nd, 2019 at Von Kármán,” Tilley said ahead of the Sunday orbital manoeuvre, with further updates to come following analysis of the orbital plane of Chang’e-4.

Tilley offers the caveat that, “Given the fact the radio signal’s nature is constantly modified by communication with Earth stations a more accurate estimate isn’t really possible at this point.”

Chang’e-4 mission background

The mission is a repurposed backup to the Chang’e-3 spacecraft, which set down on Mare Imbrium in December 2013 and became the first spacecraft since Luna 24 in 1976 to soft-land on the Moon.

Chang’e-4 entered a roughly 100 x 400 km altitude orbit upon lunar orbit insertion on December 12, following its launch from Xichang, southwest China on December 7.

Queqiao has since June been in a halo orbit around the second Earth-Moon Lagrange point some 65,000-85,000 km beyond the Moon, ready to facilitate communications between the Chang’e-4 spacecraft and terrestrial tracking stations.

The relay satellite, which features a 4.2 metre parabolic antenna, is required for a mission to the lunar far side as that hemisphere of the Moon never faces the Earth due to the phenomenon of tidal locking.

An animation showing Chang'e-4 in lunar orbit and communicating with the Queqiao relay satellite at the second Earth-Moon Lagrange point.

An animation showing Chang’e-4 in lunar orbit and communicating with the Queqiao relay satellite at the second Earth-Moon Lagrange point. CAST

Target: Von Kärmän crater

The selected landing site is the 186-kilometre-diameter Von Kármán crater,which offers a relatively flat area within the South Pole-Aitken (SPA) basin, a huge and ancient impact structure which may contain exposed material from the lunar upper mantle and clues to the development and history of the Moon.

If landing is successful Von Kármán will offer the chance to analyse the variations of composition of the lunar surface and SPA melt sheet with the Visible and Near-Infrared Imaging Spectrometer (VNIS), while also returning unprecedented images with cameras on both the lander and rover.

An image of the Von Kármán crater from a mosaic created by the Lunar Reconnaissance Orbiter Camera.

An image of the Von Kármán crater from a mosaic created by the Lunar Reconnaissance Orbiter Camera. NASA/GSFC/Arizona State University

A number of subsequent impact craters in the surrounding region, such as Finsen, are expected to have delivered ejecta to the floor of Von Kármán, potentially providing a rich sample of the SPA basin and far side geologic history.

The Lunar Penetrating Radar (LPR) and low‐frequency radio spectrometer (LFS) payloads will also return unique data from the landing region and its subsurface, while the Lunar Lander Neutrons and Dosimetry (LND) instrument and Advanced Small Analyser for Neutrals (ASAN) will provide insight into the space environment and interactions with the surface.

It is notable that Theodore Von Kármán, for whom the crater was named, was a mentor to Qian Xuesen, known as the Father of Chinese Rocketry, who studied and worked under him at Massachusetts Institute of Technology (MIT) and California Institute of Technology (Caltech).

While the mission rover was due to be named in October following a public contest and vote, much like Yutu (‘Jade Rabbit’) for Chang’e-3, the name has not yet been revealed, leading to the rumour that a successful landing could see the rover named for Qian.