The first laser retroreflector on the lunar far side onboard China's Chang'e-6 lander

The lander–ascender combination of China’s Chang’e-6 mission successfully touched down at the designated landing site within the Apollo Basin, located in the South Pole–Aitken Basin on the lunar farside, on 2 June 2024. The Chang’e-6 mission carries four international payloads, among which the lander is equipped with a next-generation lunar laser retroreflector, designated INRRI (Instrument for landing-Roving laser Retroreflector Investigations), jointly developed by the Frascati National Laboratory of the Italian National Institute for Nuclear Physics (INFN-LNF) and the Aerospace Information Research Institute, Chinese Academy of Sciences (AIRCAS). The INRRI onboard Chang’e-6 represents the first laser retroreflector absolute control point ever deployed on the lunar farside by humanity. It is expected to support lunar surface positioning and mapping efforts, as well as orbit determination and navigation for future lunar orbiters equipped with laser altimeters. In a newly published research article in Space: Science & Technology, an international team of researchers from the Aerospace Information Research Institute, Chinese Academy of Sciences; the Italian National Institute for Nuclear Physics (INFN); the China Academy of Space Technology; the National Astronomical Observatories, Chinese Academy of Sciences; the Yunnan Observatories, Chinese Academy of Sciences; and the China University of Geosciences jointly presents a detailed account of the successful deployment of the INRRI laser retroreflector on the lunar farside by the Chang’e-6 mission.

First, the authors introduce the configuration of the INRRI onboard the Chang’e-6 lander. The laser retroreflector comprises eight cube-corner retroreflectors (CCRs) fabricated from Corning high-purity fused silica glass 7980, with a total mass of 25 g and a near-hemispherical dome structure, as illustrated in Fig. 1. Each CCR features a front face with a diameter of 1/2 inch and three mutually perpendicular reflecting surfaces, such that incident light within the wavelength range from 350 to 2000 nm is reflected back parallel to the incoming beam. The field of view (FOV) of the INRRI is the combined FOV of the eight CCRs, with a maximum FOV of 120° (full-cone aperture). The INRRI is mounted on the top panel of the Chang’e-6 lander to avoid or minimise the effects of the ascent plume, and is secured by a bracket with a height of 51 mm and a mass of 52 g to minimise FOV occlusion. Analysis indicates that the INRRI FOV is unobstructed up to a half-cone angle of ±42°. The authors further calculate the relationship between velocity aberration and the azimuthally averaged optical cross section (OCS) intensity in the far-field diffraction plane of the INRRI, for various angles between the micro-reflector plane normal and the optical axis. At a velocity aberration of 10 μrad, the OCS values of the INRRI for the micro-reflector plane normal–optical axis angles of 0°, 10°, 20°, and 30° are approximately 0.50 × 10⁶ m², 0.40 × 10⁶ m², 0.22 × 10⁶ m², and 0.11 × 10⁶ m², respectively.

Subsequently, the authors present the quality tests performed on the INRRI. The INFN-LNF had previously conducted a series of tests on the same model of INRRI for ESA’s ExoMars EDM mission, including sinusoidal and random vibration tests, thermal shock tests, thermal vacuum tests, as well as peel tests on the CCRs. Following the integration of the mounting bracket by AIRCAS for the Chang’e-6 INRRI, the collaborative team also carried out mechanical and thermal performance tests on the Chang’e-6 unit. These mechanical tests comprised acceleration tests, shock tests, random vibration tests, and sinusoidal vibration tests. The thermal vacuum test was conducted at a vacuum level better than 6.65 × 10^-3 Pa, with a temperature ramp rate exceeding 1 °C/min, a dwell time of 5.5 h, and a temperature range from –130 °C to +130 °C. A total of 3.5 thermal cycles were performed throughout the entire test, with a total duration of 38 h. In addition, after integration with the lander, the Chang’e-6 INRRI participated in the mechanical tests of the complete spacecraft.

Finally, the authors describe the pre- and post-launch status inspections. The INRRI was equipped with a remove-before-flight (RBF) protective cover to shield it from dust and contamination. Prior to the closure of the rocket fairing, the RBF cover was removed, after which the INRRI was cleaned and its final status was verified, as shown in Fig. 2. Following landing on the lunar farside, photographs of the INRRI were acquired using the camera on the robotic arm to check its condition, as illustrated in Fig. 3. Moreover, the INRRI on the lander was also observed by the Lunar Orbiter Laser Altimeter (LOLA) onboard NASA’s LRO satellite, which completed its first successful observation of the Chang’e-6 INRRI on 31 July 2024. In the near future, additional instruments are expected to be capable of observing the Chang’e-6 INRRI. The successful deployment of the Chang’e-6 INRRI represents a milestone achievement, as it establishes the first permanent high-precision control point on the lunar farside, which can support lunar surface mapping, orbit determination, and navigation for future lunar missions.