Lunar Orbital VLBI Experiment: motivation, scientific purposes and status

TL;DR

The paper presents the Lunar Orbital VLBI Experiment (LOVEX) as a space VLBI component of the Chinese CLEP, detailing its architecture, instrumentation, and first in-flight results. LOVEX combines QueQiao-2’s relay capabilities with an ad hoc VLBI payload to realize a baseline of about $B \approx 3.8 \times 10^{5}$ km, enabling fringe detections and tests of near-field VLBI on Earth–space baselines. It reports on pre-launch and in-flight verifications, system-temperature calibrations, pointing corrections, and initial interferometric fringes for both astronomical sources (e.g., AO 0235+164) and deep-space probes (e.g., Chang’E-6 at L2), establishing the feasibility of ultra-long baseline SVLBI, space navigation, and astrometric applications. The results highlight LOVEX as the world’s fourth SVLBI mission, featuring a 512 MHz observing bandwidth, a 2 Gbps data rate, and a space-qualified passive hydrogen maser, with broad implications for high-resolution radio astronomy and future deep-space exploration.

Abstract

The Lunar Orbital VLBI Experiment (LOVEX) is a scientific component of the Chinese Lunar Exploration Project (CLEP) Chang'E-7. The spaceborne component of LOVEX is implemented onboard the relay satellite QueQiao-2, which was launched on 2024 March 20, and later placed into an elliptical selenocentric orbit. The LOVEX-specific payload consists of an X-band cryogenic receiver, a hydrogen maser frequency standard, and VLBI data formatting and acquisition electronics. Several components of the QueQiao-2 nominal onboard instrumentation, such as the 4.2-meter antenna, the data storage device, and the downlink communication system, contribute to the overall spaceborne VLBI instrumentation. This allows us to form a space radio telescope capable of co-observing with Earth-based radio telescopes in VLBI mode. In this space VLBI system, the length of the baseline extends up to approximately 380,000 km. This paper presents the LOVEX scientific objectives, architecture, instrumentation, pre-launch tests, in-flight verification and calibration, and the first in-flight detections of interferometric response (''fringes'') achieved through observations of the quasar AO 0235+164 and the Chang'E-6 orbital module, positioned at the Sun-Earth Lagrange point L2. These initial results demonstrate the successful performance of LOVEX, verifying its capability for both astronomical and spacecraft tracking observations at ultra-long VLBI baselines.

Figures (18)

• Figure 1: A schematic configuration of LOVEX with its spaceborne radio telescope on the QueQiao-2 relay satellite on a selenocentric orbit. Together with Earth-based radio telescopes, this VLBI system has a baseline length comparable to the Earth-Moon distance, 380,000 km.
• Figure 2: The QueQiao-2 satellite at the assembly facility. The 4.2-m parabolic antenna is equipped with a dedicated LOVEX feed and wave guide (shown in bright green).
• Figure 3: The LOVEX block-diagram. VLBI-specific payload is shown by blue boxes enclosed within the beige rectangle.
• Figure 4: The desktop integrated test of the LOVEX flight models. (a) X-band cryogenic receiver, (b) Hydrogen maser, (c) cryocooler, (d) cryocooler control unit, (e) payload management unit, and (f) frequency conversion and data acquisition backend.
• Figure 5: Geometrical configurations of LOVEX observations. In observations of natural celestial radio sources S1 and S2, the interferometer, consisting of an Erath-based radio telescope and QueQiao-2, operates in "traditional" VLBI mode, with the target of observations located at an infinitely large distance (i.e., at a distance much larger than the Fraunhofer criterion, $B^2/\lambda$). In these two cases, the ray paths to both elements of the interferometer are parallel. The difference between the cases S1 and S2 is in the projection effect: in the S1 case, the target is located close to the direction of the baseline vector, thus providing a shorter projected baseline which are more suitable for imaging observations in combination with Earth-only baselines. In the case of S3, the target (a spacecraft) is located in the near field (closer than the Fraunhofer criterion $B^2/\lambda$), thus the ray paths to the two elements of the interferometer are not parallel.