Apophis will make an extremely close flyby of Earth on April 13, 2029, at a distance of approximately 31,029 km—an event unprecedented for an asteroid of this size—providing a valuable natural experiment for studying tidal interactions, spin-state variations, and surface material migration between the asteroid and Earth. A large number of ground-based observations and space missions are expected to participate, but most observations will be concentrated from Earth's perspective, leaving observational blind spots as Apophis rapidly recedes from Earth's view after the flyby. LUMIO (Lunar Meteoroid Impacts Observer) is a 12U CubeSat of the European Space Agency, planned to be deployed in a quasi-halo orbit around the Earth–Moon L2 point; its primary mission is to observe meteoroid impact flashes on the lunar farside, with a launch expected in 2028. However, three critical questions remain unanswered: whether LUMIO-Cam—a camera designed for observing impact flashes—can detect Apophis as a sub-pixel target, whether it can resolve fluctuations in its light curve, and whether it can observe for a sufficiently long duration to obtain complete spin-period information.
In a recent study published in Space: Science & Technology , researchers from Politecnico di Milano established a CCD radiometric model for LUMIO-Cam, comprehensively calculating the signal strength of sunlight directly reflected by Apophis as well as Earthshine and lunar albedo reflected via Apophis, while simultaneously accounting for multiple noise sources including dark current, readout noise, background stars, and stray light. Detectability is evaluated through signal-to-noise ratio (SNR) analysis, and three evaluation metrics are defined: detection window, sensitivity window, and scientific window. The results indicate that under optimal camera settings, the detection window of LUMIO-Cam in the visible channel can last up to nearly one month, far exceeding the time required to observe a full spin period. By appropriately selecting gain and exposure time, saturation can be avoided; the scientific window can cover several days before and after the flyby, with multiple spin periods observable before the flyby and approximately 1.5 spin-period windows still available after the flyby. The study also notes that if LUMIO were to adopt a CMOS detector in the future instead of CCD, shorter exposure times and higher readout speeds could further enhance observational performance. This study demonstrates that LUMIO can provide complementary observational data on the Apophis flyby from the unique perspective of cislunar space, offering valuable reference for improving the planetary defense and near-Earth asteroid flyby scientific observation framework.
First, this paper focuses on the feasibility study of observing the 2029 Apophis flyby from the Earth–Moon L2 point using the LUMIO CubeSat. Apophis will make an extremely close flyby of Earth on April 13, 2029, at a distance of approximately 31,029 km, offering a valuable opportunity to study tidal interactions, spin-state variations, and surface material migration between the asteroid and Earth. LUMIO (Lunar Meteoroid Impacts Observer) is a 12U CubeSat of the European Space Agency, planned to be deployed in a quasi-halo orbit around the Earth–Moon L2 point. Its primary mission is to observe meteoroid impact flashes on the lunar farside, with a launch expected in 2028; if the mission is extended to April 2029, it could become a unique observing platform for the Apophis flyby. As illustrated in Fig. 1, Apophis will encounter the Moon at close range approximately 19 hours after its Earth flyby, with LUMIO positioned near the Earth–Moon L2 point, favorably located for observation. Fig. 2 presents the observation geometry, in which the key parameters include phase angle, distance, and field of view. The study raises three core questions: whether LUMIO-Cam can detect Apophis as a sub-pixel target, whether it can resolve fluctuations in its light curve, and whether it can observe for a sufficiently long duration to obtain complete spin-period information.
Second, this paper elaborates on the radiometric modeling methodology, including the signal model, noise model, and the definition of observation windows. Fig. 3 presents the camera quantum efficiency and lens transmittance as functions of wavelength. The signal model accounts for three sources: direct sunlight reflected by Apophis toward the camera, Earthshine reflected by Apophis, and lunar albedo reflected by Apophis. The noise model incorporates dark current noise, readout noise, photon shot noise, quantization noise, background star noise, and stray light. Fig. 4 validates the accuracy of the model by comparing the radiometric model calculations with synthetic images generated from a physically based rendering engine. To evaluate observational feasibility, three evaluation windows are defined: the detection window requires a signal-to-noise ratio (SNR) of no less than 10; the sensitivity window requires that the signal difference corresponding to the upper and lower size limits exceeds the number of electrons equivalent to one gray level of the camera, ensuring that fluctuations in the light curve can be resolved; and the scientific window is defined as the intersection of the detection window and the sensitivity window.
Finally, this paper presents quantitative results for the observation windows under different camera parameters through systematic simulations. Fig. 5 shows the signal-to-noise ratio (SNR) curves for the visible and near-infrared channels under the settings of gain 1000 and exposure time 2 seconds; the detection window in the visible channel can last up to approximately 30 days before the flyby, and the near-infrared channel also approaches 30 days, far exceeding the time required to observe a full spin period of Apophis (approximately 30.56 hours before the flyby and approximately 21 hours after the flyby). Fig. 6 illustrates the variation of detection window length with gain and exposure time, with gain 1000 and exposure time 2 seconds identified as the optimal settings. Sensitivity analysis indicates that although the camera may saturate before and after the flyby under high gain settings, saturation can be avoided and the ability to resolve light-curve fluctuations can be maintained by appropriately selecting gain 1 and exposure time 2 seconds. Fig. 7 presents the variation of scientific window length with gain and exposure time: before the flyby, the scientific window under optimal settings can reach several hundred hours; after the flyby, the visible channel can still provide approximately 1.5 spin periods, equivalent to about 31.5 hours of observation window, at gain 1000 and an exposure time of approximately 0.06 seconds. The study notes that if LUMIO were to adopt a CMOS detector in the future instead of CCD, shorter exposure times and higher readout speeds could further enhance observational performance. This study demonstrates that LUMIO can provide valuable complementary observational data on the Apophis flyby from the unique perspective of cislunar space.
Space: Science & Technology
Observing Apophis's 2029 Flyby from Earth-Moon L2 with LUMIO
29-Jun-2026