The composition of the ultraviolet unknown absorber in the cloud layers at an altitude of 50?70 km in Venus atmosphere has remained enigmatic for decades. To facilitate the identification and high-precision detection of this absorber, this paper proposes a conceptual design for a Venus cloud layer ultraviolet spectral atmospheric detection scientific mission.

The proposed detection system employs three ultraviolet spectrometers with varying spectral resolutions: an ultraviolet spectrometer with a spectral range of 190?450 nm and a resolution of 0.5 nm, enabling rapid full-spectrum detection at lower resolution within this band. An ultra-high-resolution ultraviolet spectrometer achieves fine detection with resolutions of 0.05 nm in the 190?220 nm band, 0.1 nm in the 280?300 nm band, and 0.1 nm in the 370?400 nm band. The 190?220 nm and 280?300 nm ultraviolet bands detect the absorption bands of SO and SO₂, providing vertical profiles of atmospheric density and temperature, revealing cloud structure and atmospheric dynamics. The 370?400 nm band can resolve unknown ultraviolet absorber and specific sulfur compounds such as OSSO and the presence of FeCl₃, identifying their concentrations and characteristic features. An ultraviolet spectral imager achieves imaging detection with a resolution of 5 nm in the 190?500 nm band, enabling the spatial localization of ultraviolet absorbers in the Venusian cloud layers. Furthermore, it produces wide-field images with lower wavelength stray light during imaging, resulting in superior wide-field ultraviolet images of Venus.

The “two wide and one fine” design concept is proposed and the Venus atmosphere detection spectrometer system is designed. The detection system consists of three spectrometers, namely the ultraviolet spectrometer, the ultraviolet super-fine spectrometer and the ultraviolet spectral imager. To reduce the volume of the instrument, the spectrometers are designed in an integrated manner. The ultraviolet spectrometer adopts the Offner structure design, consisting of a pre-telescope and a spectrometer part,as shown in Fig. 1. The pre-telescope system adopts an off-axis three-mirror structure to simultaneously receive the spectral signal to be detected without chromatic aberration. After passing through the slit, the convex grating disperses the signals of different wavelengths. The dispersed spectral mirror is focused by the concave mirror and imaged on the ultraviolet detector, achieving a coarse resolution of 0.5 nm in the 190?450 nm band. The three channels of the ultraviolet super-fine spectrometer share a set of spectrometers and also adopt the Offner structure design, consisting of a pre-telescope and a spectrometer part. One channel uses the second-order spectrum of the grating to achieve a spectral resolution of 0.1 nm in the 190?220 nm band, and the other two channels use the first-order spectrum of the grating to achieve a spectral resolution of 0.05 nm in the 275?305 nm band and 0.1 nm in the 370?400 nm band, respectively, to obtain high-resolution fine spectra of unknown absorbers. As shown in Fig. 3, the pre-optical system also adopts an all-reflective structure integrated design. The depolarization element is placed at the entrance pupil of the pre-telescope system to optimize the depolarization effect. The ultraviolet spectral imager mainly uses the small volume and light weight linear gradient filter spectrometer technology to provide wide-area monitoring of the Venus atmosphere. It adopts an off-axis three-mirror structure to achieve spectral imaging with a resolution of 5 nm in the 190?500 nm band, as shown in Fig. 7. In-orbit spectral calibration is accomplished through the instrument’s built-in mercury lamp observation mode, utilizing characteristic spectral lines at 253.728 nm, 296.815 nm, 365.120 nm, and 404.778 nm to achieve calibration precision better than 0.01 nm. The solar Fraunhofer lines serve as supplementary calibration references, enabling verification of the mercury lamp calibration method’s accuracy and facilitating calibration of spectral lines beyond those of the mercury lamp. Radiometric calibration is performed using solar spectral irradiance and a diffuse transmission panel as the standard source, with periodic long-term monitoring of the primary diffuse transmission panel’s variations conducted using a backup panel. The in-orbit radiometric calibration of the ultraviolet hyperspectral Venus spectrometer is realized through the integration of the nadir solar calibration mode and the limb solar calibration mode. During the initial orbital phase, comprehensive testing of all in-orbit calibration modes is conducted, transitioning ground calibration results to in-orbit calibration while establishing a foundational dataset. The initial solar spectral calibration data serves as the basis for long-term monitoring and correction of the system’s transfer characteristics. Taking into account the payload observation efficiency, fuel consumption costs for orbit braking and maintenance, as well as lighting conditions, a scientific observation orbit with an inclination of 30° and a periapsis depression angle of 90° has been selected. This orbit is characterized as a highly elliptical orbit of 300 km×72000 km with an orbital period of 25.6097 h (Fig. 10). Analysis indicates that the satellite’s angle β variation remains within ±30° (Fig. 11). Influenced by the geometric relationship changes in the orbits of Venus and Earth around the Sun, the Earth?satellite link distance varies with a period of approximately 590 d, ranging from a minimum of about 40 million km to a maximum of approximately 260 million km (Fig. 12). Due to Earth’s rotation, ground stations maintain visibility of Venus for approximately 6?14 h daily (Table 4). The relative geometric positions of Earth and Venus with respect to the Sun result in two solar conjunction events occurring approximately every 590 d, during which Earth?satellite communications may be disrupted by solar electromagnetic radiation, with simulations indicating a maximum duration of 16 d. Simulation analysis results for the Venus exploration window from July 1, 2025, to January 1, 2032, reveal a Venus exploration window cycle of approximately 560 d, with long transfer schemes requiring about 200 d and short transfer schemes about 100 d. The optimal launch window is determined to be November 7, 2029, with the possibility of launching within an appropriate timeframe around this date (Fig. 14).

This paper designs a scientific mission for high-spectral ultraviolet detection of the Venus atmosphere, aiming to observe and study the unknown ultraviolet absorbers in Venus cloud layers. A Venus atmosphere detection ultraviolet spectrometer system is designed based on the proposed “two wide and one fine” design concept. The detection system consists of three spectrometers, namely the ultraviolet spectrometer, the ultraviolet super-fine spectrometer, and the ultraviolet spectral imager. To reduce the volume of the instrument, an integrated design of the spectrometers is carried out. The on-orbit calibration of the ultraviolet spectral detection system, the instrument observation mode, and the satellite orbit are also designed and analyzed.