Ultraviolet (UV) spectroscopy is one of the most powerful tools used in a wide range of scientific fields from planetary science to astronomy. We propose a future UV space telescope, LAPYUTA (Life-environmentology, Astronomy, and PlanetarY Ultraviolet Telescope Assembly), selected as a candidate for JAXA’s 6th M-class mission in 2023. Launch is planned for the early 2030s. LAPYUTA will accomplish the following four objectives related to two scientific goals: understanding (1) the habitable environment and (2) the origin of structure and matter in the universe. Objective 1 focuses on the subsurface ocean environments of Jupiter's icy moons and the atmospheric evolution of terrestrial planets. Objective 2 characterizes the atmosphere of the exoplanets around the habitable zone and estimates their surface environment by detecting their exospheric atmosphere. In cosmology and astronomy, Objective 3 tests whether the structures of presentday galaxies contain ubiquitous Ly-α halos and reveals the physical origins of Ly-α halos. Objective 4 elucidates the synthesis process of heavy elements based on observations of ultraviolet radiation from hot gas immediately after neutronstar mergers. LAPYUTA will perform spectroscopic and imaging observations in the far-UV range of 110-190 nm with an effective area of >300 cm2 and a high spatial resolution of 0.1 arcsec. The apogee is 2,000 km, and the perigee is 1,000 km to avoid the influence of the geocorona when observing oxygen and hydrogen atoms and the Earth's radiation belt.
A small UV imager named HI (Hydrogen Imager) is under development to observe the hydrogen coma of a long-period comet or Interstellar object from space. The instrument will be aboard one of the probes for ESA's Comet Interceptor mission, scheduled for launch in 2029. This mission will remain at the Sun-Earth Second Lagrange Point (SEL2) until an optimal target (a reachable long-period comet or interstellar object) is detected through ground observation. During the cruise from SEL2 to the target, HI will observe the cometary hydrogen coma, which emits Lyman-alpha (wavelength 121.6 nm) through the resonance scattering of solar light. Additionally, during the closest approach phase, lasting several tens of hours, HI will measure Lyman-alpha emissions from both hydrogen and deuterium in the coma using switchable gas filters. The optical design, filters, and detector of HI are optimized for observing Lyman-alpha, with dimensions smaller than 100 mm × 100 mm × 250 mm and power consumption less than 9 W. The mirrors are coated with Al/MgF2 to enhance UV reflectance and maintain reflectivity during ground operations before launch. Two gas filters containing hydrogen and deuterium molecules, respectively, are installed to deduce the hydrogen/deuterium brightness ratio of the coma. A Z-stacked MCP detector assembled with a resistive anode is used, without a photocathode, to prevent degradation during ground operations.
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