The Earth 2.0 (ET) space mission has entered its phase B study in China. It seeks to understand how frequently habitable Earth-like planets orbit solar-type stars (Earth 2.0s), the formation and evolution of terrestrial-like planets, and the origin of free-floating planets. The final design of ET includes six 28 cm diameter transit telescope systems, each with a field of view of 550 square degrees, and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees. In transit mode, ET will continuously monitor over 2 million FGKM dwarfs in the original Kepler field and its neighboring fields for four years. Simultaneously, in microlensing mode, it will observe over 30 million I < 20.5 stars in the Galactic bulge direction. Simulations indicate that ET mission could identify approximately 40,000 new planets, including about 4,000 terrestrial-like planets across a wide range of orbital periods and in the interstellar space, ~1000 microlensing planets, ~10 Earth 2.0s and around 25 free-floating Earth mass planets. Coordinated observations with ground-based KMTNet telescopes will enable the measurement of masses for ~300 microlensing planets, helping determine the mass distribution functions of free-floating planets and cold planets. ET will operate from the Earth-Sun L2 halo orbit with a designed lifetime exceeding 4 years. The phase B study involves detailed design and engineering development of the transit and microlensing telescopes. Updates on this mission study are reported.
In terms of optical requirements and launch costs, large-diameter mirror should not only ensure fine surface accuracy, but also pursue high the rate of lightweight. Starting with material selection and shape design, the structure design of the 2 m mirror of a space remote sensor is carried out, and the preliminary mirror body is obtained. Then, combined with a platform of design optimization called Isight that integrated modeling software, finite element analysis software, data processing and analysis software, we optimized the key structural parameters of the mirror in detail, obtained a SiC mirror with the mass of 178 kg, its the rate of lightweight was as high as 90.9% and the RMS of surface shape accuracy under gravity deformation is 2.2 nm. On this basis, we designed and simulated the flexible support and other mirror components. The results indicated that the first-order natural frequency of the mirror components was 113.8 Hz, the RMS of surface shape accuracy was 8.1 nm under gravity deformation when the optical axis is horizontal, and 8.2 nm under the condition of 2°C temperature change, which were better than λ/60, could meet the requirement of the design index completely.
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