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.
The Earth 2.0 (ET) mission is a Chinese next generation space mission designed to find thousands of terrestrial-like planets including habitable Earth-like planets orbiting solar type stars (Earth 2.0s) through the transiting method, and cold and free-floating low-mass planets through the microlensing method. The mission will monitor 1.2M FGKM dwarf stars for patterns of transits with a differential photometry precision of 34 ppm for a G = 13.5 mag solar type star in a 6.5-hr exposure. ET will be operated at the Earth-Sun L2 halo orbit with a designed lifetime longer than 4 years. To increase the probability of discovering Earth 2.0s, wide field-of-view (FoV) and ultra-high photometry precision are two key features of this mission. The wide field transiting telescope design offers 500 square degrees of FOV. High photometry precision is achieved by the scientific payload design as well as high stable spacecraft pointing in both short term (jitters) and long-term (drifts). According to our photometry simulations and analysis, the ET spacecraft stability requirement is not the usual relative pointing error (RPE) applied in most space missions, but the forward sum stability, in which both high frequency jitters and low frequency drifts are critical for high precision photometry measurements. Therefore, the spacecraft design needs to not only deal with high frequency jitters, but also the thermal-elastic effects of scientific payloads, including long-term thermal stability of the telescope structure, cameras, fine guiding camera, and mounting plate. This paper presents the pointing stability definition suitable for the ET mission. Simulations of high precision photometry observations with different pointing stability scenarios are presented. Approaches to the high stability are also discussed.
The Earth 2.0 (ET) mission is a Chinese next-generation space mission to detect thousands of Earth-sized terrestrial planets, including habitable Earth-like planets orbiting solar type stars (Earth 2.0s), cold low-mass planets, and freefloating planets. To meet the scientific goals, the ET spacecraft will carry six 30 cm diameter transit telescopes with each field of view of 500 square degrees, and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees, monitor ~1.2M FGKM dwarfs in the original Kepler field and its neighboring fields continuously while monitoring over 30M stars in the Galactic bulge direction. The high precision transit observations require high photometry precision and pointing stability, which is the key drive for the ET spacecraft design. In this paper, details of the overall mission modeling and analysis will be presented. The spacecraft orbit, pointing strategy, stability requirements are presented, as well as the space-ground communication analysis. The ET spacecraft adopts an ultra-high photometry precision & high stable platform, largely inherited from other space science missions. The preliminary design of spacecraft which meets mission requirements is introduced, including the spacecraft overall configuration, observation modes, avionics architecture and development plan, which pays great attention to the pointing stability and huge volume science telemetry download.
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