The effect of pointing stability on Earth 2.0 space mission’s photometry precision

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.