Space Interferometry Mission


Space Interferometry Mission

(SIM) A spacebased astrometric observatory planned for launch by NASA in 2009. Placed in an Earth-trailing orbit around the Sun, SIM is designed to function as an optical Michelson interferometer operating over a wavelength range of 0.4–0.9 μm on a 10-meter baseline. The telescopic aperture is to be 0.3 meter. SIM is intended to measure the positions and distances of stars with unparalleled accuracy. With its silicon CCD detector it aims to achieve a precision of 1 microarcsecond (μas) in one measurement over a narrow field of view 1° in diameter. In this mode, SIM will search for planets orbiting nearby planets by detecting whether the stars ‘wobble’ relative to reference stars close to the same line of sight and by how much. For stars more widely separated, SIM aims to measure their absolute positions to an accuracy of 4 μas. These wide-angle measurements will be made relative to an astrometric grid of reference stars covering the whole sky. Construction of this grid is accomplished by repeatedly measuring the angle of separation between stars in overlapping grid boxes or ‘tiles’, each tile being 15° wide. The angle of 15° represents the area of sky accessible to SIM by retargeting the optical components without changing the attitude of the spacecraft. SIM aims to achieve a limiting magnitude of 20.