TESS Orbit DesignArising from its measurement requirements, NASA’s Transiting Exoplanet Survey Satellite (TESS) will operate from a previously unused highly elliptical orbit varying between 17 and 59 Earth radii in altitude which will provide the spacecraft with long, uninterrupted observation arcs to conduct its exoplanet hunt while also creating a benign thermal and radiation environment as it keeps out of the Van Allen Belts. But reaching this orbit will represent one of the largest challenges for the Explorer-class mission given its limitations in maneuverability due to the constrained spacecraft mass.
The operational orbit chosen for TESS is a highly eccentric, 2:1 lunar resonance orbit of 107,000 by 376,300 Kilometers at an inclination of 37 degrees, timed so that the spacecraft reaches apogee with the moon at a phasing of 90 degrees. This phasing is of particular importance because in this case the Moon’s gravity will be acting on TESS from one direction for half a sidereal month and from the other direction for half a sidereal month, thus essentially canceling each other out and only leaving solar pressure as an external source of perturbations experienced by TESS. (This setup also keeps the Moon out of the instrument field of view.)
This particular orbit was chosen for TESS for a number of factors, considering the operational aspects of the mission but also trading off between mission cost and science return. The operational orbit brings TESS sufficiently close to Earth for high-data-rate communications while also possessing a period sufficient for long, uninterrupted observations that are needed to maximize the odds of detecting exoplanet transits. Furthermore, the chosen orbit offers a very benign radiation environment since it is fully outside the Earth’s radiation belts and the spacecraft will not experience any major thermal changes which is a key requirement for its detection sensitivity.
This exact orbit, known as a P/2 orbit for its resonance characteristics, has never been used before, but the fundamentals are well understood and practical knowledge exists from the Interstellar Boundary Explorer mission that operated from a P/3 orbit.
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