What word does the first T in NASA's TESS mission name stand for?

The first T in NASA’s TESS mission name stands for “Transiting.” TESS expands to Transiting Exoplanet Survey Satellite, describing its search for planets that pass across the faces of their host stars.

During a transit, a planet blocks a very small fraction of its star’s light. TESS measures these periodic dips with four wide-field cameras, allowing scientists to identify candidate planets and estimate properties such as their orbital periods and sizes.

NASA launched TESS on 18 April 2018 aboard a SpaceX Falcon 9 rocket. Rather than concentrating on a small, distant patch of sky like the original Kepler survey, TESS was designed to examine about 85% of the sky, especially bright, nearby stars that are easier to study with ground-based telescopes and the James Webb Space Telescope.

“Transiting” is sometimes confused with “telescope” because TESS is a space telescope, but the acronym names the detection method and the survey itself. The mission’s target is not merely to photograph exoplanets; it finds them through the repeating shadows they cast.