On 26 September, 2018, the comet 46P/Wirtanen underwent an outburst, brightening by a factor of 60% over the course of just a few hours… and TESS saw the whole thing.
TESS — the Transiting Exoplanet Survey Satellite — is a space-based observatory that observes huge chunks of the sky for weeks at a time, measuring the brightness of everything in its field of view with exceptional accuracy. Its primary mission is to look for changes in brightness from stars when exoplanets transit them, causing a tiny drop in their light. But it looks over such a large area of sky it sees all manners of things, including comets.
46/P Wirtanen is a comet that orbits the Sun on an ellipse that takes it from just outside Earth's orbit to just inside Jupiter's every 5.4 years. It passed Earth in December 2018 by less than 12 million kilometers and was intensely studied as it did.
By happenstance, it was in TESS's field of view from 20 September to 17 October, 2018. During that period it went from about 250 to 200 million kilometers from the Sun, and closed its distance to Earth from roughly 90 to 50 million kilometers. Happily, the ...