Comets are one of those great question marks in observational astronomy. Though we can plot their orbits thanks to Newton and Kepler, just how bright they'll be and whether or not they will fizzle or fade is always a big unknown, especially if they're a dynamic newcomer from the outer solar system just visiting the inner solar system for the first time.
We had just such a surprise from a cosmic visitor over the past few weeks, as comet C/2017 S3 PanSTARRS erupted twice, brightening into binocular visibility.
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