Hyperbolic Comet Nishimura

August 18, 2023

A hyperbolic comet is falling into our solar system. Japanese amateur astronomer Hideo Nishimura discovered it just a few days ago in the constellation Gemini. Although it is relatively dim right now (magnitude +9), Comet Nishimura (C/2023 P1) could soon brighten more than 100-fold to become a naked-eye object in mid-September.

A sky map with comet position for Aug. 19th. (Source: https://spaceweather.com)

A "hyperbolic comet" is a comet with too much energy to remain trapped inside the solar system. It will visit us only once, with the sun acting as a gravitational slingshot, sending the comet hurtling back into deep space after its flyby. Does that mean Comet Nishimura is an interstellar comet? Not necessarily. It might have come from the Oort Cloud. Indeed, that is more likely.

Because this is Comet Nishimura's first trip to the inner solar system, it is extra unpredictable. On Sept. 18th, the comet will make its closest approach to the sun deep inside the orbit of Mercury. Anything could happen when intense sunlight touches the comet's pristine surface for the first time. Possibilities range from dramatic brightening to a disappointing fizzle. Standard models suggest a peak brightness of 3rd magnitude. This would make it visible to the naked eye from rural areas.

Monitoring is encouraged. Comet Nishimura can be found in the pre-dawn sky using backyard telescopes larger than 6 inches. A date of particular interest is Aug. 25th when the comet lines up with Gemini's brightest stars, Castor and Pollux, making it particularly easy to find. Sky maps: Aug. 18, 19, 20, 21, 22, 23, 24, 25. Ephemeris: from JPL.