Its name is C/2023 P1 Nisimura.
This space view, from a viewpoint 35° north of the ecliptic plane, is worth enlarging (see the note at end). Grid lines on that plane are 1 AU (astronomical unit, Sun-Earth distance) apart. The comet’s path in 2023 is shown, with stalks to the plane at 1-month intervals. The line marked with a ram’s-horns symbol is the vernal equinox direction. Red lines connect comet and Earth, and comet and Sun, at the moments when they are nearest to each other.
This comet was discovered on August 11 by Japanese amateur Hideo Nishimura (co-discoverer of C/1994 N1 Nakamura-Nishimura-Machholz). It has come from the south and is in a hyperbolic orbit, meaning that it is from outside the solar system. It has never been seen before and will (if it survives) return to interstellar space.
The orbit is steeply inclined to Earth’s and in a retrograde direction (contrary to the orbits of the planets), and the comet will dive through a perihelion only 0.221 AU from the sun, closer in than Mercury’s 0.39; so its whirl through the tight inmost curve of its vast orbit is very swift.
It climbed into the north hemisphere of our sky (crossed the equatorial plane) on March 28; was at conjunction with the Sun on June 15; ascended northward through the ecliptic on Aug. 22; will be farthest north (declination about 24°) on Sep. 4; nearest to Earth (0.85 AU), Sep. 12; at greatest latitude north of the ecliptic, Sep. 16; brightest probably, Sep. 17; at perihelion, Sep. 18.
As for how bright, and how long its tail: observations so far are promising. It could well reach magnitude 3 – fairly easy for the naked eye – or even brighter.
But, as we always have to warn about a comet: its early activity may not last; or it could break up as it passes the Sun. And a fuzzy object is not as easy to see as a star of the same magnitude.
Though the comet is not yet at its nearest and brightest, it may be most findable, with binoculars, in the pre-dawn skies of August.
At the time and location of this scene, the comet is 20° high. Arrows through the moving bodies, including the comet, show their movement from 2 days previously to 2 days later, in relation to the starry background. The crescent of Venus is exaggerated 150 times in size.
As it swings tightly around through perihelion into the evening sky, the comet will be awkwardly low to the sunset horizon.
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