Comet Pons-Brooks

In January we took an eagle-eye look forward over five known comets likely to be observable this year, and described the first two of then, 62P Tsuchinshan and C/2021 S3 PANSTARRSS. The third, 12P Pons-Brooks, is aiming for its perihelion, the innermost point of its orbit, on April 21.

Its designation means that it was the 12th comet added to the list of those known to be periodic – returning in times of under 200 years. It was discovered in 1812 by Pons and re-discovered in 1883 by Brooks.

Jean-Louis Pons (1761-1831) worked as a janitor in observatories, mostly at Marseilles but later near Lucca and Florence in Italy. Self-taught, and using small telescopes of his own devising, he managed to see three quarters of the comets of his time, and discovered at least 37 – more than any other individual in history. Only a few of them proved to be of short period, so that they will be seen again. And many others do not bear his name, because they were pre-discoveries of comets later identified as periodic by others, such as the famous comets Encke and Crommelin.

William Robert Brooks (1844-1921), American born in Britain, discovered 27 comets, making him second only to Pons.

12P Pons-Brooks is in an orbit steeply inclined (75°) to the ecliptic and with a period of about 71 years, almost as long as Comet Halley’s 76. The orbit is highly elliptical (eccentricty 0.955, where 0 is a circle and 1 is a parabola), coming to a perihelion inside Earth’s orbit, 0.78 AU (astronomical unit, Sun-Earth distance, about 150 million km or 93 million miles) from the Sun. So the comet’s only observed visit, between the discovery and re-discovery and the present, was that of 1953-54.

The nucleus or solid body of this comet is thought to be large: about 30 kilometers wide. So it is intrinsically one of the brightest of the periodic comets.

On Feb. 21 the comet was farthest north in our sky (declination 38°). On Apr 11 it descended through the ecliptic plane, close inside our orbit but near the December part of it, hence at a distant 1.6 AU from us.

On April 21 it will be at perihelion, and it is expected around this time to reach a peak brightness of magnitude 4.4 – above the naked-eye threshold of about 5 or 6 (depending on sky conditions).

There has to be the usual caveat for comets: their actual behavior can depart wildly from prediction. They can flare, or shed so much of their cloudy material that they become like asteroids, or can break into several bodies, or disintegrate and disappear. And a fuzzy object like a comet or nebula has lower surface brightness, therefore is less easy to discern, than a star of the same magnitude.

On May 5, Pons-Brooks will cross the equator into the southern celestial hemisphere. On June 2 it will be nearest to Earth, about 1.55 AU, but by now dimmed to magnitude 6 or 7. On Sep. 22 it will be southernmost, declination -47#, magnitude 13 or 14. And on Nov. 26 its performance in our sky ends as it passes behind the glare of the Sun.

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