Facts about comet 12P/Pons-Brooks

For around nine months now, comet 12P/Pons-Brooks has been visible through a telescope in the night sky : it is a periodic comet, the twelfth to be recognized as such. Hence its official name, supplemented by the names of its discoverers.

First discovered by the Frenchman Jean-Louis Pons in 1812, this comet was rediscovered by William R. Brooks in 1883, when it made its next appearance.

A "cousin" of Halley’s comet

Although comet 12P/Pons-Brooks is much less famous than Halley’s comet, it does share some similarities.

Both originate from the Oort cloud and have similar periods :

• 71 years for Pons-Brooks ;
• 76 years for Halley.

Their perihelia are respectively 0.78 AU (nearly 120 million kilometers) and 0.59 AU from the Sun ;

Their inclinations are fairly high : 74° for Pons-Brooks and 162° for Halley.

Of comet 12P/Pons-Brooks, we know neither its size (only an upper limit of 34+-12 km in diameter has been deduced from its rediscovery at 12 ua from the Sun in 2020), nor its shape, mass or volume.

All we know is that its core is probably a little larger than Halley’s, - as its outgassing rate this winter was three times higher than that measured in 1986 for Halley, at the same distance from the Sun, exceeding 30 tons of water per second, at 1 AU from the Sun, this month.

However, as its minimum distance from the Earth is always greater than 0.2 AU, it does not give rise to a swarm of shooting stars.

A hyperactive comet !

Comet 12P/Pons-Brooks has a very special characteristic : multiple bursts of activity observed during its previous appearances. Since June 2023, this singular behavior has manifested itself on several occasions ;

Brightness bursts can reach a factor of between 20 and 100 in 24 hours, and are followed by rapid decay. Almost daily monitoring since November 2023 at the Large Radio Telescope in Nançay has also revealed, on more than three occasions, sudden increases in its outgassing rate.

"After Hale-Bopp in 1997, 12P/Pons-Brooks is one of the most active comets to have been observed within 1 AU of the Sun at Nançay for decades", says Nicolas Biver, CNRS researcher at the Laboratory for Space Studies and Astrophysical Instrumentation (LESIA) at Paris Observatory - PSL.

Independently of its bursts, the comet becomes progressively brighter as it approaches the Sun ; it will reach its perihelion on April 21, 2024, but at a distance of 1.60 AU from the Earth (i.e. 240 million kilometers), its magnitude will be of the order of 4.5.

It will pass through perigee on June 2, at 1.55 AU from the Earth, so observation conditions are not optimal.

In mainland France, the comet is currently visible in the early hours of the morning. At the spring equinox (Wednesday March 20, at 4 h 06 min (TLF)), it becomes an evening star, observable one hour after nightfall. Around April 10, it will fade into twilight.

On June 2, it will tip over into the southern hemisphere, where it will be visible in the early evening.

For several months now, LESIA’s "Comets" team has been studying the comet closely using several radio telescopes. Observations will continue at the Nançay radio telescope, IRAM, Noema, ALMA (Chile)... The aim is to learn more about its size, composition and the physical processes responsible for its activity.