Usually, comet 220P/McNaught is so dim that to see it requires a telescope. During 2026, this comet underwent two outbursts on May 31 and August 5 that produced an increase in its apparent brightness 16,000 times brighter than usual. The Institute of Space Sciences (ICE-CSIC) has characterised the photometric evolution of the outbursts in order to estimate the physical properties of the comet. The results have been published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

Jupiter-family comet 220P/McNaught orbits the Sun with a period of 5,5 years and it made its closest pass to Earth on October 12, at a distance of 1.04 astronomical units (156,030,579.14 Km). However, what has captured astronomers attention are the unexpected brightness of the outbursts from this normally dormant comet. 

The study presents calibrated photometric observations obtained shortly after the first outburst on May 31 and combines them with publicly available data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey in order to get constraints on the nature and physical properties of 220P/McNaught comet. The outburst amplitude, rise time, coma morphology, dust expansion velocity, ejected mass, kinetic energy, and fractional mass loss were derived from photometric measurements and analytical modelling. 

“Jupiter Family Comets are extremely interesting objects to study because they originate primarily from the Kuiper Belt and the scattered disk region, later migrating inward until Jupiter captures them into short‑period orbits. They are apparently fragile but they are worthy of study because their orbits often cross the near-Earth region. In fact, these comets follow asteroids in the ranking of potential impact risk”, points out Josep M. Trigo-Rodríguez, leading author of the study and researcher scientist at ICE-CSIC and the Institute of Space Studies of Catalonia (IEEC).

Comet 220P/McNaught, photographed on August 7, 2026, at 03:19 UT. The image shows the comet followed by courtains of small detached pieces scattered over its multiple tails. It was taken with a 315mm Dall Kirkham telescope (f/8) and a QHY600M camera. It is a stack of 35 x 60-second exposures taken on a Paramount mount at the remote observatory in Fregenal de la Sierra, Spain (MPC Z10). Credits: Peter Carson.

Nature and behaviour of comets

Comets can increase their magnitude of brightness by millions of times, just because of their ability to suddenly release dust into the coma. The particles released are aggregates of micron-sized dust, organic matter and ices, so little by little they sublimate and disrupt around the nucleus, increasing the coma efficiency to scatter sunlight. Those evolving curtains of dust increase the number of micron-sized particles surrounding the comet, then increasing the magnitude of the comet coma in the visible range. 

The international team, also integrated by Marcin Wesolowski, Maria Gritsevich, and Peter Carson, made observations of comet 220P from Montseny Astronomical Observatory (Catalonia, Spain) and Fregenal de la Sierra Observatory (Extremadura, Spain) for its study.

“We have concluded that the comet detached localized parts of its ‘crust’, creating a dust-rich envelope that increased exponentially in 250 times the comet brightness in half a day”, says Trigo-Rodríguez, referring to the first outburst on May 31. As for the second outburst on August 5, it exhibited a brightness’ increase of eight magnitudes from July 28 to August 6, that is about 16.000 times brighter. “At that time it was widely accessible to amateur telescopes and astrophotographers”, he adds. 

These results point to the ability of these comets to release the regolith accumulated in their surfaces during more frequent approaches to the Sun. The mechanism is probably the quick sublimation of volatile ices, embedded in a fine-grained porous structure. Once these ices are exposed to sunlight the overpressure of the gas released has the ability to break the entire ‘crust’, releasing dust-rich aggregates into the coma that contribute to the energetic outbursts observed in this comet. “In this regard, it reminds us of other outbursts experienced by Jupiter Family Comets, such as the massive outburst of comet 17P/Holmes in 2007,” he notes.

Observing the distant behaviour of comets allows us to gain insights on the size and inner nature of Jupiter Family comets. Currently, the team keeps analysing comet 220P focusing on its photometric behaviour and searching for further evidence on a significant fragmentation of the nucleus, not yet noticed. 

“Particularly, we are interested in the tensile strength and nature of comets in the framework of our studies on impact hazard. The consistency of these bodies is the key to quantifying the hazard associated with asteroid and comet impacts. Comets formed far away from the Sun, and many ended as extremely fragile bodies that sometimes disrupt in front of our electronic eyes ”, says Josep M. Trigo-Rodríguez. “Some comets seem determined to demonstrate that they are fragile enough, and perhaps give us the chance to invoke disruptive approaches that might be substantially different to the ones already tested or proposed for asteroids”, he adds.