Cosmic outburst: C/2025 R2 (SWAN) surprises with an unexpected brightness surge

Newly discovered long-period comet brightens skies, poised for potentially visible to the naked-eye in October

Photo courtesy of Wikimedia Commons

Comet C/2025 R2 (SWAN) may be making its debut in our northern horizon night skies for a short period of time. This is a rare and spectacular opportunity to see a comet with the naked eye, a sight which many astronomers wait decades to experience. 

A comet is defined by astronomers at NASA as “cosmic snowball[s] of frozen gases, rock, and dust that orbit the Sun.” When a comet orbits close to the Sun, “it heats up and spews dust and gases into a giant glowing head larger than most planets.”

According to EarthSky, this comet takes more than “22,000 years to orbit the sun, making it a true once-in-a-lifetime visitor.” Referred to as SWAN was originally supposed to only be visible in the southern hemisphere. As the days progress, astronomers are saying it is more likely that the northern hemisphere will also get a glimpse of this spectacular green comet, although it will be difficult to see in our hemisphere.

According to NASA, astronomers use magnitude to determine brightness of comets, stars, and any other celestial object in the sky. The lower the number is, the brighter the object, while the higher the number, the dimmer the object is. The brightest celestial objects have a magnitude of 0 or +1. The limit for naked-eye visibility under a dark, non-polluted sky is considered to be magnitude +6.5. The SWAN comet has a current apparent magnitude of around +5.9. 

This means that the SWAN comet is just below what most people can see with the naked eye under very dark skies with no moon and almost no light pollution. Usually, naked‐eye visibility is up to a magnitude of about +6.0 (sometimes +6.5 in ideal conditions). At +5.9, it’s likely too faint for most people to see without optical help, such as a telescope.

For a short time during the northern hemisphere sunset, this comet will appear, with a visible green hue. The coloring is caused by “different elements in a comet [that] can contribute to the green hue it appears to have for Earthly observers,” as stated by howstuffworks.

Peak visibility of the comet will be from Oct. 12 to Oct. 20. This is because the comet is starting to leave perihelion, which is the closest point in its orbit to the sun. As it is leaving perihelion, the comet is approaching Earth, becoming more visible. Since the comet is at perihelion now, more gases burn up spanning across miles of gas, which is why the coma of the comet becomes visible. The coma is the part of the comet that surrounds the nucleus when it approaches the sun. The heat from our sun turns the ice of that comet into gases, forming the trail and coma which we can see.

Your ability to find this comet in the night sky will vary based on your location. In New York, it will be shortly behind Mars on the ecliptic, which is the imaginary plane of Earth’s orbit around the Sun. To see it viewers must use a telescope or binoculars, as they will not be able to see it with the naked eye.

Astronomer Vladimir Bezugly discovered the SWAN comet on Sept. 11; comets are crucial to the study of complex chemistry of life and how it possibly led to the creation of life on Earth. A theory that NASA supports is that, “some of life's ingredients [were] formed in space and were delivered to Earth long ago by meteorite and comet impacts.”