A green-hued comet with a long tail is streaking across a star-filled sky. NASA on the 14th released an image of comet C/2025 R3 (PanSTARRS) as the Astronomy Picture of the Day. The comet's green coma (nucleus) is seen at lower left, and behind it appears a sky-blue tail formed as volatile material is heated by sunlight. The photo was taken on the 12th in Rhode Island.
◇ Gas heated by the sun forms a long tail
C/2025 R3 was first detected in September 2025 by the Pan-STARRS telescope near the summit of the dormant volcano Haleakalā on Maui, Hawaii. Pan-STARRS is the English abbreviation for the Panoramic Survey Telescope and Rapid Response System.
Like asteroids, comets are small bodies that orbit the sun along long ellipses, but they differ in that they have tails made of volatile material. The most recently discovered comet in the solar system, C/2025 R3, is estimated to have a tail with a diameter of several kilometers.
A comet's tail begins at an invisible icy nucleus. The nucleus is heated by sunlight and releases a cloud of gas. Ionized gas is pushed away by the solar wind—the stream of high-energy particles from the sun—forming a sky-blue glowing tail. The ion tail is thin and narrow because the structure of the solar wind changes constantly.
Comet C/2025 R3 will be best seen in the eastern sky before dawn over the next week, and after the 25th it will be visible in the southern sky. The comet has brightened significantly as it approaches the inner solar system. It is expected to brighten further ahead of its perihelion passage, its closest approach to the sun, on the night of the 19th.
◇ Perihelion, closest to the sun, on the 19th
According to observations recorded in comet observation databases, C/2025 R3 has already reached an estimated magnitude of +4.7. Magnitude is a value astronomers use to measure the brightness of celestial objects in the night sky; the lower the number, the brighter the object. The current magnitude is bright enough to be seen as a faint point with the naked eye under a dark sky.
The human eye can see objects up to magnitude 6.5 under ideally dark skies with no artificial light. Through 10×50 binoculars, the bright nucleus of C/2025 R3 can also be observed. 10×50 binoculars provide 10x magnification and have 50 mm objective lenses. They let you see an object 7 meters away as if it were 10 times closer than viewing it from 70 meters away.
To see the comet, find a location with a clear view of the eastern horizon and set up 90 minutes before sunrise. First, look for the Great Square formed by four bright stars in Pegasus rising in the east. Comet C/2025 R3 appears 5 degrees above Algenib, the southernmost of the four stars. That corresponds to roughly the width of three middle fingers at arm's length.
After perihelion, comet C/2025 R3 will move into the Southern Hemisphere night sky, following a path away from the sun through Pisces, Cetus, Eridanus, and Orion. After that, it will not be seen again in our lifetimes.