A green-hued comet with a long tail is streaking across a star-filled sky. The National Aeronautics and Space Administration (NASA) on the 14th (local time) released an image of comet C/2025 R3 (PanSTARRS) as the Astronomy Picture of the Day. The comet's green coma (nucleus) appears at lower left, and behind it is a sky-blue tail formed as volatile materials are heated by sunlight. The photo was taken on the 12th in Rhode Island, United States.
◇ Gas heated by the sun forms a long tail
C/2025 R3 was first captured in Sep. 2025 by the PanSTARRS telescope near the summit of the dormant volcano Haleakalā on the island of Maui, Hawaii. PanSTARRS is the English acronym for the "Panoramic Survey Telescope and Rapid Response System."
Like asteroids, comets are small celestial bodies that orbit the sun along elongated ellipses, but they differ in that they have tails composed of volatile materials. 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. Electrically charged ionized gas is pushed away by the solar wind, a stream of high-energy particles from the sun, forming a sky-blue glowing tail. The ion tail takes on a thin, narrow shape 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's brightness has increased markedly 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. The magnitude scale is a value astronomers use to measure the brightness of night-sky objects; 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.
Human eyes can see objects as faint as magnitude 6.5 under ideally dark skies without artificial light. With 10×50 binoculars, the bright nucleus of C/2025 R3 can also be observed. 10×50 binoculars magnify at 10 power (10x) and have 50 mm objective lenses (50). They let you see an object at 7 m as if it were 10 times closer than seeing it from 70 m away.
To see the comet, find a location with a clear view of the eastern horizon and set up 90 minutes before sunrise. First, locate the Great Square formed by four bright stars of Pegasus rising in the eastern sky. Comet C/2025 R3 will appear 5 degrees above Algenib, the southernmost of the four. That corresponds to about the width of three middle fingers when you stretch out your arm and point at the sky.
After passing perihelion, comet C/2025 R3 will move into the Southern Hemisphere night sky, traveling away from the sun along a path past Pisces, Cetus, Eridanus, and Orion. After that, it will not be seen again in our lifetimes.