A team of astronomers has captured the sharpest image yet of the unexpected interstellar comet 3I/ATLAS using NASA’s Hubble Space Telescope.

Hubble is one of several NASA missions observing this comet to gather detailed information about its size and physical properties. While the comet poses no threat to Earth, observations from NASA’s space telescopes support efforts to find, track, and better understand near-Earth objects.

Hubble’s observations have allowed scientists to more accurately estimate the size of the comet’s solid, icy nucleus. Researchers report that the nucleus could be as large as 3.5 miles (5.6 kilometers) in diameter or as small as 1,000 feet (320 meters).

Although Hubble has provided tighter constraints on the nucleus size compared to earlier ground-based measurements, the solid core of the comet cannot be directly seen even by Hubble. Additional observations from other NASA missions, including the James Webb Space Telescope, TESS (Transiting Exoplanet Survey Satellite), and the Neil Gehrels Swift Observatory, as well as from the W.M. Keck Observatory, will help scientists refine their knowledge of the comet’s composition.

Hubble also captured images of a dust plume being ejected from the side of the comet warmed by the Sun, along with a faint dust tail trailing behind the nucleus. The data indicate a dust loss rate similar to what is seen in comets first detected at roughly 300 million miles from the Sun. This behavior is consistent with comets originating within our solar system.

The key difference is that 3I/ATLAS is an interstellar visitor that came from another solar system somewhere in the Milky Way galaxy. The comet is moving through our solar system at about 130,000 miles (209,000 kilometers) per hour, the fastest speed ever recorded for a visitor from outside the solar system. This high velocity suggests the comet has drifted through interstellar space for billions of years. Its speed increased over time due to gravitational interactions with stars and nebulae it passed along the way.

“No one knows where the comet came from. It’s like glimpsing a rifle bullet for a thousandth of a second. You cannot project that back with any accuracy to figure out where it started,” said David Jewitt of the University of California, Los Angeles, the science team leader for the Hubble observations.

The findings will be published in The Astrophysical Journal Letters and are already available on Astro-ph.

Jewitt noted that 3I/ATLAS is part of a growing population of interstellar objects being discovered thanks to powerful sky surveys. “This is now possible because we have powerful sky survey capabilities that we didn’t have before. We have crossed a threshold,” he said.

The comet was discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS), a NASA-funded early warning system developed by the University of Hawai’i. At discovery, the comet was about 420 million miles from the Sun.