哈勃望远镜刚刚拍摄到一颗罕见的星际彗星的迄今最清晰图像

Hubble's observations allow astronomers to more accurately estimate the size of the comet's solid, icy nucleus. The upper limit on the diameter of the nucleus is 3.5 miles (5.6 kilometers), though it could be as small as 1,000 feet (320 meters) across, researchers report. Though the Hubble images put tighter constraints on the size of the nucleus compared to previous ground-based estimates, the solid heart of the comet presently cannot be directly seen, even by Hubble. 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 NASA's partnership with the W.M. Keck Observatory, will help further refine our knowledge about the comet, including its chemical makeup.

Hubble also captured a dust plume ejected from the Sun-warmed side of the comet, and the hint of a dust tail streaming away from the nucleus. Hubble's data yields a dust-loss rate consistent with comets that are first detected around 300 million miles from the Sun. This behavior is much like the signature of previously seen Sun-bound comets originating within our solar system.

The big difference is that this interstellar visitor originated in some other solar system elsewhere in our Milky Way galaxy.

3I/ATLAS is traveling through our solar system at a staggering 130,000 miles (209,000 kilometers) per hour, the highest velocity ever recorded for a solar system visitor. This breathtaking sprint is evidence that the comet has been drifting through interstellar space for many billions of years. The gravitational slingshot effect from innumerable stars and nebulae the comet passed added momentum, ratcheting up its speed. The longer 3I/ATLAS was out in space, the higher its speed grew.

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

The paper will be published in The Astrophysical Journal Letters. It is already available on Astro-ph.

New Evidence for Population of Wandering Space Relics

"This latest interstellar tourist is one of a previously undetected population of objects bursting onto the scene that will gradually emerge," said Jewitt. "This is now possible because we have powerful sky survey capabilities that we didn't have before. We've crossed a threshold."

This comet was discovered by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) on July 1, 2025, at a distance of 420 million miles from the Sun. ATLAS is an asteroid impact early warning system developed by the University of Hawai'i.

In the meantime, other NASA missions will provide new insight into this third interstellar interloper, helping refine our understanding of these objects for the benefit of all. 3I/ATLAS should remain visible to ground-based telescopes through September, after which it will pass too close to the Sun to observe, and is expected to reappear on the other side of the Sun by early December.