According to an analysis of the new images from the NASA/ESA Hubble Space Telescope, the nucleus of 3I/ATLAS is between 320 m (1,000 feet) and 5.6 km (3.5 miles) across.
Hubble captured this image of 3I/ATLAS on July 21, 2025, when the comet was 446 million km (277 million miles) from Earth. Image credit: NASA / ESA / David Jewitt, UCLA / Joseph DePasquale, STScI.
3I/ATLAS was discovered by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) on July 1, 2025.
Its orbit is the most dynamically extreme of any object yet recorded in the Solar System.
The new Hubble observations allowed astronomers to more accurately estimate the size of the comet’s solid, icy nucleus.
“The upper limit on the diameter of the nucleus is 5.6 km, though it could be as small as 320 m across,” University of California, Los Angeles astronomer David Jewitt and his colleagues said in a statement.
“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 Webb, TESS, and the Neil Gehrels Swift Observatory, as well as 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.
The new data yield a dust-loss rate consistent with comets that are first detected around 480 million km (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 a planetary system elsewhere in our Milky Way Galaxy.
3I/ATLAS is traveling through our Solar System at a staggering 209,000 km (130,000 miles) 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,” Dr. Jewitt said.
“You can’t project that back with any accuracy to figure out where it started on its path.”
“This latest interstellar tourist is one of a previously undetected population of objects bursting onto the scene that will gradually emerge.”
“This is now possible because we have powerful sky survey capabilities that we didn’t have before. We’ve crossed a threshold.”
The team’s paper will be published in the Astrophysical Journal Letters.
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David Jewitt et al. 2025. Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS. ApJL, in press; arXiv: 2508.02934