In response to an evaluation of the brand new pictures from the NASA/ESA Hubble Area Telescope, the nucleus of 3I/ATLAS is between 320 m (1,000 toes) and 5.6 km (3.5 miles) throughout.
Hubble captured this picture of 3I/ATLAS on July 21, 2025, when the comet was 446 million km (277 million miles) from Earth. Picture credit score: NASA / ESA / David Jewitt, UCLA / Joseph DePasquale, STScI.
3I/ATLAS was found by the NASA-funded Asteroid Terrestrial-impact Final Alert System (ATLAS) on July 1, 2025.
Its orbit is probably the most dynamically excessive of any object but recorded within the Photo voltaic System.
The brand new Hubble observations allowed astronomers to extra precisely estimate the scale of the comet’s strong, icy nucleus.
“The higher restrict on the diameter of the nucleus is 5.6 km, although it could possibly be as small as 320 m throughout,” College of California, Los Angeles astronomer David Jewitt and his colleagues mentioned in a press release.
“Although the Hubble pictures put tighter constraints on the scale of the nucleus in comparison with earlier ground-based estimates, the strong coronary heart of the comet presently can’t be immediately seen, even by Hubble.”
“Observations from different NASA missions together with Webb, TESS, and the Neil Gehrels Swift Observatory, in addition to W.M. Keck Observatory, will assist additional refine our data in regards to the comet, together with its chemical make-up.”
Hubble additionally captured a mud plume ejected from the Solar-warmed aspect of the comet, and the trace of a mud tail streaming away from the nucleus.
The brand new information yield a dust-loss charge per comets which might be first detected round 480 million km (300 million miles) from the Solar.
This habits is very like the signature of beforehand seen Solar-bound comets originating inside our Photo voltaic System.
The massive distinction is that this interstellar customer originated in a planetary system elsewhere in our Milky Method Galaxy.
3I/ATLAS is touring by means of our Photo voltaic System at a staggering 209,000 km (130,000 miles) per hour, the best velocity ever recorded for a photo voltaic system customer.
This breathtaking dash is proof that the comet has been drifting by means of interstellar area for a lot of billions of years.
The gravitational slingshot impact from innumerable stars and nebulae the comet handed added momentum, ratcheting up its pace.
The longer 3I/ATLAS was out in area, the upper its pace grew.
“Nobody is aware of the place the comet got here from. It’s like glimpsing a rifle bullet for a thousandth of a second,” Dr. Jewitt mentioned.
“You possibly can’t undertaking that again with any accuracy to determine the place it began on its path.”
“This newest interstellar vacationer is considered one of a beforehand undetected inhabitants of objects bursting onto the scene that can steadily emerge.”
“That is now potential as a result of we have now highly effective sky survey capabilities that we didn’t have earlier than. We’ve crossed a threshold.”
The workforce’s paper shall be revealed within the Astrophysical Journal Letters.
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David Jewitt et al. 2025. Hubble Area Telescope Observations of the Interstellar Interloper 3I/ATLAS. ApJL, in press; arXiv: 2508.02934
