Recent analysis of new images from the NASA/ESA Hubble Space Telescope reveals that the nucleus of 3i/Atlas measures between 320 m (1,000 ft) and 5.6 km (3.5 miles).
Hubble captured this image of 3i/Atlas, which was located 446 million km (277 million miles) from Earth on July 21, 2025. Image credits: NASA/ESA/David Jewitt, UCLA/Joseph Depasquale, Stsci.
3i/Atlas was discovered on July 1, 2025, by the Asteroid Land Impact Trastor Alt System (ATLAS) funded by NASA.
Its orbit is the most dynamically extreme among recorded solar system objects.
New observations from Hubble have provided astronomers with a more precise estimation of the comet’s solid icy nucleus size.
“The maximum diameter of the nucleus is 5.6 km, but it averages around 320 m,” stated David Jewitt, an astronomer from UCLA, along with his colleagues.
“Hubble’s images offer tighter constraints on the nucleus’s size compared to earlier ground-based measurements, although the solid core of the comet is not currently visible directly even with Hubble.”
“Data from other NASA missions, like Webb, TESS, Neil Gehrels Swift Observatory, and WM Keck Observatory, will further enhance our understanding of comets, including their chemical composition.”
Hubble also detected indications of dust plumes ejected from the sun-warmed side of the comet, along with dust tails drifting away from the nucleus.
The new findings indicate a dust loss velocity that aligns with the first comet observed at roughly 480 million km (300 million miles) from the Sun.
This behavior closely resembles the characteristics of a previously seen sun-bound comet observed within our solar system.
A notable difference is that this interstellar visitor originates from planetary systems beyond our Milky Way galaxy.
3i/Atlas speeds through the solar system at an astonishing rate of 209,000 km (130,000 miles) per hour, marking the highest speed ever recorded for visitors to our solar system.
This remarkable velocity indicates that comets have been traversing interstellar space for billions of years.
The gravitational pull from countless stars and nebulae has contributed to its increased speed.
The longer 3i/Atlas journeys through space, the faster it moves.
“No one knows the origin of this comet; it’s like catching a glimpse of a bullet for a thousandth of a second,” Dr. Jewitt observed.
“You can’t accurately trace that back to determine where you commenced your path.”
“This latest interstellar traveler represents a previously undetected group of objects, enriching the narrative that has gradually unfolded.”
“This achievement is now possible due to the powerful Sky Survey capability we previously lacked. We’ve reached a new milestone.”
The team’s paper will be published in the Astrophysical Journal Letters.
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David Jewitt et al. 2025. Observations of the Hubble Space Telescope of Interstellar Interloper 3i/Atlas. apjl in press; Arxiv: 2508.02934
Source: www.sci.news
