heic2509 — Photo Release
Hubble makes size estimate of interstellar comet
7 August 2025
A team of astronomers has taken the sharpest-ever picture of the unexpected interstellar comet 3I/ATLAS, using the crisp vision of the NASA/ESA Hubble Space Telescope.
Hubble's observations are allowing 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 kilometers, though it could be as small as 320 metres across, researchers report. Though the Hubble images put tighter constraints on the nucleus size compared to previous ground-based estimates, the solid heart of the comet presently cannot be directly seen, even by Hubble. Observations from other observatories, including the NASA/ESA/CSA James Webb Space Telescope, will help 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 480 million kilometres from the Sun. This behaviour 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 roughly 210,000 kilometres 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.
This comet was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) on 1 July 2025 at a distance of 675 million kilometres from the Sun. 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.
The paper will be published in The Astrophysical Journal Letters. It is already available here.
More information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
Image Credit: NASA, ESA, D. Jewitt (UCLA), J. DePasquale (STScI)
Links
Contacts
Bethany Downer
ESA/Hubble Chief Science Communications Officer
Email: [email protected]
About the Release
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