Interstellar

Hubble Makes Size Estimate Of Interstellar Comet 3I/ATLAS

By Keith Cowing

Press Release

ESA

August 7, 2025

Filed under 3I/ATLAS, astronomy, ATLAS (Asteroid Terrestrial-impact Last Alert System), Comet, ESA, Hubble, imaging, Interstellar, JWST

Hubble Makes Size Estimate Of Interstellar Comet 3I/ATLAS — This image of interstellar comet 3I/ATLAS was captured by the Hubble Space Telescope’s Wide Field Camera on 21 July 2025. The scale bar is labeled in arcseconds, which is a measure of angular distance on the sky. One arcsecond is equal to an angular measurement of 1/3600 of one degree. There are 60 arcminutes in a degree and 60 arcseconds in an arcminute (the full Moon has an angular diameter of about 30 arcminutes). The actual size of an object that covers one arcsecond on the sky depends on its distance from the telescope. The north and east compass arrows show the orientation of the image on the sky. Note that the relationship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above). This image shows visible wavelengths of light. Credit: NASA, ESA, D. Jewitt (UCLA); Image Processing: J. DePasquale (STScI)

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. Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS

Astrobiology, Astrochemistry, Interstellar,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

Follow on Twitter