Interstellar Comet 3I/ATLAS | Hubble Space Telescope

Interstellar Comet 3I/ATLAS | Hubble Space Telescope

A team of astronomers has taken the sharpest picture of the unexpected interstellar comet 3I/ATLAS so far, using the crisp vision of the NASA/European Space Agency Hubble Space Telescope. This visible light image was captured by the Hubble Space Telescope’s Wide Field Camera on July 21, 2025.

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 meters across, researchers report.

The Hubble images put tighter constraints on the nucleus size compared to previous ground-based estimates. However, the solid heart of the comet cannot be directly seen by Hubble at the present time.

3I/ATLAS is traveling through our Solar System at roughly 210,000 kilometers 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.

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 kilometers from the Sun. This behavior is much like the signature of previously seen Sun-bound comets originating within our Solar System. Of course, the major difference is that this interstellar visitor originated in another Solar System in our Milky Way galaxy.

This comet was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) on July 1, 2025, at a distance of 675 million kilometers from the Sun. 3I/ATLAS should remain visible to ground-based telescopes through September 2025, before it will pass too close to the Sun to be observed. It is expected to reappear on the other side of the Sun by early December.

The scale bar is labeled in arcseconds. This 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).

Credit: NASA, ESA, D. Jewitt (UCLA)

Image Processing: J. DePasquale (STScI)
Release Date: Aug. 7, 2025

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