Key Takeaways:
- Hubble Space Telescope observations have provided size estimates for the nucleus of interstellar comet 3I/ATLAS, ranging from 320 meters to 5.6 kilometers in diameter, though the nucleus itself remains unresolved.
- Hubble imagery reveals a dust plume emanating from the sunward side of 3I/ATLAS and a faint dust tail, with a dust-loss rate consistent with comets initially detected at similar solar distances.
- 3I/ATLAS exhibits an exceptionally high velocity of 209,000 km/hr, attributed to its long interstellar journey and gravitational interactions with stars and nebulae.
- Further characterization of 3I/ATLAS's composition and origin will leverage data from other NASA missions, including the James Webb Space Telescope, TESS, and the Neil Gehrels Swift Observatory.
Using the Hubble Space Telescope, a team of astronomers has imaged an interstellar comet. This icy body was discovered by NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS) on July 1, 2025, when it was 420 million miles (676 million kilometers) from the Sun. ATLAS is an asteroid impact early warning system developed by the University of Hawai’i. After its discovery, the comet was assigned the designation 3I/ATLAS.
The data collected by Hubble has enabled astronomers to estimate the size of the comet’s solid, icy nucleus. The research team pegged the upper limit on the diameter as 3.5 miles (5.6 kilometers). A lower limit on the nucleus’ size is 1,000 feet (320 meters) across, according to the researchers. Although these limits are sound, even Hubble cannot see the solid nucleus.
Hubble also imaged a dust plume ejected from the sunward side of the comet, and a faint dust tail streaming away from the nucleus. Hubble’s data, collected when the comet was 277 million miles (446 million km) from the Sun, shows that it has a dust-loss rate consistent with comets that are first detected around 300 million miles (483 million km) from the Sun. So, it acts like normal comets. The big difference is that this one originated in some other solar system.
A fast mover
3I/ATLAS is speeding along at 130,000 mph (209,000 km/hr), the highest velocity ever recorded for a solar system visitor. Its velocity shows that the comet has been drifting through interstellar space for billions of years. The gravitational slingshot effect from stars and nebulae the comet passed has increased its speed.
Observations from other NASA missions including the James Webb Space Telescope, the Transiting Exoplanet Survey Satellite, and the Neil Gehrels Swift Observatory will help further refine our knowledge about the comet, including its chemical makeup.
“No one knows where the comet came from. It’s like glimpsing a rifle bullet for a thousandth of a second. You can’t project that back with any accuracy to figure out where it started on its path,” said David Jewitt of the University of California, Los Angeles, science team leader for the Hubble observations. He further stated, “This is now possible because we have powerful sky survey capabilities that we didn’t have before. We’ve crossed a threshold.”
