“What is the difference between a mountain and a pebble when it’s whizzing by at 210,000 kilometers per hour?” For astronomers that have been monitoring the interstellar comet 3I/ATLAS, the reply is in the Hubble Space Telescope’s optics accuracy. In July, Hubble provided the clearest images to date of this unusual visitor, reducing earlier size estimates for its frozen nucleus from a wide 10–20 kilometers to an upper limit of only 5.6 kilometers and potentially as low as 320 meters. Even at this lower limit, it would compare to or surpass the size of the first two detected interstellar objects, 1I/’Oumuamua and 2I/Borisov, which had dimensions of about 100 meters and 500 meters, respectively.
The solid core of the object is concealed in a thick coma of dust, ice, and gas, but Hubble’s resolution enables scientists to estimate its size by simulating the brightness profile of the coma. David Jewitt of UCLA, leader of the Hubble science team, pointed out that “no one knows where the comet came from. It’s like glimpsing a rifle bullet for a thousandth of a second.” Its path is hyperbolic, unmistakable evidence of a birth place outside the Sun’s gravitational influence, and its speed the fastest ever recorded for a solar system visitor has been accelerated over billions of years through gravitational passes by stars and nebulae. Hubble’s images also showed a dust plume emanating from the Sun-side of the nucleus, as well as a faint tail streaming off under solar radiation pressure.
The orientation of the plume, towards the Sun instead of away, is characteristic of comets still far from perihelion, when early solar heating finds its way into subsurface ice pockets. Such activity has been seen in solar system comets at distances greater than Mars, where volatile ices like carbon monoxide or carbon dioxide sublimate long before water ice becomes active.
For 3I/ATLAS, the rate of dust loss is estimated between 6 and 60 kilograms per second equivalent to expelling the mass of a small automobile every few minutes. The comet’s activity now at almost four astronomical units from the Sun has raised scientists’ curiosity. Spectroscopy with the Neil Gehrels Swift Observatory found water vapor in its tail at distances where solar warming would otherwise be too weak to sublimate water ice. To achieve the recorded output, about 20 percent of the surface would have to be active much greater than normal for solar system comets.
This may imply that 3I/ATLAS existed in a more ancient, frigid star system where ice is relatively more abundant with respect to other volatiles, or that its surface still carries pristine, unprocessed material from the periphery of its birth disk. The imaging technical challenge of such a rapidly moving target is no trivial one. Hubble’s tracking system adjusted for the comet’s apparent motion against background star streaks in the processed images. A blue filter was employed to optimize contrast within coma and tail structures. These data, coupled with ground-based photometry, allow for size constraints through scattering models that adopt low albedo around 4 percent, like charcoal characteristic of cometary nuclei.
NASA has coordinated a concerted effort to test 3I/ATLAS’s makeup and shape prior to its disappearing into interstellar space. The James Webb Space Telescope will pinpoint its infrared spectrum to look for molecular signatures, while the Transiting Exoplanet Survey Satellite (TESS) has already taken serendipitous pre-discovery images showing initial activity. Keck Observatory adaptive optics will improve nucleus size and rotation determinations, and the Neil Gehrels Swift Observatory will keep observing gas production rates.
Comparisons with its antecedents show the variety of interstellar visitors. ‘Oumuamua had no visible coma but displayed non-gravitational acceleration, implying outgassing of volatiles that were invisible to optical telescopes. Borisov, on the other hand, acted very much like a run-of-the-mill long-period comet from the Oort Cloud. 3I/ATLAS seems to straddle these extremes morphologically like solar system comets, but chemically and dynamically distinctive through its extraterrestrial origin. With its closest approach to the Sun expected around October 30, 2025, at 1.4 astronomical units, 3I/ATLAS will soon be subjected to peak solar heating. This period will be critical for spectroscopic studies, as elevated activity could expose deeper layers of material, offering a direct look at the molecular inventory of a comet forged in another star’s planetary nursery. It will disappear from view by early 2026, beyond the capabilities of most telescopes, leaving behind a treasure of data that will take years to completely decipher.
