Comet 3I/ATLAS shows unusual dust patterns as it zips through our solar system

A rare visitor from beyond our neighborhood is moving through the inner solar system, and astronomers have turned the Hubble Space Telescope toward it. The object is 3I/ATLAS, only the third interstellar body ever confirmed, and it is racing in on a hyperbolic path at roughly 58 km per second, about 130,000 miles per hour.

Hubble caught 3I/ATLAS on July 21, 2025, from a distance of about 277 million miles and recorded a compact, teardrop-shaped cloud of dust surrounding an unseen core.

3I/ATLAS has an unusual path

David Jewitt of the University of California, Los Angeles (UCLA), is leading the Hubble analysis, and his team is focused on what the comet’s structure and activity reveal about its history.

Their work centers on how sunlight energizes the surface and lifts dust into space, even far from the Sun.

An interstellar object is one that formed around another star and is not gravitationally bound to the Sun.

A hyperbolic orbit means the object’s path is open, so its speed at great distance remains above escape velocity and it will never return.

The point of closest approach to the Sun, known as perihelion, comes in late October 2025. The trajectory is highly tilted, nearly retrograde, which underscores how different this traveler is from most solar system comets.

Hubble sees dust cloud and tail

“The object is clearly active at 3.8 au pre-perihelion,” wrote Jewitt. That activity produces a coma, the fuzzy envelope of dust that hides the nucleus and feeds a faint tail.

From the surface brightness profile, the team capped the nucleus radius at 2.8 kilometers (1.74 miles), implying a diameter under about 5.6 kilometers (3.5 miles), with most of the light coming from dust lofted off the warm, Sun-facing side.

The Hubble images show a broad fan of material directed sunward and a weaker tail pushed away by radiation pressure, the force of sunlight on small grains. The pattern points to intense activity on the day side, while the night side contributes little.

Dust moves from 3I/ATLAS surface

Modeling of the coma indicates a dust mass loss rate on the order of 6 to 60 kilograms per second, depending on typical grain size.

The numbers line up with modest, steady sublimation, where sublimation is ice turning directly into gas that drags dust with it.

Speeds inferred from the Hubble data are small compared with gas thermal speeds, which suggests weak coupling or larger particles.

The analysis finds dust launched toward the Sun at tens of meters per second and spreading more slowly perpendicular to the orbit plane, consistent with a broad sunward fan and a diffuse, pressure-swept tail.

The shape of the surface brightness profile steepens with distance from the center, which can happen if fragile ice-rich grains sublimate or break apart as they drift outward. That behavior has been seen in many comets and fits an object warming for the first time in ages.

Many telescopes track 3I/ATLAS

Between June 21 and July 7, the Rubin Observatory spotted 3I/ATLAS in its images and tracked how bright and active it was.

Based on those observations, Colin Orion Chandler from the NSF-DOE Vera C. Rubin Observatory reported that the object likely has a nucleus about 5.6 kilometers (3.48 miles) across, give or take 0.7 kilometers (0.43 miles), and shines with a brightness equivalent to an absolute V-band magnitude of about 13.7.

NASA’s Transiting Exoplanet Survey Satellite provided pre-discovery coverage, catching the object between May 7 and June 2 and showing a slow brightening over that period. Those data extend the light curve and improve the orbit before the official July 1 discovery.

A Monthly Notices Letters report summarizes discovery details, photometry, and the object’s interstellar classification, building a baseline for what observers should expect as activity grows near the Sun.

How 3I/ATLAS differs from other visitors

The first visitor, 1I/‘Oumuamua, showed no visible coma and exhibited an excess speed of about 26 km per second, an unusual case that still sparks debate about its composition and outgassing. Its light curve and color set it apart from typical small bodies.

The second, 2I/Borisov, behaved much more like a comet, with clear gas and dust and an excess speed near 32 kilometers per second (19.88 miles per second) as measured from production rates and imaging across perihelion. That object provided the first sustained look at interstellar volatiles.

3I/ATLAS is different again, with a higher inbound speed and a coma that already dominates the optical cross section. The mix of sunward dust emission and a faint pressure tail hints at large particles and patchy activity.

What scientists hope to learn next

Spectroscopy over the coming months can nail down the ices that drive activity and how they vary with distance from the Sun.

The big question is whether the coma is mostly water-driven or if more volatile species like carbon monoxide or carbon dioxide are doing the heavy lifting.

Rotational signals may emerge if jets or asymmetric activity modulate brightness on short timescales.

Any secure period or pole direction would help explain the sunward fan and tell us how heat flows into the surface layers after eons in the deep cold.

The study is published in The Astrophysical Journal Letters.

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