Comet 3I/ATLAS, the third confirmed interstellar object ever detected passing through our solar system, was caught shedding dust in a dramatic teardrop-shaped cocoon as it traveled through the inner solar system. NASA’s Hubble Space Telescope captured the dust plume on July 21, 2025, just weeks after the comet was first reported on July 1, 2025. The observation has helped drive a coordinated campaign across space-based and ground-based instruments, yielding an unusually detailed look at an alien comet’s behavior inside our neighborhood.
Hubble Reveals a Dusty Cocoon Around an Alien Visitor
The Hubble Space Telescope’s image of 3I/ATLAS showed a teardrop-shaped cocoon of dust lifting off the comet’s nucleus, a clear sign that volatile ices were sublimating and dragging particles into space. NASA researchers described the early dust activity as notable compared with prior interstellar visitors. The first known interstellar visitor, 1I/’Oumuamua in 2017, showed no detectable coma at all, while the second, 2I/Borisov in 2019, behaved more like a conventional comet but was already close to the Sun when observers caught it. Comet 3I/ATLAS stands apart because it was spraying material well before reaching perihelion, which raises questions about the composition and resilience of its embedded ices.
The comet follows a hyperbolic path, meaning it will never loop back into the solar system for a second pass. Its closest approach to the Sun came around October 30, 2025, and its nearest pass by Earth was roughly 1.8 astronomical units, about 1.7 times the distance between Earth and the Sun. That geometry limited how bright 3I/ATLAS could get for backyard telescopes, but it was close enough for professional instruments to extract real science. As only the third known object from outside our solar system, the comet offered researchers a rare chance to study material forged around a distant star, carried across interstellar space for potentially millions of years.
Mars Orbiters and STEREO-A Join the Chase
While Hubble provided the sharpest optical view, NASA’s fleet at Mars added a different vantage point. On October 2, 2025, the Mars Reconnaissance Orbiter’s HiRISE camera captured high-resolution images of the comet’s dusty coma from Mars orbit, and the MAVEN ultraviolet spectrograph detected hydrogen consistent with ongoing outgassing. NASA’s Perseverance rover on the Martian surface also attempted to image the comet, turning its cameras skyward in a coordinated effort. The hydrogen detection matters because it implies 3I/ATLAS was actively releasing water, not just dry dust, even at a significant distance from the Sun, hinting that its surface may be unusually rich in easily vaporized ices.
Separately, NASA’s STEREO-A heliophysics spacecraft observed the comet using its HI1 wide-field instrument. Stacked images taken between September 11 and September 25, 2025, showed 3I/ATLAS as a bright, fuzzy source intensifying over that two-week window. The stacking technique, which combines many short exposures to pull a faint signal out of noise, confirmed the comet was growing more active as it approached the Sun. That brightening arc gave scientists a baseline to compare against later observations from Hubble and Mars-based instruments, helping them model how the dust production rate changed with solar distance and how the outflowing material shaped the comet’s extended tail.
Ground Telescopes Detect Unusual Dust Properties
According to an early analysis posted to arXiv, the Vera C. Rubin Observatory recorded pre-discovery observations of 3I/ATLAS. The paper reports commissioning-phase observations from June 21 through July 20, 2025, capturing early imaging and photometry that provided quantitative measures of activity, including dust-to-nucleus scattering ratios and multi-band color data. Those pre-discovery frames are valuable because they document the comet’s behavior at greater solar distances, before heating intensified. The Rubin data showed the comet was already active weeks before the ATLAS survey flagged it, which means whatever volatiles drive its dust production sublimate at relatively low temperatures and can loft dust long before the comet reaches the inner solar system.
Polarimetry observations from the Very Large Telescope’s FORS2 instrument and other facilities added another layer of insight. Researchers measured extreme negative polarization in the light scattered by the comet’s dust grains, a pattern that differs sharply from what solar system comets typically produce. Negative polarization at certain phase angles can indicate unusual grain sizes, shapes, or compositions. For 3I/ATLAS, the extreme values suggest the dust particles may have physical properties unlike anything seen in comets born in our own Sun’s protoplanetary disk. If confirmed by follow-up spectroscopy and modeling, that finding would be direct evidence that the raw building materials of another star system differ in measurable ways from the solids that formed planets and comets here.
Water Activity Far From the Sun Challenges Models
One of the most striking aspects of 3I/ATLAS is that it was releasing water at distances where solar heating alone should not easily drive sublimation of ordinary water ice. The hydrogen glow detected by MAVEN’s ultraviolet spectrograph, together with the dust production seen by Hubble, STEREO-A, and Rubin, points to sustained outgassing well beyond the region where typical comets become strongly active. That behavior challenges standard thermal models, which predict that water-dominated ices should remain largely frozen until a comet ventures closer to the Sun. Researchers are now exploring whether super-volatile species such as carbon monoxide or carbon dioxide could be heating subsurface layers, indirectly liberating water, or whether the comet’s structure allows sunlight to penetrate more deeply than usual.
This unexpected activity has broader implications for how interstellar comets form and evolve. If 3I/ATLAS can drive water outgassing at large heliocentric distances, it may have an unusually porous interior or a composition enriched in low-temperature ices that act as a trigger. That, in turn, would suggest the comet formed in a cold, outer region of its home planetary system, perhaps analogous to our own Oort Cloud, before being ejected into interstellar space. By comparing its behavior with that of 2I/Borisov, which more closely resembled a typical solar system comet, scientists hope to map out the diversity of building blocks that different stellar nurseries can produce and to refine models of how often such objects might wander through our neighborhood.
A Glimpse of Other Solar Systems’ Building Blocks
Because 3I/ATLAS is on a one-time, hyperbolic flyby, astronomers have only a narrow window to gather data before it fades into the dark. The coordinated response from Hubble, Mars orbiters, heliophysics missions, and major ground-based observatories reflects lessons learned from earlier interstellar visitors, when key opportunities were missed due to late discovery or limited coverage. This time, observers mobilized quickly to track the comet across multiple wavelengths, from ultraviolet signatures of escaping water to optical measurements of dust color and polarization. Each data set captures a different facet of the comet’s physical state, and together they form a composite portrait of an object born around another star.
Agencies and research teams are now working to synthesize these observations into a coherent picture of 3I/ATLAS’s size, composition, and activity history. As with many complex campaigns, the analysis will unfold over years, with new results emerging as teams reprocess images, refine models, and compare the comet to both interstellar predecessors and local analogs. For the broader public, the comet offers a tangible reminder that our solar system is not isolated but embedded in a dynamic galactic environment, occasionally visited by fragments of distant worlds. For mission planners at organizations such as NASA, it also underscores the scientific payoff that a dedicated, rapid-response mission to a future interstellar object could deliver, if engineers can design spacecraft ready to launch on short notice when the next alien traveler appears.
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*This article was researched with the help of AI, with human editors creating the final content.
