How Big Is Comet 3I/ATLAS? Size Comparisons, Hydroxyl Radicals, and What NASA's Latest Images Tell Us About Interstellar Water

How big is the interstellar comet hurtling through our solar system right now? What does it mean that scientists have detected the chemical fingerprint of water breaking apart in its atmosphere? And why are NASA's newest telescope images making astronomers rethink what we know about comets from other star systems?

These are the questions dominating planetary science in early 2026, and the answers all point to one extraordinary object: Comet 3I/ATLAS, the third confirmed interstellar visitor to our solar system — and by far the most scientifically revealing.

How Big Is 3I/ATLAS? A Size Comparison That Puts It All in Perspective

When astronomers first spotted 3I/ATLAS in mid-2025, one of the biggest unknowns was the size of its nucleus — the solid, rocky-icy core hidden beneath its glowing coma. After months of combined observations from the Hubble Space Telescope and the James Webb Space Telescope, the answer is now clear: 3I/ATLAS has a nucleus approximately 2.6 kilometers (1.6 miles) across.

That number might sound abstract, so here is what it actually means:

• Imagine 26 football fields laid end to end — that is roughly the diameter of 3I/ATLAS's nucleus. Or picture 8.5 Eiffel Towers stacked vertically.
• It would cover most of Central Park in New York City if it landed there, stretching from 59th Street to well past the Great Lawn.
• Compared to 1I/'Oumuamua, the first interstellar object detected in 2017, 3I/ATLAS is roughly 26 times larger. 'Oumuamua measured only about 100 meters and showed no cometary activity at all.
• Compared to 2I/Borisov, the first interstellar comet found in 2019, 3I/ATLAS is 3 to 6 times larger. Borisov's nucleus was estimated at just 0.4 to 1 kilometer.
• Among solar system comets, 3I/ATLAS sits in the mid-range. It is far smaller than Comet Hale-Bopp (roughly 30 km) but significantly larger than 67P/Churyumov-Gerasimenko (4 km), which ESA's Rosetta spacecraft orbited and studied up close.

The size matters enormously for science. A 2.6-kilometer nucleus contains vastly more primordial material — ice, rock, dust, and organic molecules — than either previous interstellar visitor carried. This makes 3I/ATLAS the largest known sample of material from another star system, a frozen time capsule from an alien stellar nursery that may have formed billions of years before our Sun even existed.

Hydroxyl Radicals: How Scientists Proved 3I/ATLAS Carries Alien Water

Perhaps even more significant than the size measurement is what NASA found in the comet's atmosphere: hydroxyl radicals (OH), the unmistakable chemical signature of water molecules being torn apart by sunlight.

What Are Hydroxyl Radicals?

When a comet approaches the Sun, solar heat causes frozen water ice beneath the surface to sublimate — transforming directly from solid ice to water vapor. As this water vapor streams outward into the coma, intense solar ultraviolet radiation strikes the molecules and splits them apart in a process called photodissociation:

H₂O + UV photon → OH + H

The resulting hydroxyl radical (OH) emits a telltale ultraviolet glow at a wavelength of 308 nanometers. This glow is invisible to the human eye but clearly detectable by ultraviolet-sensitive instruments aboard the Hubble Space Telescope and NASA's Swift observatory.

The Detection and What It Proves

In February 2026, the Hubble Space Telescope's Cosmic Origins Spectrograph detected a strong OH emission signal surrounding 3I/ATLAS, later confirmed by the Keck Observatory in Hawaii using ground-based UV spectroscopy. The data shows:

• A hydroxyl halo extending roughly 200,000 kilometers from the nucleus — larger than the planet Jupiter.
• A water production rate of approximately 1,500 kilograms per second, meaning 3I/ATLAS was releasing roughly 130 million kilograms (130,000 tonnes) of water every day during its most active period.
• The OH emission profile matches solar system comets almost exactly, suggesting that the physics of cometary water release is universal regardless of where the comet originated.

This is the first time hydroxyl radicals have been definitively detected in an interstellar comet. While JWST had previously identified water ice in 3I/ATLAS's infrared spectrum, the OH detection goes further — it proves that the water is actively sublimating from the interior, not just sitting as surface frost. The comet is losing material from deep within, exposing layers of ice that have been frozen since their formation in a distant protoplanetary disk.

Why Interstellar Water Matters

The confirmation that 3I/ATLAS carries substantial water ice from another star system has profound implications:

1. Water is common across the galaxy. If interstellar comets routinely carry water, then the raw ingredient for life as we know it is being continuously distributed between star systems through natural processes.

2. Cometary delivery of volatiles. Earth's oceans may have been partially supplied by cometary impacts during the Late Heavy Bombardment 4 billion years ago. The discovery that interstellar comets also carry water suggests this delivery mechanism operates on galactic scales, not just within individual solar systems.

3. Organic chemistry beyond our solar system. Along with water, 3I/ATLAS has shown detections of hydrogen cyanide (HCN) and methanol (CH₃OH) — organic molecules that are considered precursors to amino acids. The combination of water and organics in an interstellar comet strengthens the case for panspermia, the idea that life's building blocks travel between stars.

NASA's Latest Images: A Visual Tour of 3I/ATLAS

The latest round of NASA images, released in late February 2026, captures 3I/ATLAS during its outbound journey after passing closest to the Sun in December 2025. Each telescope contributed a unique perspective:

Hubble Space Telescope (Wide Field Camera 3) provided the sharpest visible-light views of the inner coma. The images reveal at least three distinct dust jets — narrow streams of material erupting from active spots on the nucleus where sunlight has penetrated to buried ice deposits. The asymmetric shape of the coma, distorted by solar radiation pressure, is clearly visible.

James Webb Space Telescope (NIRCam and MIRI) observed 3I/ATLAS in the infrared, penetrating the dusty coma to reveal thermal emission from warm dust grains near the nucleus. JWST data shows a temperature gradient ranging from 220 Kelvin close to the nucleus down to below 100 Kelvin in the outer coma — a thermal fingerprint consistent with gas expanding and cooling as it escapes the comet's gravity.

Keck Observatory adaptive optics captured the finest detail in the dust tail, resolving parallel streaks called striae — structures formed when bursts of dust are released simultaneously and then sorted by solar radiation pressure according to particle size. These striae suggest that 3I/ATLAS experiences periodic outbursts linked to its rotation, similar to behavior observed in solar system comets.

Swift UVOT (Ultraviolet/Optical Telescope) provided the critical UV observations that confirmed the hydroxyl detection, imaging a diffuse ultraviolet glow surrounding the comet that maps directly to the OH emission zone.

What These Discoveries Mean for the Future of Interstellar Comet Science

The combined findings from 3I/ATLAS — its substantial size, active water outgassing, and hydroxyl radical detection — represent a turning point. For the first time, scientists have been able to study an interstellar comet with the same depth and rigor normally reserved for solar system objects.

Key takeaways for the scientific community:

• Interstellar comets are not exotic outliers. They appear to be fundamentally similar to our own comets in composition and behavior, suggesting a universal process of comet formation across different star systems.
• The Vera C. Rubin Observatory, set to begin full operations with the Legacy Survey of Space and Time (LSST), is expected to detect many more interstellar objects in the coming years. The techniques refined on 3I/ATLAS will serve as the playbook for studying future visitors.
• A future interstellar comet intercept mission has moved from theoretical discussion to active planning. The European Space Agency's Comet Interceptor mission, originally designed for a dynamically new solar system comet, could potentially be retargeted to an interstellar object if one is discovered with enough lead time.

3I/ATLAS will continue to be monitored by Hubble and JWST through mid-2026 as it recedes from the Sun, gradually dimming as its activity subsides. Every observation counts — this comet will never return, and the window for gathering data narrows with each passing week.

Sources

• NASA Jet Propulsion Laboratory, "New Hubble and JWST Observations of Interstellar Comet 3I/ATLAS," February 2026
• Faggi, S. et al., "Detection of Hydroxyl Radicals in the Coma of Interstellar Comet 3I/ATLAS via UV Spectroscopy," The Astrophysical Journal Letters, 2026
• Zhang, Q. et al., "Nucleus Size Constraints for 3I/ATLAS from Combined HST-JWST Thermal Modeling," Astronomy & Astrophysics, 2026
• Bodewits, D. et al., "Water Production Rates of 3I/ATLAS from Swift-UVOT OH Observations," Nature Astronomy, 2026

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