Interstellar comet 3I/ATLAS is bursting with methanol

Overview

Updated Sep 9

3I/ATLAS is only the third object ever confirmed to pass through our solar system from another star system. New ALMA observations show it is one of the most methanol-rich comets ever measured, with alcohol-to-hydrogen-cyanide ratios of roughly 70 and 120.

The methanol is a chemical fingerprint of how another planetary system made its icy bodies. The data also show the alcohol outgasses from tiny grains in the coma, not just the comet's core — the first time such behavior has been traced in an interstellar object.

Why it matters

Every interstellar comet is a direct sample of another star system. 3I/ATLAS shows they can be chemically unlike anything in ours.

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Key Indicators

70 and 120

Methanol-to-HCN production ratio

Two observing runs found ratios of roughly 70 and 120, among the highest ever measured in a comet.

Confirmed interstellar objects in our solar system

3I/ATLAS is the third, after 1I/'Oumuamua and 2I/Borisov.

>258 km

Coma-scale methanol source size

Statistical analysis shows methanol also comes from icy grains in the coma, at 99% confidence.

Voices

Curated perspectives — historical figures and your fellow readers.

People Involved

Nathan Roth

Research scientist at the U.S. National Radio Astronomy Observatory, led the ALMA analysis

Lead author of the study reporting 3I/ATLAS's methanol-rich chemistry

Organizations Involved

ALMA – Citizens for Cyprus

Observatory

Conducted the observations of 3I/ATLAS

A 66-antenna radio array in Chile's Atacama Desert that detected methanol and hydrogen cyanide in 3I/ATLAS.

National Radio Astronomy Observatory (NRAO)

Federal research center

U.S. partner in ALMA; lead authors of the study

The U.S. partner in ALMA whose researchers led the methanol analysis of 3I/ATLAS.

Timeline

August 2025 September 2026

4 events Latest: September 9th, 2026 · 2 months ago

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September 2026
Methanol-rich chemistry reported
Latest Research publication

Researchers publish methanol-to-HCN ratios of about 70 and 120, ranking 3I/ATLAS among the most methanol-rich comets ever measured.
September 9th, 2026
October 2025
Observation campaign closes
Observation

The final methanol run places 3I/ATLAS at 1.7 astronomical units from the Sun, with methanol production rising sharply toward perihelion.
October 2025
September 2025
Hydrogen cyanide detected in the coma
Observation

ALMA measures HCN, whose production traces direct sublimation from the comet's nucleus.
September 15th, 2025
August 2025
ALMA begins methanol observations of 3I/ATLAS
Observation

The Atacama Compact Array first detects methanol in 3I/ATLAS while the comet sits 2.6 astronomical units from the Sun.
August 28th, 2025

Scenarios

ALMA and JWST detect additional molecules in 3I/ATLAS

Possible Resolves by End of 2028

Discussed by: National Radio Astronomy Observatory researchers following the comet

As 3I/ATLAS recedes from the Sun, its coma will dim but may still reveal other parent molecules, such as carbon monoxide or water. A detection would test whether the methanol richness reflects the comet's formation environment or chemical processing during its journey through interstellar space.

3I/ATLAS fades without further chemical detections

Possible Resolves by End of 2028

Discussed by: Observers who study comets at large heliocentric distances

The comet's outgassing will decline as it moves away from the Sun. If the coma thins before new observing time is scheduled, the methanol data will stand as the object's complete chemical record, and the formation-versus-processing question stays open.

A fourth interstellar object is discovered before 3I/ATLAS fades

Possible Resolves by End of 2028

Discussed by: Vera C. Rubin Observatory survey planners

The Vera C. Rubin Observatory's decade-long survey is designed to find more interstellar visitors. A fourth object would give astronomers another data point on whether 3I/ATLAS's methanol-rich chemistry is typical of comets from other planetary systems or an outlier.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

October 2017

1I/'Oumuamua (2017)

The Pan-STARRS telescope spotted a fast-moving object on October 19, 2017, later named 1I/'Oumuamua — Hawaiian for "scout." It was the first object confirmed to come from beyond our solar system, with an elongated shape and no visible coma.

Then

Its odd acceleration and absence of outgassing sparked years of debate over whether it was an asteroid, a comet, or something stranger.

Now

It broke the assumption that comets must look like comets and established a naming system for interstellar objects.

Why this matters now

1I/'Oumuamua showed interstellar objects can defy easy classification; 3I/ATLAS now shows they can also be chemically extreme.

August 2019

2I/Borisov (2019)

Amateur astronomer Gennady Borisov found the second interstellar object, 2I/Borisov, on August 30, 2019. Unlike 'Oumuamua, it developed a proper coma, and telescopes detected hydrogen cyanide and carbon monoxide.

Then

Its chemistry resembled typical solar system comets, easing fears that interstellar objects were fundamentally strange.

Now

It set a baseline expectation that interstellar comets behave like local ones — a baseline 3I/ATLAS now challenges.

Why this matters now

The extreme methanol enrichment in 3I/ATLAS breaks the pattern Borisov established.

2016–2018

Comet C/2016 R2 (Pan-STARRS)

C/2016 R2 is a solar system comet from the Oort Cloud distinguished by extraordinary carbon monoxide and ion-driven chemistry. It held the highest methanol-to-HCN ratio measured in any comet until 3I/ATLAS.

Then

It became a test case for how comet chemistry varies with formation conditions.

Now

It remains the sole local reference point for extreme methanol enrichment.

Why this matters now

The only comet with a higher methanol-to-HCN ratio than 3I/ATLAS is this local oddball, sharpening the view that 3I/ATLAS formed under unusual conditions.