3I/ATLAS: The Oldest Thing We've Ever Seen Is About to Buzz Jupiter

# 3I/ATLAS: The Oldest Thing We've Ever Seen Is About to Buzz Jupiter

In thirteen days, a rock between 7.6 and 14 billion years old — older than Earth, possibly older than the Sun — will make its closest approach to Jupiter. And we might have a spacecraft in position to photograph it.

What 3I/ATLAS Is

3I/ATLAS (C/2025 N1) is the third interstellar object ever detected passing through our solar system. Discovered on July 1, 2025, by the ATLAS survey telescope in Chile, it arrived from the direction of Sagittarius at 58 km/s with an orbital eccentricity of 6.14 — the most hyperbolic trajectory ever observed. For context, 1I/'Oumuamua had an eccentricity of 1.2. 2I/Borisov had 3.36. This thing is moving.

At 2.6 kilometers across (confirmed by Hubble in January 2026), it's 40 times more massive than 2I/Borisov and roughly 20,000 times more massive than 'Oumuamua. It's not a pebble. It's a mountain from another star system, and we've had eight months to study it.

What We've Learned

The JWST first observed 3I/ATLAS on August 6, 2025, and immediately found something unusual: a CO2-dominated coma with a CO2/H2O ratio of approximately 8:1. That ratio is 4.5 standard deviations above the trend for solar system comets. This object formed far from its parent star, well beyond the CO2 frost line.

Then it got weirder.

In December 2025, JWST detected methane in the coma. This was the first time methane has ever been detected in an interstellar object. The problem is that methane is hyper-volatile — it should have started sublimating before less volatile species like carbon monoxide. Yet methane was absent in all pre-perihelion observations and only appeared after the comet rounded the Sun on October 29. Nobody has a clean explanation for this. Avi Loeb has written about it. The planetary science community is still working on it.

In February 2026, scientists confirmed the first-ever detection of water from an interstellar object. A faint UV glow of hydroxyl gas showed the comet blasting water at approximately 40 kilograms per second. 1I/'Oumuamua showed no water. 2I/Borisov had no confirmed water detection. 3I/ATLAS is spraying it across the solar system.

The pre-perihelion nickel-to-iron ratio was 3.2 — more than three times the solar system average of 1.0. By January 2026, after months of solar heating had processed the outer layers, it had declined to 1.1. The original surface composition of this object is fundamentally different from anything that formed around our star.

Hubble captured a quad-jet structure in January 2026 during a rare opposition alignment: two jets pointing sunward and two anti-sunward, creating an anti-tail. The spin period is 16.16 hours.

The age estimate, derived from Galactic kinematics, puts 3I/ATLAS at 7.6 to 14 billion years old with 68% confidence. It comes from the thick Galactic disk population. The universe is 13.8 billion years old. This rock may have been drifting between stars for nearly the entire history of the cosmos.

The Jupiter Encounter

On March 16, 2026, 3I/ATLAS will pass Jupiter at 0.358 AU — roughly 53.5 million kilometers. This matters for two reasons.

First, Jupiter's tidal field may trigger fresh outbursts or even nucleus fracturing. Comet Shoemaker-Levy 9 broke apart approaching Jupiter at a comparable distance. 3I/ATLAS is far more massive and on a very different trajectory, but the gravitational stress could crack open subsurface reservoirs and expose fresh material. If the delayed methane appearance is related to layered composition — volatile-poor crust over volatile-rich interior — Jupiter might literally peel the lid off.

Second, NASA's Juno spacecraft is still in Jovian orbit. The Minor Planet Center is refining 3I/ATLAS's trajectory to determine whether Juno can image the comet during the encounter. Juno is low on fuel and has limited maneuvering capability, but even low-resolution JunoCam imagery would be historic — the first close-range observation of an interstellar body. ESA's JUICE already captured the comet from 66 million kilometers in November 2025 using five science instruments. The data, downlinked in February after JUICE emerged from solar conjunction, showed a bright coma and extended tail. Juno could do better. Maybe.

The Spacecraft That Already Looked

JUICE's observation deserves more attention than it's gotten. ESA pointed a Jupiter-bound spacecraft at an interstellar comet and captured images with JANUS (camera), MAJIS (spectrometer), SWI (submillimeter), PEP (particle detector), and UVS (ultraviolet spectrograph). Five instruments, one interstellar target. The images released in February 2026 are the first spacecraft observations of any interstellar object, period.

Breakthrough Listen pointed the Allen Telescope Array at 3I/ATLAS across 1-9 GHz in July 2025. The Green Bank Telescope — the world's largest steerable radio dish — ran the most sensitive technosignature search ever conducted on an interstellar object across 1-12 GHz in December. MeerKAT observed at 900-1670 MHz with sensitivity down to 0.17 watts. No artificial signals detected.

TESS observed from January 15-22, 2026. SPHEREx tracked brightening through February. Hubble has been on it continuously. The Gran Telescopio Canarias, Palomar, Apache Point, and SOAR have all contributed spectroscopy. This is the most observed interstellar object in history, and it's not close.

What Happens After Jupiter

Ground-based observability continues through approximately May 2026, when 3I/ATLAS enters a second solar conjunction. After re-emerging around September 2026, the comet will be beyond Saturn's orbit and fainter than magnitude 20 — effectively unreachable for most telescopes.

There have been proposals for direct spacecraft missions to intercept interstellar objects, but none can be launched in time for 3I/ATLAS. The object is simply moving too fast. At 58 km/s, it will leave the solar system and never return. Every observation between now and May is irreplaceable.

Why This Matters

Three interstellar objects in eight years. The first was a dry, disk-shaped anomaly with unexplained acceleration. The second was a relatively normal small comet. The third is a 2.6-kilometer CO2-dominated mountain with delayed methane outgassing, water spray, anomalous nickel-iron ratios, and an estimated age that spans most of cosmic history.

The sample size is three. That's enough to know the galaxy is full of these things — models estimate 10 trillion interstellar objects per cubic parsec of the Milky Way — but not enough to draw conclusions about what's typical. 'Oumuamua was weird. Borisov was normal. 3I/ATLAS is weird in completely different ways than 'Oumuamua was weird. Every new detection rewrites assumptions.

The Jupiter encounter on March 16 is the next inflection point. If the nucleus cracks, we get fresh interior composition data from an object that predates our solar system. If Juno can image it, we get the first close-range portrait of something born around another star. If nothing happens, we still have the most comprehensive dataset ever collected on interstellar material — CO2, water, methane, nickel, iron, dust, jets, spin rate, age — all from a single visitor that will be gone by autumn.

Thirteen days.

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*Her name was Renee Nicole Good.*

*His name was Alex Jeffery Pretti.*