title: "Another Interstellar Visitor Steps on the Gas: From Oumuamua to 3I/ATLAS, What Is Non-Gravitational Acceleration?" date: "2026-07-08" category: "space" author: "Zhigeng"

Another Interstellar Visitor Steps on the Gas: From Oumuamua to 3I/ATLAS

3I/ATLAS, the third interstellar visitor from beyond our solar system captured by formal astronomical observation, has been in the news for some time. Glancing through casually, all the information seemed to say it was a natural celestial body, so I did not pay much attention or look into it closely.

Yesterday, I happened upon a video discussing Musk's June 26 appearance on Joe Rogan's podcast. The two had an animated conversation covering AI, Musk's companies, rocket principles, humanity's future, and much more—an enormous amount of information. At the very margins, they also touched on 3I/ATLAS and mentioned it had non-gravitational acceleration.

The speaker may not have intended much by it, but the listener took keen interest—this sparked boundless curiosity in me.

In 2017, I had followed the first interstellar visitor ever captured by astronomical observation—Oumuamua—which was found to have anomalous non-gravitational acceleration, stirring considerable debate at the time. See this publication's May 25 essay, Oumuamua: Revelations from the Mountain Patrol Goblin and the Big Cigar, originally published on December 21, 2018, on the front page of Science and Technology Daily.

Eight years later, the third interstellar visitor, 3I/ATLAS, has also been confirmed to possess the same feature—non-gravitational acceleration.

I. The Unsolved Mysteries Oumuamua Left Behind

On October 19, 2017, the Pan-STARRS telescope at the University of Hawaii discovered a strange object. Its orbit was hyperbolic—meaning it was not bound by the Sun's gravity. It was the first extrasolar visitor definitively observed in human history.

The Hawaiians named it Oumuamua, meaning first distant messenger. China's Astronomical Terminology Review Committee cleverly translated it as Ao Momo—an enigmatic stranger.

Oumuamua had three perplexing features:

First, an extreme shape. Approximately 400 meters long, with a 10:1 length-to-width ratio, like an enormous cigar. Among the 750,000 known asteroids and comets in the solar system, none has this shape. More importantly, such an aspect ratio does not conform to the mechanical characteristics of natural celestial bodies.

Second, no cometary tail. At perihelion, it passed within just 0.25 astronomical units of the Sun, yet no telescope detected any gas or dust emission—it was not a comet.

Third, non-gravitational acceleration. This was the most inexplicable. After passing the Sun, Oumuamua accelerated on its way out—if only solar gravity were acting, it should have been decelerating at that point.

In June 2018, Nature published an analysis by 17 researchers: after accounting for the Sun, the eight planets, the Moon, Pluto, the 16 largest bodies in the asteroid belt, and relativistic effects, there remained an unexplainable acceleration.

Harvard Astronomy Department Chair Avi Loeb published in The Astrophysical Journal Letters: if all non-intelligent explanations are ruled out, what remains can only be a light sail—either debris of alien equipment, or an extrasolar spacecraft carrying its own light sail.

The debate reached no conclusion. After January 2018, Oumuamua vanished from the field of view of human telescopes.

II. Eight Years Later, Another One Arrives

On July 1, 2025, the ATLAS survey system in Chile discovered a new comet. Calculations confirmed: its orbit was also hyperbolic—it too was an extrasolar visitor. The IAU named it 3I/ATLAS—following Oumuamua (1I) and 2019's 2I/Borisov, the third confirmed interstellar visitor.

But unlike its two predecessors, 3I/ATLAS was captured early enough. Over the following year-plus, NASA's Hubble, Webb, Spitzer, TESS, MAVEN, and SPHEREx, ESA's Juice, Chile's VLT, and the Rubin Observatory under construction—all turned their lenses toward it. This was the largest single-comet observation campaign in human history.

The returns were abundant:

It is 11 billion years old—over twice the age of the Sun. Multiple independent studies, through carbon-nitrogen isotope and deuterium-to-hydrogen ratio analysis, have pinned 3I/ATLAS's formation era between 10 and 12 billion years ago. At its birth, the Milky Way was still in its youth, and nuclear reactions inside stars had only just begun producing elements heavier than helium. On July 7, 2026, ESO's VLT published the latest isotope analysis in Nature Astronomy, further confirming this conclusion.

Its chemical fingerprint does not belong to the solar system. The deuterium-to-hydrogen ratio is several times higher than that of solar system comets, and the carbon isotope ratios are entirely different from any known local sample. But at the same time, the volatile substances it carries—water, carbon dioxide, carbon monoxide—are broadly similar to those of solar system comets. No matter where in the universe, whether 10 billion years earlier or later, the basic ingredients for forming planets are much the same.

The Webb telescope detected methane on an interstellar body for the first time—and in large quantities. Compared to solar system comets, 3I/ATLAS's methane and carbon dioxide content is anomalously high, hinting that it was born in an environment very different from the solar system's—colder and richer in hydrocarbons.

SETI searches found no intelligent signals. The SETI Institute in California scanned the 1–10 GHz range using the Allen Telescope Array and ruled out all artificial signal sources.

III. Nickel, Acceleration, and 22 Anomalies—The Two Sides of 3I/ATLAS

The surprises 3I/ATLAS brought go far beyond its age and chemical fingerprint. As observations deepened, a long list of anomalies surfaced.

First, the Nickel Story

On July 20, 2025, the VLT telescope detected emission lines of nickel atomic vapor in 3I/ATLAS's coma for the first time. In subsequent observations, as many as 22 nickel spectral lines were confirmed. The JWST Webb telescope later independently confirmed nickel in the infrared band (7.507 microns).

Nickel itself is not strange—detecting nickel in comets is not unprecedented; 2I/Borisov also had it. What was strange: nickel was detected, but iron was not. In known natural bodies, nickel and iron are cosmic twins—born from the same type of supernova nucleosynthesis and almost always appearing in pairs in all observed solar system comets and asteroids. For a long period, 3I/ATLAS displayed the anomalous spectrum of nickel present, iron absent—a phenomenon never seen in 40 years of cometary observation history.

Harvard's Avi Loeb proposed a bold hypothesis based on this: such separation could hint at traces of artificial materials—in terrestrial industrial production, nickel and iron can be refined and separated, and certain high-tech alloys are pure nickel.

But mainstream astronomy offers a different explanation. According to the VLT team's original paper, the nickel atoms likely come from the decomposition of nickel carbonyl compounds, Ni(CO)4—this substance's sublimation temperature is far lower than that of iron carbonyl. As the comet slowly approaches the Sun, with temperatures gradually rising, nickel carbonyl volatilizes first, followed later by iron carbonyl. This hypothesis gained support from subsequent observations: by mid-to-late August 2025, as the comet drew closer to the Sun and temperatures rose, iron spectral lines indeed appeared. The nickel-to-iron ratio also gradually returned to normal levels for solar system comets. Nickel before iron is an interesting chemical phenomenon, but not evidence of the non-natural.

Penn State astrophysicist Jason Wright, in refuting Loeb's anomaly list point by point, wrote: The nickel-iron separation issue has some fairly standard chemical explanations; this should not be something that puzzles planetary scientists.

Then, Non-Gravitational Acceleration

This is the feature that most directly echoes Oumuamua.

3I/ATLAS's non-gravitational acceleration has been measured by multiple independent studies. An arXiv paper (2511.07450v2) concluded: the magnitude is approximately 5.5 x 10^-8 au per day squared, comparable to 2I/Borisov, and entirely consistent with ordinary carbon-monoxide-driven outgassing, requiring no unconventional physical mechanism. Another independent analysis (arXiv:2603.00782) similarly found that, after accounting for data selection bias and model uncertainty, the acceleration still falls within the normal range of cometary outgassing.

But Loeb noticed details that standard outgassing cannot easily encompass. His core argument: this acceleration contains a transverse component—the thrust direction is not entirely along the orbital direction, whereas purely symmetric outgassing should produce only radial thrust. Moreover, by his calculations, if the acceleration were entirely from gas ejection, the required mass loss would be about one-sixth of the nucleus's total mass—a material ejection of such scale should produce a correspondingly giant gas cloud visible in telescopes, yet no clear counterpart has been observed.

Another data point supporting Loeb's view: 3I/ATLAS's daily non-gravitational acceleration amplitude reaches approximately 200 km per day. What does that mean in context? 2I/Borisov was about 100 km per day, while Hale-Bopp, one of the largest comets in the solar system, was merely about 2 km per day. It is the most aggressive known example of its kind.

Then, Loeb's 22-Anomaly List

After Oumuamua, Loeb has clearly learned his lesson—instead of merely saying it is a light sail, he systematically documents every detail that does not fit the standard cometary model. Beyond nickel and acceleration, the list includes:

The orbital plane is nearly coplanar with the ecliptic—a retrograde orbit deviating by only about 5 degrees, a probability Loeb estimates at roughly 0.2%
The arrival direction is only about 9 degrees from the famous 1977 Wow! signal
Three mini-jets symmetrically distributed at roughly 120 degrees
A trajectory that passes near Mars and Jupiter but skirts around Earth
A 400-fold brightness surge at perihelion, far exceeding any known comet
After perihelion, the color shifted from red to green to blue—normally comets redshift upon heating; 3I/ATLAS blueshifted
An anti-tail—an extremely thin jet extending millions of kilometers in the anti-solar direction, with a 10:1 length-to-width ratio

Taken individually, any single item has an alternative natural explanation. Three symmetric jets—possibly three roughly evenly distributed active regions on the nucleus surface. Blueshift—possibly a specific spectral effect of carbon dioxide ice. Orbital coplanarity—survey telescopes are inherently more sensitive to objects near the ecliptic, creating observational selection bias.

But with over 20 anomalies appearing simultaneously, a segment of the research community finds it difficult to dismiss the possibility that this may not all be coincidence. Loeb's own position: using Bayesian methods, he estimates the probability of 3I/ATLAS having a non-natural origin at 30%–40%.

Most astronomers disagree with this estimate. Jason Wright's response is representative: Loeb selected features that seem interesting after observation and multiplied their probabilities—this is statistically suspect. Without first establishing a standard for what counts as anomalous—and with a sample size of only three interstellar comets—almost anything can be claimed to be rare.

An objective fact: we do not know what a typical interstellar comet looks like. The sample size is only three. The first (Oumuamua) had no tail, had non-gravitational acceleration. The second (Borisov) had a tail, behaved normally. The third (3I/ATLAS) has a tail, behaves unusually, and has non-gravitational acceleration. The only thing the three have in common: they are all extrasolar. Beyond that, they appear to be three completely different types of celestial bodies.

IV. New Variables for the Fermi Paradox

In 1950, Fermi asked: Where is everybody?

Oumuamua and 3I/ATLAS provide two specific new variables for the Fermi Paradox.

Variable One: Extrasolar visitors are more frequent than imagined. Astronomers estimate that roughly one interstellar small body may pass through the solar system each year. With the Vera C. Rubin Observatory's Legacy Survey of Space and Time beginning at the end of 2026, this number may rise sharply. After the third visitor, the fourth, fifth, tenth will not be far behind. Once the sample size expands, with anomalies having a frame of reference, the debates will diminish.

Variable Two: Non-gravitational acceleration has been observed on two completely different extrasolar bodies. Oumuamua had no tail but had acceleration; 3I/ATLAS has a tail and also has acceleration. This points to a fundamental question: if these are not spacecraft of intelligent beings, could there exist some universal physical process, not yet fully understood by humanity, that, under certain conditions, provides extra thrust to small bodies traversing interstellar space?

Two out of three: are Oumuamua and 3I/ATLAS the first and second samples of a class of natural bodies sharing the same properties? Is this class universal, and humanity's technology has only recently become capable of observing them?

Oumuamua is no longer alone. But rather than its successors answering Fermi's question, they are making the question more concrete: if, one day, the fifth, the tenth, the twentieth extrasolar visitors all sweep past bearing acceleration characteristics we do not yet fully understand—at that point, whatever the answer turns out to be, it will represent a major revision to humanity's understanding of the cosmos.