3I/ATLAS vs Oumuamua vs Borisov: Interstellar Visitors

Our solar system is no longer a gated community. For generations, astronomers assumed rogue rocks from other star systems routinely drifted through our neighborhood, yet we lacked the instruments to catch them. Today, three confirmed interstellar wanderers — 1I/’Oumuamua, 2I/Borisov, and 3I/ATLAS — have rewritten our understanding of galactic history and alien planetary chemistry.

What makes an object “Interstellar”?

How can scientists tell whether a comet belongs to our Sun or came from light-years away? The dead giveaway is its orbital shape and speed.

Everything native to our solar system moves in closed, elliptical loops bound by the Sun’s gravity. Comets from the distant Oort Cloud take millions of years to orbit, but they still loop back eventually. Interstellar objects do not play by those local rules.

They arrive on hyperbolic trajectories — open-ended, sling-shot paths with an orbital eccentricity greater than 1.0. They enter the solar system faster than the Sun’s escape velocity, swing around our star once, and fling back out into deep space forever.

When an object receives an official “I” designation from the International Astronomical Union (IAU), it formally confirms that this traveler was born around an alien star and was kicked out into the cosmos.

Three interstellar visitors

Hyperbolic trajectory of 3I/ATLAS. Source: NASA/JPL-Caltech / via REUTERS

1I/’Oumuamua

When Pan-STARRS 1 spotted a dim streak in October 2017, astronomers scrambled. It had already zoomed past the Sun and was heading away at blistering speed. Its Hawaiian name, ’Oumuamua, translates roughly to “a scout reaching out from the distant past.”

It looked like nothing ever observed in our solar system:

• Extreme Shape: Light curve measurements showed its brightness swung by a factor of 10 every 7.3 hours. That meant it was tumbling end-over-end, shaped either like an elongated cigar or a flat, pancake-like shard with an aspect ratio of 6:1 or higher.
• No Visible Tail: Even under extreme magnification by the Hubble Space Telescope, ’Oumuamua showed zero dust, gas, or classic cometary glow.
• The Non-Gravitational Kick: As ’Oumuamua pulled away, it accelerated slightly faster than gravity alone predicted. Without a visible dust tail pushing it like a rocket thruster, how did it gain speed?

What caused the push?

Astronomers debated this puzzle intensely. Harvard astrophysicist Avi Loeb famously suggested it might be an ultralight solar sail built by an extraterrestrial intelligence.

Mainstream planetary scientists proposed natural explanations:

1. Molecular Hydrogen Ice: A frozen iceberg of pure $H_2$ evaporating invisibly under solar radiation.
2. Entrapped Hydrogen Bubbles: Water-rich ice blasted by galactic cosmic rays during billions of years in interstellar space, releasing trapped $H_2$ gas when heated by the Sun.

’Oumuamua was weird, quiet, and left us with far more questions than answers.

2I/Borisov

If ’Oumuamua was a cosmic enigma, 2I/Borisov brought astronomers back to familiar ground. Discovered in August 2019 by Crimean amateur astronomer Gennadiy Borisov using a custom-built 0.65-meter telescope, this visitor looked exactly like a traditional comet.

It had a fluffy coma, a sweeping tail millions of kilometers long, and a solid icy nucleus roughly one kilometer wide.

What made borisov special?

When the Atacama Large Millimeter/submillimeter Array (ALMA) and Hubble inspected Borisov’s vapor, they detected staggering amounts of carbon monoxide ($CO$) — between 9 and 26 times higher than the average comet in our own solar system.

Carbon monoxide freezes only at ultra-low temperatures below -250°C (-418°F). This distinct chemical fingerprint revealed that Borisov likely condensed in the extreme outer fringes of an alien star system, perhaps around a cool M-dwarf star, before gravitational tugs from a giant exoplanet hurled it into the void.

3I/ATLAS

On July 1, 2025, the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile, flagged an inbound object moving at 61 kilometers per second (about 137,000 mph). Designated 3I/ATLAS (and cataloged as C/2025 N1), it officially claimed the title of our third confirmed interstellar guest.

3I/ATLAS blew past previous records:

• Record-Breaking Speed: Entering the inner solar system faster than both ’Oumuamua and Borisov, 3I/ATLAS accelerated to nearly 250,000 km/h (69 km/s) at perihelion on October 29, 2025.
• Hyper-Active Coma: Ground observatories and space assets — including the James Webb Space Telescope (JWST), the Very Large Telescope (VLT), and the ESA Juice spacecraft — tracked massive outbursts of dust and gas as it crossed Mars’ orbit.
• Extreme Negative Polarization: Light bouncing off 3I/ATLAS’s dust envelope displayed unusual negative polarization. Astronomers usually see this signature on trans-Neptunian objects in our own Kuiper Belt, indicating a unique blend of fine water ice and ultra-dark, carbonaceous organics.

Interstellar Velocity Hierarchy (Speed entering the Solar System)

What telescopes found

Studying an interstellar comet lets us sample materials from another planetary system without launching a multi-century interstellar probe. How do these three wanderers compare under the spectrograph?

Key chemical insights:

• Carbon Dioxide Dominance in 3I/ATLAS: JWST revealed that 3I/ATLAS produces heavy amounts of carbon dioxide ($C{O}_2$), mixed with modest water vapor, carbonyl sulfide ($OCS$), and methane ($C{H}_4$). This differs markedly from 2I/Borisov’s carbon-monoxide-heavy profile.
• Heavy Metal Vapors: Spectrographs on the VLT detected atomic nickel vapor drifting out of 3I/ATLAS. While surprising to find metal gas in a freezing comet, solar system comets also emit tiny traces of iron and nickel at low temperatures through metal-carbonyl complexes.
• The Ice Ratio Spectrum: These variations prove that protoplanetary disks around distant stars come in drastically different recipes depending on star type, disk temperature, and radiation exposure.

Where did they come from? galactic origins

Did these visitors form around young stars near our Sun, or did they cross thousands of light-years across the Milky Way?

Astronomers trace an interstellar object’s origin by mapping its speed and direction against the Local Standard of Rest (LSR) — the average circular motion of stars around the galactic center.

1. ’Oumuamua’s gentle galactic drift

’Oumuamua was traveling very close to the LSR before falling toward our Sun. That suggests it is relatively young (perhaps one to two billion years old) and likely came from a nearby star-forming cluster or young stellar association in the Milky Way‘s thin disk.

2. Borisov’s outer-rim slingshot

Borisov approached from the direction of Cassiopeia. Its kinematics match the Milky Way’s thin disk, pointing to a standard extrasolar planetary birth before a giant planet swung it out into the void.

3. 3I/ATLAS: A relic from the ancient thick disk?

The staggering entrance velocity of 3I/ATLAS (~61 km/s relative to our local stellar neighborhood) places it in rare company.

High orbital velocity relative to the Sun often correlates with older stellar populations. Over billions of years, passing giant molecular clouds and gravitational disturbances continuously scatter older stars and their debris, kicking up their speeds.

Astrophysicists estimate that 3I/ATLAS could trace its heritage back to the Milky Way’s thick disk. If this hypothesis holds, this comet might be 7 to 11 billion years old — meaning it formed around an ancient star long before our own Sun and Earth even existed.

Every planet, asteroid, and comet in our solar system coalesced from the same original nebula 4.6 billion years ago. We are trapped with one single data point.

Interstellar interlopers change everything:

1. Universal Planet-Building: The presence of volatile gases, ice, silicates, and organics on Borisov and 3I/ATLAS proves that the basic building blocks of planets and comets are universal across our galaxy.
2. Dynamic Chaos in Alien Systems: For an object to escape its parent star, massive planets must migrate and trigger violent gravitational instability, kicking billions of icy bodies into deep space.
3. Cross-Seeding the Galaxy: Trillions of rogue comets drift between the stars. Could these frozen travelers transport complex organic compounds or water across stellar systems over billions of years?

The next era: catching interstellar visitors live

Why did we discover three interstellar objects in less than a decade after centuries of seeing none?

Our discovery rate is surging because large survey telescopes have become exponentially more sensitive. The commissioning of the Vera C. Rubin Observatory in Chile, powered by its 3,200-megapixel Legacy Survey of Space and Time (LSST) camera, promises to detect dozens of interstellar objects every single year.

Space agencies are already preparing interceptor missions. The European Space Agency’s (ESA) Comet Interceptor mission will park a spacecraft at the Sun-Earth Lagrange point $L_2$, waiting in hibernation until astronomers spot the next pristine interstellar target. When a new traveler appears, the probe can fire its engines, perform a close flyby, and take our very first high-resolution pictures of an alien world up close.