'Oumuamua vs Borisov vs 3I/ATLAS: Comparing the Three Interstellar Objects

Since 2017, humanity has confirmed the detection of three interstellar objects — visitors from other star systems passing through our solar neighborhood. Each one has told us something profoundly different about the universe beyond our Sun. From the enigmatic 1I/'Oumuamua to the familiar-looking 2I/Borisov to the spectacular 3I/ATLAS, these cosmic wanderers represent a new frontier in astronomy.

Here's a comprehensive comparison of all three interstellar objects, what each one taught us, and what the future holds for detecting more visitors from the stars.

1I/'Oumuamua (2017): The Mysterious Pioneer

On October 19, 2017, the Pan-STARRS1 telescope in Hawaii detected an unusual object moving on a hyperbolic orbit — meaning it wasn't bound to our Sun. Designated 1I/'Oumuamua (Hawaiian for "a messenger from afar arriving first"), it became the first confirmed interstellar object in history.

'Oumuamua was deeply strange:

• Shape: Extremely elongated, estimated at roughly 100–1,000 meters long but only about one-sixth as wide — often described as cigar-shaped or pancake-shaped
• Appearance: No coma, no tail, no visible outgassing — it looked like a bare asteroid, not a comet
• Mysterious acceleration: As it left the solar system, 'Oumuamua exhibited a subtle non-gravitational acceleration that couldn't be explained by solar radiation pressure alone
• Composition: Reddish surface, possibly organic-rich, but no direct spectral matches to known solar system materials
• Debate: The lack of outgassing combined with the anomalous acceleration sparked wild speculation — from hydrogen iceberg to alien light sail (proposed by Harvard astronomer Avi Loeb)

'Oumuamua was discovered after its closest approach, already heading away from the Sun. Astronomers had only a few weeks to study it before it faded beyond the reach of even the largest telescopes. We were left with more questions than answers.

2I/Borisov (2019): The Familiar Comet

On August 30, 2019, amateur astronomer Gennady Borisov discovered a comet from his personal observatory in Crimea. Follow-up observations confirmed its hyperbolic orbit: this was 2I/Borisov, the second interstellar object and the first interstellar comet.

Unlike 'Oumuamua, Borisov looked reassuringly normal:

• Size: Nucleus estimated at roughly 0.4–1 km in diameter
• Appearance: Classic cometary coma and tail — it was clearly outgassing as it approached the Sun
• Composition: Spectroscopy revealed it was rich in carbon monoxide (CO) — in fact, it had 2–3 times more CO than typical solar system comets
• Water: Detected at levels broadly consistent with solar system comets
• Cyanide (CN): Detected, again consistent with known comets
• Discovery timing: Found inbound, months before perihelion, giving astronomers time to plan observations

2I/Borisov's closest approach to the Sun (perihelion) was 2.01 AU on December 8, 2019. It passed Earth at a minimum distance of about 1.9 AU. The comet later showed signs of fragmentation as it headed back out of the solar system.

The key takeaway from Borisov was reassurance: interstellar comets can look a lot like the comets in our own solar system. The building blocks of comets — water ice, carbon monoxide, dust — appear to be universal.

3I/ATLAS (2025): The Spectacular Giant

On July 1, 2025, the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey in South Africa detected what would become the third interstellar object: 3I/ATLAS (formally C/2025 N1). It was the most spectacular of the three by far.

3I/ATLAS shattered records:

• Size: Estimated nucleus diameter of up to 5.6 km (3.5 miles) — roughly 5–10 times larger than Borisov and orders of magnitude larger than 'Oumuamua
• Appearance: Bright, expansive coma and a long, dramatic tail visible even to amateur astronomers with modest telescopes
• Composition: The James Webb Space Telescope detected carbon dioxide (CO2) as a dominant volatile — unusual for comets, which tend to be water-dominated. Hydroxyl radicals (OH) were also detected, indicating water ice sublimation
• Prebiotic molecules: Hydrogen cyanide (HCN) was detected, a molecule considered important for the origin of life
• Activity: Powerful cometary jets showed wobbling patterns tied to the nucleus rotation, indicating complex surface activity
• Speed: Entered the solar system at roughly 30 km/s relative to the Sun on a hyperbolic trajectory
• Closest approach to Earth: 1.8 AU (270 million km) on December 19, 2025

You can explore 3I/ATLAS's full journey through our solar system on the interactive timeline.

Head-to-Head Comparison

Property	1I/'Oumuamua (2017)	2I/Borisov (2019)	3I/ATLAS (2025)
Discovery date	Oct 19, 2017	Aug 30, 2019	Jul 1, 2025
Discoverer	Pan-STARRS1 (Hawaii)	Gennady Borisov (amateur)	ATLAS survey (South Africa)
Estimated size	100–1,000 m (elongated)	0.4–1 km	~5.6 km
Type	Ambiguous (no coma)	Comet (coma + tail)	Comet (bright coma + tail)
Dominant volatiles	None detected	CO-rich, water	CO2-rich, OH, HCN
Coma/Tail	None observed	Yes (classic comet)	Yes (spectacular, large)
Non-gravitational acceleration	Yes (unexplained)	Minor (outgassing)	Under study
Perihelion distance	0.26 AU	2.01 AU	1.36 AU
Closest to Earth	0.16 AU (post-perihelion)	~1.9 AU	1.8 AU
Hyperbolic excess velocity	~26 km/s	~32 km/s	~30 km/s
Discovery timing	After closest approach	Months before perihelion	Months before perihelion
Key telescopes used	Gemini, VLT, Spitzer	Hubble, Keck, VLT	JWST, Hubble, Keck, VLT
Fragmentation	None observed	Yes (post-perihelion)	Under observation

What Each Object Taught Us

'Oumuamua: Interstellar Objects Exist — and They're Weird

Before 2017, interstellar objects were theoretical. 'Oumuamua proved they exist and pass through our solar system. But its bizarre properties — no outgassing, anomalous acceleration, extreme shape — challenged our assumptions. It showed us that not all interstellar visitors would look like what we expected.

Borisov: Comets Are Universal

2I/Borisov demonstrated that the basic recipe for comets — ices, dust, and volatile gases — is not unique to our solar system. Other stars produce comets that look remarkably like our own. The elevated CO levels hinted that Borisov may have formed farther from its parent star than typical solar system comets form from our Sun.

3I/ATLAS: Interstellar Chemistry Is Diverse and Complex

3I/ATLAS revealed that interstellar comets can be very different from each other and from solar system comets. Its CO2-dominated composition, combined with prebiotic molecules like HCN, suggests it formed in a chemically distinct environment — perhaps around a different type of star or at a different distance from its host star than Borisov's birthplace. Learn more about these findings on the science page.

How Detection Has Improved: 2017 to 2025

The progression from 'Oumuamua to 3I/ATLAS tells a story of rapidly improving astronomical capabilities:

2017 ('Oumuamua): Detected after closest approach, already outbound. Astronomers had only weeks of useful observations. Limited spectroscopic data. No space telescope observations were possible in time.

2019 (Borisov): Detected inbound, months before perihelion. Hubble, Keck, and other major facilities were scheduled in advance. Much richer dataset, including composition measurements and morphological studies over many months.

2025 (3I/ATLAS): Detected inbound with even more lead time. The James Webb Space Telescope — not available for the previous two objects — provided unprecedented infrared spectroscopy. Ground-based networks coordinated globally. Amateur astronomers contributed valuable observations. The dataset is the most comprehensive ever assembled for an interstellar object.

Key factors driving this improvement:

• Survey coverage: ATLAS, Pan-STARRS, and ZTF now cover more sky more frequently
• Processing pipelines: Automated detection algorithms flag unusual orbits faster
• Follow-up networks: Global coordination means rapid confirmation and characterization
• JWST: Infrared spectroscopy from space reveals molecular compositions invisible from the ground

What to Expect: 4I and Beyond

With three interstellar objects confirmed in eight years, astronomers estimate that at least one interstellar object per year passes through the inner solar system at detectable brightness. Most go unnoticed because current surveys still don't cover the entire sky continuously.

That's about to change. The Vera C. Rubin Observatory in Chile, with its 8.4-meter mirror and enormous 3.2-gigapixel camera, will begin its Legacy Survey of Space and Time (LSST) and scan the entire visible southern sky every few nights. Simulations suggest Rubin could detect several interstellar objects per year, potentially finding them even earlier in their approach.

Future interstellar object science may include:

• Rapid-response missions: The European Space Agency's Comet Interceptor (launching ~2029) is designed to wait at the L2 Lagrange point and redirect to intercept a long-period or interstellar comet with minimal warning
• Dedicated flyby missions: If an interstellar object is found early enough, a fast spacecraft could be launched to perform a close flyby, capturing high-resolution images and in-situ measurements
• Sample return: The ultimate goal — collecting material from another star system. Current technology makes this extremely challenging, but concepts are being studied
• Statistical studies: With dozens of detections, astronomers will be able to study the population of interstellar objects — their size distribution, compositions, and origin systems

Each new detection adds to our understanding of how planetary systems form across the galaxy and whether the chemical ingredients for life are truly universal.

Conclusion: Three Visitors, One Expanding Frontier

From 'Oumuamua's cryptic flyby to Borisov's comforting familiarity to 3I/ATLAS's spectacular display, each interstellar object has expanded our understanding of the universe. They've shown us that other star systems eject material into the galaxy, that comet-building chemistry is widespread, and that we're getting better at spotting these cosmic visitors every year.

The age of interstellar object astronomy is just beginning. With better telescopes, faster detection, and planned intercept missions, the next interstellar visitor — 4I and beyond — may teach us even more about the alien worlds where they were born.

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Explore 3I/ATLAS's trajectory in real time on the 3D Orbit Tracker, dive into the science behind interstellar comets, and follow the full observation timeline to see how this historic discovery unfolded.