In October 2017, astronomy changed forever. For the first time in history, we knowingly observed an object from another star system passing through our own cosmic backyard. Designated 1I/ʻOumuamua, a Hawaiian name meaning “a messenger from afar arriving first,” it was a fleeting visitor that baffled scientists. It was too fast, accelerated inexplicably, and had a shape unlike anything we’d ever seen. It sparked a firestorm of debate, with theories ranging from a new class of natural object to, most tantalizingly, a piece of alien technology. To dive deeper into its initial discovery and strange characteristics, read ‘Oumuamua Explained: The Full Story of Our First, Mysterious Interstellar Visitor.

Then, just two years later, it happened again. A second interstellar traveler, 2I/Borisov, was detected. But this visitor was different. It was familiar. By being so wonderfully, beautifully normal, Borisov provided the perfect cosmic counterpoint to ʻOumuamua’s strangeness. It didn’t solve the mystery of our first guest, but it grounded the conversation in data, turning a single anomaly into the beginning of a brand-new field of science: interstellar astronomy. This is the story of what we learned when we finally had two data points instead of one. For a broader overview of these cosmic wanderers, see A Guide to Interstellar Objects: Understanding Visitors from Other Stars.

The Anomaly: Recapping the Bizarre Case of 1I/ʻOumuamua

When the Pan-STARRS1 telescope in Hawaii first spotted ʻOumuamua, it had already whipped around the Sun and was speeding away, never to return. The discovery was itself a surprise; Pan-STARRS is an automated survey designed to find near-Earth asteroids that could pose a potential impact threat. Discovering an object that originated from another star, and was already on its way out, was an extraordinary fluke. Our observations were frantic and limited, but what they revealed was a laundry list of oddities.

First and foremost was its trajectory. Everything native to our solar system is gravitationally bound to the Sun, traveling in closed, elliptical orbits. ʻOumuamua, however, was moving at a blistering 87 kilometers per second relative to the Sun. This was far beyond the Sun’s escape velocity, meaning it was on a one-way trip. Its path, what astronomers call a hyperbolic trajectory, was the smoking gun for its interstellar origin. It had come from the void and was destined to return to it. For more on how such objects find their way to us, explore Galactic Origins: Where Do Interstellar Visitors Like ‘Oumuamua Come From?

A diagram illustrating the difference between a bound, elliptical orbit (like a planet) and an unbound, hyperbolic trajectory of an interstellar object passing through the solar system.
A diagram illustrating the difference between a bound, elliptical orbit (like a planet) and an unbound, hyperbolic trajectory of an interstellar object passing through the solar system.

A Messenger with No Coma

The initial assumption was that ʻOumuamua must be an interstellar comet, a chunk of ice and rock knocked loose from another star system. But it lacked the defining features of a comet. As comets approach the Sun, their ices heat up and sublimate (turn directly from solid to gas), creating a visible fuzzy atmosphere called a coma and a long, glowing tail. ʻOumuamua had neither. Deep observations from powerful instruments like the Spitzer Space Telescope searched for the tell-tale infrared signature of carbon-based gas and dust but found nothing. It was a crisp, star-like point of light.

This led to the next logical conclusion: it must be an asteroid. But that theory didn’t hold up either, thanks to its most perplexing behavior.

The Unexplained Acceleration and the Alien Probe Debate

As astronomers tracked ʻOumuamua’s escape, they noticed something impossible for a simple rock. It was accelerating, pushing away from the Sun with a tiny but measurable force that wasn’t due to gravity. This is normal for a comet; the outgassing of sublimating ice acts like a gentle, natural rocket engine. But how could ʻOumuamua be accelerating like a comet without any visible outgassing?

This single fact opened the door to a universe of speculation. Harvard astrophysicist Avi Loeb famously argued from a principle of simplicity: instead of inventing a new class of invisible-outgassing natural objects, we should consider a known technology. He proposed that the most straightforward explanation might be that ʻOumuamua was an artifact of an extraterrestrial civilization—perhaps a solar sail, a thin membrane designed to be pushed by starlight. The object’s strange shape, inferred from its dramatically pulsing brightness, also fueled this idea. The light curve suggested it was either an extremely elongated, cigar-like object or a very thin, pancake-shaped one, tumbling through space—both highly efficient shapes for a solar sail.

More conventional, though equally exotic, natural theories emerged. The leading contender is the “nitrogen iceberg” hypothesis, which suggests ʻOumuamua was a fragment chipped off a nitrogen-ice-covered exoplanet, similar to our own Pluto. This nitrogen ice would sublimate when warmed by the Sun, producing an invisible thrust that would explain the acceleration. Challenges to this idea remain, however, as it’s unclear if such a fragile object could survive a long, harsh journey through interstellar space. Whatever the cause, ʻOumuamua remained a profound enigma. To delve into these fascinating hypotheses, read Natural Phenomenon or Alien Probe? The Competing Theories for ‘Oumuamua’s Strange Behavior.

Enter 2I/Borisov: The Grounding Discovery That Changed Everything

For two years, ʻOumuamua was a cosmic ghost story - a single, inexplicable data point. Then, on August 30, 2019, everything changed. An amateur astronomer in Crimea, Gennadiy Borisov, pointed his custom-built 0.65-meter telescope at the sky and spotted something new. It was faint and fuzzy, and its motion was unusual. When professionals trained their powerful instruments on it, they confirmed what Borisov suspected: it was another interstellar visitor.

A conceptual image of a lone amateur astronomer on a clear night, looking through a powerful personal telescope at a vast, starry sky, symbolizing the moment of discovery.
A conceptual image of a lone amateur astronomer on a clear night, looking through a powerful personal telescope at a vast, starry sky, symbolizing the moment of discovery.

A Familiar Face: Why Borisov Looked and Acted Like a “Normal” Comet

Unlike ʻOumuamua, 2I/Borisov was comfortingly familiar. It was, without a doubt, a comet. It had a prominent coma and a beautiful dust tail that grew as it approached the Sun. It behaved exactly as a comet should, and its composition looked remarkably similar to the comets born in our own solar system’s outer fringes.

This discovery was monumental. It proved that ʻOumuamua wasn’t a complete fluke. Interstellar objects were not just a theoretical possibility but a real, observable population of cosmic travelers. More importantly, Borisov provided the crucial baseline that had been missing. We now had a “normal” interstellar object to compare with our first, deeply weird one.

A Tale of Two Visitors: The Head-to-Head Comparison

The differences between 1I/ʻOumuamua and 2I/Borisov couldn’t be more stark. Comparing them side-by-side finally allowed scientists to put the anomalies of ʻOumuamua into context.

A simple, clean infographic comparing 1I/‘Oumuamua on the left (depicted as a reddish, flat, tumbling object) and 2I/Borisov on the right (depicted as a classic comet with a blueish coma and tail), with text labels highlighting key differences like 'No Coma' vs. 'Visible Coma' and 'Anomalous Acceleration' vs. 'Cometary Acceleration'.
A simple, clean infographic comparing 1I/‘Oumuamua on the left (depicted as a reddish, flat, tumbling object) and 2I/Borisov on the right (depicted as a classic comet with a blueish coma and tail), with text labels highlighting key differences like 'No Coma' vs. 'Visible Coma' and 'Anomalous Acceleration' vs. 'Cometary Acceleration'.
  • Appearance: ʻOumuamua was a sharp point of light with no tail. Borisov was a classic comet, complete with a diffuse coma and a long tail of dust and gas.
  • Shape: ʻOumuamua's light curve suggested a highly unusual, non-spherical shape (cigar or pancake). Borisov's nucleus was too obscured by its coma to determine its exact shape, but it behaved like a typical, roughly spherical comet nucleus.
  • Composition: We could only guess at ʻOumuamua's makeup. With Borisov, we could perform detailed spectroscopy on its coma. It contained water, dust, and cyanogen—all common in our own comets. However, crucial observations from the Atacama Large Millimeter/submillimeter Array (ALMA) revealed an unusually high concentration of carbon monoxide (CO), far more than any comet from our solar system. This was a tantalizing chemical fingerprint of its alien birthplace.
  • Origin Story: The high level of CO, which vaporizes at extremely cold temperatures, suggests Borisov was a pristine comet. It likely formed in the frigid outer disk of its home star system and was ejected into interstellar space without ever getting close to its star. This chemical signature hints that its parent star might have been a cool red dwarf. ʻOumuamua's origin remains a mystery, with leading theories pointing to it being a fragment of a larger, violently disrupted body.

The Key Lessons Learned from Our First Interstellar Guests

Having two visitors to study, even briefly, revolutionized our understanding of the galaxy. Here are the four biggest takeaways.

Lesson 1: Interstellar Objects Are More Common Than We Thought

Before 2017, the existence of interstellar objects was purely theoretical. Finding two in two years was a shock. Based on these detections, models now estimate that several such objects pass inside Earth’s orbit every single year, and our solar system may host thousands of them at any given time.

A stunning, artistic rendering of a vast protoplanetary disk of gas and dust swirling around a young, bright star. Small planetesimals can be seen forming within the disk, representing the birthplace of comets and other interstellar objects.
A stunning, artistic rendering of a vast protoplanetary disk of gas and dust swirling around a young, bright star. Small planetesimals can be seen forming within the disk, representing the birthplace of comets and other interstellar objects.

Lesson 2: Other Solar Systems Eject Familiar Building Blocks

2I/Borisov’s most profound lesson was its normalcy. The fact that a comet from another star looks and acts so much like one of our own is incredible. It implies that the fundamental processes of planet formation—the slow accretion of ice and rock in a protoplanetary disk—are likely universal. Solar systems across the galaxy may be built from a very similar chemical toolkit. For more on their origins, see Galactic Origins: Where Do Interstellar Visitors Like ‘Oumuamua Come From?.

Lesson 3: ʻOumuamua Is the Weird One, Not the Norm (Probably)

With only one data point, it was impossible to know if ʻOumuamua represented a common class of object we had simply never seen before. Borisov’s arrival strongly suggests the opposite: that ʻOumuamua is the true outlier. Whatever it is—a nitrogen iceberg, a hydrogen iceberg, or something else entirely—it appears to be far less common than conventional interstellar comets.

Lesson 4: We Can Chemically Analyze Other Star Systems from Home

Borisov proved that we don’t need to send probes across light-years of space to study the chemistry of other solar systems. The systems will send samples to us. By analyzing the light from Borisov’s coma, we got a direct measurement of the raw materials present in its formation disk. Each new interstellar visitor will be a free sample from a different part of the galaxy, allowing us to build a chemical map of our galactic neighborhood.

A beautiful, scientifically accurate rendering of the interstellar comet 2I/Borisov, showing its glowing blueish coma and a long, faint dust tail stretching out against the blackness of deep space, dotted with distant stars.
A beautiful, scientifically accurate rendering of the interstellar comet 2I/Borisov, showing its glowing blueish coma and a long, faint dust tail stretching out against the blackness of deep space, dotted with distant stars.

The Next Wave: How We’re Preparing for 3I, 4I, and Beyond

The discoveries of ʻOumuamua and Borisov were just the opening act. We are now entering a new era where detecting these visitors will become routine.

The Vera C. Rubin Observatory: Opening the Floodgates

The upcoming Vera C. Rubin Observatory in Chile is poised to be the ultimate interstellar object hunter. Its massive 8.4-meter mirror and the largest digital camera ever built for astronomy - a 3,200-megapixel behemoth - will survey the entire visible sky every few nights with unprecedented depth. This survey is expected to discover not just one or two interstellar objects per year, but potentially dozens. This will provide the statistical power to understand their populations, compositions, and origins, finally revealing just how strange ʻOumuamua truly is. For more on detection methods, read Cosmic Watch: How We Detect, Track, and Study Objects from Deep Space.

The futuristic dome of the Vera C. Rubin Observatory at sunset on a remote mountaintop. The sky above is filled with the first stars, and long-exposure light trails suggest the telescope's rapid, continuous scanning of the heavens.
The futuristic dome of the Vera C. Rubin Observatory at sunset on a remote mountaintop. The sky above is filled with the first stars, and long-exposure light trails suggest the telescope's rapid, continuous scanning of the heavens.

Project Lyra: Could We One Day Intercept an Interstellar Visitor?

The ultimate dream is not just to see these objects, but to touch them. While chasing down ʻOumuamua is impossible, scientists are already planning for the future. Initiatives like Project Lyra are developing mission concepts for a high-speed spacecraft that could be launched on short notice to intercept a future interstellar object. Such a mission could use complex orbital mechanics, like a powered “Jupiter Oberth Maneuver,” where the craft fires its engine at its closest, fastest point to the giant planet to gain the incredible speeds needed for the chase. Getting up-close images and data from one of these messengers would be one of the greatest scientific achievements in human history.

A sleek, futuristic space probe, reminiscent of concepts for Project Lyra, maneuvering in deep space as it approaches a mysterious, rocky interstellar object. Earth and the Sun are tiny, distant points of light in the background.
A sleek, futuristic space probe, reminiscent of concepts for Project Lyra, maneuvering in deep space as it approaches a mysterious, rocky interstellar object. Earth and the Sun are tiny, distant points of light in the background.

From Anomaly to a New Frontier

The tale of our first two interstellar visitors is a perfect illustration of the scientific process. We began with a single, baffling anomaly that challenged our understanding of the cosmos. Then came a second, more conventional discovery that provided the context needed to frame the right questions. ʻOumuamua remains a mystery, but thanks to Borisov, it’s no longer an isolated one. It is now the first strange chapter in a much larger story that is just beginning to unfold. You can explore the full range of these cosmic phenomena in A Guide to Interstellar Objects: Understanding Visitors from Other Stars.

We stand on the cusp of a new frontier, one where the galaxy sends its messengers directly to our doorstep. Each new visitor will carry with it the chemical secrets of its home star, giving us a clearer picture of our place in the universe and the incredible diversity of worlds that lie beyond our own.

What do you think is the most compelling explanation for ʻOumuamua’s strange behavior? Was it a natural phenomenon we don’t understand yet, or do you think the alien probe hypothesis holds any water? Share your thoughts in the comments below!