How rare interstellar comet 3I/ATLAS is whizzing through our solar system

Though comets are commonplace, a visitor from another star system is an exceptional occurrence. 3I/ATLAS marks only the third interstellar visitor ever detected in human history. The first such object, Oumuamua (formally designated 1I/2017 U1), was spotted in 2017, followed by comet 2I/Borisov in 2019.

Prompted by the alert, astronomers worldwide directed their telescopes toward the object, tentatively named 3I/ATLAS or C/2025 N1 (ATLAS). Initial observations revealed that the object is moving in interplanetary space with a bizarre, highly eccentric hyperbolic orbit, unlike any other object in the solar system. Additionally, it exhibited uncertain signs of cometary activity, including a faint coma and a short tail, which further deepened the mystery.

The discovery triggered an immediate global response. Nearly 97 observatories worldwide are directing their telescopes toward the mysterious object. Researchers meticulously examined archival data and made a significant finding — the Weizmann Astrophysical Observatory had actually detected the object much earlier, on May 21, 2025. By compiling 319 observations recorded up to July 4, 2025, astronomers connected the dots, mapped the object’s trajectory, and determined its speed.

Three key characteristics of this comet strongly indicate its interstellar origin. First is its extraordinary speed. At the time of discovery, the comet was moving at 2,21,000 kilometres per hour (61 kilometres per second), a velocity that will increase further as it approaches the Sun. This immense speed clearly exceeds the Sun’s gravitational pull, meaning the object is not bound to our solar system.

Second, unusual hyperbolic trajectory. While most solar system objects follow circular or elliptical orbits — and some long-period comets may adopt parabolic paths due to gravitational perturbations — this comet follows a distinctly hyperbolic path. Unlike closed orbits, a hyperbolic trajectory indicates the object is merely passing through our solar system without being captured.

Third, the comet is found to have no elements beyond helium. “As the thread, so the cloth,” goes the saying. Likewise, comets are remnants of star formation, and hence, their material composition is likely to be the same. Unlike the Sun, which is a second-generation star, the earliest stars formed in the universe would have contained only hydrogen and helium, with few elements beyond helium in the periodic table. Thus, these comets must have originated around the early stars and not be part of the solar system.

From the direction of the arrival of a football, we can estimate its path. Likewise, astronomers analysed 3I/ATLAS’s reconstructed path using computer simulations to trace its possible origin. Their findings indicate that the comet most likely originated from the Milky Way’s ‘thick disk’ — a region of ancient stars situated above and below our galaxy’s central stellar disk, where our Sun resides.

Since stars in this zone formed billions of years before our solar system, and comets essentially represent preserved remnants of planetary formation around young stars, 3I/ATLAS may share this extraordinary antiquity. If confirmed, this would make it the most ancient comet ever documented, with rocky material and water ice nearly 7 billion years old, older than our Sun. The simulation also confirmed that the interstellar visitor will continue its journey beyond our solar system, never to return.

The mysterious celestial object exhibited not just unusual orbital characteristics but also behaviour that was inconsistent with typical comets. Early images did not show signs of dust release or a faint tail. Still, it had unexpected reddish brightness — features not commonly seen in typical comets.

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In popular imagination, comets are often envisioned as fiery objects with long, dramatic tails. In reality, however, a comet is primarily a nucleus made up of rock, water ice, and frozen volatile compounds, earning it the nickname “dirty snowball”. As a comet nears the Sun, the heat causes the ice and volatiles to vaporise, forming a hazy envelope called a coma.

When the comet gets close enough, the solar wind pushes the released dust and gas away, forming a tail that always points opposite to the Sun. Additionally, ionised gas aligns along the Sun’s magnetic field lines, creating a separate plasma tail. These features are not permanent but arise from the interaction between solar radiation and the comet’s surface material. Early images of the comet did not show any of these telltale signs.

Based on these initial observations, Harvard astrophysicist Avi Loeb and his colleagues speculated that 3I/ATLAS might not merely be an interstellar visitor. They suggested that it could potentially be an artificial object, an extraterrestrial probe, or a spacecraft. 

This speculation sparked considerable media attention in the early days. However, subsequent observations have since clarified the nature of the enigmatic object. Recent images from the Hubble Space Telescope reveal a distinct dusty coma spanning about 26,400 km, along with a faint anti-sunward tail and dust plumes facing the Sun.

Detections of water ice, vapour, and hydroxide ions confirm the sublimation of volatile materials as the object approaches the Sun — a behaviour typical of a water-rich comet. Recent images of 3I/ATLAS show the gradual development of a tail as the comet moves closer to the Sun, as one would expect.

Spectroscopic studies conducted using NASA’s Infrared Telescope Facility (IRTF) in Hawaii and the Gemini South Telescope in Chile, led by astronomer Bin Yang, have detected signatures of water ice, organic molecules, silicates, and carbon-based minerals on 3I/ATLAS.

Astronomers worldwide are closely monitoring the third known interstellar object to visit our solar system, using both ground-based and space telescopes. From its initial detection in May until August 6, 2025, researchers have recorded 2,663 observations of this celestial visitor. Currently, the comet has passed Jupiter’s orbit as it journeys toward its closest approach to the Sun.

The object will make its nearest pass by Mars on October 2, 2025, coming within about 30 million kilometres of the Red Planet. Later, on October 29-30, 2025, it will reach its closest point to the Sun at approximately 210 million kilometres. On its way out of the solar system, the comet will make its closest approach to Earth on December 19, 2025, maintaining a safe distance of about 270 million kilometres — posing no danger to our planet. Nonetheless, due to this considerable distance, the comet will remain, sadly, too faint for naked-eye observation. Before departing our solar system, the interstellar traveller will pass Jupiter on March 16, 2026, at a distance of roughly 53.5 million kilometres.

As the comet 3I/ATLAS approaches its closest point to the Sun, its path will take it behind our star, temporarily blocking Earth-based observations. In response to this challenge, astronomers are considering innovative ways to study this rare interstellar object by repurposing existing spacecraft.

Two promising candidates at the top of the list are the European Space Agency’s JUICE mission, currently en route to Jupiter, and NASA’s Psyche spacecraft, headed toward the asteroid 16 Psyche. Another proposal suggests redirecting NASA’s Juno spacecraft, which is currently orbiting Jupiter, to intercept the comet in March 2026. Some have even proposed redirecting Mars orbiters, such as the Mars Reconnaissance Orbiter and Mars Odyssey, for a flyby mission.

However, as Luca Conversi, an astronaut at the European Space Agency (ESA), points out, the reality of orbital mechanics is far more complex than what science fiction portrays, making spacecraft redirection extremely challenging. While these ambitious plans may or may not materialise, the ESA is proactively preparing for future opportunities with its Comet Interceptor mission.

This spacecraft will be stationed at the Sun-Earth Lagrange Point 2 (L2), ready to spring into action when either a distant solar system comet or another interstellar object enters our neighbourhood, providing scientists with unprecedented close-up data of these elusive cosmic visitors.

NASA’s Hubble Space Telescope obtained crucial images of the interstellar comet 3I/ATLAS on July 21, 2025, when it was approximately 446 million kilometres away from Earth. The observations revealed a teardrop-shaped dusty envelope surrounding the comet’s solid, icy core. These detailed measurements have enabled astronomers to refine their estimates of the nucleus size with greater accuracy.

Current analysis suggests that the comet’s diameter could be up to 5.6 kilometres at its largest, making it the largest interstellar object detected so far. However, accounting for observational uncertainties, NASA indicates the nucleus may be no smaller than 320 metres across. Upcoming observations by the James Webb Space Telescope and other advanced instruments are expected to provide more exact measurements, offering a clearer understanding of this intriguing celestial visitor.

Astronomers know that stars can explode unexpectedly anywhere in the sky or produce mysterious gamma-ray bursts (GRBs). Similarly, unstable space rocks from the outer solar system can suddenly plunge toward the Sun. Such unpredictable celestial events are challenging to forecast and observe. However, with the advancement of robotic telescopes — even those with modest one-meter diameters — scientists have discovered that they can effectively track these transient celestial phenomena in real-time.

By repeatedly capturing images of the same sky regions and comparing them through specialised software, researchers gained the ability to detect sudden brightness changes from supernovae or positional shifts of moving objects, such as asteroids and comets. These automated systems then notify the global astronomy community, enabling follow-up observations with more powerful telescopes. 

This capability has created what essentially functions as an automated astronomical survey of transient events and an early warning system for Earth-threatening objects, as part of the international cooperation for Planetary Defence.

NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS) exemplifies this technology, comprising five strategically placed telescopes: two in Hawaii (Haleakala and Mauna Loa), one at South Africa’s Sutherland Observatory, another at Chile’s El Sauce Observatory in Rio Hurtado, and a fifth at Spain’s Teide Observatory. It was the Chilean ATLAS telescope that first alerted to this peculiar object on July 1, 2025.

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The object’s abnormal trajectory immediately drew attention, triggering coordinated tracking efforts by multiple observatories. After confirming preliminary details, astronomers issued the Minor Planet Electronic Circular the following day, formally alerting the worldwide astronomical community about this remarkable discovery.

Comets are typically named after their discoverers — in this case, the ATLAS survey team — and are also designated through a numbering system. Since many astronomers suspected this comet originated from beyond our solar system, they included the letter ‘I’ for ‘interstellar’, marking it as an object from another star system. At the same time, the number ‘3’ indicated it was only the third such interstellar visitor ever identified. Thus ‘3I/ATLAS’.

However, in the initial stages, not all researchers were convinced of its interstellar origin. Following standard naming conventions, it was also designated as C/2025 N1 — where ‘C’ stands for comet, ‘2025’ denotes the year of discovery, and ‘N1’ signifies it was the first object detected in the second half of July (letters A to N represent the first and second halves of each month sequentially from January). The Minor Planet Committee of the International Astronomical Union may eventually assign it a formal name, as was done in the earlier two cases, after further confirmation and analysis.