Why Scientists Are Racing to Study Interstellar Comet 3I/ATLAS

Why 3I/ATLAS Has Astronomers' Attention

Astrophysicists worldwide are urgently studying an ancient comet that arrived from another star system and passed through our solar system this year. Named 3I/ATLAS, the object poses no threat to Earth or neighboring planets, but it is only the third confirmed interstellar visitor ever recorded—making it a rare opportunity to study material formed around another star.

Discovery and context

3I/ATLAS was first spotted in July by Larry Dennau of the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope team in Rio Hurtado, Chile. ATLAS is funded by NASA and operated by the University of Hawaii Institute for Astronomy. The two previous interstellar visitors were 1I/’Oumuamua (discovered in 2017) and 2I/Borisov (discovered in 2019).

Trajectory and recent flybys

In early October the comet swept past Mars, coming within roughly 29 million km (18 million miles) of the planet while traveling at about 310,000 km/h (193,000 mph). It reached perihelion (closest approach to the Sun) at the end of October and is expected to make its nearest approach to Earth in December, at about 270 million km (170 million miles)—still far beyond the Earth–Sun distance of roughly 150 million km.

Why this matters

Because 3I/ATLAS follows a hyperbolic trajectory, it is not bound to the Sun and originated elsewhere. Studying its size, composition and activity can reveal how other planetary systems form and evolve. Hubble images from July show a teardrop-shaped dust envelope streaming from a solid, icy nucleus; estimates place the nucleus between about 440 meters and 5.6 kilometers in diameter.

What spacecraft are observing it

Multiple telescopes and spacecraft have been tasked to observe the visitor while it remains observable, and many instruments have contributed data that confirmed its cometary nature and composition. Key assets include:

• Hubble Space Telescope — captured detailed images of the coma and nucleus.
• Mars rovers — Perseverance (operating since 2021) and Curiosity (operating since 2012) have provided additional viewpoints while the comet passed near Mars.
• Mars Reconnaissance Orbiter — contributed observations from orbit around Mars.
• Parker Solar Probe and SOHO — solar observatories that have monitored the comet when geometry allows.
• PUNCH — the Polarimeter to Unify the Corona and Heliosphere mission (launched in 2025) provides continuous solar-science vantage points useful for tracking objects near the Sun.
• Europa Clipper, Lucy and Psyche — deep-space missions that have opportunistically observed the interloper while en route to their primary targets.
• ESA's JUICE — has also monitored the object during November.

Composition clues

Spectroscopic observations indicate the coma is rich in carbon dioxide, suggesting the comet formed in a very cold, distant region of its parent star system. Such volatiles offer clues about the chemical environments of other planetary systems.

What’s next

The comet is currently behind the Sun from Earth's perspective and untrackable for a short period; NASA expects it to reappear by early December 2025. Continued observations from space- and ground-based telescopes will refine estimates of its size, rotation, composition and activity before it departs our system.

Bottom line: 3I/ATLAS offers a rare, time-limited chance to sample material from another star system remotely. Rapid, coordinated observations across many spacecraft and telescopes are giving scientists a valuable glimpse into the building blocks of distant planetary systems.