3I/ATLAS Explained: What We Know About the Interstellar Comet Visiting the Solar System

What is 3I/ATLAS?

3I/ATLAS is the third confirmed interstellar object ever observed passing through our Solar System. First detected by the Chile-based ATLAS telescope and reported to the Minor Planet Center, follow-up observations indicate it is an active comet — a body with an icy nucleus that releases gas and dust as it warms.

How was it identified as interstellar?

Measurements of the object's speed and trajectory show it follows a hyperbolic path that is not bound to the Sun, indicating it originated outside our Solar System and was likely ejected from another star system. The designation "3I" signals it is the third confirmed interstellar interloper after ʻOumuamua (2017) and Comet Borisov (2019).

Why scientists are studying it

Because 3I/ATLAS formed around a different star, studying its composition and behavior gives astronomers a rare opportunity to compare planetary material from elsewhere with material from our own system. Its escape trajectory means it will eventually return to interstellar space, so observations while it remains visible are time-sensitive.

Key observations and instruments

• Space telescopes: Hubble and the James Webb Space Telescope (JWST) have returned images and data.
• Ground-based: Gemini South in Chile captured images of the comet and its tail.
• Spacecraft: Several orbiters, including ESA craft near Mars, have recorded data that helped refine the comet's trajectory. ESA’s JUICE mission planned observations during a post-perihelion active phase; those data are expected to be delivered after the spacecraft’s downlink schedule allows.

Size, speed and orbit

Size estimates remain uncertain. Agencies provide a wide range: general estimates span from a few hundred feet up to a few miles across. Hubble-based analyses narrowed the likely diameter of the solid, icy nucleus to roughly 1,400 feet (about 0.27 km) up to about 3.5 miles (about 5.6 km).

When discovered, 3I/ATLAS was moving at roughly 137,000 miles per hour and models predicted it could reach about 153,000 mph as it approached the Sun. It reached perihelion (closest point to the Sun) on Oct. 30 at about 130 million miles, just inside Mars’ orbit. As it recedes, the Sun’s gravity will slow it; once it leaves the Solar System its speed relative to the Sun will match its inbound value.

Will it hit Earth?

No. Current orbital solutions show no impact risk. The comet is expected to make its closest approach to Earth on Dec. 19 at approximately 170 million miles — about twice the Earth–Sun distance — and then continue outward into interstellar space.

Public reaction and scientific caution

Public fascination has produced speculation, including suggestions from some researchers that the object’s origins should be scrutinized for all possibilities. Most astronomers regard 3I/ATLAS as a natural comet based on observed activity (coma and tail). NASA and other agencies emphasize there is no evidence it poses a threat to Earth.

Why this matters

Interstellar visitors like 3I/ATLAS provide direct samples — via remote sensing — of material formed around other stars. Each such encounter deepens our understanding of planetary system formation and the diversity of small bodies in the galaxy.

Note: Observational details and size estimates may be refined as additional data are analyzed and published by the teams working with Hubble, JWST, ground telescopes and interplanetary spacecraft.