The Comet 3I/Atlas, as seen by the  International Gemini Observatory/NOIRLab/NSF/AURA/K. Meech (IfA/U. Hawaii). Image Processing: Jen Miller & Mahdi Zamani (NSF NOIRLab). The spectral artifacts are revealed in a timelapse exposure detailing various wavelengths of visible light as the comet sweeps across the sky.

On 1 July 2025 Comet 3I/ATLAS was discovered by the ATLAS telescope in Chile. It is an interstellar object, meaning it comes from outside our Solar System.The prefix “3I” designates it the third confirmed interstellar object, following 1I/Oumuamua in 2017 and 2I/Borisov in 2019. Its shape and behavior show that it is a comet: an icy object releasing clouds of dust and gas, with one or more ‘tails’ of charged particles and dust streaming away.

This unusual object inspired some to propose an immediate new space mission to land on the comet. Mayby it was an alien spaceship! But there was a better plan: point dozens of different kinds of telescopes, from Earth to Mars, at the comet. They gathered many kinds of observations from every angle and quickly discovered 3I is a comet from another star. It has a hyperbolic trajectory past the Sun, meaning it is just passing through the solar system and is already speeding away, back to interstellar space.

3I/ATLAS was already speeding at 240,000 km/hr when discovered, strong evidence that it was not from our Solar system. The Sun’s comets need to get close to the Sun to pick up speed. It’s faster than the two previous interstellar comets.

Its closest approach to Earth, 270 million km or 1.8 AU, was on 19 December 2025. Perihelion occurred on 29 October, 203 million km or 1.36 AU from the Sun. During its closest approach to the Earth, it was on the other side of the Sun. It poses no danger to our planet or any other planets in the Solar System. (Asteroid 2024 YR4 has a small chance of impacting the Moon in 2032, but that’s another article).

Thought to originate from a region of the Milky Way populated by older stars, 3I/ATLAS may be over 7 billion years old — older than our Solar System.

The brief presence of these bodies in the solar system offers a unique glimpse into the chemical makeup around other stars, with the objects coming to us across the light years.

As the comet continued to approach the Sun, it lost more mass as frozen gases transform into vapor, carrying dust and ice into space. Observations in late August by the NASA/ESA/CSA James Webb Space Telescope of the coma, or ‘halo’, surrounding the comet revealed carbon dioxide, water, carbon monoxide, carbonyl sulphide and water ice being released as the comet heated up. These are all common molecules in our familiar solar comets.

3I Atlas has an unusually high level of carbon dioxide, and much of the tail, or coma, is made of this. It implies the comet came from another system that may be colder than ours. Or maybe the comet was formed far away from its star and was easily knocked into deep space.

At its peak activity, near our Sun, the comet was spraying the equivalent of an Olympic swimming pool of water every 2 seconds. This tells us that after traveling interstellar space for millions of years, 3I Atlas was still similar to a solar comet. This also supports an important concept of modern astronomy, the Universe is homogeneous. Made of the same stuff (atoms and energy) everywhere. 

As soon as the comet was close enough to accurately measure its size and composition claims that it was made by aliens were backpedaled.

Space missions were briefly repurposed to observe the comet, including Perseverance, on the surface of Mars! It’s amazing we can see an interstellar comet from the surface of another planet. NASA invented the best technology they could, and were ready when the moment came.

An x-ray telescope launched 20 years ago was in the right position to catch the X-rays caused by the comet interacting with our solar wind. New discoveries require a global effort.

It’s also a reminder that there are things we don’t know about the Universe, from giant meteors we can’t see yet, to dark matter, to actual Alien civilizations, that could have an enormous impact on Earth. We have to keep exploring in every direction. Who knows when we’ll make an amazing, planet saving discovery?