Astronomers track 3I/ATLAS, an interstellar object

 

On July 1, 2025, ATLAS—the Asteroid Terrestrial-impact Last Alert System—spotted something weird in the night sky over Río Hurtado, Chile. It was faint, moving fast, and about to shake up what we know about visitors from deep space. This thing got the name 3I/ATLAS: the third interstellar object ever caught passing through our Solar System.

So, what sets it apart? Most comets and asteroids loop around the Sun in bound, elliptical orbits. Not this one. 3I/ATLAS is on a hyperbolic path, which basically means it’s a drifter from beyond the Sun’s reach. It came from outside the Solar System and it’s not staying long.

Here’s what we know so far: When ATLAS first saw it, 3I/ATLAS was already about 670 million kilometers from the Sun. It’s booking it at around 60 kilometers per second (that’s about 221,000 km/h) compared to the Sun. And don’t worry, it’s not coming anywhere near Earth—closest it gets is about 1.8 astronomical units, which puts it 270 million kilometers away.

Why’s everyone excited? These interstellar visitors are incredibly rare. Only two confirmed before: 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Now, 3I/ATLAS joins this exclusive club and gives us another shot to study stuff that didn’t form in our own Solar System.

The discovery itself was a bit of detective work. Not just the ATLAS telescope, but also pre-discovery images from old archives helped piece together where it came from. Turns out, it was visible as early as June 14, maybe even May. Once astronomers nailed down its hyperbolic trajectory, it officially became “3I” (third interstellar) and got the comet label C/2025 N1 (ATLAS).

Even though it’s an interstellar wanderer, it’s acting like a regular comet: kicking off a coma and dust tail as it gets closer to the Sun. That means it’s got volatile ices boiling off—just like the comets we know.

Looking ahead, 3I/ATLAS will swing closest to the Sun (perihelion) around October 30, 2025, at about 1.4 AU, just inside Mars’ orbit. After that, it’ll head out again, passing through Jupiter’s neighborhood by early 2026 as it leaves the Solar System for good. Telescopes on the ground and in space will track it while they can. At some point, it’ll slip behind the Sun and disappear from view, but then it’ll pop back out for a bit before vanishing into interstellar space.

For astronomers everywhere—India included—this is a big deal. It’s a rare chance to study something made outside our Solar System, and it’s coming close enough for us to really take a look.

So, what have we learned so far? One of the biggest surprises: 3I/ATLAS has a coma loaded with carbon dioxide compared to water. Way more than you usually see in comets from our neighborhood. According to recent data, the CO₂ to H₂O ratio is about 8 to 1—one of the highest ever seen. That hints at a couple of things: maybe this object formed in a colder part of its home system, closer to where CO₂ ice could pile up, or maybe it spent time somewhere with a lot of radiation, which let CO₂ escape more easily than water.

There’s more. Ultraviolet observations picked up OH emissions (a sign of water breaking apart) even when the comet was still 3.5 AU from the Sun. Normally, water ice doesn’t evaporate much that far out, so something’s really cooking on its surface.

About its size: Hubble data from July 2025 puts the upper limit on the nucleus at about 3.5 miles (5.6 km) across, though it could be smaller—down to around 0.44 km—depending on how reflective it is. Early guesses in the media went as high as 15 miles, but those got knocked down as better data came in.

And then there’s the dust. Polarimetry—basically looking at how the dust and coma scatter sunlight—shows that 3I/ATLAS doesn’t behave like most comets. The polarization signature is different, which suggests the dust grains themselves might be a totally different mix, or shaped in ways we don’t usually see.

Why does any of this matter? Because 3I/ATLAS is a piece of another star system. It’s carrying clues about how planets, ices, and dust form somewhere else, not just here. That’s gold for scientists trying to figure out how our Solar System stacks up against the rest of the galaxy.