Visitors · 6 of 7

A Visitor from Another Star

How do we know 3I/ATLAS came from another star?

3I/ATLAS, the third object ever seen from beyond the Solar System: where it was found, its best night from where you are, and the speed that proves it is not the Sun's.

You’ll be able to tell an interstellar visitor from a comet that belongs to the Sun.

8 steps · about 6 min

Before this: Understanding Star Brightness (10 min), Racing and Crawling (4 min)

Where and which night

Working out when this is best placed from where you are…

Where

Which night

Date

Time

Kept for this guide only, until you change it here — your map settings, and every other guide, stay as they are.

Print, play outside or present

Present on a big screen

Scan with a phone's camera, or open

Sources

JPL Small-Body Database — the orbit of C/2025 N1 (ATLAS), solution of 2026 February 19, and Horizons ephemerides it is tested against
Seligman et al. 2025, "Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS" — discovery on 1 July 2025, its arrival speed and direction, and those of ʻOumuamua and Borisov
COBS Comet Observation Database — the brightness observations the map's magnitude law for 3I is fitted to
Hopkins et al. 2025, "From a Different Star: 3I/ATLAS in the Context of the Ōtautahi–Oxford Interstellar Object Population Model" — its likely age, older than the Sun
NASA Science: Comet 3I/ATLAS — facts and figures
ESA: ExoMars and Mars Express observe comet 3I/ATLAS — the images from Mars orbit, about 30 million km away, in October 2025
Vera C. Rubin Observatory — the survey expected to find many more interstellar objects

← GUIDES

More below ↓

With a keyboard, focus the sky, aim it with the up and down arrow keys, and press Enter to drop the pin at the centre of the view; or press Show me.

You are here

Settings

Screen

Night mode Turns every page red, to keep your eyes adjusted to the dark.

High contrast Bolder constellation lines, larger and brighter names on the sky, and stronger text and edges everywhere. Starts on if your device asks for more contrast.

Reduce motion The camera cuts to where it is going instead of flying there, and panels and sheets appear without sliding. Starts on if your device asks for less motion.

Battery saver Draws the sky thirty times a second at most: smoothness traded for battery.

Night brightness Dims the screen below your phone’s lowest setting, down to a fifth, while night mode is on.

Clock and voice

Clock How every time is written. Until you choose, your language’s own clock decides.

D describes What D says about the view on the sky: one sentence, or in full with the field, the constellation and five names.

Sounds Quiet tones as you use the map: a tick as you drag the clock past a mark or dusk, a chime for the star you pick — higher for hotter stars — and a low swell while a figure is out of the dome. Made in your browser, and off until you turn them on.

All settings and your progress

3I/ATLAS was found in the ring on 1 July 2025 by the ATLAS survey telescope in Chile: a dot of magnitude 17 in Sagittarius, far too faint for the eye. Within a day its path showed something new.

Its best night from where you are: a fuzzy spot in a small telescope, a comet made round another star. The line is its last two weeks, a night at a time — about 0.6° a night, more than the full Moon's width, across stars that do not move.

From outside, to scale: the rings are the orbits of Mercury, Venus, Earth and Mars. 3I's path is open. Follow the arrows: in once, round the Sun the opposite way to every planet, bent just 19°, and out for good.

The falling curve is escape speed: any faster, that far from the Sun, and nothing brings you back. Halley runs just under it, so it returns. 3I swung past the Sun at 68 km/s, far over it, and still did 61 km/s at Jupiter's distance.

Run four months. 3I passes Mars at 30 million km — orbiters there photographed it in October 2025 — sweeps past the Sun on its far side from us, then climbs out. The line from the Earth through it lands where the map showed it.

The ring is where it came from, 22° from where it was found: it was heading nearly straight in. That is a direction, toward the Milky Way's centre, not a birthplace. It came in at 58 km/s, faster than ʻOumuamua's 26 or Borisov's 32: old stars get stirred faster.

And the ring is where it is going, in Gemini, at 58 km/s for good. The Rubin Observatory should find many more.

The whole visit once more, to scale. Sky surveys are already looking for the fourth.

Found in Sagittarius on 1 July 2025
An open path, in once and out for good
In from toward the galaxy's centre, out to Gemini