Interstellar comet 3I/ATLAS leaves Solar System at 58 km/s after Sagittarius approach

September 2, 2026 at 01:30 AM2 min

This digest was compiled by AI from multiple sources — links to the originals are below.

Interstellar comet 3I/ATLAS is departing the Solar System permanently, retaining about 58 kilometres per second relative to the Sun after its gravitational encounter. Its trajectory is hyperbolic, with no return date. The comet arrived from the direction of Sagittarius at roughly 208,600 kilometres per hour.

Key Facts

3I/ATLAS will retain close to 58 kilometres per second relative to the Sun after the Sun's gravitational braking is complete.
The comet arrived at roughly 208,600 kilometres per hour and accelerated to approximately 246,000 kilometres per hour near the Sun.
The incoming asymptote pointed towards right ascension of about 295 degrees and declination near minus 19 degrees, in Sagittarius.
The original discovery analysis found an incoming hyperbolic excess speed of 57.942 kilometres per second, with a formal uncertainty of 0.049 kilometres per second.

Hyperbolic Trajectory

3I/ATLAS is leaving the Solar System without entering an orbit that will ever carry it back. The Sun is slowing it now, just as the Sun accelerated it on the way in. Yet even after that gravitational accounting is complete, 3I/ATLAS will retain close to 58 kilometres per second of speed relative to the Sun. Its trajectory remains open, its orbital energy positive and its departure permanent. There is no distant turnaround point and no return date to place in a calendar.

Arrival Direction

The comet arrived at roughly 208,600 kilometres per hour, was moving at about 221,000 kilometres per hour when astronomers discovered it, and accelerated to approximately 246,000 kilometres per hour near the Sun. In the discovery team's preliminary characterisation, the incoming asymptote pointed towards right ascension of about 295 degrees and declination near minus 19 degrees. On Earth's sky, that lies in Sagittarius, not far from the direction of the Milky Way's centre. Constellations are two-dimensional patches on our sky, not physical containers. Stars that appear close together in Sagittarius can sit at very different distances.

Speed Comparison

An object speeds up as it falls into the Sun's gravitational field. That means a speed measured near Mars's orbit cannot be compared cleanly with one measured much closer to the Sun. The useful common quantity is hyperbolic excess speed, usually written as v-infinity. For 3I/ATLAS, the original discovery analysis found an incoming value of 57.942 kilometres per second, with a formal uncertainty of 0.049 kilometres per second.