Comet: Astronomers determine the nature of the third interstellar object

The third interstellar object ever discovered, which will approach the Sun in October 2025, is a comet. This was reported by NASA.

Interstellar comet in an artist’s impression. Source: ESO/M. Kormesser

On July 1, the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) detected an object that arrived in the Solar System from the constellation Sagittarius. Initial calculations showed that it was moving along a hyperbolic (open) trajectory. This meant that the celestial visitor had not come from the Oort Cloud, but from interstellar space.

Following this initial report, astronomers searched the archives of various telescopes for earlier images of the object taken before its official discovery. The oldest of them are dated June 14. The analysis shows that the eccentricity of the celestial body’s orbit is 6.15, which is much higher than for the asteroid Oumuamua and the comet Borisov. So, it’s totally an interstellar wanderer.

Initially, it was unclear whether the third interstellar object was a comet or an asteroid. However, further observations revealed that it had a coma and a small tail. After that, the interstellar visitor was officially designated 3I/ATLAS.

Currently, 3I/ATLAS is located approximately 670 million km from the Sun. On October 29, the comet will approach the Sun at a minimum distance of 136 AU (203 million km), passing inside the orbit of Mars. At that moment, it will be moving at a heliocentric speed of 68 km/s. After that, 3I/ATLAS will begin to leave the Solar System.

The interstellar comet poses no threat to Earth. It will not come closer to Earth than 1.8 AU (270 million km). At the same time, before passing perihelion, it will fly at a distance of about 0.4 AU (60 million km) from Mars. In theory, an interstellar comet could be photographed by the MRO spacecraft orbiting the Red Planet. And on March 16, 2026, 3I/ATLAS will fly by Jupiter at a distance of 0.35 AU.

The flight path of interstellar comet 3I/ATLAS. Source: NASA/JPL-Caltech

At present, astronomers do not know the size and physical properties of the interstellar comet. According to some rough estimates, the diameter of its core could reach 20 km. More accurate estimates are likely to appear in the near future.

3I/ATLAS should remain visible to ground-based telescopes throughout September, after which it will come too close to the Sun to be observed. The comet is expected to reappear on the other side of the Sun in early December, allowing observations to resume.

According to NASA

Advertising