RARE INTERSTELLAR VISITOR 3I ATLAS

Comet C/2025 N1 ATLAS was discovered in the constellation of Sagittarius by the Asteroid Terrestrial-Impact Last Alert System located in Chile on 2025 July 1.

What was most unusual about the discovery is that the initial and follow up observations did not seem to fit an object bound to our Solar System.
It was travelling at a very fast speed of 58 km/s or about 210,000 km/h and at a hyperbolic eccentricity of 6.1,
much higher than the maximum eccentricity of 1 for solar system objects. It had to be arriving from interstellar space!

The Central Bureau of Electronic Telegrams made the announcement on July 3 under CBET 5578 and assigned an interstellar designation 3I ATLAS.
Only 2 other interstellar objects have been identified prior to ATLAS. 1I Oumuamua, detected in 2017 by the PANSTARRS survey,
was a cigar-shaped object exhibiting weak non-gravitational forces, which are indicative of cometary activity.
However, some speculated that these characteristics were evidence of intelligent design!
2I Borisov was discovered in 2019 by an amateur astronomer located in Russia which showed clear cometary activity as it sported a coma and dust tail.

Although 3I ATLAS was quite faint at discovery, of magnitude 18, it was determined that the object, only showing weak cometary activity through large telescopes,
would approach the Sun on 2025 October 29 at a distance of 1.35 astronomical units, just inside the orbit of Mars.
In fact, it would have a very close approach to Mars on 2025 October 3 at 0.19AU offering the opportunity for Mars orbiting spacecraft to get a close look at it.
Unfortunately for Earth’s inhabitants, the comet will be unobservable when located on the opposite side of the Sun when at perihelion passage during October.
However, it will be observable either side of this time. The comet will be closest to Earth on 2025 December 19 at a rather distant 1.80AU.

The orbital inclination of the comet is 175 degrees, which means its motion is retrograde, opposite to that of the planets,
and situated not more than 5 degrees from the ecliptic plane. At perihelion, the comet should reach a maximum heliocentric speed of approximately 68 km/s
 and the Suns gravity will only deflect the orbital path somewhat.

At the time of writing, the comet's brightness development was uncertain.
It may only be visible as a faint object through telescopes or could become bright enough for binocular viewing. It is unlikely to become visible to the unaided eye.

One thing will be certain; many telescopes will be directed at it to analyse its extrasolar chemical composition.
One of the most interesting early findings is that the comet is redder than most Solar System comets but similar to the surface colour of some Trans-Neptunian Objects or Centaurs.

Where did the comet originate from? We know that it arrived from the vicinity of Sagittarius, in the thick disk of the Milky Way,
but it is too early to pinpoint exactly what star system it was ejected from. Once we map the precise orbit amongst the 3D map of the galaxy,
we may get a better idea. It may be the most ancient visitor ever detected, potentially older than our solar system itself.

How big is it? Zwicky Transient Facility images from May 22 set the comet's maximum diameter at 16 km, but likely under 10 km.
In late July, Hubble Space Telescope observations of the comet worked out its diameter at approximately 5 kms and clearly showed a coma.  

OBSERVING GUIDE

nterstellar visitor is expected to interest scientists but probably won't be visually spectacular for the public. Photographic methods are the best way to observe it, and advances in smart telescopes have During September 2025, the comet will be favourably situated for southern observers and may be spotted in telescopes during evening skies
in the constellation of Libra, brightening as it approaches the Sun.

On September 1, the magnitude 14 comet can be seen near Gamma Librae (Zuben Elakrab).

On September 7, it locates 35’ North of magnitude 12 galaxy NGC 5878.

After September 8, moonlight will not interfere with viewing for a couple of weeks.

On September 20-21, it will be within 30’ of a faint galaxy pair NGC 5741/5742.

On September 25, the comet has brightened to magnitude 13 and the crescent Moon now appears to its South.

By early October, the comet becomes harder to spot as it quickly moves into evening twilight.

From early to mid-November, comet 3I ATLAS will then reappear in the morning sky, at peak brightness of potentially magnitude 9 to 11.
Northern observers in dark locations will have the best view, while those in the Southern Hemisphere may struggle due to its northerly position.
Viewing conditions slowly improve during November.

On the morning of November 23, it will be adjacent to magnitude 12 galaxy NGC4544 in Virgo and may appear of similar brightness.

On November 26, it lies 30’ south of Eta Virginis (Zaniah)

By December 4, the now magnitude 13 comet lies within 30’ of beta Virginis (Zavijah) but moonlight now interferes with viewing.