In addition to 3I/ATLAS, what are the other interstellar comets that visited our solar system?

Together with him 3I/ATLASonly two confirmed objects from other stars were observed by both the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) passing through our solar system: the 1I/’Oumuamua and the 2I/Borisov.

What is Oumuamua? The pioneer messenger

1I/’Oumuamua was the first confirmed object coming from another star system. Astronomers from the Pan-STARRS project in Hawaii discovered it on October 19, 2017. Initially, it was classified as a comet, then as an asteroid, since observations showed no signs of comet activity after passing by the Sun.

However, the object deviated slightly from the trajectory it would follow if only gravitational forces had influenced it. This phenomenon, known as non-gravitational accelerationled to the conclusion that the emanation of gases from its surface caused the minuscule variations in its trajectory, finally identifying it as a comet.

Despite this reclassification, the team did not detect dusty material or chemical signatures typical of comets in the deep images captured by the Hubble Space Telescope from NASA.

What does «oumuamua» mean in Hawaiian?

The official name of the object is 1I/2017 U1. The Pan-STARRS team nicknamed him ‘Oumuamuaa Hawaiian word that means «a messenger who arrives first from afar». The «I» in its official designation (1I) stands for «Interstellar.»

‘Oumuamua is a rocky and elongated object, with a somewhat reddish tint. Its shape, similar to a cigar, is highly elongatedperhaps 10 times as long as it is wide. This proportion is greater than that of any asteroid or comet seen to date in our solar system.

The object showed variation in its brightness by a factor of 10 as it rotated on its axis every 7.3 hours. Scientists concluded that it was a dense object, composed of rock and possibly metals, and lacking water or ice. The irradiation of cosmic rays for hundreds of millions of years in interstellar space reddened its surface. The object had a diameter calculated at just several hundred meters.

Where is Oumuamua currently?

It is estimated that ‘Oumuamua wagon through the Milky Waydetached from any star, for hundreds of millions of years before its encounter with our system.

Tour of ‘Oumuamua upon entering the solar system. Photo: ESA/Hubble, NASA, ESO, M. Kornmesser.

It reached a speed of 87.3 kilometers per second after its perihelion, the closest point to the Sun, the September 9, 2017. The object passed the orbit of Mars around November 1, 2017 and surpassed the orbit of Jupiter in May 2018.

It traveled beyond Saturn’s orbit in January 2019. As it leaves our solar system, ‘Oumuamua heads towards the Pegasus constellation. Its exit trajectory is located about 20 degrees above the plane of the planets.

What is Borisov? The active comet

He Comet 2I/Borisov It was the first interstellar comet whose classification was confirmed. It was discovered by the amateur astronomer Gennady Borisov -from Crimea- on August 30, 2019. Unlike ‘Oumuamua, which looked like a rock, Borisov was very active, behaving more like a normal comet.

Observations from NASA’s Hubble Space Telescope revealed that the heart of the comet was a loose agglomeration of ice and dust particles.

Hubble took this image on October 12, 2019, when comet 2I/Borisov was about 418 million kilometers from Earth. Photo: NASA, ESA and D. Jewitt (UCLA).

As it approached the Sun, it developed a coma, which is a cloud of gas and dust surrounding the core, formed by solar heating of ice. It was estimated that he had a diameter of about 975 meters. In March 2020, scientists using Hubble noticed that a fragment broke off from the core.

What does Borisov mean?

The designation «2I» indicates that it is the second known interstellar object. The name follows the tradition of naming comets after their discoverer, Gennady Borisov.

Comet Borisov quickly moved away from our Sun and finally returned to interstellar space, with no possibility of return. Photo: ESA.

The comet was moving at a speed of about 177,000 kilometers per hour. Although the Sun’s gravity slightly altered its trajectory, its high speed and path prevented the Sun from capturing it in orbit. The comet quickly moved away from our Sun and finally returned to interstellar space, with no possibility of return.

What is 3I/ATLAS?

The comet 3I/ATLAS It is the third known object originating from outside our solar system. Astronomers categorize it as interstellar because its orbital path is hyperbolic, that is, it does not follow a closed path around the Sun.

By tracing its orbit back into the past, the comet clearly originates outside our solar system. The object is active, has an icy core and a coma, a bright cloud of gas and dust, which classifies it as a comet.

The designation is numbered «3» because it is the third known interstellar object. The «I» is for «interstellar.» «ATLAS» refers to the program that operates the telescope that first detected it: the Asteroid Terrestrial-impact Last Alert Systemfunded by NASA and located in Río Hurtado, Chile.

Astronomers don’t know its exact size, but Hubble Space Telescope observations on August 20, 2025 indicate that the diameter of Its core is located between 440 meters and 5.6 kilometers.

Hubble captured a teardrop-shaped envelope of dust emanating from its icy, solid core. When it was discovered, it was traveling at about 221,000 kilometers per hour. Its speed increases as it approaches the Sun. The James Webb Space Telescope observed that the comet releases carbon dioxide, water, carbon monoxide, carbonyl sulfide and water ice as it warms.

Where is Comet 3I Atlas today?

Comet 3I/ATLAS is approaching from the general direction of the constellation Sagittarius. It passed near Mars on October 3, 2025about 30 million kilometers away. It reached its closest point to the Sun around October 30, 2025, at a distance of 1.4 astronomical units.

The clearest image of interstellar comet 3I/ATLAS captured by the Hubble Space Telescope. Photo: NASA, ESA, David Jewitt (UCLA).

Does not pose a danger to the Earthsince its closest point to our planet is approximately 270 million kilometers -1.8 times the Earth-Sun distance-, a fact that will occur on December 19, 2025, when it will be hidden behind the Sun.

After its perihelionwill continue its journey into interstellar space. The comet will pass in front of Jupiter in March 2026 and will never be seen again.