In 2025, astronomers watched a visitor from another star system cross the inner solar system. The object, named 3I/ATLAS, was only the third interstellar object ever found. NASA’s published facts about it explain what it was, how fast it moved, how close it came to Earth and why scientists cared.
What is 3I/ATLAS?
3I/ATLAS is an interstellar comet. The “3I” means it is the third interstellar object found, and “ATLAS” credits the survey that discovered it. The telescope that spotted it belongs to the Asteroid Terrestrial-impact Last Alert System (ATLAS), a NASA-funded survey in Rio Hurtado, Chile, which is part of NASA’s planetary defense network. The discovery was reported to the Minor Planet Center on July 1, 2025, when the object was about 410 million miles (670 million kilometers) from the Sun, within the orbit of Jupiter.
Why is it called interstellar?
Interstellar means the object came from outside our solar system, from the space between stars. Scientists identified it as interstellar from its very high speed and its path. The path was hyperbolic: the comet moved too fast to be bound by the Sun’s gravity, so it did not orbit the Sun. It passed through and left. In orbital terms, a hyperbolic path has an eccentricity greater than one, a concept explained in our entry on orbital eccentricity. That last link is our own explanatory connection and is not drawn from the NASA page.
How fast did it travel?
- At discovery: about 137,000 miles per hour (221,000 kilometers per hour)
- At closest approach to the Sun: about 153,000 miles per hour (246,000 kilometers per hour), after the Sun’s gravity sped it up
- On departure: a speed matching its arrival speed
How close did it come?
The comet’s closest approach to the Sun was on October 30, 2025, at about 130 million miles, or 1.4 astronomical units (AU), just outside the orbit of Mars. The NASA page lists the distance in kilometers as “210 kilometers,” which is a typographical error. The correct figure is about 210 million kilometers, since 130 million miles is about 210 million kilometers. At that time Earth was on the opposite side of the Sun. The comet came no closer to Earth than about 170 million miles (270 million kilometers), or 1.8 AU. The FAQ cites a distance of about 1.8 AU from Earth on December 19, 2025. NASA states that the comet posed no danger to Earth.
How big is it?
The Hubble Space Telescope observations as of August 20, 2025 constrain the nucleus diameter to not less than 1,400 feet (440 meters) and not greater than 3.5 miles (5.6 kilometers). That is a wide range, and NASA states that the exact size is unknown. The nucleus is hidden inside a bright cloud of gas and dust, so it cannot be measured directly.
Why is it a comet and not an asteroid?
Telescope observations showed that the object was active, with an icy nucleus and a coma, a bright cloud of gas and dust that grows as a comet nears the Sun. That activity is what distinguishes a comet from an asteroid. Gas released near the Sun caused small changes to its trajectory, and observations matched this effect.
Where did it come from?
NASA says the comet formed in another star system and was ejected into interstellar space. It drifted for millions or billions of years before reaching ours, and it approached from the general direction of the constellation Sagittarius, toward the center of the Milky Way. NASA does not identify its home star. Because it formed around another star, its makeup may reveal how other systems form, which is why the page calls its differences from solar system comets scientifically interesting. For the study of worlds around other stars, see our entry on exoplanets.
Which missions observed it?
NASA lists the Hubble, James Webb and SPHEREx space telescopes among the observers. It also lists several spacecraft at Mars, including Perseverance, MAVEN and the Mars Reconnaissance Orbiter, which may have spotted it during its October 3, 2025 pass by Mars. Other missions on the list include Parker Solar Probe, PUNCH, SOHO, STEREO, Lucy and Psyche. NASA’s Eyes on the Solar System tool shows its path. For an earlier chapter of NASA exploration, see our article on the Apollo 11 landing.
The first two interstellar objects
- ‘Oumuamua: discovered in 2017, the first known interstellar object
- 2I/Borisov: discovered in 2019, the second
How to read the facts
Two cautions apply. First, NASA’s page is a facts-and-FAQ sheet, so it reports measurements and not full analysis, and the chemical composition is not given in detail. Second, NASA classifies the object as a natural comet, and we follow that classification. The comet has now left the inner solar system, so the remaining work is in analyzing the data collected.
Glossary: terms used here
- Perihelion: the point in an orbit or path closest to the Sun
- Astronomical unit (AU): the average distance from Earth to the Sun, about 93 million miles
- Coma: the cloud of gas and dust around a comet’s nucleus
- Nucleus: the solid, icy core of a comet
- Hyperbolic trajectory: an open path that does not return, taken by objects moving faster than the escape speed
These definitions are general ones and are not quoted from the NASA page.
Frequently asked questions
When was 3I/ATLAS discovered?
It was reported to the Minor Planet Center on July 1, 2025 by the ATLAS survey telescope in Chile.
Did it threaten Earth?
No. It came no closer than about 170 million miles, and NASA states it posed no danger.
How many interstellar objects are known?
Three: ‘Oumuamua (2017), 2I/Borisov (2019) and 3I/ATLAS (2025).
How fast was it moving?
About 137,000 miles per hour at discovery and about 153,000 miles per hour at its closest approach to the Sun.