Astronomers have discovered that the interstellar comet known as 3I/ATLAS is significantly larger than initially estimated. The Hubble Space Telescope captured an image of the comet on July 21, 2025, when it was 277 million miles from Earth. This interstellar body, which has been traveling toward the inner solar system, now appears to be more massive than previously thought.
According to Harvard astrophysicist Avi Loeb, 3I/ATLAS likely has a weight exceeding 33 billion tons, with a nucleus diameter of more than 3.1 miles, making it the largest interstellar object detected so far. This observation was based on analyzing the comet’s trajectory and calculating mass loss using data from the James Webb Space Telescope, which observed it on August 6.
The comet’s composition seems to primarily consist of carbon dioxide, with a notable presence of carbon dioxide gas in its coma and water ice in its nucleus. 3I/ATLAS was first discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, an early warning network funded by NASA. The object, originating from interstellar space, was noted as traveling from the direction of the Sagittarius constellation.
While 3I/ATLAS poses no threat to Earth, its trajectory is expected to bring it near Jupiter, Mars, and Venus. On October 3, the comet is predicted to pass within 1.67 million miles of Mars’ orbit. This event marks only the third occasion in history where an interstellar object has entered our solar system.
The comet is anticipated to pass too close to the sun to remain visible by the end of September but is expected to reappear in December. Avi Loeb has detailed these findings in a recent paper, pending peer review, providing new insights into the characteristics and trajectory of this interstellar traveler.