Earth will witness a rare cosmic event on December 19, 2025 as 3I/ATLAS, an interstellar comet that originated from outside the solar system, makes its closest approach to the planet. It will pass Earth at a distance of about 1.8 astronomical units (AU) [roughly 170 million miles] with no threat to the planet. NASA’s Asteroid Terrestrial-IMPACT Last Alert System (ATLAS) survey telescope discovered it on July 1, 2025 in Chile which was later confirmed by astronomers that it is the only third known interstellar object to enter the solar system after 1I/’Oumuamua in 2017 and 2I/Borisov in 2019.
Timeline: Key Moments in the Journey of Comet 3I/ATLAS
- May-June 2025: Precovery observations identified earlier images of the comet
- July 1: ATLAS telescope in Chile officially discovered the object and reported it as an interstellar visitor
- July 21: Hubble Space Telescope captured detailed images of the comet’s coma helping estimate the comet’s size
- September-October: As the comet approached the Sun, increased monitoring was observed by multiple space missions
- October 29-30: The comet reached perihelion (closest point to the Sun) and sped through the inner solar system
- November 11: Reappeared after passing behind the Sun, more active and brighter
- Mid-late November: Brightening of green coma observed
- December 19: Will see the comet’s closest approach to the Earth at 1.8 AU
- Early 2026: Expect the comet to exit the Solar System permanently following its trajectory eventually passing near Jupiter’s orbit
Scientific Mysteries
- Hyperbolic trajectory
Unlike typical comets which loop around the Sun and return decades or centuries later, the 3I/ATLAS has a hyperbolic trajectory meaning it is a one-time visitor and is just passing through our solar system.
- Extreme Velocity
The comet is travelling at a lightning speed of around 130,000 miles (209,000 kilometres) per hour, the highest velocity ever recorded of any solar system object. Such a speed cannot be explained by Solar System dynamics alone, another key indicator of its interstellar origin.
- Unusual composition
Made up of icy water, carbon compounds and dust grains, mysterious nickel gas has also been detected in the comet’s gas plume inviting curiosity regarding its chemistry at enormous distances from the Sun. Observations show that it has begun displaying typical cometary features, i.e. the glowing coma surrounding its nucleus and a tail stretching away from the Sun. What is more fascinating is its bright green hue that maybe caused due to some chemical reaction with the Sun sparking worldwide observation.
While observations about the 3I/ATLAS remain consistent with known cometary behaviour, some features like the intensity of its activity and the composition of its gases has sparked controversies amidst studies. Scientists note that the conclusions derived are based on limited comparisons and that observations are underway as the coming months are expected to clarify these uncertainties and further help with the scientific understanding of 3I/ATLAS.