(Photo: NASA, ESA, David Jewitt (UCLA); Image Processing: Joseph DePasquale (STScI))
An interstellar comet believed to be billions of years older than the Sun passed its closest point to Earth on Friday, giving astronomers a fleeting opportunity to study what may be the oldest physical object ever detected in the solar system.
The object, known as 3I/ATLAS, swept past Earth at a distance of about 168 million miles, or roughly 1.8 astronomical units, according to data from NASA and international observatories. The comet poses no threat to Earth, but its age, speed and chemical makeup have made it a high priority for researchers seeking clues about the earliest history of the Milky Way.
First detected on July 1 by the Asteroid Terrestrial-impact Last Alert System in Chile, 3I/ATLAS immediately stood out because of its extreme trajectory. Its hyperbolic orbit, with an eccentricity of 6.13, is the highest ever recorded for an object entering the solar system, confirming it originated beyond the Sun's gravitational influence.
Astronomers calculate that 3I/ATLAS is traveling at roughly 42 miles per second, or about 68 kilometers per second, fast enough to escape the solar system entirely after its brief passage. The comet entered from the direction of the constellation Sagittarius, a path that researchers say is consistent with an origin in the Milky Way's "thick disk," a region populated by some of the galaxy's oldest stars.
Matthew Hopkins, an astronomer at the University of Oxford, said the comet's trajectory and velocity suggest it could be more than 7.6 billion years old, with some models placing its age closer to 11 billion years. By comparison, the solar system is about 4.5 billion years old, making 3I/ATLAS a relic from a much earlier chapter of galactic evolution.
The comet's physical behavior has reinforced the conclusion that it is a natural object rather than something artificial. Observations from the Gemini North telescope in Hawaii detected a faint green glow around the nucleus, a hallmark of outgassing as solar heat causes volatile materials to evaporate from the icy surface.
Additional data from the James Webb Space Telescope, NASA's Psyche mission and the European Space Agency's Mars Trace Gas Orbiter show that the comet is accelerating as it departs the inner solar system. Scientists attribute that acceleration to jets of vaporized gas acting as natural propulsion, a process commonly observed in comets.
Chemical analysis has revealed an unusual composition. Webb data show that 3I/ATLAS is rich in carbon dioxide and contains high levels of methanol-nearly four times the concentration typically seen in comets native to the solar system-along with traces of nickel and cyanide gas. Researchers say those ingredients indicate the comet formed in a different stellar environment but from the same fundamental materials found throughout planetary systems.
The nucleus of 3I/ATLAS is estimated to be between 0.3 and 5.6 kilometers in diameter, placing it among the larger known interstellar visitors. It is only the third confirmed object from another star system detected passing through the solar system, following 1I/'Oumuamua in 2017 and 2I/Borisov in 2019.
