“It looks and behaves like a comet, and all evidence points to it being a comet. But this one came from outside the solar system, which makes it fascinating, exciting and scientifically very important,” NASA Associate Administrator Amit Kshatriya told journalists in November. This particular comet has made 3I/ATLAS one of the most studied celestial bodies ever.
On the 19th of December, 2025, the 3I/ATLAS passed just 168 million miles from our planet, approximately twice the average distance from our planet to the Sun, giving astronomers a brief chance to study the composition of material formed around another star. The third interstellar object, discovered on the 1st of July, 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, has a hyperbolic path which indicates it has never been bound by the Sun’s gravitational pull and will never again be in our solar system. It follows the interstellar path of the 1I/‘Oumuamua in 2017 and the 2I/Borisov in 2019 but is distinguished by its activity and the vast amount of data.
The NASA Europa Clipper mission, traveling to explore Jupiter’s moon Europa, snapped ultraviolet images of 3I/ATLAS on November 6 from a distance of 103 million miles away to gather information about its two tails and a coma saturated with oxygen and hydrogen, in addition to others from dust. This discovery adds to previous evidence of a comet experiencing heavy outgassing in the aftermath of perihelion in October. The technology utilized in this study, which separates ultraviolet light, is also what will be used by mission Europa Clipper in analyzing chemical compositions in possible water vapor plumes at Europa. For 3I/ATLAS, this is an unprecedented view of elemental composition in a coma that would not otherwise have been observable through telescopes from Earth.
Additionally, the encounter offered a twin X-ray observation of an interstellar object for the first time in history. This was conducted by the XRISM satellite from Japan, which observed 3I/ATLAS for a period of 17 hours in late November, identifying a 250,000-mile-wide X-ray glow caused by collisions between solar wind charged particles and gases escaping from its nucleus. Carbon, nitrogen, and oxygen signatures were detected, allowing researchers to gain information on its composition beyond what is possible with ground-based telescopes. This was followed by a 20-hour observation conducted by the XMM-Newton observatory from the European Space Agency, allowing scientists to identify X-ray intensities in order to understand where there would be interaction areas for solar wind particles in the comet’s area around the Sun.
Ground-based optical and radio facilities contributed additional insights, the researchers write. One such facility was the Two-meter Twin Telescope, operated from the Teide Observatory on the island of Tenerife. It detected a faint and irregular jet of gas and dust that confirmed an estimated rotation period of 14 to 17 hours the first direct measurement ever made on an interstellar comet. The jet’s behavior not only indicates that the alien rock has an “extraordinarily normal” process triggered by the Sun’s radiation but also indicates that the alien rock acts like normal solar system comets.
It is vital that the spectrum is performed across various wavelengths, including optical, infrared, ultraviolet, and X-ray. These capabilities of instruments such as the Europa-UVS, which is well suited for detecting certain emissions, such as diatomic carbon (C₂), which gives the comet its green color, or other volatiles such as carbon dioxide ice, combined with orbital mechanics, give information that suggests that comet 3I/ATLAS has a median age of 4.6 billion years, which is older than our solar system. Such a rapidly moving target demanded an orchestrated ground observation network. When at its nearest point, 3I/ATLAS was moving away from the Sun at 144,100 mph, already leaving the orbit of Mars.
The Eyes on the Solar System service offered by NASA, amateur astronomer records, and live webcast images from the Virtual Telescope Project offered real-time locations. The various observations made it possible to miss none of the observation opportunities before the comet transits past Jupiter in early 2026, Saturn and Uranus in 2028, and enters interstellar space. However, for anyone with an interest in space, 3I/ATLAS has presented a unique intersection of need and ability ultraviolet imagery provided by a passing satellite on a separate mission, X-ray mapping provided by foreign observatories, and rotation information provided by ground-based telescopes on our planet. Each piece provides a unique insight into the differences between comets in other systems compared to our own, as well as those differences that might be found in our galaxy.
