Why in news?
NASA’s Swift observatory detected hydroxyl gas around the interstellar comet 3I/ATLAS, signalling the presence of water molecules. At the same time, a separate mission called SPHEREx found a vast cloud of carbon dioxide gas and water ice surrounding the comet. These discoveries mark the first time water’s chemical fingerprint has been observed on a comet from another star system.
Discovery and orbit
3I/ATLAS was discovered on 1 July 2025 by the Asteroid Terrestrial‑ impact Last Alert System (ATLAS) telescope in Chile. The “3I” prefix indicates that it is the third known interstellar object, following ‘Oumuamua in 2017 and 2I/Borisov in 2019. Its hyperbolic orbit shows that it came from outside our solar system and will leave again forever. The comet reached its closest point to the Sun on 30 October 2025 at about 1.4 astronomical units (AU) and will not approach Earth closer than 1.8 AU (about 270 million kilometres).
Composition and observations
- Water signature: Swift detected ultraviolet emissions of hydroxyl (OH), produced when sunlight breaks water molecules apart. This indicates that 3I/ATLAS contains water ice despite being billions of years old and far from the Sun.
- Carbon dioxide coma: The SPHEREx spacecraft found that the comet’s coma – the gas envelope around its nucleus – is rich in carbon dioxide and extends hundreds of thousands of kilometres. Observations were made in August 2025 when the comet was about 470 million kilometres from the Sun.
- Size and appearance: Hubble Space Telescope images show a teardrop‑shaped cloud of dust streaming from a small icy nucleus. Estimates suggest the nucleus could be anywhere from 440 metres to 5.6 kilometres in diameter.
- Colour changes: As it neared the Sun, 3I/ATLAS brightened rapidly and exhibited a bluish hue not typical of solar‑system comets. Scientists suspect that gas emissions rather than dust scattering are responsible for the blue colour.
- Visibility: The comet was hidden behind the Sun at perihelion but became visible again from early November 2025. Astronomers in the Northern Hemisphere can observe it with medium telescopes just before dawn. It will remain faint and poses no threat to Earth.
Why it matters
- Clues to other worlds: Finding water and carbon dioxide on 3I/ATLAS suggests that planetary systems around other stars may have similar chemical building blocks to our own. Water ice preserved for billions of years hints that life‑supporting ingredients travel between stars.
- Testing models: Each interstellar object has defied expectations – ‘Oumuamua was dry and elongated, Borisov had abundant carbon monoxide, and ATLAS carries water and carbon dioxide. These differences challenge scientists to refine theories of comet formation and interstellar chemistry.
- Scientific opportunity: Several NASA missions (Hubble, Webb, Swift, SPHEREx) and ground telescopes are coordinating observations to learn about the comet’s structure, rotation and composition. Its passage offers a rare chance to study material from beyond the solar system before it disappears.
Source: Economic Times – water discovery, SPHEREx – CO2 coma, NASA – comet overview