Physics World Stories Podcast · Physics World

Cosmic time capsules: the search for pristine comets

·52 min·1 clip

Scientists discovered something strange about this interstellar comet - it has a tail pointing toward the sun, which shouldn't exist.

The episode focuses on comet 3i Atmos, an interstellar object discovered in July 2025 that is traveling through our solar system at 58 kilometers per second. Dr. Tracy Becker from the Southwest Research Institute explains how her team quickly repurposed the Europa Clipper mission to observe this ancient visitor. Europa Clipper, launched in October 2024, is primarily designed to study Jupiter's moon Europa and its subsurface ocean when it arrives in 2030. The spacecraft carries a comprehensive suite of instruments including cameras, spectrometers, radar, and UV sensors. When the interstellar comet was discovered, scientists had only two weeks to plan and execute observations using the spacecraft's ultraviolet spectrometer. The UV instrument, designed to study Europa's thin atmosphere, proved ideal for analyzing the comet's composition by detecting light signatures from molecules breaking apart in the comet's coma. The observations revealed high levels of hydrogen and oxygen, as expected, but also showed unusually high carbon content compared to comets from our solar system. Most intriguingly, the team discovered a mysterious tail structure pointing toward the sun, contrary to typical comet behavior where tails point away from the sun due to solar wind pressure. This sunward tail has been observed by other instruments as well, but Europa Clipper's unique viewing angle from the night side of the comet provides a different perspective. The comet likely originated from an ancient solar system and has been traveling for billions of years, potentially preserving pristine materials from the early universe. Its high carbon-to-water ratio suggests it formed in a different chemical environment than our solar system. The fact that it began sublimating materials when still far from the sun indicates it may never have been close to a star before, making it scientifically invaluable. The episode also introduces Michelle Kueppers from ESA, who discusses the Comet Interceptor mission. This ambitious project will park a spacecraft at the L2 Lagrange point, 1.5 million kilometers behind Earth, and wait for a pristine comet to arrive. Unlike previous comet missions that studied weathered, short-period comets that have made multiple passes around the sun, Comet Interceptor aims to study a long-period comet fresh from the outer solar system. The mission represents a new approach to comet exploration, essentially lying in wait rather than chasing a predetermined target. Dr. Becker explains that Europa Clipper's observations were enhanced by simultaneous data collection from the JUICE mission, providing multiple viewing angles of the same comet. This multi-perspective approach allows scientists to better understand the comet's dust particle distribution and composition. The episode emphasizes how UV observations reveal information invisible to human eyes and regular cameras, detecting auroral emissions from different materials in the comet's atmosphere. The discussion touches on the broader implications of studying interstellar objects for understanding solar system formation and the distribution of materials throughout the galaxy. The comet's ancient origins and unique composition provide insights into how other solar systems form and evolve compared to our own.

As heard by us

An interstellar comet proves more compelling than the headlines.

This episode treats 3i Atmos as the more interesting story: an interstellar comet first spotted in July 2025, it draws the alien-probe chatter but stands out for an unusually high carbon-to-water ratio.

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You want the real story behind 3i Atmos, the interstellar comet the internet turned into a mystery.

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