A Green Wanderer: City-Sized Comet on a One-Way Trip Past Earth

A rare “green” comet, C/2024 E1 (Wierzchos), is rapidly approaching Earth, promising to be one of the most striking astronomical events of the month. The celestial body, comparable in size to a small city with a diameter of about 13.7 km (around 8.5 miles), is currently increasing in brightness, displaying a characteristic emerald glow and a long tail. It will reach its maximum brightness for Earth-based observers this coming Tuesday, February 17, when it passes at a distance of 151 million km (nearly 94 million miles) from Earth.

A Celestial Emerald’s Fleeting Glow

The comet’s distinctive green hue is not just a beautiful spectacle but also a clue to its chemical composition. This glow is not from the ice itself but is created when sunlight interacts with diatomic carbon (C2) molecules in the coma-the gas and dust cloud surrounding the comet’s nucleus. As the comet nears the Sun, its heat causes organic material on the nucleus to vaporize, forming this gaseous envelope. The Sun’s ultraviolet radiation excites the dicarbon, causing it to glow green. However, this same solar radiation also breaks down the unstable dicarbon molecules, which is why the vibrant green color is confined to the comet’s head and does not extend into its tail.

A Journey of No Return

The key feature of Comet Wierzchos is its “open” hyperbolic orbit. Scientists believe it has entered the inner part of the Solar System for the first time in millions of years, likely originating from the Oort Cloud-a vast, distant sphere of icy bodies. Unlike periodic comets such as Halley’s, C/2024 E1’s trajectory is not a closed loop. After it swings around the Sun, a gravitational slingshot will impart it with enormous speed, forever ejecting the icy body into interstellar space. Like the interstellar comet 3I/ATLAS, C/2024 E1 will become a cosmic wanderer, destined to drift endlessly between the stars.

Webb’s Gaze and Future Implications

C/2024 E1 was discovered by Polish astronomer Kacper Wierzchos in March 2024 at the Mount Lemmon Observatory in Arizona. Following its discovery, the James Webb Space Telescope (JWST) turned its powerful instruments toward the object. JWST’s observations revealed that the comet’s activity is primarily driven by unusually high concentrations of carbon dioxide gas. This finding is significant as it provides insights into the comet’s origins, suggesting it formed in an extremely cold region of our solar system. While initial size estimates were around 13.7 km, subsequent analysis suggests the nucleus is between 2 and 10 km in diameter. This flyby represents the only opportunity for humanity to study this pristine relic from the dawn of the Solar System, offering a rare glimpse into the building blocks of planets before it departs on its eternal journey into the galactic void.