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The second interstellar guest, comet 3I/ATLAS, shines unusually due to rusting metal grains in its icy core. As it gets closer to the Sun, it behaves like an old tin can tossed into a fire: the heat melts the ice, water seeps into the metal, and vigorous corrosion begins. In its depths lies water ice, carbonaceous compounds, and metallic dust — a legacy of distant stars.

Corrosion kicks off a chemical conveyor belt: simple gases give birth to complex hydrocarbons. This "rusty soup" makes the glowing envelope unusual. As it heats up, the comet erupts with icy geysers, spewing this mixture into space. Using spectroscopy (breaking light into colors) and photometry (measuring brightness), astronomers determined that the comet's chemical fingerprint matches the composition of ancient meteorites found in Antarctic ice.

Interstellar wanderers are rare. They offer a glimpse into the past: similar icy bodies lurk beyond Neptune and in the Oort cloud. Cosmic dust, from which all this was born, is still full of chemical mysteries.