This is Part 3 of a series on interstellar comets. Read Parts 1 and 2. So why should we expect interstellar comets like 3I/ATLAS and 'Oumuamua and even to some extent Borisov to be different-different? Well, for one, they came from somewhere else. They were literally born in a foreign solar system. And not all solar systems are created equal. The star they formed around might have been hotter or cooler than the Sun. The protoplanetary disk — the spinning cloud of gas and dust that eventually clumps into planets and comets — would have had its own unique recipe. Different ratios of hydrogen to helium to metals. Different temperatures at different distances from the star. Different amounts and kinds of dust grains acting as seeds for everything that came after. A comet born in a system with a higher metal content is going to carry that signature with it forever, like an accent you never quite lose. So when ATLAS shows up loaded with nickel, that might just be where it's from.…