New observations of the interstellar comet 3I/ATLAS include the first measurement of the abundance of deuterated water relative to ordinary water in an interstellar object. Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) discovered that the interstellar comet 3I/ATLAS is made of an astonishingly high ratio of semi-heavy water relative to water, indicating that its system of origin likely formed under conditions far colder than our own. The findings are published in Nature Astronomy.