Free Astronomy Magazine July-August 2026

4 JULY-AUGUST 2026 assistant professor Teresa Paneque- Carreño, who served as Principal In- vestigator of the ALMA Director’s Discretionary Time program that made these observations possible. The data were obtained with ALMA’s Atacama Compact Array (ACA) just six days after 3I/ATLAS reached its closest point to the Sun — a narrow observing window made possible by ALMA’s unique ability to point toward the solar di- rection, unlike most optical tele- scopes. “Our new observations show that the conditions that led to the forma- tion of our Solar System are much different from how planetary sys- N ew observations from the At- acama Large Millimeter/sub- millimeter Array (ALMA) have yielded the first-ever measurement of deuterated water — also known as semi-heavy water — in an inter- stellar object. The discovery reveals that the interstellar comet 3I/ATLAS contains at least 30 times the pro- portion of semi-heavy water found in comets from our own Solar Sys- tem, providing a direct chemical window into the frigid conditions under which its home star system formed. The research was led by PhD student Luis E. Salazar Manzano at the Uni- versity of Michigan, working with 3I/ATLAS formed in a far colder world than our own by ALMA Observatory Bárbara Ferreira T his artist’s impression compares the semi-heavy water content of the interstellar comet 3I/ATLAS (left) and Earth (right). Insets illustrate the relative abundance of deuterated water (HDO) molecules, showing that 3I/ATLAS contains over 30 times more HDO than is found in Earth’s oceans. This elevated ratio suggests the comet formed in an extremely cold environment, very different from the conditions that shaped our Solar Sys- tem. [NSF/AUI/NSF NRAO/M.Weiss] tems evolved in different parts of our Galaxy,” said Salazar Manzano. Comets are often nicknamed dirty snowballs, in part because of their