A comet is racing into our solar system from interstellar realms at approximately 152,000 miles per hour. The comet, designated 3I/ATLAS, was identified by the NASA-backed ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Rio Hurtado, Chile, on July 1. Initially, when the object was detected, it was presumed to be one of the many familiar inhabitants of our solar system. However, just a few hours later, astronomers recognized that the U.S. National Science Foundation (NSF)-sponsored Zwicky Transient Facility (ZTF) had previously observed the object on June 28 and 29. Those “pre-discovery” observations refined the comet’s trajectory, generating excitement throughout the astronomical community.
“The remarkably hyperbolic, or open, trajectory could only be accounted for by this being an interstellar traveler,” explains George Helou, a co-investigator of ZTF and a research professor of physics at Caltech.
Comet 3I/ATLAS marks the third known interstellar entity identified so far; the other two are asteroid ‘Oumuamua and comet 2I/Borisov, discovered in 2017 by the University of Hawai’i’s Pan-STARRS1 survey and in 2019 by amateur astronomer Gennadiy Borisov, respectively.
ZTF is an automated survey camera located at Caltech’s Palomar Observatory near San Diego. It surveys the entire night sky every three nights, detecting any movement or variation in brightness in the night sky. In addition to discovering and cataloging over 10,000 supernovae and other intriguing cosmic artifacts, it consistently identifies near-Earth asteroids and comets. For example, in 2020, it found the closest known asteroid to pass by Earth and the first asteroid known to completely orbit within Venus’s path.
In the early hours of July 1, the ATLAS team identified a seemingly slow-moving object in the Sagittarius constellation and communicated its new finding to the International Astronomical Union Minor Planet Center, a central hub for tracking small celestial bodies. Shortly afterward, Quanzhi Ye of the University of Maryland, a member of the ZTF collaboration, was engaged in his routine task of documenting ZTF observations of comets and asteroids. He employs a computer program that detects new candidate small-body discoveries and then reviews ZTF data to see if the camera captured those objects. Ye subsequently submitted the ZTF data to the Minor Planet Center.
Included in his data batch that day were ZTF readings taken of comet 3I/ATLAS on June 28 and 29. However, at that point, neither he nor anyone else realized that the comet had interstellar origins, so Ye returned to his other responsibilities.
Everything shifted a few hours later when an email from Robert Seaman, an engineer at the Catalina Sky Survey of the University of Arizona, circulated among astronomers indicating that ZTF’s pre-discovery images had prompted an update to 3I/ATLAS’s trajectory, suggesting potential interstellar origins. The orbital arc—the segment of the comet’s path recorded by observers—had expanded from a three-hour span to three days.
“The new three-day arc indicated a strongly hyperbolic orbit, which implies that the comet is merely passing through our solar system and will not return. This led to conjecture on community mailing lists that the object could be interstellar,” states Ye, who stayed up until 2 a.m. on July 2, verifying the ZTF data, including additional pre-discovery images dating back to May 22.
“I was anxious because I didn’t want to make a mistake that might suggest the body was interstellar. There were many intense email dialogues. We were all extremely excited.”
The object 3I/ATLAS poses no danger to Earth and is currently about 4.5 astronomical units (approximately 416 million miles) from the Sun. 3I/ATLAS will achieve its closest approach to the Sun around October 30, at a distance of 1.4 astronomical units (roughly 130 million miles), just within the orbit of Mars.
Regarding the increasing discovery of interstellar objects by astronomers, Helou states, “there are numerous ground-based large-format cameras on telescopes now surveying the sky—including ZTF, which is designed to locate moving objects across the entire sky.”
Caltech’s ZTF is financed by the NSF and an international consortium of partners. Additional backing comes from the Heising-Simons Foundation and Caltech. The ZTF data is processed and archived by IPAC, an astronomical center at Caltech. NASA supports ZTF’s quest for near-Earth objects through the Near-Earth Object Observations program.