Zwicky Transient Facility Nabs an Interstellar Visitor

A comet is hurtling into our solar system from interstellar space at about 152,000 miles per hour. The comet, named 3I/ATLAS, was discovered by the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Rio Hurtado, Chile, on July 1. When the object was first spotted, it was assumed to be one of the many usual denizens of our solar system. But just a few hours later, astronomers realized that the U.S. National Science Foundation (NSF)-funded Zwicky Transient Facility (ZTF) had previously observed the body on June 28 and 29. Those "pre-discovery" observations refined the comet's orbit, sparking excitement throughout the astronomical community.

"The extremely hyperbolic, or open, orbit could only be explained by this being an interstellar visitor," says George Helou, a co-investigator of ZTF and a research professor of physics at Caltech.

Comet 3I/ATLAS is the third known interstellar object discovered to date; 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 a robotic survey camera based at Caltech's Palomar Observatory near San Diego. It scans the whole night sky every three nights, detecting anything that moves or changes in brightness in the night sky. In addition to discovering and classifying more than 10,000 supernovae, among other interesting cosmic specimens, it regularly spots near-Earth asteroids and comets. For instance, back in 2020, it discovered the closest known asteroid to fly by Earth as well as the first asteroid known to orbit entirely within the orbit of Venus.

In the early morning on July 1, the ATLAS team detected a seemingly slow-moving object in the constellation of Sagittarius and reported its new discovery to the International Astronomical Union Minor Planet Center, a hub for tracking small bodies. A few hours later, Quanzhi Ye of the University of Maryland and member of the ZTF collaboration, was busy going about his usual job of logging ZTF observations of comets and asteroids. He has a computer program that checks for new candidate small-body discoveries and then scans ZTF data to see if the camera imaged the objects. Ye then submitted the ZTF data to the Minor Planet Center.

Included in his batch of data on this day were ZTF measurements taken of comet 3I/ATLAS on June 28 and 29. But at that point, neither he nor anybody else had realized the comet was interstellar in origin, so Ye went back to his other duties.

Everything changed a few hours after that when an email from Robert Seaman, an engineer at the Catalina Sky Survey of the University of Arizona went out to astronomers indicating that ZTF's pre-discovery images had led to an update of 3I/ATLAS's orbit that suggested possible interstellar origins. The orbital arc—the portion of the comet's orbit recorded by observers—had changed from covering a period of three hours to three days.

"The new three-day arc suggested a strongly hyperbolic orbit, which means the comet is just passing through our solar system and will not come back. This prompted speculation on community mailing lists that the object might be interstellar," says Ye, who stayed up that night until 2 a.m. July 2 double checking the ZTF data, including additional pre-discovery images dating back to May 22.

"I was trembling because I didn't want to make a mistake that prompted speculation of the body being interstellar. There were a lot of intense email exchanges. We were all very excited."

The object 3I/ATLAS poses no threat to Earth and is currently about 4.5 astronomical units (about 416 million miles) from the Sun. 3I/ATLAS will reach its closest approach to the Sun around October 30, at a distance of 1.4 astronomical units (about 130 million miles), just inside the orbit of Mars.

As to why astronomers are discovering more interstellar objects than ever before, Helou says "there are a lot of ground-based large-format cameras on telescopes that are surveying the sky now—including ZTF, which is designed to find moving objects across the whole sky."

Caltech's ZTF is funded by the NSF and an international collaboration of partners. Additional support comes from the Heising-Simons Foundation and from Caltech. ZTF data are processed and archived by IPAC, an astronomy center at Caltech. NASA supports ZTF's search for near-Earth objects through the Near-Earth Object Observations program.