“Green Comet” Viewing Event Brings Community Together

After dark on Thursday, February 2, nearly five hundred Caltech and Pasadena community members flocked to the field behind the Cahill Center for Astronomy and Astrophysics to catch a glimpse of C/2022 E3 (ZTF), also known as “the green comet.”

Lines of stargazers snaked across the field, anticipating their turn to peer into the six telescopes operated by volunteer scientists from the Caltech astronomy department’s outreach group. One telescope offered a view of Jupiter, magnifying the gas giant’s stripes and four largest moons. Another pointed at the Pleiades star cluster. But the longest lines led to the three telescopes focused on the “star” of the night: the green comet, making its closest approach to Earth in 50,000 years.

The C/2022 E3 (ZTF) comet was discovered in March 2022 at the Caltech-led Zwicky Transient Facility (ZTF) at Palomar Observatory. After a routine scan of the night sky, the ZTF system flagged a moving object that was later determined to be a comet. As the comet’s orbit brought it closer to the sun, observers from Earth spotted its green halo or “coma,” the result of fluorescing diatomic carbon (two bonded carbon atoms) and cyanide in its chemical structure.

Many of the event participants also attended a lecture in the Cahill Center auditorium given by Cameron Hummels, Caltech’s director of astrophysics outreach. Hummels explained how the green comet originated in the Oort Cloud, a spherical layer of icy objects that orbit in the outermost section of the solar system. Rocking from side to side, he demonstrated how the comet was likely destabilized by another object and knocked into a new orbit, pulled in by the sun’s gravity.

Hummels described the comet as a “tiny snowball” relative to the vast expanse of space, but in reality, “the size of a neighborhood.” He went on to explain that as the comet approaches the sun, its ice evaporates and creates distinctive tails of dust and charged atoms, or ions, which can be millions of miles long. He also made a point to correct misinformation about the comet’s visibility, noting that although February 2 was its closest approach to Earth, the comet will still be perceptible in coming weeks, slowly getting dimmer as its orbit takes it deeper into space.

For those who missed the event, Hummels’ lecture can be seen on YouTube, and there are more opportunities to catch the green comet in person. Caltech astronomers will be taking their outreach efforts to the upcoming Death Valley Dark Sky Festival from February 10–12. “While the comet will be slightly dimmer in the sky as it travels farther out into the solar system, the skies will be much better for viewing the comet with zero light pollution and the absence of the moon,” says Hummels. “I’ve been working with the Death Valley National Park Rangers for the last six months to plan and organize this event, and I’m excited it aligns so well with the appearance of the comet!”

Closer to home, the Caltech astronomy department’s outreach group organizes monthly programming, from the Stargazing Lecture Series held on campus to Astronomy on Tap, where scientists give informal talks at the Dog Haus Biergarten in Old Town Pasadena. Both series are free and open to all ages.