C/2022 E3 (ZTF)

Lore & Background

The comet was initially identified as an asteroid, but subsequent observations revealed a condensed coma, confirming it as a comet. It was later detected in pre-discovery images from Pan-STARRS 1 on 10 July 2021, when it had magnitude 23, and had also been photographed unnoticed by ZTF in October and November 2021. Upon discovery, the comet had magnitude 17.3 and was about 4.3 AU from the Sun.

As it approached the Sun between July and October 2022, the dust tail and coma grew, with dust production rising from 241±3 kg/s to 476±9 kg/s. By early November 2022 it had brightened to magnitude 10, exhibiting a green coma, yellowish dust tail, and faint ion tail. On 19 December it showed a greenish coma, short broad dust tail, and a long faint ion tail spanning 2.5 degrees. The comet reached perihelion on 12 January 2023, and first naked-eye observations occurred on 16 and 17 January at magnitudes 5.4 and 6.0. A coronal mass ejection caused a disconnection event of the ion tail on 17 January. An anti-tail became visible on 22 January, pointing toward the Sun due to particles on the orbital plane. XMM-Newton observed the comet faintly on 23 January.

During its closest approach to Earth on 1 February 2023, the comet was near the north celestial pole in Camelopardalis, with magnitude about 5 and a coma reported about 20 arcminutes across. The central coma region measured about 4 arcminutes across (50,000 km) with two jets. The waxing gibbous moon hampered viewing. On 5 February it passed 1.5 degrees from Capella, on 6 February near comet C/2022 U2 (ATLAS), on 10–11 February 1.5 degrees from Mars, and on 13–15 February in front of the Hyades star cluster.

Reader's Guide

C/2022 E3 (ZTF) gained significance as a rare naked-eye comet from the Oort cloud, offering a vivid green hue due to diatomic carbon (C2) excited by solar ultraviolet radiation, which emits at 518 nm. The green glow appears in the comet's head but not the tail, as C2 photolyzes within about two days. Comet researcher Matthew Knight noted that such green color is not unusual for gas-rich comets, but they rarely approach Earth closely enough for such good observation; similar colors were seen in C/2021 A1 (Leonard).

Scientifically, the comet's nucleus rotates every 8.5 to 8.7 hours. Spectroscopy detected emission lines of NH2, C2, and [OI] between 5000 and 7000 Å, and later lines of CN, diatomic and triatomic carbon, and possibly OI. The C2-to-CN ratio was about 0.6, lower than most Solar System comets but not depleted. Other detected species include hydrocyan, hydrogen isocyanide, CH3CN, isocyanic acid, formamide, methanol, formaldehyde, formic acid, acetaldehyde, H2S, carbon monosulfide, carbonyl sulfide, ethanol, and CH2OH2. The comet was depleted in hypervolatiles (low CO and H2S relative to water) and had relatively low methanol abundance (1.8% relative to water) compared to the mean of other comets.

Its outbound trajectory is uncertain: pre-perihelion models showed a bound orbit to the Sun+Jupiter system but with an unrealistic maximum distance of 270,000 AU. Post-perihelion, the orbit is only weakly bound; at 200 AU from the Sun, the escape velocity is 2.98 km/s and the comet will be going 2.97 km/s. It may leave the Solar System or return in many millions of years, depending on outgassing and perturbations from the galactic tide and passing stars.

Did You Know?

• The comet was discovered by astronomers Bryce Bolin and Frank Masci using the Zwicky Transient Facility at Palomar Observatory on 2 March 2022.
• Its green glow is caused by diatomic carbon (C2) excited by solar ultraviolet radiation, emitting at 518 nm.
• A coronal mass ejection caused a disconnection event of the comet's ion tail on 17 January 2023.
• The comet's nucleus rotates every 8.5 to 8.7 hours and has an estimated radius of 0.81–2.79 km.

The 2022 Astronomical Discovery Landscape

In the same year that C/2022 E3 (ZTF) entered the public consciousness, the astronomical community was experiencing an extraordinary wave of discoveries that reshaped our understanding of the cosmos. In April 2022, astronomers announced the identification of HD1, a galaxy so distant that its light had traveled for 13.5 billion years to reach Earth, placing it just 330 million years after the Big Bang. This made it the earliest and most remote galaxy known at the time, with a present proper distance of 33.4 billion light-years due to cosmic expansion. Days later, researchers reported GNz7q, a starburst galaxy described as a "missing link" connecting supermassive black holes to quasar evolution. Meanwhile, U.S. Space Command confirmed the detection of what it called the first known interstellar object, CNEOS 2014-01-08, a meteorite that struck Earth in 2014 on a hyperbolic trajectory indicating an origin beyond our Solar System. This finding predated the more widely recognized interstellar visitors ʻOumuamua and 2I/Borisov by several years.

A Year of Fundamental Breakthroughs

The year 2022 also delivered landmark results in fundamental physics and biology that contextualize the era in which C/2022 E3 (ZTF) was observed. Physicists working with the Collider Detector at Fermilab measured the mass of the W boson to a precision of 0.01%, a result that hinted at a potential flaw in the Standard Model of particle physics. On the biology front, biochemists reported completing the full sequence of the human genome, a milestone that closed a long-standing gap in genomic knowledge. In another striking advance, bioresearchers demonstrated an in vitro reprogramming method called MPTR, in which fibroblast skin cells temporarily shed their cell identity, showing promise for transcriptome and epigenome rejuvenation. Additionally, a study revealed that among mammal survivors of the dinosaur-era extinction, body sizes were the first trait to evolutionarily increase, with brain size expansion following later during the Eocene epoch.

Climate Science and Environmental Urgency

The 2022 scientific record surrounding C/2022 E3 (ZTF)'s discovery was dominated by an urgent climate narrative. In early April, the IPCC released the final volume of its Sixth Assessment Report, issuing a stark warning that greenhouse gas emissions must peak before 2025 and fall by 43 percent by 2030 if there is to be a reasonable chance of limiting warming to 1.5 degrees Celsius. That same month, researchers studying cacti found that 60 percent of species would face a significant reduction in favorable climate conditions between 2050 and 2070, with epiphytic forms bearing the greatest exposure to rising temperatures. A separate study linked air pollution in rapidly growing tropical cities to approximately 500,000 premature deaths in 2018 alone, projecting a substantial further increase and calling for regulatory action targeting newly emerging anthropogenic pollution sources. These findings painted a picture of accelerating ecological pressure that defined the broader scientific mood of the year.

Medical and Pandemic Science

Amid the astronomical and climate stories, 2022 was also a pivotal year for medicine and public health, forming part of the broader scientific landscape in which C/2022 E3 (ZTF) captured public attention. A preclinical study from the Medical University of Vienna reported that a newly developed vaccine showed effectiveness against all SARS-CoV-2 variants known at the time, including Omicron. In Alzheimer's research, scientists identified 42 new genes associated with elevated disease risk, proposed sleep disturbance as a potential early-stage mechanism of neurodegeneration, and pinpointed genes linked to lifetime brain-structure changes that could serve as therapy targets. A separate study confirmed the antidepressant potential of psilocybin-based protocols, using fMRI data to show that the likely major effect mechanism involves global increases in brain network integration. These advances collectively signaled a maturing of neuroscience and pharmacology in the same year the comet entered the scientific record.