The last time that the green comet was visible, there were rhinoceros-size wombats living in Australia, along with some ginormous kangaroos. Though some comets swing by Earth only once, many visit periodically. Halley’s Comet comes by every seventy-five years or so. The comet that’s approaching us now—it will be closest to Earth between the first and second of February—is uncatchily named C/2022 E3 (Z.T.F.), and it visits every fifty thousand years. (If it gets ejected from the solar system before its next loop, however, it won’t return.) We didn’t know about this comet until recently. It was discovered last March, when it was only a distant speck. Now it has passed as close to the sun as it will get and is heading back toward its place of origin, in the Oort Cloud, a part of our solar system so distant that Voyager 1, which was catapulted into space in 1977, won’t reach it for a few more centuries.
On a clear night, between fifty and a hundred comets can be seen through telescopes of a sufficient size. But last weekend humans were already able to go outside, look beyond the eye of the constellation of Draco, and see this comet, with its sweet little tail, for themselves. “Humans have been seeing comets for forever, which is a cool way to connect to our history as humans,” Carrie Holt, a Ph.D. student who has been studying the comet since its discovery, said.
But what are comets? The etymology of the term in English—from the Greek word “komētēs,” meaning “long-haired”—reminds us that they were once seen as long-tressed stars. For much of human history, comets were less than celebrated. Martin Luther called them “harlot stars,” for their wanton behavior. A Lutheran bishop, in 1578, described them as “the thick smoke of human sins, rising every day, every hour, every moment full of stench and horror, before the face of God, and becoming gradually so thick as to form a comet, with curled and plaited tresses, which at last is kindled by the hot and fiery anger of the Supreme Heavenly Judge.” A scholar countered that this theory didn’t account for why we saw comets only occasionally. In 1607, shortly after watching a fireworks display, the astronomer Johannes Kepler saw a comet that remained visible for a month. He wrote a pamphlet suggesting that comets were created from fatty globules in the ether; he compared the process of cometary formation to the way that oceans spontaneously generate whales. Kepler said that space was as full of comets as the sea was full of fish but that we could see them only when, guided by a special spirit, they came close to Earth.
What a comet portended was also of great interest. A silk book found in a Han dynasty tomb in China, dating from around 168 B.C., detailed how the shapes of comets were linked to different portents. A mulberry-shaped comet meant war and worry, a waterweed-shaped comet heralded disease, and a pheasant-shaped comet meant something different depending on the season of its appearance: good harvest in spring, drought in summer, flood in autumn, and, in winter, minor battles. Pliny the Elder also categorized comets by their appearance—bearded, shaped like a torch, or vibrating like a javelin—and also wrote that a comet’s location and tail direction could indicate where an imminent disaster might occur. Tracts from the Middle Ages offered their own elaborations: a comet travelling west to east foretold a foreign invader; a comet appearing at a certain cardinal point meant a leader would soon die. “When beggars die, there are no comets seen,” Caesar’s wife, Calpurnia, says, in Shakespeare’s “Julius Caesar.” As it happened, there was a tremendously bright comet that was visible in the sky for seven days not long after Caesar’s assassination. Ovid wrote that a Roman coin featuring the “Julian star” was circulated by Caesar’s great-nephew, Octavian (who renamed himself Augustus Caesar when he became emperor), as a way to solidify his power.
Isaac Newton was cheekier. As a child, he put candles inside paper lanterns and affixed them to the tails of kites so they would look like comets, frightening those around him. He wasn’t afraid of comets—he thought that they were necessary to replenish “planetary fluids spent on vegetation and putrefaction.” In his thirties, he observed the comet of 1680 and devised a way to calculate the parabolic orbit of what he had seen. (He used Kepler’s laws of planetary motion to do so.) Soon afterward, Edmond Halley used Newton’s method to deduce that the comets that had been seen in 1531 and 1607 were the same comet that he was observing in the skies of 1682. (The comet famously seen over the Battle of Hastings, in 1066, was also the same comet.) Halley made a marvellously straightforward prediction—that the comet would be back in 1758. Shortly after that came to pass, the comet was named for him.
“In this case, we found the comet in our twilight survey,” Tom Prince, one of the lead investigators of the Zwicky Transient Facility (Z.T.F.), a Caltech-led project, said. For the Z.T.F.’s night-sky surveys, the facility uses a special camera to take images through the Palomar Observatory’s forty-eight-inch telescope. It records an image of the night sky every thirty seconds or so; in the course of two nights, it collects data on the heavenly dome. The facility also takes images when the sky isn’t entirely dark—right after sunset and right before sunrise, when areas of the night sky that are not seen as well in the night surveys can be captured. Prince said, “We were lucky, because this comet was out towards Jupiter, but sort of across the solar system from us, kind of sunward”—and so still very far away.
Since the Z.T.F. produces too many images for a human or a team of humans to sift through, machine-learning algorithms teach a computer how to detect a moving object. (The discovery is largely attributed to the scientists Frank Masci and Bryce Bolin.) “On March 2nd, all we knew was that we had found a moving object,” Prince said. “We reported it to the Minor Planet Center—they are the clearing house for these things.” The Minor Planet Center, housed at the Smithsonian Astrophysical Observatory, ultimately determines whether a moving object is already known or a new discovery, and also what its orbit is. Other observatories then looked for the same object that the Z.T.F. had found, and so, piecing together results, the astronomers soon discovered that it had an orbit. Once they saw the orbit, they knew not only that it was a comet but that it was coming from the Oort Cloud. “These are visitors from the most distant reaches of our solar system,” Prince said.
“A bright comet is always exciting,” Mike Kelley, an astronomer at the University of Maryland and a comet expert, said. The brightness is more than pretty; it allows astronomers to use spectroscopy to measure gases that are emitted. The comet’s green color is caused by diatomic carbon (C2), but more detail can also be worked out. Detecting molecules such as chloroacetophenone and hydroxide, for example, gives clues as to the amount of water ice on the comet. “When a comet passes the sun, that’s an opportunity for change,” Kelley said. “We start to see a tail.” The energy of the sun makes the ice turn directly into gas and makes the dust scatter. “By studying that process of cometary change, we can work backward to what the original composition of the comet was and get a clue about the early solar-system soup,” Kelley said.
“Comets are the cats of the solar system,” Bryce Bolin, one of the comet’s discoverers and an astronomer at NASA’s Goddard Space Flight Center, said. Scientists have reasons to think it will be pretty bright, but comets do what they want. “For now, we’re assuming a stable brightening model,” Bolin said.
Comets don’t maintain their orbits forever. Occasionally, they crash into the sun; occasionally, they are moved off their course by other gravities, like that of Jupiter. Although comets tend to orbit stars, recently, a comet named Oumuamua hopscotched from another solar system to our own, spawning speculation that it might be a sign of intelligent alien life. (The speculation spawned, in turn, very serious doubt.) Oumuamua was studied by Karen Meech, an astrobiologist from the University of Hawaii, who did her Ph.D. thesis on comets, including Halley’s. “I’m a cold-weather person,” she said, of living in Hawaii. “But this is a great place to do astronomy, because of the telescopes.” Those telescopes are situated high on old volcanoes, in an environment that is relatively free from light pollution. Meech is known for her work investigating the formation of habitable planets. She has studied where, for example, Earth’s water, a prerequisite for life, came from. There is reason to suspect that some of it came from comets. “Studying comets tells us a little about what things were like when planets were forming,” Meech said. A comet can have material that is largely unchanged from its makeup four billion years ago, which might be one step in understanding how habitable worlds form elsewhere.
We know that comets change, which is to say age, but scientists are still working out the progressions of these stellar cats. We also know that, in small but real ways, they alter the lives of earthlings. When Kepler was a young boy, his mother took him to a nearby hill to better view the great comet of 1577—perhaps an influence. Kelley, of the University of Maryland, said the comet that was most formative for him was Hale-Bopp—the bright one that I remember seeing from my dorm room, in 1997, and which won’t return again until the forty-fourth century. “There are a bunch of us in comet science who are about the same age,” Kelley said. “Our hypothesis is that Hale-Bopp set us on this course.” ♦