A green visitor is going to make its closest approach to Earth this week.
Comet C/2022 E3 (ZTF) is visible in binoculars or a telescope from Canada and depending on what happens in the coming days, it may enter naked-eye visibility. Comets are notoriously difficult to predict, however, as luminosity (inherent brightness) depends on factors such as how much gas and dust streams off their surfaces, how close they are to the sun and to Earth, and their composition, among numerous other complex factors.
The comet’s closest approach to the sun, or perihelion, was on Jan. 12 when it reached a distance of just 1.11 astronomical units or sun-Earth distances. It is expected to make its closest approach to Earth at 0.28 AU on Feb. 1, according to NASA. The comet is currently visible in the constellation Camelopardalis, but Feb. 10 will present perhaps the best viewing opportunity for amateurs; the comet should be relatively close to Mars in the night sky, making it easier to find.
Numerous amateur observers have remarked on the comet’s green colour, which occurs as sunlight interacts with the molecules diatomic carbon and cyanogen streaming from the comet’s surface. The comet was discovered by an automated telescope, the Zwicky Transient Facility, at the Palomar Observatory at the California Institute of Technology in March 2022. The telescope had been looking for near-Earth asteroids and spotted the comet as a part of the survey.
Western University, which was interviewed for this story, does not have science investigations of the comet as of yet – but it may partner with other facilities in the future. Paul Wiegert, a professor who studies small bodies at the university, said scientists in general are interested in the comet due to its relatively close approach to us and its green colour, which is unusual among comets and points to a different composition.
The comet originated in a zone of icy objects far out in the solar system known as the Oort Cloud, which NASA says is located approximately 2,000 to 100,000 AU from the sun. To be clear, this region is theoretical and matches predictions of long-period comets such as C/2022 E3, which last visited Earth about 50,000 years ago. Scientists have not made any direct observations of the Oort Cloud. It is possible that the unusual composition of C/2022 E3 may help scientists better understand the conditions under which the Oort Cloud formed, especially if they are able to better understand the comet’s orbit and to match it with others like it.
Wiegert added that scientists are interested in comets, asteroids and other small bodies in general as the ingredients of gas and dust making up those tiny worlds are the same that were eventually used by planets. The dominant model of how the solar system formed suggests that planets and moons grew by accreting smaller bodies; in other words, our solar system was once made up nearly fully of small bodies. As such, comets provide a sense of what our solar system used to look like before planets and moons were formed.
“The analogy we use sometimes is the Earth as a chocolate cake,” Wiegert said. “A chocolate cake doesn’t look very much like the ingredients that it was made up of. It doesn’t look like the butter, the flour and the sugar and so forth that went into it. If you want to learn more about chocolate cakes, you can look at the chocolate cake, of course – but there’s a lot to be learned by looking at the recipe, and the ingredients that it was made up of.”
Planetary processes were identified as one of four astronomy priorities by the Canadian Astronomical Society / Société Canadienne d’Astronomie (CASCA) in its 2020 long-range plan. The community is interested in knowing about “how planets form and evolve, the composition of exoplanets and their atmospheres, and the study of protoplanetary disks,” CASCA said at the time.
The report also highlighted the role of the 10-year-old Canadian Near Earth Object Surveillance Satellite (NEOSSat) in studying comets from space, where the atmosphere of Earth presents less of a problem to observations. The satellite also has a range of view nearby the sun, which allows for observations of comets at close points to our star. The satellite can also provide “direct parallax measurements of nearby objects due to its orbit around Earth,” CASCA wrote, referring to a method of gauging distance accurately.
In general, comet researchers are not only interested in the composition of these small bodies, but also in how they played a role in transporting key “life ingredients” such as carbon, water and organic molecules. How much water these comets have – and how much of a role they played in delivering water to Earth – remain open questions.Some comets have vastly different proportions of hydrogen and deuterium, which is an isotope of hydrogen with an added neutron, than what is found in the oceans of Earth. A notable study was that of Comet 67P/Churyumov-Gerasimenko by the European Space Agency’s Rosetta mission, which found that the ratio of these two elements was 5.3 × 10-4 for 67P and just 1.56 × 10-4 for Earth’s oceans, which is a threefold difference.