Comet C/2022 E3 (ZTF) - January 26, 2023 - Closeup

This is one of a series of five photographs I took of this comet (C/2022 E3 (ZTF)) as it was on the cusp of its closest approach to Earth, which occurred on February 1, 2023 when it was 26 million miles away from us moving across the Ursa Minor constellation. Working at the Zwicky Transient Facility, a collaborative managed by CalTech, Bryce Bolin and Frank Masci discovered C/2022 E3 on March 2, 2022.

The coma—the area around the nucleus—glows green because of sunlight’s effect on chemicals in the coma, namely carbon and nitrogen-oxygen (diatomic carbon and cyanogen) compounds.

C/2022 E3 made a very long journey to get this close to us. Its origins are in the Oort Cloud—a gigantic spherical bubble of millions of icy cometary objects surrounding the solar system at a spectacular distance from the sun. The inner edge of the Oort Cloud is thought to be between 186 billion miles to 465 billion miles away. The outer edge has been estimated at 900 billion to 9 trillion miles away (which would be more than a light year!).

It is classified as a non-periodic comet. Its trip from the Oort Cloud took the better part of 50,000 years. And while it may return to our area sometime in the distant future—perhaps during the fourth administration of Donald J. Trump MCVXXXII—its recent pass near the sun may have helped it achieve escape velocity from the solar system.

The name C/2022 E3 (ZTF) looks unwieldy and unintelligible. But as with all classification systems in astronomy there is a logical, albeit convoluted, explanation for it. The “C” denotes that it is a non-periodic comet. Periodic comets are those that will return in less than 200 years or comets that have made more than one trip around the sun. Those are denoted with a “P” instead of a “C” before the slash. The number after the slash indicates the year the comet was discovered. The letter that follows, in this case E, indicates not only the month the comet was discovered, but also in which half of that month it was discovered. So “A” would denote the first half of January, “B” the second half of January, and so on. In this case, “E” denotes the first half of March. The number attached to that, “3” in this case, indicates that it was the third discovery of this type made in the first half of March. Finally, the letters in parenthesis indicate what institution is responsible for the discovery, in this case “ZTF”—the Zwicky Transient Facility.

Comets (and similar bodies like asteroids) can be very difficult for amateurs like me to photograph and process. To capture any detail, we need to use long exposures. Comets, even brighter ones, have very faint features. But comets move perceptibly in a short period of time. So we will have to keep our exposure times as low as possible—usually around 60 seconds instead of the usual three, five, ten, or twenty minutes. Expose too long, and the comet will smear in the frame.

Aside from exposure time, tracking the sky is a difficult issue. To get a good picture, we need to shoot many exposures and stack them together. But to stack them, we need to use a reference point. Normally, when shooting things like galaxies and nebulas, we use the stars to line up the multiple exposures and stack them together. We could do that with a comet image and the stars would look great, but then the comet would be smeared across the frame. So instead, we use the center of the head of the comet as the alignment point. That yields a sharp image of the comet, but then all the stars are smeared across the frame. So we actually do both these things. We stack an image for the stars and remove the comet, then we stack an image for the comet and remove the stars, then we combine the results to get what you see here.

I put together a little animation showing an hour of 60-second exposures aligned to the stars. You can see how far the comet moves across the frame in that time, thus creating all these challenges: