A comet named ... C/2022 E3 (ZTF)! 💩

 tl;dr - On 2 March 2022 the observatory "Zwicky Transient Facility" discovered a new comet. The icy wanderer reached its closest approach to earth on 1 February 2023. It did not quite put on a show as comet Neowise in 2020, nevertheless we didn't leave the visitor unattended. Besides taking pictures, we also decided to acquire a spectrum.

It is difficult to tell when the best time for amateur astronomy was. I might have been the early 90s, when digital cameras (CCDs) became affordable. It was the time when one could simply walk out into the backyard, take some images through a medium size telescope and discover a previously unknown asteroid or comet. A comet would inherit the name of its discoverer - that's the rule that still applies today!

Unfortunately, human discoverers of comets belong to a species in the way of extinction. Automated search programs are the state of the art now and they are shockingly effective. The Zwicky Transient Facility (ZTF) has a telescope with 16 (sic!) imaging chips in its focal plane, able to cover the entire northern sky in three nights only. And that is just the beginning. The Vera C. Rubin observatory stands ready to raise this benchmark by the factor of 10. Its first light is scheduled for December 2023.

For the scientific community, this development means a bonanza of data - one which, however, comes with a cruel drawback: The era of melodious names of comets is over. Lovejoy, McNaught, Halley, Tuttle, Schwassmann-Wachmann and not to forget the legendary Churyumov-Gerasimenko ... all you hear now is ATLAS, PanSTARRS, LINEAR, NEAT, SWAN, or worst of all ZTF.

Well, despite its ugly name but with a heavy heart, we decided against ghosting the comet. On the contrary, we took images on 25 January 2023 and even acquired a spectrum on the 29. With the latter, we were able to gain insight into its chemical composition. 

Dark sky fans hunting a comet:
Gregor, Samuel, Alex and the teacher Christof Wiedemair (from right)

The target: Comet C/2022 E3 (ZTF) with green coma, ion and dust tail.

Starless version of the comet's inner region taken with the telescope.
Three types of tails are visible: The broad, fan-shaped dust tail and the narrow gas tail
point upwards while the anti-tail points to the lower right.

Although we love the dark, plain daylight has it "bright sides", too.
Gregor preparing our spectrograph (DADOS) for its nightly assignment.

We took six single spectra, each with 3 min exposure time.

In the sum of all scans, we could identify some of the "ingredients" of the comet.

The spectrum we obtained has a wavelength range from < 4000 A up to > 8000A, so it is covering the entire visible range. The bulge of the light comes from dust particles reflecting sunlight but on top of that, there are several emission lines and bands from the comet's gaseous outflows. The green cast of the coma comes from the so-called swan bands between 5000 A and 5750 A, produced by diatomic carbon. The Hg-line originates from local street light pollution. Another remarkable feature is the strong emission of Cyanogen below 4000 A in the ultraviolet regime. If our eyes and cameras were more susceptible to this wavelengths, the comet might show a somewhat violet cast. That would certainly be an eye-catcher!

Now we probably all concur that this comet had some interesting aspects and that its weird name C/2022 E3 (ZTF) does not fit the bill at all. But who are we to change it? To stand up against the mighty International Astronomical Union? Is this the moment for a rebellion to break loose? Wait! We might compromise to a nickname like "Zwicky" or "Snowball" or henceforth speak of you as "He-Who-Must-Not-Be-Named". I guess it's time to start an online voting and to write a petition! 

Either way: Farewell, and tell your friends to pass by. Earth is just round the corner. 

Christof Wiedemair

Correction: In the original spectrum the CN line at 4208 Angstrom was labeled with CO+. A weak CO+ feature might be present at 4260 Angstrom. Thank you Hugh Allen for the hint!


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