Geez… only antlersoft gets it. AmeliaGrace wants the effective cross-sec of the comet , not a nucleus , which would be straightforward. A comet is a phenomenon of nucleus plus coma plus tail plus whatever adnexa. (Ulysses showed that tails, at multiple comets, are ridiculously long. A human notion of “tail” is limited by our eyesight, not what’s actually there or not.)
The extent of a coma, let alone tail, is inherently arbitrary- Rosetta flying through 67P/C-G’s coma was “filthy” by spacecraft-engineering standards but still cleaner than a cleanroom on Earth. The definition/calculation of coma radius/tail length then arbitrates with instrument aperture, exposure time, etc. And that was 67P/C-G. ‘Oumuamua was dust-poor, gas-rich to the point that its very comethood could only be showed mathematically, not directly observed (imaging) at all. And there is every shade of grey between these two extremes- 2/Encke, for one, is considered “dust-poor” because its solid emission is mainly macroscopic grains, which violate the standard for “dust” and show up poorly in most telescopes.
Faced with a continuum debate, the simplest path forward is to define some line in the sand, then proceed with that (arbitrary) standard, acknowledging its limited domain. AmeliaGrace even started down this trail in the woods:
I was thinking that I could calculate the area of the comet seen from the Earth
Assuming visible-light telescopes/detectors, this is somewhat straightforward. If there’s an issue, it’s an arbitrary line separating faint coma from noise/hot pixels, but we do that already, no choice but to. Even then, that line is drawn at different radii, for active comets versus comets still in the outer Solar System versus comets just not releasing much.
However, I have no idea how to calculate the effective area of the comet.
Given a comet, go ahead and take the image and draw your radius. But for comet s , you cannot give the radii of their whole class .