Comet SWAN, Comet ATLAS and Comet ISON. Just getting tired friends and family about a new comet
that has a good possibility of being bright and then disintegrates. They just don't make comets like they used to. I have fond memories of three others; Hale-Bopp, Holmes and Hyakutake
There's a distinction to be made between having some remote possibility of being bright and a good possibility of being bright. Neither C/SWAN nor ATLAS ever had much chance of being particularly impressive, even with fairly optimistic brightness extrapolations, unless you were looking ones extrapolating outbursts forward as though they would continue forever. They both had a good chance of being decent, faint naked eye comets, which C/SWAN did achieve. Of the many comets that have any apparent chance of being very bright and impressive (usually get one every year or so), it is guaranteed to be hyped up by people who look at a (usually very badly fit) line on a plot without any clue as to where that line comes from, or what it means. This means the vast majority of hyped up comets will always be duds. This is probably true now more than ever, with the growing social media presence---it now only takes one person who doesn't know what they're looking at to share a completely ridiculous plot and have it spread like wildfire, while no one shares the boring but realistic projections because, well, they're not very exciting...
The upcoming C/NEOWISE has a better chance of being "bright" than either of the above ever did, but where "bright" is most likely only faintly naked eye in twilight at best. (Starting to see the hype building here too, but perhaps more subdued as there's no outburst to extrapolate, just badly interpreted data) For what its worth, every now and then, there will be similar comets that, due to unusual structure and/or composition, are very faint at discovery, but turn out to be much bigger comets than are typical of that brightness and subsequently brighten much more rapidly than normal. C/2006 P1 (McNaught) comes to mind. Most comets (despite guaranteed hype) don't do this though, and picking these out from the rest is next to impossible without very high quality data beyond what's normally available soon after discovery.
I will also point out that at its peak, C/ISON was brighter than both Hale-Bopp and Hyakutake at their respective peaks. You probably missed it because this happened ~4 deg away from the Sun (lasting for only an hour or so), and so was only seen with coronagraphs. This was always going to be the case regardless of it survived perihelion---even it had, it would still not have been anything close to Hale-Bopp or Hyakutake by the time it reached dark skies, nor would it have had an impressively long tail like the latter. Brightness isn't everything.
Edited by Octans, 30 May 2020 - 04:22 AM.