Thanks for the reply. It’s good to know that I’m not alone.
My reaction would be to query what the point of publishing these absolute magnitude values is, if the numbers produce such wild predictions. Surely, the only reason to publish absolute magnitudes is for people like me to use them to predict the future brightnesses of comets. Isn’t it inviting naive people like me to start telling the world that Blanpain will be a fifth magnitude comet?
In the past I’ve heard people express exasperation when comets get over-hyped, only to disappoint. And so, wouldn’t it be better to publish absolute magnitudes at the conservative end, rather than ones that lead to wildly optimistic forecasts? It would help if there was any kind of error bar quoted, to signify which absolute magnitudes are vaguely trust-worthy.
I should add that the absolute magnitudes on the BAA Comet Section pages are much more reliable than those on the MPC website – I suspect in part because the BAA only publishes values for moderately well-behaved comets. The BAA does not, for example, publish any values for Blanpain.
Even then, by my calculations, the BAA’s published absolute magnitude for 2017 T2 puts it at magnitude -1 next May, which is about 5-6 magnitudes brighter than most people seem to be expecting.