Tell me about comets. What do we know?

[Laie]

Trying to figure out how to find comets and what to expect when I meet them, this is what I found so far. Please add more information if you have it.

It seems that perhaps one in ten objects discovered in the tracking station is a comet. They're quite obvious: while asteroids are clustered near Kerbin, comets turn up ...elsewhere. I could spend a hundred words describing it, but they really stick out when they show up.

So far I found one class E (1500t, in the ballpark of an E class Asteroid), one class G (40,000t when I visited), and one class H (191kt).

Unlike Asteroids, their mass appears to be fixed at the moment they fist show up in the tracking station: I got exactly the same mass on several visits to the same comet. With asteroids, you can saveload: they only get their mass when you first get into physics range -- or at any rate, it was such when I last caught an asteroid in KSP v1.7.

I see resource concentrations in the usual range, and this stat is subject to saveloading: the H class mentioned above showed resource concentrations of 85%, 87% and 92% on three visits.

On my first attempt I couldn't mine the resources (perhaps the drill not getting deep enough?), on the next try with a different vessel it worked.

Strangely enough, vessel mass increases while mining. Here are masses (in tons) before/after mining 1500u (15t) of Ore:

Asteroid Mass	Resources	Vessel mass
191,946.377	176,589.442	191981.142
191,931.377	176,574.442	191,996.142
-15	-15	+15

Asteroid Mass and Resources mass were taken from the PAW, vessel mass as reported by MJ (in-game I only see it down to the nearest 100kg, but within that accuracy I get the same result). The PAW shows the appropriate mass loss, but can it be that the actual mass remains constant? Otherwise I don't know how gross vessel mass could increase by the amount of ore mined.

Edited July 26, 2020 by Laie