orbits had been determined quite precisely, yet comets were at times recovered "off-schedule," by as much as days. Early comets could be explained by a "resisting medium"- such as "the aether", or the cumulative action of meteoroids against the front of the comet(s). But comets could return both early and late. Whipple argued that a gentle thrust from asymmetric emissions (now "nongravitational forces") better explained comet timing. This required that the emitter have cohesive strength- a single, solid nucleus with some proportion of volatiles. Lyttleton continued publishing flying-sandbank works as late as 1972. The death knell for the flying sandbank was Halley's Comet. Vega 2 and Giotto images showed a single body, emitting through a small number of jets. "Icy dirtball" It has been a long time since comet nuclei could be imagined as frozen snowballs. Whipple had already postulated a separate crust and interior. Before Halley's 1986 apparition, it appeared that an exposed ice surface would have some finite lifetime, even behind a coma. Halley's nucleus was predicted to be dark, not bright, due to preferential destruction/escape of gases, and retention of refractories. The term dust mantling has been in common use since more than 35 years. The Halley results exceeded even these- comets are not merely dark, but among the darkest objects in the Solar System Furthermore, prior dust estimates were severe undercounts. Both finer grains and larger pebbles appeared in spacecraft detectors, but not ground telescopes. The volatile fraction also included organics, not merely water and other gases. Dust-ice ratios appeared much closer than thought. Extremely low densities (0.1 to 0.5 g cm-3) were derived. The nucleus was still assumed to be majority-ice, perhaps overwhelmingly so. Modern theory Three rendezvous missions aside, Halley was one example. Its unfavorable trajectory also caused brief flybys at extreme speed, at one time. More frequent missions broadened the sample of targets, using more advanced instruments. By chance, events such as the breakups of Shoemaker-Levy 9 and Schwassmann-Wachmann 3 contributed to our understanding. Densities were confirmed as quite low, ~0.6 g cm3. Comets were highly porous, and fragile on micro- and macro-scales. Refractory-to-ice ratios are much higher, at least 3:1, possibly ~5:1, ~6:1, or more. This is a full reversal from the dirty snowball model. The Rosetta science team has coined the term "mineral organices," for minerals and organics with a minor fraction of ices. Comets and active asteroids in the outer