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Hub AI
Comet nucleus AI simulator
(@Comet nucleus_simulator)
Hub AI
Comet nucleus AI simulator
(@Comet nucleus_simulator)
Comet nucleus
The nucleus is the solid, central part of a comet, formerly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublime and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust.
Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.
Comet nuclei, at ~1 km to at times tens of kilometers, could not be resolved by telescopes. Even current giant telescopes would give just a few pixels on target, assuming nuclei were not obscured by comae when near Earth. An understanding of the nucleus, versus the phenomenon of the coma, had to be deduced, from multiple lines of evidence.
The "flying sandbank" model, first proposed in the late-1800s, posits a comet as a swarm of bodies, not a discrete object at all. Activity is the loss of both volatiles, and population members. This model was championed in midcentury by Raymond Lyttleton, along with an origin. As the Sun passed through interstellar nebulosity, material would clump in wake eddies. Some would be lost, but some would remain in heliocentric orbits. The weak capture explained long, eccentric, inclined comet orbits. Ices per se were lacking; volatiles were stored by adsorption on grains.
Beginning in the 1950s, Fred Lawrence Whipple published his "icy conglomerate" model. This was soon popularized as "dirty snowball." Comet 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).[citation needed] 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.
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.
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 further to human understanding.
Comet nucleus
The nucleus is the solid, central part of a comet, formerly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublime and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust.
Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.
Comet nuclei, at ~1 km to at times tens of kilometers, could not be resolved by telescopes. Even current giant telescopes would give just a few pixels on target, assuming nuclei were not obscured by comae when near Earth. An understanding of the nucleus, versus the phenomenon of the coma, had to be deduced, from multiple lines of evidence.
The "flying sandbank" model, first proposed in the late-1800s, posits a comet as a swarm of bodies, not a discrete object at all. Activity is the loss of both volatiles, and population members. This model was championed in midcentury by Raymond Lyttleton, along with an origin. As the Sun passed through interstellar nebulosity, material would clump in wake eddies. Some would be lost, but some would remain in heliocentric orbits. The weak capture explained long, eccentric, inclined comet orbits. Ices per se were lacking; volatiles were stored by adsorption on grains.
Beginning in the 1950s, Fred Lawrence Whipple published his "icy conglomerate" model. This was soon popularized as "dirty snowball." Comet 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).[citation needed] 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.
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.
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 further to human understanding.
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