The vast scientific campaign associated with the 1986 return of Halley’s Comet has greatly improved and expanded our knowledge of comets. An overview of the first results is presented here with emphasis on the large-scale structure, the chemistry, and the nucleus. Biermann and Alfven’s basic large-scale picture involving the interaction with the solar wind was confirmed. The interaction extends over very large distances and involves the draping of magnetic field lines from the solar wind around the head region. The near-nuclear region is essentially free of magnetic field. The cometary environment is a rich plasma physics laboratory as well as the site of spectacular disconnection events. As Whipple proposed, the chemical composition of the nucleus is largely water, and the breakup of the water molecule produces the large hydrogen-cloud surrounding the comet. Minor constituents with high molecular mass have been observed in the comet. The composition of the dust generally resembles carbonaceous chondrites enriched in the elements H, C, N and O. The interest in the cometary chemistry stems from the belief that cometary material is probably the best remnant of the solar nebula’s original composition. The nucleus is monolithic, as predicted by Whipple’s icy-conglomerate model. Far from spherical, the nucleus is irregular and peanut- or potato-shaped. The surface is very dark, and the emission of gas and dust occurs in jets on the sunward side. Irregular erosion of the surface, which is covered by a dust crust, could lead to many interesting possibilities for outbursts or splitting. Even with our current enhancement of knowledge, comets will continue to excite scientific curiosity. Future research on comets should be very fruitful.
Lunar Plasma Environment in Cases with Extreme Solar Wind Conditions: First Results from3-D Hybrid Kinetic Modeling and Comparison with ARTEMIS Observations
