Pieces of a fluffy comet.

Pieces of a fluffy comet

Measurements during spacecraft encounters with Comet Halley (SN: 5/24/86, p.327) revealed that dust shed by the comet contained elements such as hydrogen, carbon, oxygen, silicon and magnesium. Those observations indicate the dust particles probably consist largely of water ice, silicates and various hydrocarbons. J. Mayo Greenberg of the University of Leiden in the Netherlands suggests these materials are layered within tiny, capsule-like grains. Each grain has a silicate core, surrounded by an inner mantle of hydrocarbon material and an outer mantle of ice flecked with minute, dark particles. Hundreds of these grains stick together to form a single dust particle, producing a fluffy aggregate, about 80 percent empty space, with an average diameter of 5 microns.

"Such a model would explain practically all that we observed in the dust of [Comet] Halley," says vassily I. Moroz of the Space Research Institute of the USSR in Moscow. Whether the dust from Comet Halley is typical of interstellar dust is unknown. Researchers have proposed a number of alternative models for interstellar dust (SN: 6/18/88, p.396).