文档介绍：CLBE001-ESS2E November 9, 2006 15:37
568 Encyclopedia of the Solar System
FIGURE et Arend–Roland on 25 April 1957 showing
the sunward spike. (Photo C UC Regents/Lick Observatory.)
FIGURE et Hale–Bopp on 17 March 1997 showing Sodium gas tails were observed in ets, and
well-defined striae in the dust tail at right. The plasma tail is at
comet Hale–Bopp displayed a dramatic example. Figure 17
left. (Courtesy of Kurt Birkle, Max-Planck-Institute fur ¨
Astronomie, Heidelberg, Germany.) shows the long, narrow sodium tail. There is also a wide
sodium tail superimposed on the dust tail. The source for
the narrow tail is probably sodium-bearing molecules in the
sunward direction. If the Earth is close to the plane of the a that are dissociated. The source for the wide tail
comet’s orbit, a sunward spike is observed. If the Earth is is probably the dust tail itself.
away from the orbital plane but reasonably close, a sunward Sodium tails may well be mon feature ets.
fan is observed. Comet Hale–Bopp’s nucleus was very large, with a diame-
The most famous sunward spike of the 20th century ter 60 ± 20 km. Estimates for the total gas production rate
was observed et Arend–Roland during April 1957
(Fig. 15). Comets Kohoutek (December 1973/January
1974) and Halley (February 1986) also showed sunward
spikes. Some of these are produced by large ejection speeds
in the sunward direction, but most only appear to be sun-
ward in projection.
The neck-line structure is a long, narrow dust feature
observed when et is past perihelion and the Earth
is close to et’s orbital plane. Dust particles emitted
from et at low speeds are, in fact, in orbit around
the Sun. These orbits return to the orbital plane to produce
a dust concentration. The neck-line structure has been ob- FIGURE et Hale–Bopp on 6 June 1997 showing the
served ets t, Halley, and Hale–Bopp ( Fig. 16). neck-line structure, the narrow feature extending to the left from
The ne