568 Encyclopedia of the Solar System
FIGURE 14 Comet Hale–Bopp on 17 March 1997 showing
well-defined striae in the dust tail at right. The plasma tail is at
left. (Courtesy of Kurt Birkle, Max-Planck-Institute f ̈ur
Astronomie, Heidelberg, Germany.)
sunward direction. If the Earth is close to the plane of the
comet’s orbit, a sunward spike is observed. If the Earth is
away from the orbital plane but reasonably close, a sunward
fan is observed.
The most famous sunward spike of the 20th century
was observed in comet 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 the comet is past perihelion and the Earth
is close to the comet’s orbital plane. Dust particles emitted
from the comet 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-
served in comets Bennett, Halley, and Hale–Bopp ( Fig. 16).
The neck-line structure in comet Halley was stable and was
a major feature for over a month in May and June 1986.
FIGURE 15 Comet Arend–Roland on 25 April 1957 showing
the sunward spike. (Photo©CUC Regents/Lick Observatory.)Sodium gas tails were observed in earlier comets, and
comet Hale–Bopp displayed a dramatic example. Figure 17
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
inner coma that are dissociated. The source for the wide tail
is probably the dust tail itself.
Sodium tails may well be a common feature of comets.
Comet Hale–Bopp’s nucleus was very large, with a diame-
ter 60±20 km. Estimates for the total gas production rateFIGURE 16 Comet Hale–Bopp on 6 June 1997 showing the
neck-line structure, the narrow feature extending to the left from
the head. (Image taken by G. Pizarro, European Southern
Observatory.)