  Even though the orbital motions of most long-period comets are found to
comply with the gravitational law, a rapidly increasing minority of these
objects is found to display detectable outgassing-driven deviations. The
systematic research of nongravitational effects in the motions of long-period
comets began with Hamid & Whipple's work aimed to support Whipple's
icy-conglomerate comet model in the 1950s, profited from fundamentally new
results for fragments of the split comets in the 1970s, and has been expanding
ever since the development of the Style II nongravitational orbit-determination
model by Marsden et al. (1973). It has also thrived thanks to a dramatic
improvement in the quality of astrometric observations in the past decades. A
set of 78 long-period comets with derived nongravitational accelerations and
perihelion distances smaller than 2 AU has been assembled and its cumulative
distribution shown to mirror the distribution of nuclear dimensions, once the
bias toward bright comets and comets with large nongravitational effects is
accounted for. As a rule, long-period comets with detectable deviations from
the gravitational law appear to have small, subkilometer-sized nuclei. Comet
C/1995 O1 Hale-Bopp and perhaps some others remain a challenge.
