  Context: Increasingly, Oort Cloud comets are being discovered at great
distances from the Sun and tracked over ever wider ranges of heliocentric
distances as observational equipment improves. Aims: To investigate in detail
how the original semimajor axis for near-parabolic comets depends on the
selected data arc and the assumed form of the non-gravitational (NG)
acceleration. Methods: Among currently known Oort Cloud comets with large
perihelion distances ($q > 3$ au), we selected 32 objects observed over the
widest ranges of heliocentric distances in orbital legs before and after
perihelion. For each of them, we determined a series of orbits using at least
three basic types of data sets selected from available positional data (pre-
and post-perihelion data and the entire data set), and a few forms of NG
acceleration representing water ice or CO sublimation. Results: We found that
the motion of comets is often measurably affected by NG forces at heliocentric
distances beyond 5 au from the Sun. The most spectacular example is C/2010 U3
(Boattini), whose perihelion distance is 8.44 au. NG effects are detectable for
19 of the 32 comets within the positional data. For five comets, we found
asymmetric effects of NG forces - in three cases significantly greater before
perihelion than afterward (C/2017 M4, C/2000 SV$_{75}$, and C/2015 O1), and in
two others the opposite (C/1997 BA$_6$ and C/2006 S3). We also find that the
well-known systematic effect of finding more tightly bound original orbits when
including the NG acceleration than in purely gravitational solutions may be
related to the specific form of the standard $g(r)$ function describing the
sublimation of ices.

Earth and Planetary Astrophysics