C/2012 S1 ISON
more info
Comet ISON was discovered on 21 September 2012 with International Scientific Optical Network (ISON) near Kislovodsk, Russia, that is a year and about two months before its perihelion passage. C/2012 S1 was a comet with an extremely small perihelion distance of 0.0125 au (about 3 solar radii). It was intensely followed by numerous observers until its decay. Sekanina and Kracht (2014) studied the disintegration process of this object in detail, suggesting that the comet fully disintegrated hours before perihelion passage. The first minor outburst occurred 16 days before perihelion (beginning of November) at a distance of 0.7 au from the Sun. However, an unforeseeable fading of the originally very active comet was reported by observers many weeks before this event.
Therefore, two orbits worth noting here are based on data taken at large heliocentric distances (more than 3.4 au from the Sun, solutions: pa, pc) and before the two-month gap starting from 9 June 2013 (see picture). The first orbit is purely GR and gives original 1/a as nearly zero, whereas the second one is an NG orbit obtained using the g(r)-like formula describing CO sublimation. Here this last orbit was taken as preffered orbit.
Comet would have its closest approach to the Earth on 26 December 2003 (0.431 au), about a  month after its perihelion passage.
See also Sekanina 2019 and Królikowska 2020.

solution description
number of observations 4056
data interval 2011 09 30 – 2013 06 08
data arc selection data generally limited to pre-perihelion (PRE)
range of heliocentric distances 9.39 au – 3.41au
type of model of motion NC - non-gravitational orbits for symmetric CO-g(r)-like function
data weighting YES
number of residuals 8059
RMS [arcseconds] 0.35
orbit quality class 1b
orbital elements (barycentric ecliptic J2000)
Epoch 1717 04 10
perihelion date 2013 11 28.72899334 ± 1.15383462
perihelion distance [au] 0.01238150 ± 0.00000069
eccentricity 0.99999972 ± 0.00000017
argument of perihelion [°] 345.525311 ± 0.000323
ascending node [°] 295.768066 ± 0.000485
inclination [°] 61.809065 ± 0.002448
recip semi-major axis [10-6 au-1] 22.39 ± 12.63
file containing 5001 VCs swarm
2012s1pc.bmi
Time distribution of positional observations with corresponding heliocentric (red curve) and geocentric (green curve) distance at which they were taken. The horizontal dotted line shows the perihelion distance for a given comet whereas vertical dotted line – the moment of perihelion passage
Six 2D-projections of the 6D space of original swarm including 5001 VCs. Each density map is given in logarithmic scale presented on the right in the individual panel.