[11] Mars Reconnaissance Orbiter (MRO) observations suggested the nucleus was smaller than 0.8 kilometers (0.5 mi) in diameter.

Reports on 28 November 2013 (the day of perihelion passage)[13][14] indicated that Comet ISON had partially or completely disintegrated due to the Sun's heat and tidal forces.

However, later that day CIOC (NASA Comet ISON Observing Campaign) members discovered a coma-like feature, suggesting a small fragment of it may have survived perihelion.

Shortly after their session, Nevski processed data using CoLiTec, an automated asteroid discovery software program.

In analysis he noted an unusually bright object with slow apparent movement, indicating a position outside the orbit of Jupiter based on the use of four 100-second CCD exposures.

[a][28] The group reported their discovery to the Central Bureau for Astronomical Telegrams as an asteroidal object, which was subsequently forwarded to the Minor Planet Center.

Precovery images from Mount Lemmon were first taken on 28 December 2011 and indicated that the comet had an estimated apparent magnitude ranging from 19.5 to 19.9.

[32][b] Near perihelion, generic heliocentric two-body solutions to the orbital eccentricity suggested that the comet could be either bound or unbound to the Sun.

[37] When Earth passed near the orbit of Comet ISON on 14–15 January 2014, it was predicted that micron-sized dust particles blown by the Sun's radiation might cause a meteor shower or noctilucent clouds;[38][39] however, both events were considered unlikely.

Because Earth only passed near Comet ISON's orbit, not through the tail, the chances that a meteor shower would occur were slim.

[40] In addition, meteor showers from long-period comets that make just one pass into the inner solar system are very rare, if ever recorded.

[41] The possibility that small particles left behind on the orbital path—almost one hundred days after the nucleus has passed—could form noctilucent clouds is also slim.

In April and May 2013 the Hubble Space Telescope (HST) measured Comet ISON's size, and the color, extent, and polarization of its emitted dust.

[51] October 2013 images of Comet ISON displayed a greenish tint, probably attributable to the release of cyanogen and diatomic carbon.

[53] On 14 November 2013, Comet ISON was reported to be visible to the naked eye by experienced observers located at dark sites.

[48] On 17–18 November, when Comet ISON was brighter and much closer to the morning twilight, it passed the bright star Spica in the constellation Virgo.

[56] But due to the full Moon and glow of twilight, Comet ISON had not become bright enough to be seen without optical aid by the general public.

Originally, media sources predicted that it might become brighter than the full Moon,[30][31] but based on more recent observations, it was only expected to reach around apparent magnitude −3 to −5, about the same brightness as Venus.

[64] If it had survived its perihelion passage intact, Comet ISON should have been well-placed for observers in the northern hemisphere during mid to late December 2013.

[66] On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).

Therefore, under the International Astronomical Union's comet-naming guidelines, Comet ISON was named after the team that discovered it, rather than the individual discoverers.