A comet is a “dirty snowball” composed of ice and rock. They are thought to have originated in the Oort Cloud, a spherical vast cloud of icy bodies surrounding the Solar System beyond the Kuiper belt. Cometary nuclei are at most only a dozen or so kilometers across, but can develop a coma far larger when they approach the Sun. A comet’s heated passage through the Solar System can also form a brilliant tail, even multiple ones.

Comet C/2009 P1 (Garradd)

Comet C/2009 P1 (Garradd), unusually large and bright for its distance at the time; in September 2011, it brightened to 7th magnitude and maintained that brightness for several months afterward; had it been on the “right side” of Earth’s orbit when it reached perihelion, it would have likely qualified for “Great Comet” status     [Source]

Comets can be sorted into two categories: periodic and non-periodic. Periodic comets orbit the Sun in ellipses, with periods ranging from only 3 years (in the case of Comet 2P/Encke) to thousands of years. Non-periodic comets have unbound parabolic or hyperbolic trajectories, meaning that their visits to the Solar System are their first and last.

They are fascinating objects to track. Not only are they temporary apparitions, but their appearance can noticeably change on a day-to-day basis. They are also wildly unpredictable, the best example of the century likely being the ill-fated Comet C/2012 S1 (ISON), a sungrazing comet that ultimately disintegrated in November 2013 during its passage not even 1 solar diameter away from the Sun’s surface!

In 2014, Comet 67P/Churyumov–Gerasimenko became the 6th comet to be visited by a spacecraft

In 2014, Comet 67P/Churyumov–Gerasimenko became the 6th comet to be visited by a spacecraft     [Source]

Posts on short-period comets (≲200 years):


Posts on long-period and non-periodic comets:

* Spotted with the naked eye


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