Oort Cloud
Oort Cloud
[Three asteroids float in space.] ISON: Have you noticed that bright dot in the distance? Asteroid: Yeah. What's the deal with it?
ISON: Dunno. I'm gonna go check it out.
[Pause while ISON checks it out off screen.]
ISON (broken up, with multiple tails): Wow. Do NOT go over there.

The Oort cloud is a hypothesized sphere containing many small Solar System bodies , reaching out to roughly 50,000 AU (astronomical units) or nearly one light-year from the sun. Gravitational forces from passing stars or collisions with other objects sometimes perturb one of these bodies enough to let it fall into the inner solar system. When it gets closer to the Sun, which is just a bright dot at that far distance, it warms up and some of its mass is lost as gas and dust, making it more visible as an object commonly referred to as a comet. This is what has happened to a comet called C/2013 UQ4 , AKA Comet Catalina. And although this is not what will happen to Catalina, comets that get close enough to the sun may break up entirely.
There seems to be no definitive astronomical definition of the word "comet", and definitions can be challenging and problematic [1] , but in general terms a comet is a celestial object consisting of a nucleus containing a huge amount of ices and dust which, when near the sun, has an atmosphere (called coma ) and perhaps a 'tail' of ionized gas and dust particles pointing away from the sun.
The comet pictured here upon its return strangely resembles the unusual asteroid P/2013 P5 . That object sported six comet-like tails, but it's not a comet. Rather, the six comet-like tails were suspected to be caused by rapid spinning of that object.
Randal has drawn the hapless Oort Cloud object with its tails generally left of frame, i.e. away from the sun. Comet tails point away from the sun regardless of their direction of movement, as they are blown out by the solar wind which moves much faster than the comet. As neither of the other two objects have tails, this lends the picture a comical cartoon-like quality, as when Yosemite Sam is blasted by his own gun and it leaves his moustache tails statically pointing away from the direction of the blast.
Comet ISON presumably came from the Oort cloud and reached its closest approach to the sun ( perihelion ) on the day before this comic was published. The comet passed very close to the sun, at a distance of 1,860,000 kilometers or 1,150,000 miles from the centre of the sun. It was thus within one sun-diameter of the surface of the sun itself (diameter of sun = 1,391,000 km). At that distance the temperature, at approx. 2,700 degrees Celsius, vaporizes rock as well as ice and can break the comet apart entirely.
The broken-up object here is presumed to be ISON, and is labeled as such in the transcript, even though Randall hasn't unambiguously identified it. Note that it's not realistic that ISON still would have a tail so far away from the sun.
On December 2, 2013 NASA released a statement that ISON did not survive its close perihelion with the sun. The Comet ISON Observing Campaign posted a delightful biographical sketch (In Memoriam Comet C/2012 S1 (ISON) Born 4.5 Billion BC, Fragmented Nov 28, 2013, age 4.5-billion yrs old) which touches on its early years, retreat to the Oort Cloud, career as a Sungrazer, "dynamic and unpredictable life, alternating between periods of quiet reflection and violent outburst", delicate inner working, and its tragic demise.
The closest approach of ISON to the earth was predicted for December 27, 2013 at a distance at approx. 60 million kilometers or 37 million miles, 170 hundred times more than the moon. The Hubble Space Telescope looked for it on December 18 but saw nothing.
This video shows an animation of the encounter at the sun: ISON 28.11.2013 .
Some more information about comets will help put the comic in perspective:
[Three asteroids float in space.] ISON: Have you noticed that bright dot in the distance? Asteroid: Yeah. What's the deal with it?
ISON: Dunno. I'm gonna go check it out.
[Pause while ISON checks it out off screen.]
ISON (broken up, with multiple tails): Wow. Do NOT go over there.
