Explain xkcd: It's 'cause you're dumb.
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. If it gets close enough to the sun it may break up entirely.
There seems to be no definitive astronomical definition of the word "comet", and definitions can be challenging and problematic, 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.
- [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.
Some more information about comets will help put the comic in perspective:
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- The surface of cometary nuclei does reflect less sunlight than asphalt does. Telescopes can't identify or find them until they do have a coma.
- An object at a distance of one light-year would only have an orbital speed about 100 meters per second, the speed of the Earth is about 30 kilometers per second.
- One revolution at that distance would last approximately 20 million years.
Reading the Wikipedia page on the Oort Cloud didn't help me understand the joke. I don't know if it has anything to do with comets, or the asteroids getting smashed up by them. 22.214.171.124 05:15, 29 November 2013 (UTC)
- The asteroid becomes a cost after being severely burnt by the sun. It warns the other asteroid not to go over there. 126.96.36.199 (talk) (please sign your comments with ~~~~)
- Which, the title text indicates, is a warning that's utterly ignored... (Also being "right back" indicates a slower perceived thought process. As is probably the case for anything out there in such cold(-ish) depths of space.) 188.8.131.52 11:05, 29 November 2013 (UTC)
- What does "becomes a cost" mean? is that a slang expression? 12:02, 29 November 2013 (UTC)
- I think it's supposed to be "comet". 184.108.40.206 14:09, 29 November 2013 (UTC)
- No, I think he means ghost. All thats left is a faint image after all. 220.127.116.11 (talk) (please sign your comments with ~~~~)
At least according to my freshman year science teacher, the Oort Cloud is just a theory, and hasn't been proven. Perhaps that should be made more clear? Wasda (talk) (please sign your comments with ~~~~)
- @Wasda, A theory is something which is proven, if not its a hypothesis or a speculation. Mocking on "theories" is typical for laymen. 15:12, 30 November 2013 (UTC)~
- Theory is the wrong term. Gravity is a theory. Evolution is a theory. The oort cloud is "hypothesized". Omega Talk • Contribs 06:21, 30 November 2013 (UTC)
What's here looks exactly in line with current comet theory: A comet is "perturbed" by interactions with other objects out there, and at that distance the sun is a very bright dot, no more. On return (chancy, based on both/either burning up or being in a no return hyperbolic orbit), what has come back is fragmented and with two tails. What I'm not seeing is the second level joke - it's in the movie "I'm going to check out x" form, but I don't get the specific quote. FractalgeekUK (talk) 13:55, 29 November 2013 (UTC)
According to http://www.nasa.gov/content/goddard/timeline-of-comet-ison-s-dangerous-journey, ISON's journey to earth from the Oort cloud started "At least a million years ago." So I'm going to edit "many thousand years later" to "several million years later". In other news, I recently played "Das Rad" (http://www.youtube.com/watch?v=n-7y3B8DjGw) for my kids and this comic really reminded me of that Oscar-nominated short. I hate lichen! :-) Tovodeverett (talk) 15:10, 29 November 2013 (UTC)
What's with the "Several million years later" context in the transcription?!? The distances might be vast and all, but i doubt the timeframe is that long... -- 18.104.22.168
- See the comment right above yours. Diszy (talk) 18:03, 29 November 2013 (UTC)
- You are discussing editorial elements included in the transcription that have no relationship to the original comic. Regardless of any implied passage of time the comic does not directly state a time-frame that passes and the transcript should only include information contained in the original comic. Any discussion of time-frame should take place in the explanation. The panels imply a pause of unspecific time. The transcript should read the same. Mrarch (talk) 16:18, 2 December 2013 (UTC)
- A comic I did predict yesterday
Uhhh, while I observed that close encounter of ISON at the sun yesterday live I was sure Randall would do a comic on this matter. And he did. That's just funny for me. --Dgbrt (talk) 21:28, 29 November 2013 (UTC)
- When is a comet a comet?
Wikipedia says "A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail." I see no clear suggestion on Wikipedia that a small Solar System body might change from being an asteroid to being a comet each time it passes by the sun and starts to outgas. It seems to me that SSSB's that contain ices and other volatiles that would outgas given enough heat are comets, but I haven't seen that question really addressed clearly by any authoritative sources. For now I'm switching the references to distant comets from "asteroid" to "comet nucleus", which is used on wikipedia and seems like a neutral term for an icy object that would show a cometary atmosphere and tail when close to the sun. Also, the oort cloud is hypothesized to have both a spherical and a disk-like structure, and is part of the solar system. Please discuss further related changes here. Thanks! Nealmcb (talk) 16:34, 1 December 2013 (UTC)
- The most false understanding is: A comet is a dirty snowball. In fact it's just a snowy rock, most of it is still rock, otherwise it would break up at a distance between Earth and Venus. And all the frozen gases are not only H2O. Furthermore a comet is a comet when we can see its shape, unless that tail is shown it's just a small object at our solar system, not more.--Dgbrt (talk) 22:17, 1 December 2013 (UTC)
- The wikipedia references in the article talk of objects out in the Oort cloud as "comet nuclei". Unless you can find better sources, that's what we should go with. Nealmcb (talk) 14:56, 2 December 2013 (UTC)
Diameter of the sun: 1,391,000 km
Distance of ISON from sun at perihelion: 1,860,000 km
ISON went within one sun diameter
One of the above statements is false. All are in the explanation. --Zagorath (talk) 18:37, 2 December 2013 (UTC)
Nothing is false. ISON's distance is from the CENTER of the sun. ISON went within one sun diameter distance from the SURFACE. So ISON went closer than 1.5 diameters from the center. 1,391,000 * 1.5 = 2,086,500 which is more than 1,860,000. --22.214.171.124 19:28, 2 December 2013 (UTC)
- A definition for a comet doesn't exist
I did mark the comic as incomplete again. We have to explain that there is no real definition for comets.
- Look at this really big one: 2060 Chiron.
- Wiktionary says: "A celestial body consisting mainly of ice, dust and gas in a (usually very eccentric) orbit around the Sun and having a "tail" of matter blown back from it by the solar wind as it approaches the Sun." 
- IAU says: "All other objects (except planets and dwarf planets), except satellites, orbiting the Sun shall be referred to collectively as Small Solar-System Bodies". Look for RESOLUTION 5A
- And an interesting blog entry at livejournal: What's the difference between a comet and a planet?
--Dgbrt (talk) 20:19, 2 December 2013 (UTC)
- Maybe the incomplete tag could be removed again, but this is not my decision because there are maybe still some questions. --Dgbrt (talk) 22:37, 3 December 2013 (UTC)