Explain xkcd: It's 'cause you're dumb.
The six words are "It works in Kerbal Space Program".
The Kuiper Belt is a region of the outer Solar System beyond the orbit of Neptune consisting of numerous small icy bodies, including the dwarf planets Pluto and Eris. An Oberth maneuver is a spaceflight maneuver, specifically an engine burn performed during the flyby of a celestial body. The point of this is to optimize usable energy, because rocket burns are more effective to perform at high speeds than at low speeds. The more massive the body and the lower the altitude, the higher the flyby speed will be, and the greater the performance gain due to the Oberth effect. The theoretical way to use rocket fuel most efficiently is therefore to execute the burn during a flyby of the most massive celestial body available, as close as possible.
Cueball is proposing to implement an "Oberth Kuiper Maneuver", and the proposal diagram shows the spacecraft using gravity assists to travel first towards (presumably) Venus for a first boost, then towards Jupiter for another swing by, aiming it back towards the Sun, the most massive Solar System body, to perform an Oberth maneuver at the point of closest approach, as indicated by a small dot along the spacecraft trajectory. It is possible that the diagram might only be a simplified outline of a more complex flight plan. For example, the trajectory from Earth to Venus would require two separate burns in deep space. The first burn would occur immediately after escaping Earth's sphere of influence. The second burn would occur about halfway to Venus. Alternatively, Cueball may have gotten it wrong, or Randall may simply not have concerned himself with such things for the purpose of a webcomic sketch.
An Oberth maneuver in the close vicinity of the Sun, while theoretically possible and extraordinarily effective at the speeds the spacecraft would reach, would however be very difficult to carry out in real life, because the neighborhood of the Sun is an extremely hostile environment and the spacecraft could be incinerated during a too-close flyby.
Cueball's argument for why the maneuver will work in real life is that it works in Kerbal Space Program (KSP), a sandbox spaceflight simulator. While KSP does take into account quite a bit of the physics of space flight, it is (necessarily) simplistic in its modeling of orbital dynamics. KSP does not make any attempt to deal with the N-body problem (for which there is no complete solution), or even simpler 2-body mechanics. Instead celestial bodies are "on rails" and have fixed Spheres of Influence (SOI); a craft within a particular SOI feels the gravitational influence of only that body, and transitions are abrupt when a craft crosses the edge of an SOI. This means there are no Lagrangian points, Lissajous orbits, or, critically for this context, orbital perturbations by distant bodies or other factors. Therefore, orbits modeled using KSP would poorly reflect the actual orbital behavior of a probe traveling through the solar system on a multi-year mission involving multiple fly-by maneuvers. KSP does however have a big modding community and one of its members has created a mod called Principia that implements an approximation of N-body physics by using numerical integration.
The humor in referencing KSP in the comic lies in using a simple game program to simulate complex space missions which in reality take a great number of experts to plan and implement. Fly-by maneuvers, used to minimize the fuel needed to reach a destination, need to be very carefully timed - often to within seconds - so the use of Kerbal Space Program to simulate them isn't a good enough argument for NASA to agree to implement the proposal, and implies simplistic thinking on the part of the proposer; therefore one should not say it at NASA.
The title text refers to Orbiter, which is another spaceflight simulator program. The title text suggests that the argument doesn't work for NASA, not because it's not scientifically sound, but because NASA relies on the Orbiter simulator rather than the Kerbal simulator (although the proposed maneuver would appear to work in both).
Humorously, in real life a group of NASA team members "love" to play Kerbal.
- [Caption above the frame:]
- The six words you never say at NASA:
- [A diagram shows a possible trajectory path for a space probe starting at Earth and involving two slingshots around two other planets, and finally the Sun. There is a title above and a label beneath the diagram. The diagram is being presented by Cueball in front of three other Cueball-like guys. Behind Cueball Ponytail appears to be taken by surprise by his six words, and holds her hand to her mouth.]
- Title: Proposal:
- Label: Oberth Kuiper Maneuver
- Cueball: And besides—
- Cueball: It works in Kerbal Space Program.
- NASA is planning a mission to the sun at a closest distance of only 8.5 solar radii. The Solar Probe Plus will use seven Venus flybys to reach its first close approach. The maneuvers will last nearly seven years, before the real mission does start.
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I believe the "Oberth Kuiper Manuver" refers not to the exploitation of the Kuiper belt, but to its the maneuver's usefulness for crossing the belt efficiently. There are multiple various points supporting this conclusion:
1. The size and positioning of the circles strongly indicates that they represent Venus, Earth, and Jupiter (or Eve, Kerbin, and Jool).
2. Reaching the Kuiper belt (which begins at Neptune) requires a great deal of delta-v. Even if you were to slingshot around gas giants (which, in the current explanation, is not shown in the diagram), the trip would consume the majority of a spacecraft's propellant, making the extreme exploitation of the Oberth effect largely ineffective.
3. Kuiper belt objects are very small and therefore would not effectively serve to redirect or slingshot the spacecraft.
220.127.116.11 10:11, 29 July 2013 (UTC)
- "This makes no sense, as it it is vastly more expensive in terms of fuel to get to a Kuiper belt object (which is at least 10 billion km from earth) than it is to get to the sun."
- This is incorrect. To go directly to the Sun takes a delta-v about equal to Earth's orbital speed (30 km/s). Escape speed is √2 * v_c = 42 km/s; since you start with v_c, you need a delta-v of 12 km/s to get out to the Kuiper Belt. Once you're far from the Sun, a very small delta-v will put you on an orbit passing near (or into) the Sun.Wwoods (talk) 16:47, 29 July 2013 (UTC)
- Okay, that's true. But I have another objection to the interpretation: how long would it take to get a probe to the Kuiper belt? And of course there's the problem of actually finding a Kuiper belt object to slingshot around, especially when you've got much better candidates, e.g. Jupiter, Saturn, etc. available for the maneuver. -- Emurphy (talk) (please sign your comments with ~~~~)
- The term "Oberth Kuiper Maneuver" does not exist, but Randall did add the word "Kuiper" to the well known "Oberth Maneuver". So he means a Kuiper Belt object. Wwoods' statement is absolutely correct. The trip will just take a long time.--Dgbrt (talk) 18:25, 29 July 2013 (UTC)
- I have to agree with the IP address. You're not correct about the Delta_v benefit of going to the Kuiper belt, because Jupiter provides a gravity assist. The probe fires the thrusters at the closest approach to Jupiter in the direction opposite of motion, and this causes it to take a sharper turn, requiring lower Delta_v. This is not possible with the Kuiper belt objects because they are significantly less massive than Jupiter. Yes your speed will be less, but not by enough. The closest approach potential is greater for Jupiter by a factor much greater than the velocity difference. You could do the equations, and I'm sure Jupiter will be more propellant-efficient, in addition to taking less time. AlanSE (talk) 21:40, 29 July 2013 (UTC)
- Why the hell you all do TWO spaces after a sentence? Like "IP address.__You're not". But back to the discussion, as more far you are of the massive sun, the the energy for changing the speed direction is reduced, you go back to the sun at HIGH speed.--Dgbrt (talk) 22:57, 29 July 2013 (UTC)
- Because that's how you do in on a typewriter. Do you remember typewriters? --Thnidu (talk) 23:07, 29 July 2013 (UTC)
- You do not leave Jupiter with zero speed. That's the only way that your argument makes sense. It's not true, so your argument is not true. I am saying that if you sent two of these probes, one doing the incorrect Kuiper object assist, and one just doing a Jupiter assist, the Jupiter-assisted probe would be faster even as the Pluto-assisted was passing Jupiter. That is because the Jupiter-assisted probe does not grid to a screeching halt as it passes the planet like you seem to argue that it does. AlanSE (talk) 15:55, 30 July 2013 (UTC)
- Sure, Jupiter would maybe a good option, but Randall mentions the Kuiper Belt. He definitively talks not about Jupiter. The voyage would take more than a decade but the delta-v you need is much smaller than at Jupiter. Randall is just joking about this long lasting mission.--Dgbrt (talk) 19:47, 30 July 2013 (UTC)
- The idea is difficult in practice because of the proximity to the sun combined with the volatility of propellants, and is entertained seriously by NASA: . We have an sufficient explanation for the use of the word Kuiper. A more troubling detail of your theory is how it would work in Kerbal Space Program. Jool is the Jupiter analog. There is no Pluto analog. The "icy" planet in KSP is Eeloo, and it is in a resonance with Jool. In other words, it is no further than Jool. The comic makes no sense with your theory, because it would have been impossible to do that maneuver in KSP, and one of the few things we know about the maneuver is that it was done in KSP. AlanSE (talk) 20:59, 30 July 2013 (UTC)
- Your second links tells me this: "The physical characteristics of Eeloo are most likely an analogue of the ice moon Enceladus, and its orbit is similar to that of Pluto...". So we have the Kuiper Belt at this program. The first link refers to the NASA Institute for Advanced Concepts, but they are not talking about a Kuiper Belt object as an bad option. And because of their time line they should consider this. And, why Randall should rename the "Oberth Maneuver" to an non existing "Oberth Kuiper Maneuver"?--Dgbrt (talk) 21:59, 30 July 2013 (UTC)
- I see where you get the Pluto thing, and I can see where the developers were going with that. The idea is that Pluto hasn't cleared its own orbit. But Pluto crosses Neptune, whereas Eeloo crosses Jool. That's a big difference in terms of distance from the sun. The NASA study's diagrams did use Jupiter in the diagrams, but you're right in that they didn't dismiss the use of other objects. AlanSE (talk) 23:18, 30 July 2013 (UTC)
- I find the Jupiter explanation more plausible. Not only it is more consistent with the diagram (as far as body sizes and configuration, but also orbit shape, which would be a much narrower ellipse with periapsis near Venus and apoapsis in the Kuiper Belt), but also flyby around a Kuiper Belt body would be quite pointless: its low mass would make it a particularly lousy target for both gravity assist and Oberth maneuver - and yes, you only need a small velocity change at such distances to head for the Sun, but you could just as well do a deep space maneuver in empty space without any Kuiper Belt object nearby; its presence or non-presence would make little difference. - More importantly though, the whole travel so far outwards would be completely unnecessary, because Jupiter's gravity is already sufficient to send a spacecraft on a close Sun flyby trajectory (or even straight into the Sun, if desired) with a single unpowered flyby, no engine burn needed at all. Saving lots of time and/or fuel (as the unnecessary detour via Kuiper Belt would take more fuel/flybys than a simple trip to Jupiter, not even mentioning the roundtrip duration). The reference to Kuiper Belt is explained just as easily with that being the intended mission target (also following from the word order, Oberth-Kuiper, more logical for Kuiper Belt visit after the Oberth maneuver, rather than before). --18.104.22.168 12:12, 31 July 2013 (UTC)
- I think the idea is actually to use gravity assist from a Kuiper Belt object, Pluto or Eris. Of course this will be ridiculously ineffective, but that's part of the joke! Using gravity assist from Pluto for a destination that could be reached using gravity assist from Jupiter is an insane idea that could work in theory, but there are too many risks in real life. Only in Kerbal would you use this trajectory. 22.214.171.124 10:30, 16 April 2014 (UTC)
- strictly an orbiter shop
According to this  that is not true. 126.96.36.199 11:59, 29 July 2013 (UTC)
- Indeed - thanks! I added that in. Now, is there any evidence that they play Orbiter?? Nealmcb (talk) 16:55, 29 July 2013 (UTC)
I just made a few for sense (undergo → implement), punctuation (possessive it's → its), grammar (and → comma before "eventually leaving"), and logic (inserting if it succeeded,).
--Thnidu (talk) 23:16, 29 July 2013 (UTC)
a.k.a. Dr. Whom: Consulting Linguist, Grammarian, Orthoëpist, and Philological Busybody
Randall adds the transcripts sometimes later. We now know the planets are Earth and Jupiter and at the center is, of course, the sun.--Dgbrt (talk) 16:21, 12 August 2013 (UTC)
Slightly off-topic, but I just wanted to add this. You know how the ESA hosts their little Moon Mission competition and stuff? A theoretical mission to the moon whereby one researches and proposes a Moon Mission and writes out the mission details, although it won't actually happen, as far as I know. My college (well really, only me and one other girl) is collaborating with some other colleges in the US and some others from other countries (in total, 11 of us on one team) is participating in it. Part of the mission requirement is that we must be able to simulate the mission. Of course I read the details... and I absolutely died laughing when I read that Kerbal Space Program is officially a viable simulator for the "mission." I immediately thought of this comic, haha! I suppose you don't say it at NASA, but it's OK at ESA, I suppose! International Space Station (talk) 02:09, 9 October 2015 (UTC)
- Kerbal Space Program
Made some adjustments to the section about KSP. Mostly just describing which limitations of the game would make it inappropriate for use at NASA, simulating a complex orbital trajectory. As a side note: in the full release they dramatically overhauled the drag modeling and re-entry dynamics, making it somewhat more realistic. There's also a fairly significant modding community that has offered improved realism via alternate atmospheric drag modeling, life support systems, drilling for fuel (now in the game-proper), and more realistic planet size/density/separation (the default settings have bodies ~1/10th the size of similar bodies in our own solar system, with very high densities, and relatively close together, resulting in comparable surface gravity but making it much easier to reach orbit and beyond in the KSP system). As far as I know nobody has looked at N-body simulation in KSP (while there is no general solution partial ones can be determined, and complete solutions exist for 2-body and a limited 3-body system). I think Orbiter had it to some degree, as you could make use of more complex orbits and Lagrangian points. I'd love to put a refueling station at the Kerbin-Mun L1 point. --188.8.131.52 22:01, 29 October 2015 (UTC)
My friend's dad took a tour of SpaceX recently and said the designers there use Kerbal Space Program extensively. Benjaminikuta (talk) 22:57, 25 December 2015 (UTC)