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We have an explanation for all 3304 xkcd comics, the 21 extra comics, and everything xkcd-related.
Only 55 explanations (1.7%) are incomplete. Help us finish them!


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—Randall Munroe, in an email to our wiki

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Jupiter Icy Moons Explorer
"I did briefly visit Venus in August 2025, but I figured out the mistake on my own because it didn't have any moons."
Title text: "I did briefly visit Venus in August 2025, but I figured out the mistake on my own because it didn't have any moons."

    Explanation

    The Jupiter Icy Moons Explorer (JUICE) is a scientific space exploration spacecraft created by the European Space Agency (ESA). Its planned voyage began in 2023 and includes flybys of the Moon (2024), Earth (2024), Venus (2025), Earth (2026), and Earth (2029), before orbiting Jupiter (2031) and finally settling into an orbit around Ganymede in 2034. The many flybys it will make provide gravity assists (25 in total) to help it reach its final destination. Gravity assists work by using the celestial bodies' orbital and gravitational energy (instead of rocket fuel) to accelerate JUICE out of the Sun's gravity well and into Jupiter's gravity well, therefore reducing the amount of rocket fuel needed to reach its mission destination, as well as the mission costs. Each such flyby transfers momentum from the celestial body to the probe, allowing it to accelerate outwards in the solar system using less propellant than if it were to simply use some form of Newtonian propulsion. Momentum is conserved; the celestial bodies lose exactly the same amount of momentum as the probe gains. However, the probe is so much less massive than the celestial bodies that the effect on those bodies is negligible.

    This comic shows supposed interactions between JUICE and (presumably) ESA Mission Control each time JUICE is near Earth. These suggest that, rather than the gravity assist explanation, the reason for all the flybys is that the probe is inattentive or forgetful, and has to keep returning to Earth to check its instructions. It appears that the person communicating with it was unclear why so many flybys were planned, but the mission planners were expecting this behaviour from the probe, so deliberately built them in as contingency.

    The first panel is lift off. The second panel is the first flyby, with the spacecraft asking which planet it should visit. The third panel is the second flyby, with the spacecraft asking which planet was Jupiter. The final panel is the last time JUICE is expected to visit Earth, asking which moons it should visit.

    The title text extends the joke, making reference to JUICE's visit to Venus. The probe indicates that it knew Venus wasn't Jupiter, but only because of the lack of moons, rather than the extreme size and color differences. JUICE thinks its visit to Venus was a “mistake,” when in fact its flyby of Venus was carefully planned in advance. Jupiter is over 10x the diameter of Venus, and among other things currently has a storm system larger than Venus. Even its moon Ganymede, the final destination of JUICE, is half the diameter of Venus. In mitigation, distances in space are hard to judge, so the probe might have difficulty assessing the size of Venus without additional reference data.

    Transcript

    [Panel labeled "April 2023". Earth is shown, and the JUICE space probe is flying away from it, with dotted lines showing its path since liftoff.]
    Mission Control: We have liftoff!
    Mission Control: JUICE is en route to explore the icy moons of Jupiter!
    JUICE: Wheee!
    [Panel labeled "August 2024". Two images of JUICE are shown, with dotted lines showing its path through space. One is to the left of Earth, the other to the right, and the path goes below Earth.]
    JUICE: ...So which planet was I visiting again? I got distracted.
    MC: Jupiter!
    JUICE: Right, OK.
    [Panel labeled "September 2026". Two images of JUICE again, this time the path goes above Earth.]
    JUICE: Is Jupiter the one with all the rings? Or...
    MC: No, the big one near it.
    JUICE: Got it!
    [Panel labeled "January 2029". JUICE is approaching Earth from the left.]
    JUICE: And which moons am I looking at?
    Mission Control 1: Oh my God.
    Mission Control 2: Now I get why we budgeted for so many Earth flybys.

             View comic discussion



    Edit this week's pick!

    This Week's Explanation for Improvement

    As part of the Explanation Improvement Project, each week the community selects one older, stagnant, or otherwise neglected explanation to expand and polish together. This week's pick is comic 172: Skateboarding is Not a Crime, you can help us by editing its explanation!

    To get involved with the project, you can also nominate an explanation that you think should be improved, or vote on current nominations!

    Skateboarding is Not a Crime
    'Arrest me, I'm a skateboarder' is an even more obnoxious variant.
    Title text: 'Arrest me, I'm a skateboarder' is an even more obnoxious variant.

      Explanation

      Welcome to This Week's Explanation for Improvement! Anyone is encouraged to help explain comic 172: Skateboarding is Not a Crime; click here to get started! If you are new to wiki editing, you can read our editing FAQ for guidance. Here are some suggestions for improving this explanation:

        Better explain the controversy around skateboarding, the stickers, and similar occurrences. (You can learn more about the protest on Wikipedia's page for Skateboarding is not a crime)
        The last parts of the explanation read more like a transcript, so improve the explanation of the caption & title text instead of just describing what they say.
      The comic depicts multicolored lockers, often used in American high schools, gyms, or colleges, with one of them having the "Skateboarding is not a crime" sticker.

      Skateboarding culture emerged in the 1980s, especially among young people (high schoolers, college students) as a countercultural movement initially associated with the "punk" movement. In addition, because skateboarding can take place in public locations with railings and benches, many cities have adopted specific measures to either increase the difficulty of skateboarding (Skatestopper), or by trying to pass laws to prohibit the act of skateboarding in public parks. In addition to the stated public safety concerns of fast-moving skateboarders among pedestrians, many cities unfairly associate skateboarders with those who drink, do drugs, or acts of juvenile delinquency, such as truancy or underage drinking.

      As a result, the skateboarding subculture has taken up the slogan "skateboarding is not a crime" as a protest against these restrictions from cities. This slogan began in 1987, and since then, several skateboard manufacturing companies have adopted it. At one point, the slogan was a countercultural push against those who would consider skateboarding to be a crime, but as the phrase became more popular (by 2006, the time at which the comic was posted), there was likely a backlash against the slogan. Perhaps some people, both within the community of skateboarders and outside, would consider this sticker to be obnoxious, or to appear to have a persecution complex. For instance, in this comic, the slogan is the *only* sticker displayed on these lockers, perhaps put there as a way of the locker owner trying to stand out, in a "not like the other girls" manner.

      Randall apparently really does not like these stickers and states that when/if he becomes president (a common expression that is equivalent to "If I were in charge, ..."), any and all displays of stickers bearing the phrase (like the one on the locker in the comic) will be considered a felony, though skateboarding itself will still be legal. Actually implementing this would be against the first amendment and it would be unlikely to ever become a real law. However this may not always be true, pending recent developments with the current administration heavily fighting against free speech.

      The title text also shows another protest phrase, "arrest me, I'm a skateboarder", that Randall appears to hate even more than "skateboarding is not a crime", potentially for the more standoffish tone of the message. This phrase has basically the same message, so it is unsurprising that he hates both, if he hated either.

      Transcript

      [A set of lockers, that are dark pink, blue, yellow, dark pink then blue. The middle one has a white sticker stuck to it proclaiming "Skateboarding is not a crime".]
      [Outside of the Box] When I'm president, skateboarding will still bee legal, but display of those stupid stickers will be a felony.

               View comic discussion

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