Free What If? by Randall Munroe Page B

to come get us.
    8 “TPA.”

Little Planet
    Q. If an asteroid was very small but supermassive, could you really live on it like the Little Prince?
—Samantha Harper

    “Did you eat my rose?” “Maybe.”
    A. Th e Little Prince , by Antoine de Saint-Exupéry, is a story about a traveler from a distant asteroid. It’s simple and sad and poignant and memorable. 1 It’s ostensibly a children’s book, but it’s hard to pin down who the intended audience is. In any case, it certainly has found an audience; it’s among the best-selling books in history.

    It was written in 1942. Th at’s an interesting time to write about asteroids, because in 1942 we didn’t actually know what asteroids looked like. Even in our best telescopes, the largest asteroids were visible only as points of light. In fact, that’s where their name comes from — the word asteroid means “starlike.”
    We got our first confirmation of what asteroids looked like in1971, when Mariner 9 visited Mars and snapped pictures of Phobos and Deimos. Th ese moons, believed to be captured asteroids, solidified the modern image of asteroids as cratered potatoes.

    Before the 1970s, it was common for science fiction to assume small asteroids would be round, like planets.
    Th e Little Prince took this a step further, imagining an asteroid as a tiny planet with gravity, air, and a rose. Th ere’s no point in trying to critique the science here, because (1) it’s not a story about asteroids, and (2) it opens with a parable about how foolish adultsare for looking at everything too literally.
    Rather than using science to chip away at the story, let’s see what strange new pieces it can add. If there really were a superdense asteroid with enough surface gravity to walk around on, it would have some pretty remarkable properties.
    If the asteroid had a radius of 1.75 meters, then in order to have Earthlike gravity at the surface, it wouldneed to have a mass of about 500 million tons. Th is is roughly equal to the combined mass of every human on Earth.
    If you stood on the surface, you’d experience tidal forces. Your feet would feel heavier than your head, which you’d feel as a gentle stretching sensation. It would feel like you were stretched out on a curved rubber ball, or were lying on a merry-go-round with your head nearthe center.

    Th e escape velocity at the surface would be about 5 meters per second. Th at’s slower than a sprint, but still pretty fast. As a rule of thumb, if you can’t dunk a basketball, you wouldn’t be able to escape this asteroid by jumping.

    However, the weird thing about escape velocity is that it doesn’t matter which direction you’re going. 2 If you go faster than the escape speed, as long as you don’t actually go toward the planet, you’ll escape. Th at means you might be able to leave our asteroid by running horizontally and jumping off the end of a ramp.

    If you didn’t go fast enough to escape the planet, you’d go into orbit around it. Your orbital speed would be roughly 3 meters per second, which is a typical jogging speed.

    But this would be a weird orbit.
    Tidal forces would act on you in several ways. If you stretched your arm down toward the planet, it would be pulled much harder than the rest of you. And when you reach down with one arm, the rest of you gets pushed upward, which means other parts of your body feel even less gravity. Effectively, every part of your body would be trying to goin a different orbit.
    A large orbiting object under these kinds of tidal forces — say, a moon — will generally break apart into rings. 3 Th is wouldn’t happen to you. However, your orbit would become chaotic and unstable.
    Th ese types of orbits were investigated in a paper by Radu D. Rugescu and Daniele Mortari. Th eir simulations showed that large, elongated objects follow strange paths aroundtheir central bodies. Even their centers of mass don’t move in the traditional ellipses; some adopt pentagonal