681: Gravity Wells
Title text: This doesn't take into account the energy imparted by orbital motion (or gravity assists or the Oberth effect), all of which can make it easier to reach outer planets.
The xkcd page links to a much larger version.
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The comic shows the solar system gravity field of each planet and some of their satellites.
The drawing next to Jupiter is playing on the classic "Yo Mama" joke, combining the aspects of your mom being very fat (having a huge gravitational pull) and very slutty (having sex with the entire football team).
The figures on Titan are sirens, a reference to Kurt Vonnegut's The Sirens of Titan.
Megan's quote is a paraphrase of Carl Sagan's quote, "...but from a planet orbiting a star in a distant globular cluster, a still more glorious dawn awaits, not a sun-rise, but a galaxy rise."
 Main Text
- Gravity Wells scaled to Earth surface gravity
- This chart shows the "depth" of various solar system gravity wells.
- Each well is scaled such that rising out of a physical well of that depth — in constant Earth surface gravity — would take the same energy as escaping from that planet's gravity in reality.
- Each planet is shown cut in half at the bottom of its well, with the depth of the well measured down to the planet's flat surface.
- The planet sizes are to the same scale as the wells. Interplanetary distances are not to scale.
- Depth = (G × PlanetMass) / (g × PlanetRadius)
- G = Newton's constant
- g = 9.81 m/s2
 Planetary Descriptions
- To Sun, very very far down
- Earth - 5,478 km
- Moon - 288 km
- Mars - 1,286 km
- [A drawing of a "very deep" gravity well, "Your mom" at the bottom, several member of "local football team" falling down towards her.]
- Jupiter is not much larger than Saturn, but much more massive. At its size, adding more mass just makes it denser due to the extra squeezing of gravity.
- If you dropped a few dozen more Jupiters into it, the pressure would ignite fusion and make it a star.
- Two figures: Weeoooeeoooeeooo
- Megan: An even more glorious dawn awaits!
 Mars Inset
- [Mars gravity well, the Pathfinder probe on its surface, with its moons Deimos and Phobos as smaller gravity wells.]
- [Figure of a man (to scale) in Deimos's gravity well.]
- You could escape Deimos with a bike and a ramp.
- [Figure of a man (to scale) in Phobos's gravity well.]
- A thrown baseball could escape Phobos.
 Earth Inset
- [Zoomed-in view of Earth/moon gravity well, featuring the relative locations of the atmosphere, Low Earth Orbit, the International Space Station, the Space Shuttle, GPS satellites, and satellites in geosynchronous orbit.]
- Cueball: This is why it took a huge rocket to get to the moon but only a small one to get back.
- It takes the same amount of energy to launch something on an escape trajectory away from Earth as it would to launch it 6,000 km upward under constant 9.81 m/s2 Earth gravity.
- Hence, Earth's well is 6,000 km deep.