The chart below created by the xkcd team 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.

It takes the same amount of energy to launch something on an escape trajectory away from Earth as it would launch it 6,000 km upward under constant 9,81 m/s2 Earth gravity. Hence, Earth’s well is 6,000 km deep.

This is why it took a huge rocket to get to the Moon, but only a small one to get back.

Mercury, the closest planet to the Sun, has the weakest gravity of all the planets in the Solar System. Its gravity is only about 38% of Earth’s gravity, making it easier for spacecraft to escape its gravitational pull.

On the contrary, Jupiter is more than 300 times as massive as Earth and has a gravity about 2.5 times stronger than Earth’s gravity.

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.

The Sun is by far the most massive object in the Solar System, accounting for over 99.8% of its total mass. Its immense gravity holds the entire Solar System together and governs the orbits of all the planets, moons, and other celestial objects.

To learn more about the Solar System and gravity, have a look at the following books:

• National Geographic Atlas of Space
• Space Atlas, Second Edition: Mapping the Universe and Beyond

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