Watch me as I gravitate (hahahahaha). –Gorillaz
Gravity — unbelievably — is the weakest force of all. But if you get enough mass together, gravity will overwhelm even the strongest outside influence. A simple case-in-point? You take a rock that’s massive enough, and gravity will crush it into a spherical shape,
The more massive and more compact your planet is, the harder it is to get off of. Something like the Moon, which is only about 1.2% of the mass of the Earth but 27% of the Earth’s radius, is way, way easier to escape from than the Earth. To escape from the Earth’s gravity, you need to reach a speed of 40,000 km/hr (25,000 mph) from the Earth’s surface. To escape from the Moon, on the other hand, you only need to reach 8,600 km/hr (5,400 mph). This is why, to go from the Earth to the Moon, you need a rocket like this:
But to go from the Moon back to the Earth, you only need a rocket like this:
(For those of you who are physics buffs, remember that energy goes as velocity squared, which means that if you need five times the speed, you actually need 25 times the Energy!)
This is one of the biggest arguments in favor of a one-way mission to Mars. To get to Mars from the Earth requires about the same rocket power as it takes to get to the Moon. However, to get back to Earth would require an escape velocity of 18,000 km/hr (11,200 mph), which means something far more powerful than we needed on the Moon; something like this:
That’s right, we’d need to build a major launch site on Mars just to return to Earth. I’m not saying that doesn’t mean we shouldn’t do it, but if we require it, it’s going to be that much longer (and more expensive) before we get to go to Mars.
The Martian Moons, on the other hand? We could get back — in theory — with one of these.
So choose your planetary targets carefully, if you ever want to come back!