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,

like it was nothing more than a drop of water.

And here we live on the surface of one of the *smaller* gravity balls in our Solar System. XKCD has a wonderful illustration of this today (and click for full-size).

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!