A Metro Map Of Our Solar System

The almost free gravity train around the solar system

Our solar system has a metro system. Each planet is a train, whipping around the sun. We can catch them to get almost anywhere in the solar system, without burning much fuel. There’s an Express line (expensive) and a Local (almost free).

Here’s how they work.

The Express

The Express is the simplest route and the one we currently use. As you can see, it only has a few stops — the planets. It’s not free, but it’s still way cheaper than burning your own fuel. This is called a Hohmann transfer.

Picture each planet as a train, whipping around its orbit. You can ‘catch’ the train by joining its orbit, or just passing close by. Then you can jump off the train in a new direction, at a different speed.

Here’s an example from Voyager 1.

Animation of Voyager 1’s trajectory from 5 September 1977 to 30 December 1981

Voyager 1 catches the Express at Jupiter (cyan), picking up speed and executing an…