4 planets and a bunch of asteroids instead of continents. Each turn, the planets and asteroids move once round their track. Since Mercury's track only has 2 spaces, Earth's 8 and so on, this means the "continents" move closer to or farther away from each other. You can be beating the hell out of one player on Earth when suddenly Mars pulls alongside, somebody launches an invasion and you get your arse handed to you.
I never liked Risk much. A great game that's basically similar but with major improvements is Buck Rogers: Battle for the 25th Century. Here's the board:
4 planets and a bunch of asteroids instead of continents. Each turn, the planets and asteroids move once round their track. Since Mercury's track only has 2 spaces, Earth's 8 and so on, this means the "continents" move closer to or farther away from each other. You can be beating the hell out of one player on Earth when suddenly Mars pulls alongside, somebody launches an invasion and you get your arse handed to you.
Long out of print but worth tracking down.