https://www.youtube.com/watch?v=qhNkK4h1oVk&feature=youtu.be At the 1:00 mark they said all that land will be playable. Its not, and will not for the release. They said only one zone per planet.
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The zones are pretty big. I watched a fireteam on twitch explore moon for an hour and never reach a dead end. Now for moon there should only really be one zone anyways, its the moon Lol I believe other zones and possible new planets will arrive as dlc. Some people expect way to much out of games. If they concentrated on more zones but only gave us earth, moon and mars you'd bitch about that.