The best alternatives to Space Mayhem are Out There, Z-Type, and Zenith - Space Adventure. If these 3 options don't work for you, we've listed a few more alternatives below.
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Solar Idle is a free idle iOS game that players buy, build, upgrade, and colonize planets - including the sun. Not only is it an idle game, Solar Idle features a build in arcade where players can play various arcade games to increase earnings.