Here is the full list of all 14 Spacefarers! achievements.
Kill your first Zootopian.
Kill a roach!
Find and kill a Goltopian.
Find and kill a Pinktopian.
Kill a Lavtopian only using the lava gun.
Traverse into an alternate dimension.
Defeat five oppressors in a quest.
Defeat the oppressor field marshall.
Defeat the Skeleton King deep in the cave.
Wear each piece of the mage armor and then right click somewhere...
Travel to 50 different planets.
Travel to 500 different planets.
Travel to 5000 different planets.
Escape the Straton planet.