2b95aab6_Marvel_Moon___Background__On_Moonless_Earth
[0] Astronauts [On Companion Earth] Astronauts walk on the Moon during the Apollo missions, 1969-72 [Implication] Humans are not able to visit another world so early in their technological lifetime and may not be inspired to do so without a nearby goal [On Moonless Earth] 
There are no objects within the technological reach of current rockets and radiation shielding that are appropriate for manned explorations
Marvel Moon - Background
On Moonless Earth