In the 1950s, during one of humanity's early spaceflight missions, a NASA astronaut was abducted by this race, after they discover his spacecraft orbiting their homeworld. Superboy and Krypto take flight to this world to rescue the astronaut, but Superboy's super powers were stolen by a strange Energy Transference Device that they had.
Krypto battled the Super Spaceman, who got Superboy's powers and he rescued the Boy of Steel, and in turn, they both stop the Spacemen Fleet from attacking Earth, which they planned to conquer by means of an alien invasion.
The technology that the Spacemen possessed was incredibly advanced. They had a device that was capable of draining energy from superhuman beings, such as Superboy, and then transferring that energy into one of themselves.
They also had spacecraft that appeared to be more advanced than that of what Earth's was at that time. Considering that their race lived in domed cities, it seems likely that these were colonies, implying that their world wasn't necessarily their native world, and that they or the species that they evolved from could have been from another planet.
Very little is known about their physiology, but they had green skin, no hair, and had unusually strange shaped ears and faces that were somewhat featureless, like they were difficult to tell apart from one another.
They apparently could not breathe Earth's atmosphere, or they at least had difficulty doing it, as they built a machine to drain the energy of an astronaut from Earth so that they could use that energy to transfer to themselves, allowing them to adapt to the atmosphere.
Considering that they live on an asteroid so close to Earth, which was indeed in the same solar system as Mars, it was possible that their species was descendants of Green Martians. Indeed, considering their skin color was green, and their proximity to Mars, it is possible, but just a theory, which has by no means ever been suggested.
Physically, as far as strength is concerned, they didn't seem to be any superior to humans at all, which means if they were descendants of Martians then they somehow lost the abilities of their ancestors. However, when using the energy transference device, they were able to steal the superpowers of at least one Kryptonian teenager, which was Kal-El.
Much of their culture seemed to revolve around scientific achievement and military conquest. Although not much is known beyond that. Military personnel wore pink uniforms, while some wore medallions around their necks.
They had one known military force, and that was the Spacemen Fleet, a fleet of space warships equipped with missiles. They attempted to conquer the Earth with this force, but they were stopped by Superboy and Krypto.