The 'outward urge' was a factor in the Troon inheritance. Successive generations of Troons, looking up at the stars, heard the siren voices that called them out into Space. And, as the frontiers of Space receded, there was usually one Troon, if not more, out there, helping to push them back. The four exciting episodes related here deal with the parts they played in the building of the Space Station, the occupation of the Moon, the first landing on Mars, and the trouble about Venus.