In The Colossus Expansion, the fourth book of the Second World Series, deep space travel evolves as humans venture deeper into the solar system. Our existence in space is facilitated by the CASSI AI system. CASSI's consciousness and morality becomes legendary as human expansion into deep space is thwarted by ATek, a Russian asteroid mining conglomerate.

We need bigger, faster ships with artificial gravity for long voyages, something that the Colossus II settlement ships, launching November 20, 2054, can provide. The Colossus II is a huge interplanetary ship, ever so much more spacious and accommodating than the Colossus I, equipped with two hab rings rotating around the ship's central axis giving a Mars equivalent ship gravity. The Colossus II Interplanetary ship is propelled by Super Heavy booster rocket engines and ion thrusters.

Human settlement of the solar system reaches Jupiter's moon Callisto and Saturn's moon Titan while robotic probes penetrate the ice of Europa and Enceladus to explore oceans more vast than those of Earth. Discoveries there redefine the boundaries of what we imagine as life.

The Colossus Expansion saga culminates in 2079 on the Perseus Mission. Not only will this mission be the last hope of averting the Shiva disaster for Earth and Mars, the Colossus III ship Perseus secretly launches continuing outward on the most important exploration mission in human history.