Subway in the Sky — Nathan B. Dodge

Scotty and Bobby, genius electrical and computer engineers, have developed a system that sets up a stable wormhole between two points. Together with Kaley, the best technical field rep in the solar system, they and their company are establishing travel portals all over Earth, and also on the moon, Mars, Mercury, plus several of the moons of Jupiter and Saturn. Their company, Subway Solar Systems, is growing like crazy, and the future looks bright. What could go wrong?Lots of things, it turns out. An alien race from the far reaches of space has taken control of a portal on Titan and started an invasion. They are rapidly taking over the outer solar system, and Earth's military forces may not be able to stop the attack.Just to make things perfect, the personal lives of the three people who created Subway Solar Systems are a mess as well. Scotty' wife wants a divorce, Bobby's boyfriend is causing him constant trouble, and maybe Scotty's relationship with Kaley isn't just based on their jobs.