NASA's leaders today laid out an ambitious multibillion-dollar space exploration plan that calls for building a moon base over the next decade and launching a nuclear-powered probe to Mars by 2028. The space agency is also pressing the pause button on its multibillion-dollar plan to create a moon-orbiting outpost known as the lunar Gateway, and on its plan for transitioning from the International Space Station to commercial outposts in low Earth orbit. Instead, NASA says it aims to work with commercial partners to procure a government-owned Core Module for the ISS. That module would serve as the attachment point for commercial space modules that could eventually detach to become free-flying space stations. Meanwhile, the Power and Propulsion Element that was designed for the Gateway would be repurposed for the Mars probe known as Space Reactor-1 Freedom.…