To push farther out into the solar system — to the moon and beyond, to asteroids, eventually to Mars — NASA unveiled plans on Wednesday for a behemoth rocket that would serve as the backbone of its human spaceflight program for decades.

The finished rocket would be the most powerful ever to rise from the gravitational bonds of Earth.

“We’re investing in technologies to live and work in space, and it sets the stage for visiting asteroids and Mars,” the NASA administrator, Maj. Gen. Charles F. Bolden Jr., said at a news conference in Washington.

The megarocket, blandly named the Space Launch System, embodies the space agency’s enduring desire to aim far and dream big. But it also reflects a shrinking of near-term ambitions as budget cutters seek to rein in federal spending. Just two years ago, NASA had hoped to build an even larger rocket that would take astronauts back to the moon and set up an outpost there. With money more limited, the pace of progress will be much slower than during NASA’s Apollo heyday in the 1960s.

William H. Gerstenmaier, the agency’s associate administrator for human exploration, said NASA expected to devote $3 billion a year to the effort, or a total of about $18 billion over the next six years.