SEATTLE — Microsoft co-founder Paul Allen and aerospace pioneer Burt Rutan announced today they’re building a giant airplane and spaceship to zip people and cargo into orbit.

But unlike traditional rockets and government spaceships, this new commercial spaceship will drop from a high-flying airplane instead of blasting off from a launch pad.

“When I was growing up, America’s space program was the symbol of aspiration,” Allen said at a news conference. “For me, the fascination with space never ended. I never stopped dreaming what might be possible.”

Allen bemoaned the fact that government-sponsored spaceflight is waning and said his new project would “keep America at the forefront of space exploration” and give a new generation of children something to dream about.

“We have plenty and many challenges ahead of us,” he said.

Allen and Rutan join a field crowded with Silicon Valley veterans who grew up on “Star Trek” and now want to fill a void created with the retirement of NASA’s space shuttle. Several companies are competing to develop spacecraft to deliver cargo and astronauts to the International Space Station.

The duo won a prize in 2004 with a spaceship that went into space but not orbit. Their new business model likely includes tourism and satellites.

Their new plane will have a wingspan of 380 feet, longer than a football field. The plane will carry under its belly a space capsule with its own booster rocket, take off from a runway and go high into the air. Then the plane will release the spaceship, which will rocket to orbit.

This method saves money by not using rocket fuel to get off the ground. Another older rocket company, Orbital Sciences Corp., uses this method for unmanned rockets to launch satellites.

But Allen’s rockets will eventually carry people. However, the first tests, scheduled for 2016, will be unmanned. It should be about five years before people can fly on the system that Allen and Rutan are calling Stratolaunch.

The spaceship and booster will be provided by another Internet tycoon, Elon Musk of PayPal, who has built a successful commercial rocket.

Rutan will build the carrier aircraft, which will use six 747 engines. The company will bill its method of getting to space as “any orbit, any time,” officials said.

The company will be based in Huntsville, Ala. Allen left Microsoft in 1983. Since his time at the software giant he has pursued many varied interests and is the owner of the Seattle Seahawks football team as well as the Portland Trailblazers of the NBA.