(CNN) -- The space shuttle program should have come to an end a long time ago, NASA administrator Charles Bolden told CNN in an exclusive interview Wednesday.
The 30-year program has been kept on life support because the United States has not readied another vehicle to take its place.
"What is not acceptable is the fact that the most powerful nation in the world, the United States of America, finds itself in a situation that we didn't do the proper planning to have a vehicle in place to replace shuttle when it lands its last landing," Bolden said.
NASA originally planned to retire the aging shuttle fleet last September, but mission delays have pushed that date. But once Endeavour, Atlantis and Discovery are permanently grounded, Russia's aging Soyuz capsules will ferry astronauts and cosmonauts to the space station and bring what supplies can fit in the smaller craft.
And that will have to do, perhaps throughout this decade, until commercial cargo spacecraft are available.
The last shuttle landing, according to the administrator, will not be in April when Endeavour is scheduled to fly. NASA says it wants one more mission to resupply the space station. That flight would be STS-135 this summer, using the Atlantis orbiter and designated with the acronym that stands for "Space Transportation System."
But whether NASA will have the money for the flight -- about a half a billion dollars -- is in question.
"We are budgeted for 135 and unless something disastrous happens, it's our intent to fly it," said Bolden, "It's in the authorization bill signed by the president back in November, so for me it's the law and I'm excited about it because I need it, so we plan to fly 135."
That would leave three flights before the program ends.
STS-133 using Discovery sits on the launch pad ready to make its last trip into space Thursday.
In April, STS-134 with Endeavour is schedule to fly. Commanding that mission is astronaut Mark Kelly. Kelly's wife, U.S. Rep. Gabrielle Giffords, was among 19 people shot in Arizona during a political event. Giffords is going through rehabilitation in Houston as she recovers from a gunshot wound to the head.
During a news conference, Kelly said his wife would attend his launch.
"I pray that he is absolutely correct," Bolden said, "because for all in the NASA family, who have been praying with him ever since that Saturday, and all of us who love her and him, It would be just tremendous, it would be tremendous for the nation. It would give everybody a big boost."
But with those launches, the nation is seeing an end to its only means of putting humans in space. It is also seeing an end to a program that caught the public's attention nearly 30 years ago, with the launch on April 12, 1981, of the shuttle Columbia on the program's first mission.
Bolden hopes that at least two commercial space companies will emerge to transport astronauts to the International Space Station. This will allow NASA to concentrate on building a new heavy-lift rocket capable of leaving lower Earth orbit.
There had been concern whether the space agency would have enough money to develop this vehicle. Bolden said the money is there to build a vehicle that will evolve over time.
"When I say we are not going to do things the way we used to, we're not building the world's heaviest, biggest rocket right out of the chute. It's going to take us ... a decade or so to get to the point that we have the final vehicle that is going to take the first humans to Mars," said Bolden.
The White House has said it wants humans on Mars by the mid-2030s. Right now, Bolden says his priority is flying these last three shuttle missions and bringing the astronauts home safely.