(CNN) -- Will U.S. astronauts ever return to the moon?
Yes, says NASA, but maybe not in a government-built spaceship -- and maybe not any time soon.
Constellation was to follow NASA's space shuttle program, which, after about 30 years of taking astronauts to space, is scheduled to end later this year, following five more flights.
Instead of building new spacecraft of its own, NASA, under the proposal, would invest in space technology research and spend $6 billion to pay private space groups to develop and build new rockets to take astronauts into orbit.
The plan leaves many open questions about the future of U.S. space travel, including if and when NASA will ever build rockets of its own again, when astronauts will return to space and in what kind of spacecraft.
"It's almost like a reboot of NASA's human space flight program," said Tariq Malik, managing editor of Space.com.
NASA eventually will return to the moon under the proposed scenario, NASA officials said in a conference call on Monday, but the U.S. space agency has set no firm timeline for doing so if Constellation is eliminated. In addition, NASA likely would have to rely on private companies to develop the technology and rockets to send people back into space instead of doing so itself.
One big impact of the shift may be symbolic, Malik said, since canceling Constellation means that the United States will not have any government spaceships to jet astronauts into orbit or beyond.
"It's going to be gone," Malik said, noting that no government-built spacecraft could dampen the pride some Americans feel for the space program. "And there's nothing there to replace it, not even on NASA's drawing board."
The Obama budget proposal has sent confusing reverberations through the space community.
Some groups applaud the move, saying Constellation was over budget and behind schedule anyway -- so it's simply better to start anew.
Fuel for this argument comes, in part, from an independent report (PDF) on NASA's plans, released in October of last year by a panel of experts called the Augustine Committee.
The committee's review found Constellation wouldn't send astronauts to the moon until "well into the 2030s," instead of 2020, as projected. Funding would have to be increased to keep the program going. And NASA's scheduled $99 billion investment in human space exploration over 10 years "appears to be on an unsustainable trajectory," the report said.
But others say canceling NASA's moon-exploration program is the wrong move.
Members of Congress have attacked Obama's proposal on two fronts: Some say canceling NASA's moon-exploration program will put the U.S. behind other nations in terms of space exploration and technological development. Others say it is wasteful to abandon Constellation after $9 billion in federal money already has been spent on technology specifically designed for those missions.
Larry Young, a former astronaut and the Apollo Program Professor of Astronautics at MIT, said the proposal calls into question the career tracks of astronauts.
"The astronauts like to fly and anything that reduces the number of opportunities to go into orbit is going to discourage both current and future astronauts," he said.
"But we've been through dry periods before -- before the shuttle was developed and then after the [Space Shuttle] Challenger accident -- and we lived through it. Astronauts find other things to do as long as there is a future to build towards, and it sounds to me like that will be the case."
He praised the budget's reinvestment in space science research and emphasis on international cooperation on space flight, but said it saddens him that the U.S. may not longer send its own rockets into orbit.
Monday's budget proposal would increase NASA's overall budget by $6 billion over the next five years, even while it cuts the agency's moon-exploration program.
That has led groups like The Planetary Society to say the Obama Administration maintains a genuine commitment to space exploration, even as it plans to nix Constellation.
"I think the Constellation program probably fell on its own weight as opposed to any major policy change," said Louis Friedman, executive director of that group.
The change is a fresh start that puts needed emphasis on space exploration beyond the moon, he said.
Buzz Aldrin, one of the first men to set foot on the moon in 1969, also issued a statement on his Web site in support of the changes.
"The truth is that we have already been to the Moon -- some 40 years ago," he wrote. "A near-term focus on lowering the cost of access to space and on developing key, cutting-edge technologies to take us further, faster, is just what our Nation needs to maintain its position as the leader in space exploration for the rest of this century."
Sen. Bill Nelson, a Democrat from Florida, and others criticized the president for slashing NASA's moon-mission program from his suggested budget.
"If they don't push hard now for research and development of the new big rocket that'll take us out of low-Earth orbit and let us explore the heavens, then we are going to be falling behind China and Russia, and that's something I don't think will sit well with the American people," he said in an interview with CNN.
NASA Administrator Charlie Bolden tried to downplay fears about the shift.
"Imagine trips to Mars that take weeks instead of nearly a year; people fanning out across the inner solar system, exploring the Moon, asteroids and Mars nearly simultaneously in a steady stream of firsts ... That is what the president's plan for NASA will enable, once we develop the new capabilities to make it a reality," Bolden said in a conference call with reporters.
In lieu of Constellation and moon exploration, NASA's proposed budget appears to place emphasis on other programs -- both on Earth and in space.
The budget would spend more than $2 billion to monitor climate change on the Earth with orbiting satellites and to observe the Earth in other ways; invest $4.9 billion in a five-year program that would aim to send robots to the moon to study the possibility of human colonies and fund the development of space-related technology more generally; spend $3.2 billion on exploration of the solar system; and allocate $420 million over five years for a mission to the Sun.
It also extends the U.S. commitment to the International Space Station beyond 2016.
The budget would have to be approved by Congress to take effect.
The proposal will be greeted in Washington with a tough fight, said Malik, the Space.com editor, since legislators from states like Texas and Florida, which saw a flood of government investment because of Constellation, are likely to defend that program.
"Lawmakers have been really lashing out against this plan because it means a lot of the contracts these constituencies have will be dissolved," he said.