Mr. Bush announced a "vision for space exploration" in 2004, after the loss of the shuttle Columbia. He proposed returning astronauts to the moon by 2020, and then going on to Mars, but he did not give a target date.
At a presentation Sunday night on the topic, he said," It was a great personal honor to walk on the moon, but as Neil once observed, there are still places to go beyond belief. Isn't it time to continue our journey outward, past the moon?"
Less than one penny out of every dollar Americans send to the federal government in income taxes goes to support space, said Apollo astronaut Eugene Cernan who flew to the moon in 1972.
"You decide whether that's worth it," he challenged.
Cernan said that he and his colleagues were "the luckiest people in the history of the modern world," and with that gift came a responsibility.
"The response is to give that back in some way or another ? to those who follow in our footsteps. To once again see what has never been seen before," he said.
Lovell acknowledged that at the height of the Cold War, the country was unified behind the common goal of reaching the moon, and it's difficult to re-create those conditions.
But that momentum, he said, hurtled the astronauts to the moon and led to technology in use today.
Now that he and his colleagues are in their retirement years, he said, "our main mission now is to inspire the young people."
The goal of reaching Mars could unify the public the way the moon mission did in the 1960s, he said.
"It's the other moon that we now have conquered."
In 1994 a military space probe called Clementine, sent to map the moon as a way of testing sensors for possible Defense Department use, found evidence of ice in the shadowed corners of craters near the moon's south pole. In 1998 a NASA probe called Lunar Prospector was sent to confirm Clementine's findings, and as it orbited the moon it found evidence of large amounts of ice in the lunar soil.
If there is frozen water there -- H2O -- it can be used for drinking, or broken down into its components of hydrogen and oxygen for fuel, air to breathe, and myriad other uses.
The astronauts comments came as public debate about the value of the Apollo program still simmers.
"For all the trillions of dollars we have spent on the space program, all we have are some moon rocks, several tons of space junk and a dozen and a half or so dead astronauts," wrote someone commenting on a recent ABCNews.com story about a space shuttle mission.
In fact, public opinion has always been split. In a July 1967 Harris poll, two years before the first moonwalk, 43 percent of Americans favored the effort, 46 percent opposed -- hardly a rousing endorsement. And in 1970, a year after the landing, 56 percent said it had not been worth its allotted $4 billion a year for nine years.