On complex issues, as is often said, it is possible for intelligent people to disagree. That was certainly the case March 15 at the American Museum of Natural History in New York City, when five leaders of the space exploration intelligentsia met to discuss NASA's plans for human spaceflight.

The topic of the event, the 10th annual Isaac Asimov Memorial Debate, could hardly have been more timely, given the February budget request from President Obama that sought to drastically change NASA's direction for human spaceflight and the way the agency does that business. If the budget survives Congress, NASA could start hiring private corporations to launch U.S. astronauts into orbit rather than use its own hardware; Obama's plan would also scrap the existing Constellation Program, including the Ares rockets being developed to lift humans beyond low Earth orbit for the first time since the 1970s.

Effectively, Obama's budget would abandon the Constellation goal of returning to the moon in the next decade. (As many have noted, that timeline may not have been realistic anyway.) With the immediate goal of a moon landing stricken, it remains unclear where NASA astronauts would go next, and various panelists in the debate spoke up on behalf of Mars, the moon and asteroids.

Firmly in the Mars camp was an animated Robert Zubrin, founder of The Mars Society, who said the president and his advisers of "have basically endangered the human spaceflight program." (The Constellation Program was not ideal, he said, but at least one could call it a plan.) Zubrin likened an approach to manned spaceflight that lacks a destination to a couple who wants to build a home and "cruises garage sales every weekend and finds house parts that appeal to them" before asking an architect to incorporate all the parts into a coherent whole.

That gibe earned laughs from the crowd but drew a rebuke from retired U.S. Air Force Gen. Lester Lyles, a member of the Review of U.S. Human Space Flight Plans Committee (aka the Augustine Commission), the group that recently advised Obama on NASA's options for manned spaceflight. "I vehemently disagree with what Bob said," Lyles responded, defending Obama's plan. "It is not going to be the willy-nilly approach that was described by my colleague." Lyles conceded that he was somewhat concerned about the lack of a destination in Obama's proposal but said that NASA's overarching goals of inspiration and exploration would not change. Kenneth Ford, chair of the NASA Advisory Council, also communicated a desire to see a destination and a timeline for the agency's exploration but maintained that NASA must first set goals and then let those directives guide the choice of destination.

Like Zubrin, lunar scientist Paul Spudis of the Lunar and Planetary Institute expressed disappointment—albeit in a more measured way—in the proposed relaxation of NASA's lofty goals to reach the moon and beyond. Just weeks before, Spudis had presented findings showing that the moon's north pole is rich in ice deposits, which would be a boon to establishing a long-term lunar presence. "My feeling is that the moon is a logical destination because it has the resources we need," Spudis said.

Even the token "robot guy" advocated for an aggressive plan for manned exploration. "You can't send humans out to explore the solar system fast enough for me," said Cornell University's Steve Squyres, the principal investigator for the Spirit and Opportunity rovers, which are still doing science at Mars more than six years after landing there. "The best exploration and the most inspiring exploration will be done by humans." Squyres advocated for returning to the moon as a way to flex NASA's "deep-space muscles" before moving on to what he called more interesting destinations, such as near-Earth asteroids. Many people favor such a mission because a spacecraft could simply rendezvous and dock with an asteroid without requiring the complex landing systems needed to negotiate the Martian atmosphere or the gravitational fields of Mars or the moon; and because, unlike the moon, astronauts have never visited an asteroid. (Spudis noted that asteroids have their drawbacks, too—they often rotate rapidly and tend to be much farther away.)

Zubrin passionately argued that the space agency must aim for a more ambitious target, asserting that with "gutsy leadership" a visit to Mars could be closer to reality than a moon landing was at the dawn of the Apollo program in 1961.

But Spudis countered that, in the post–Cold War world, it isn't proper to frame a discussion of NASA's current situation in the terms of Apollo. The political environment is different now, as is the involvement of the military complex that helped win the space race. On the other hand, there are now a number of private entities, such as satellite television providers and GPS vendors, that rely on access to orbit for their financial well-being. Spudis said one of the driving forces in future space missions will be in developing an infrastructure that can support exploration as well as the creation of wealth. After all, he said, invoking a famous adventurer motivated by economics, "Columbus didn't come to the New World to catalogue the plants there."