Scientists often grumble about the terrific astronomy and robotic exploration they might have done with the billions of dollars NASA is budgeting to send astronauts back to the moon.
But the moon is where NASA and its budget are going. So shouldn't astronomers be on board?
About 150 of the nation's top space scientists -- among them a Nobel Prize winner and three astronauts -- debated the issue this week at the Space Telescope Science Institute in Baltimore, sorting out the unique opportunities the moon might offer if they hitch a ride on moon-bound spacecraft.
Most agreed that there are exciting possibilities for low-frequency radio astronomy on the far side of the moon -- where their dish antennas would be shielded from Earth's blaring radio "noise."
There may also be an opportunity to test Albert Einstein's theories about gravity by establishing precise laser ranging devices on the moon, some said.
But for others, it still makes more sense to launch observatories into free space. Out there, designers don't have to contend with complications posed by lunar gravity, moon dust, huge temperature swings or power and communications interruptions posed by the lunar day and night.
"The lunar surface is vastly better for telescopes than Earth is," said University of Texas astronomer Daniel Lester. But researchers working with Hubble and other space telescopes have learned to build and service orbiting instruments. "Compared with free-space operations," he said, "[the moon] is no longer as compelling as it used to be."
The researchers were interested in the transport opportunities opened by NASA's manned lunar spacecraft. The Ares rockets, Apollo-like Orion capsules and their astronauts could be enlisted to deploy and service big observatories in free space.
Princeton University astrophysicist David Spergel argued for "L-1" -- the "Lagrangian point" between the Earth and the moon where the two bodies' gravitational fields are in balance.
It's on the path to the moon, and easy for spacecraft to reach. It would even be relatively easy, he said, to bring stricken observatories back from a similar L-1 point between the sun and the Earth, and have astronauts or robots repair them at an Earth-moon L-1 "service station."
"We have an obligation, if we think there is a better place [than the moon] to go, to tell the drivers [at NASA] there's a lot of opportunity at the Earth-moon L-1," Spergel said.
Astronomy on or near the moon was a moot issue for three decades after Apollo 17 astronauts returned from the moon in 1972, without a mandate from the White House and Congress to go back.
That changed in January 2004, when President Bush reopened the lunar frontier by sketching out his Exploration Initiative for sending astronauts back to the moon, and eventually on to Mars "and beyond."
That mandate -- since endorsed by bipartisan congressional majorities -- is expected to shift NASA's resources away from the space shuttle program (now set to end in 2010) and the International Space Station after its completion in 2015. The money will be redeployed gradually to build a new fleet of manned Orion spacecraft, reminiscent of Apollo.
NASA also plans new light-duty and heavy-lift Ares launch vehicles, based on existing shuttle main engines and solid-fuel boosters.
Space scientists have complained that the president's initiative has already delayed or diverted NASA dollars for some coveted space science programs. But if the moon is where NASA's dollars are headed, many scientists realize their own futures may lie there as well.
"Yes, there definitely is an element here of, you know, 'Go where the airlines go,'" said Mario Livio, a senior astrophysicist at the space telescope institute. But "if we are going to the moon, can we think really hard and find what important scientific questions can be addressed by the initiative."
"If the answer is no, we will come out of here and say, really, this is not the best way to answer those questions," he said.
The moon is a mixed bag for astronomers. The near-vacuum means astrophysicists can conduct long-term studies of the energetic solar particles and cosmic rays that zip through the solar system, but don't make it intact through Earth's atmosphere.
A two-week lunar night provides more time to search for "near-Earth objects" -- asteroids that could one day threaten the planet.
Paul Spudis of the Johns Hopkins University Applied Physics Laboratory near Laurel said that seismically, the moon is thousands of times more stable than Earth. And scientists say that could help them detect exotic subatomic particles as they pass through the moon.
Deep polar craters could offer astronomers perpetual darkness, Spudis said, while polar highlands could provide perpetual sunlight for electricity.
On the downside, astronauts and telescope designers will have to contend with big temperature changes, high levels of radiation and the moon's fine, highly abrasive, magnetic and "sticky" dust.
Apollo-era imagery suggests the dust is regularly "levitated" above the surface, perhaps by static electricity generated by each lunar sunset. It's not clear how high or far the dust flies, or how it might degrade the performance of telescope mirrors and optics.
Even so, Spudis concluded, "I contend the moon is a benign environment" for astronomy, "like free space itself."
NASA astronaut John Grunsfeld was similarly upbeat. He has twice helped repair and upgrade the orbiting Hubble Space Telescope and is training to go again in 2008.
He told the gathering that work on Hubble and the International Space Station has proved that NASA astronauts can assemble and repair complex equipment and solve problems in harsh environments.
But telescopes and their components must be designed to be serviced, upgraded or replaced by astronauts or robots, he said. They'll need the means to recover from failure. And they should share a common lunar communications infrastructure.
Harley Thronson, chief scientist for exploration concepts applications at NASA's Goddard Space Flight Center in Greenbelt, argued that the manned space program "should not be dismissed by the people sitting here at the Space Science Telescope Institute; they [astronauts] rescued the world's greatest science telescope and continually upgraded it."
But all lunar or free-space telescopes should be designed for easy repairs and upgrades by astronauts or remotely operated robots.
He cautioned, however, that NASA "does not have a substantial, sustained robotics program to make [robotic repairs] possible. NASA has spent more on advanced propellers for airplanes than it has on space robotics."