There is no promise that NASA will ever get a directive to send astronauts back to the Moon. Nor is it clear if humans will venture to Mars. But speculation last week that President Bush might be mulling those grand plans had scientists around the world savoring the possibilities.

Many researchers believe robotic exploration is the best way to conduct most space science.

Others disagree. Only humans can properly investigate other worlds, they say, to answer the most pressing questions about the origin and fate of humans and the possibility that life exists elsewhere.

For many space visionaries and practical scientists alike, human spaceflight is about opening up profitable commercial opportunities and, perhaps more important, continuing the immutable human desire to explore.
 
Here are ten of the top reasons scientists give for going back to the Moon:
 
1. Satisfy the soul
 
Beyond the basic needs for food, shelter and clothing, we humans are a restless lot. Exploration seems to be in our bones. The quest for knowledge is not an exclusive motivator in the desire to venture across land, sea, air or cosmic frontiers.

"The practical case for manned spaceflight gets ever-weaker with each advance in robotic probes and fabricators," Sir Martin Rees, one of the world's leading theoretical astrophysicists, told SPACE.com last week as the rumors swirled.

"Indeed as a scientist I see little purpose in sending people into space at all. But as a human being, I'm nonetheless an enthusiast for space exploration -- to the Moon, to Mars and even beyond -- as a long-range adventure for (at least a few) humans."

2. Bring nations together
 
Just as the International Space Station packed explorers from previously antagonistic nations into tight quarters, an effort to return to the Moon could bring nations together in an era of increasing international tension, some analysts say.

China, with its own lunar ambitions, is a good example of a country the United States might want to work more closely with.

"I think the international public would cheer a program designed as a flagship for how space technology can be a role model for letting technicians of various countries work together in programs that would benefit all of us," said William K. Hartmann, a senior scientist at the Planetary Science Institute in Tucson.

3. Foster commercialization

There is no agreement among scientists over the role private enterprise ought to play in human spaceflight. Yet already, commercial companies help build the machines that carry astronauts into space.

Many experts think space tourism and even certain mining and manufacturing will succeed in space if only entrepreneurs are turned loose (and perhaps assisted with federal money or incentives).

"Perhaps future space probes will be plastered in commercial logos, just as Formula I racers are now," Rees says. "Perhaps 'robo-wars' in space will be a lucrative spectator sport."

4. Gather rocks
 
The Apollo era answered many questions about the Moon. But much was left undone.

Scientists see the Moon as an attic of Earth, a place where rocks long ago blasted from our planet are sitting around waiting to be studied. This history has not decayed much because there is almost no atmosphere and little geological activity on the Moon.

Earth, on the other hand, regularly recycles clues to its past, pulling material inward, and spitting it back out as unrecognizable lava.

"We are talking about finding material from the very early Earth," says John Armstrong of the University of Washington in Seattle. "Samples of the Earth 3.9 to 4.0 billion years ago could tell us a lot about the state of the early atmosphere, what the crust and surface were like, and possibly even when life began to evolve."

5. Study catastrophe
   
The Moon's nearly pristine state means billions of years of asteroid impacts are preserved, as obvious scars on the surface. These craters hold a record of how frequently and intensely the inner solar system -- including Earth -- was peppered by asteroids through time.

Studying many of these craters up close -- from an Antarctica-like outpost -- would allow scientist to figure out if mass extinctions on Earth, including the death of dinosaurs that allowed the rise of mammals, were the result of single, large asteroid impacts, flurries of smaller assaults, or neither. An answer would impact many scientific fields.

"A lunar base could be designed that would benefit not just a few of us geeky planetary scientists who want to study lunar rocks, but a wide range of sciences -- biology, paleontology, planetary science, historical geology, and even exobiology," said Hartmann. "Is evolution of intelligent life primarily a question of surviving the cosmic shooting gallery that characterizes each extra-solar planetary system?"
 
6. Look outward

Astronomers would love to set up observatories on the Moon. Optical telescopes could be placed almost anywhere on the lunar surface and, since there's almost no atmosphere to scatter light, they'd get clear, Hubble-like views of the cosmos.

Astronomers could build scopes that are far larger than Hubble, too.

Radio telescopes -- the sort used both in conventional astronomy and in the search for possible signals from other civilizations -- could be placed on the far side of the Moon. There they would be shielded from all the radio noise of Earth.

Critics argue that these telescopes could be deployed robotically and run remotely.

7. Generate power

Space is full of free solar power. There are no clouds on the Moon. Those two facts make setting up a lunar power station sensible in the minds of some futurists. The power could be beamed to Earth or to satellites for distribution around the world, 24/7.

A lunar power station won't be built in a day. But some visionaries think any scheme to return to the Moon ought to include plans to eventually route power back home.

"By mid 21st Century, enough lunar solar power can be imported Earthward to supply the world's population of 10 billion people to meet all basic human needs," says advocate David Criswell, director of the Institute for Space Systems Operations at the University of Houston in Texas.

8. Launch industry

The Moon has minerals, and many proponents of human space exploration figure lunar mining is good enough reason to go back. Gaining experience and mastering the necessary technology might then allow venturous and potentially more lucrative claims to be laid on asteroids, which are rich in metals.

Asteroids, in fact, are thought to contain everything needed to support their own colonies, from water to iron, nickel and cobalt.

The Moon may also have water, frozen in dark polar craters. If so, it could be converted to fuel, turning the Moon into a base for all other solar system exploration.

Critics say it makes more sense to go directly to asteroids and, if desired, Mars.

9. Improve technology

There are very few critics for the idea that space exploration induces technology and health spin-offs that benefit economies and society. Yet many scientists question whether the current space program, which consists of endless trips around Earth, is worth the investment.

Hartmann says "crewed missions" to the Moon and Mars would be far more beneficial than "our present crude missions" to Earth orbit.

Bigger goals are needed, scientists reason, to spur the sort of advancements in technology worthy of a multi-billion dollar annual budget.

Of course no one knows what might be invented in an effort to set up lunar colonies, reach Mars or mine for metals on an asteroid. But a lot of scientists think it's worth finding out.
   
10. Stepping stone to Mars
 
There are those who say a human mission to Mars is what NASA really needs to reinvigorate itself. Many would like to go directly, and soon. Louis Friedman, director of the Planetary Society, worries that a return to the Moon could be a detour, rather than a stepping-stone, to Mars.

But Hartmann, whose newest book is called A Traveler's Guide to Mars,sees a progression of exploration, if NASA is given any directive to leave Earth orbit:

Return to the Moon "to demonstrate our capability" and perhaps pursue the science of asteroid impact history.

Begin to explore resources of asteroids, "some of which are easier to reach and return from than the Moon."

Explore Mars "with the big theme of discovering whether life ever started there, and thus whether life starts easily on water rich planets throughout the universe."

(Xinhua News Agency December 9, 2003)