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."

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