Houston ? For the first time since 1972, the United States is planning to fly to the moon, but instead of a quick, Apollo-like visit, astronauts intend to build a permanent base and live there while they prepare what may be the most ambitious undertaking in history – putting human beings on Mars.

President Bush in 2004 announced to great fanfare plans to build a new spaceship, get back to the moon by 2020 and travel on to Mars after that.

But with NASA focused on designing a new spaceship and spending about 40 percent of its budget on the troubled space shuttle and international space station programs, that timetable may suffer. Still, NASA’s moon planners are closely following the spaceship initiative and, within six months, will outline what they need from the new vehicle to enable astronauts to explore the lunar surface.

The moon is not for the faint of heart. It is a lethal place, without atmosphere, pelted constantly by cosmic rays and micrometeorites, plagued by temperature swings of hundreds of degrees, and swathed in a blanket of dust that can ruin space suits, pollute the air supply and bring machinery to a screeching halt.

“It’s like Magellan. You send them off, and maybe they come back, maybe they don’t,” said planetary scientist Wendell Mendell, manager of NASA’s Office for Human Exploration Science, during an interview at the recently concluded Lunar and Planetary Science Conference here.

In some ways, the moon will be harder than Mars. Moon dust is much more abrasive than Mars dust; Mars has atmosphere; Mars has more gravity (one-third of Earth’s); Mars has plenty of ice for a potential water supply, while the moon may have some, but probably not very much.

Still, the moon is ultimately much more forgiving because it is much closer – 250,000 miles away, while Mars is 34 million miles from Earth at its closest point. If someone needs help on the moon, it takes three days to get there. By contrast, Mars will be several months away even with the help of advanced – and as yet nonexistent – propulsion systems.

Not having to pay as dearly for mistakes is one key reason why the moon is an integral part of the Bush initiative. The other, as even scientists point out, is that if the United States does not return to the moon, others will.

“The new thing is China, and they’ve announced they’re going to the moon. The Europeans want to go; the Russians want to go; and if we don’t go, maybe they’ll go with the Chinese,” Mars Institute Chairman Pascal Lee said in an interview. “Could we bypass the moon and go to Mars while India and China are going to the moon? I don’t think so.”