"This program is not driven by human exploration," said Ed Weiler, NASAs associate administrator for space science, in presenting the American space agencys next decade of Martian explorers 100-percent robotic, the lot of them.

The revised plan, prompted by the loss of a $300 million pair of probes last year, left pro-human exploration backers disappointed.

"This is a poor mans Mars program. We are not a poor country. We can do a lot better than this," said Robert Zubrin, president of the Mars Society, based in Indian Hills, Colorado. A strident proponent of a humans-on-Mars initiative, Zubrin called the robotic missions fine "but they dont go far enough."

"If were serious about doing science on Mars, let alone settlement, we have to send people," Zubrin said. "This plan does not take any significant steps in that direction."

Louis Friedman, executive director of The Planetary Society, the Pasadena, California-based space exploration advocacy group, echoed Zubrins criticism, saying the revised program showed no connection to either human missions or even a permanent robotic presence on Mars.

"Thats clearly absent, no one mentions it and the sequence does not allow for that," Friedman said. "Its not taking the program to its logical conclusion."

NASA has kicked around the idea of astronauts strutting about on Mars for decades, at times more seriously than at others. A 1989 study, prepared for President Bush, pegged the cost of such a multi-year mission at nearly half a trillion dollars, promptly cooling enthusiasm for the idea.

But NASA-supported scientists, under the aegis of its Human Exploration and Development of Space (HEDS) enterprise, have been plugging away on a suite of instruments that would enable a crewed mission to travel to Mars and live there safely for an extended period of time.

Scott Hubbard, NASAs Mars program director, said the HEDS payloads and other technologies, including the ability to land a spacecraft on Mars with pinpoint precision, would help get humans to Mars.

"I feel like we do pave the way," Hubbard said of the revised Mars plan. "All of this leads up to it. Its maybe more connected than [Friedman] realizes."

The various HEDS instruments, originally slated to travel in batches to Mars in 2001 and 2003, would do everything from produce breathable oxygen from the thin, carbon dioxide-rich Martian atmosphere to assess the potential threat of dust and radiation to astronauts on the planets surface.

However, the 2001 lander mission was scrapped and in 2003 NASA elected to send a twin-rover mission that will not include the HEDS payloads. That means most will have to wait until 2007 to hitch a ride to Mars. (The 2001 orbiter will carry one of the original HEDS payloads.)

"Were enthusiastic about the chance to redefine our instruments, but seven years is a long time to wait to see them on Mars," said Peter Smith, a planetary scientist at the University of Arizona's Lunar and Planetary Laboratory. Smith had hoped his experiment, designed to measure dust devils, would travel to the Red Planet in 2003.

The wait to see humans on Mars will take even longer. Nor will NASA have to grapple with just technology and science to do so funding may represent an even bigger hurdle.

"To make a commitment to humans on Mars is something that the administration and Congress would have to do," NASAs Hubbard said.

Former Congressman Robert Walker, who once headed the powerful space and aeronautics subcommittee in the House of Representatives, said dont look to Congress to shell out the dough required to plop people on Mars.

"I dont think I would want to look at any scheme that depended upon Congress to get me the money. You wont find Congress receptive to $20 billion, $30 billion, whatever the figure is," said Walker, speaking at a space tourism workshop last June.

Rather, Walker a science and technology advisor on Gov. George W. Bushs presidential bid said a humans-to-Mars enterprise needs a little Red Planet politics in the private sector. Firms involved in carrying out such a Mars project could receive tax-breaks to spark interest, he said.

"How interested would Microsoft or General Motors become if they were told, 'If you sponsor a trip to the Moon or a mission to Mars, the government will give you 50 years of tax-free status for the entire company? They would go subcontracting out with all the people that NASA now subcontracts with in order to do those missions," Walker said.