Washington ? One year after the space shuttle Columbia fell out of a blue Texas sky in blazing fragments, the people of NASA are caught in a mix of euphoria over the Mars landings, hope for a rebirth of human space exploration a decade or so in the future — and painful determination to return the shuttle to flight.

The accident, it is clear in hindsight, sounded the death knell for the aging shuttle fleet. President Bush has directed that the reusable space planes are to cease operations at the end of the decade.

And for the first time in decades, Congress and the White House have recognized the need to supply a worthy answer to the question: To what end do astronauts sacrifice their lives?

The flags of the National Aeronautics and Space Administration fly at half staff today for 17 fallen astronauts whose tragedies are clustered on the calendar: those lost aboard Columbia one year ago Sunday as they returned from space; the crew that died aboard Challenger 18 years ago last Wednesday (Jan. 28, 1986) during the ascent to orbit; and the three Apollo 1 astronauts who died 37 years ago last Tuesday (Jan. 27, 1967) in a flash fire on the launch pad.

Bush last month handed the agency a call to action that gives human spaceflight the prospect of a renewed lease on the sense of lofty purpose and exploratory excitement that attends the Mars robots today — and once drove men to the moon.

But for now, the only two humans in space, American Michael Foale and Russian Alexander Kaleri, remain locked in orbit aboard a half-built space station just 240 miles above Earth.

Indeed, the operations of the robotic Mars rovers Spirit and Opportunity, so successful on a planet 100 million miles away, have become a model of sorts for managers of the human spaceflight program, as they try to instill fundamental change in their much-criticized organizational culture.

AP Photo Space shuttle Columbia crew members strike a pose for their traditional in-flight crew portrait in the research lab aboard the shuttle in this photo from January 2003. Clockwise from left are Kalpana Chawla, David M. Brown, William C. McCool, Michael P. Anderson, Ilan Ramon, Laurel B. Clark, and Rick D. Husband. All seven crew members died on Feb. 1, 2003, when the Columbia broke up during re-entry. This picture was developed from a roll of film recovered from the shuttle debris.

Bush has called for the space agency to focus anew, for the first time since 1972, on sending humans beyond Earth’s environs to explore the solar system, beginning with the moon as a steppingstone to Mars and other destinations.

But the human spaceflight program, staggered once again last year by revelations of its failures, lapses and “blind spots,” will spend much of next year preparing its temperamental spaceships, grounded since the accident, to fly again — possibly late this year.

NASA has vowed to comply, and then some, with the list of stringent recommendations for safety improvements outlined by the Columbia Accident Investigation Board under the leadership of retired Adm. Harold W. Gehman.

The Bush plan decrees that the shuttles will operate again only for the limited purpose of completing construction of the space station.

The rear hazard- identification camera on NASA's Mars rover Opportunity shows the spacecraft's rear point of view and its tracks after it rolled off its lander and onto the soil of Mars. Scientists at NASA's Jet Propulsion Laboratory erupted in cheers as the photograph appeared Saturday on a screen in mission control.