Traveling at more than 12,000 miles per hour, the object first streaked by itself, heading west to east. Then, it seemed to split into two pieces, then four, then a dozen, then more and more. By the time the cluster of objects reached Texas, there was no more forward motion.

Objects were falling from the sky by the hundreds. Space shuttle Columbia had disintegrated during its re-entry into Earth's atmosphere. Its seven-member crew died in the disaster.

Columbia, the second shuttle to carry astronauts to their deaths, was done in because two weeks earlier, a piece of foam insulation on the spacecraft's huge external fuel tank - which is jettisoned a few minutes after liftoff - struck the leading edge of its left wing during launch. Traveling at about 500 mph, the foam punched a softball-sized hole in the wing.

During re-entry, the hole allowed super-heated gas to penetrate the wing's interior, eventually melting structural components and causing a catastrophic failure of the spacecraft.

Now, two years later, NASA is preparing to return its remaining three shuttles to flight, beginning with Discovery, which sits at the Kennedy Space Center in Cape Canaveral, Fla. It is set to take off for a construction and resupply mission to the International Space Station sometime in late May or early June.

Among the safety improvements, NASA has redesigned the insulating foam to prevent chunks from being ripped away during launch.

Engineers have refitted the wing's leading edges with sensors to warn of damage to the delicate heat-resistant ceramic tiles that protect the shuttle from the heat of re-entry. They also have attached a new extension to the spacecraft's robot arm that will allow the crew to inspect the wing edges and critical underside without requiring a spacewalk.

The space agency has taken these precautions, plus one more: Now, anytime a shuttle flies, NASA will park another one at the launch pad, ready to takeoff on short notice, in case it is required to rescue a crew that cannot attempt re-entry without risking a repeat of Columbia's fate.

For that reason, NASA is preparing shuttle Atlantis at the same time it is readying Discovery. NASA aims to have Atlantis ready to go within a month of Discovery's launch. That is the length of time engineers estimate the space station could support seven shuttle astronauts along with the regular two-person crew.

In one sense, NASA has a lot riding on its next flight. Unless the shuttle fleet can resume operations, the United States will have no way to send crew members to the space station for the foreseeable future - unless the current dispute with Russia can be resolved so U.S. astronauts can hitch rides on the Soyuz spacecraft.

That means, as Robert Zimmerman has written in UPI's Space Watch recently, the country has put its $100 billion investment in the station in jeopardy and risked seriously damaging its relationship with Russia, all to continue missions of modest utility.

In another sense, the harsh reality of the shuttle and the space station is both hugely expensive programs have always represented dead ends to some extent in terms of advancing the human exploration of space. The shuttle is much more costly to build and operate than rocket-and-capsule alternatives, much more difficult to maintain and more dangerous to fly.

This is not to impugn the motives, dedication and skill of the thousands of NASA employees now struggling to ensure that another Columbia tragedy does not happen.

As Sean O'Keefe, the outgoing NASA administrator, said Jan. 29 at a memorial service for the 17 U.S. astronauts killed during missions or mission tests: "Because of the Columbia accident we are constantly, diligently, 24-7, working at fixing it so that we know that when it comes time to fly we have done our absolute best. We will never eliminate the inherent risk, though I know each of us will work to minimize that risk."

No doubt everyone working on the shuttle program at NASA right now echoes that sentiment.

Likewise, it obviously is a point of national pride - and institutional pride at NASA - to get the shuttles flying again. It is the same motivation that drove an extraordinary bunch of naval engineers and workers to take the U.S. Pacific Fleet, decimated by Japanese bombers and torpedo aircraft at Pearl Harbor on Dec. 7, 1941, and restore many of its sunk and damaged vessels to seaworthiness within a matter of days.

Back then, there was a war to fight. Now, practically the only reason NASA has to spend billions of dollars more on the shuttles and space station is to finish what was started.

The worthy, even noble goal of sending humans back to the moon and eventually out into the solar system - which was announced by President George W. Bush on Jan. 14, 2004 - will not be served much by either reflying the shuttles or completing the station under their current mission configurations.

Arguably, one of the most valuable roles for the shuttle is growing less likely by the month: carrying a crew to repair and upgrade the Hubble Space Telescope, the most successful and popular scientific instrument ever built. The Hubble is maybe two or three years from failure, with no optical-observing replacement in sight. Thus, the Hubble may become Columbia's last casualty.

O'Keefe, who waited on the tarmac at Kennedy for the shuttle's return two years ago, and then had to face the families of the dead, has said for the past year or more he would not go through that ordeal again.

Any shuttle crew visiting the Hubble would have to do so without the possibility of escaping to the space station if necessary, and O'Keefe - and his as-yet-unnamed successor - may not be willing to take that risk. Instead, NASA will work to get the shuttles flying again and keep them flying for the next five years, traveling exclusively to and from the station.

Meanwhile, NASA attempts to re-imagine a three-decade-old concept of using disposable rockets and capsules to send humans into space a whole lot higher than the space station's orbit, something that worked back then and would have carried the space program much farther along by now had it not been abandoned.

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