Cape Canaveral, Fla. ? Few Americans knew the names of the seven astronauts on shuttle Columbia 16 days ago when the aged orbiter left Earth. Now their loss in another shocking space tragedy leaves the nation searching for answers:

How did this happen? When and how will Americans return to space? Should they return to space?

“The cause in which they died will continue,” President Bush on Saturday told a nation stunned again by sudden, saddening loss, this time of the entire crew aboard a spaceship that disintegrated over Texas. “Our journey into space will go on.”

Initial speculation about the cause of the accident focused on possible damage sustained by the shuttle’s left wing during blastoff 16 days earlier.

The astronauts — a cross section of America and an Israeli pioneer — never had a chance. Astronauts have no way to escape a shuttle as it glides to a landing without power at 13,000 miles per hour.

The crew included three U.S. military officers, one of the nation’s few black astronauts and a woman who immigrated to America from India. Five were married. Between them, the astronauts of shuttle Columbia had 12 children.

Astronauts are explorers on the frontiers of space. They depend on muscular but fragile technology. The machines failed seven of them Saturday, but they knew the risks going in.

“I take the risk because I think what we’re doing is really important,” Michael Anderson, 43, Columbia’s payload commander, said Jan. 16 before Columbia blasted off from Cape Canaveral. He was the son of an Air Force man and grew up on military bases. He was one of America’s few black astronauts..

“This day has brought terrible news and great sadness to our country,” President Bush said. “The Columbia is lost. There are no survivors …

“The crew of the shuttle Columbia did not return safely to Earth, yet we can pray they all are safely home. May God bless the grieving families.”

In addition to Anderson, aboard Columbia were:

The space shuttle Columbia crew from left, Cmdr. Rick Husband, pilot William McCool, mission specialists Israeli astronaut Ilan Ramon, David Brown, Michael Anderson, Laurel Clark and Kalpana Chawla, field questions during a news conference in Cape Canaveral, Fla., in this Dec. 20 file photo. All seven crew members were killed Saturday after Columbia disintegrated over Texas on its re-entry to Earth's atmosphere.

Rick Husband, 45, the shuttle’s commander and an Air Force colonel; co-pilot William McCool, 41, a Navy commander; mission specialists David Brown, 46, a Navy captain; Kalpana Chawla, 41, who was born in India; Laurel Clark, 41, a flight surgeon; and Ilan Ramon, 48, a colonel in Israel’s air force.

All but Brown were married. Ramon had four children, McCool had three, Husband had two, Anderson had two, and Clark had one. Ramon, McCool, Brown and Clark were space rookies.

It was the shuttle program’s 113th mission and second major disaster, eerily reminiscent of the 1986 explosion of the Challenger shuttle during liftoff, which also killed all seven astronauts aboard.

Latest information from CNNStories: ‘Columbia is lost’ Astronauts accepted risks of duty Americans stunned by shuttle disaster Mission Control calls go unanswered by shuttle Shuttle’s age wasn’t a factor, experts say Escape from craft was impossible, specialist says Future of shuttles comes into question Disaster revives town’s heartache Investigators focusing on tiles hit during liftoff Astronaut brought joy, tears to Israel Crews collecting scattered debris Flights had nearly lost mystique TV brought crash images into homes The world reactsPhotos: Photo galleryVideo: Columbia breaks apart over Texas (CNN)Links: Latest information from NASA NASA background information on the Columbia crew and its missionTexas media: Nacogdoches, Texas DailySentinel.com HoustonChronical.com DallasNews.comIsrael news: Jerusalem Post

No cause was immediately apparent, but sensors aboard Columbia reported a sudden spike of intense heat, an indication that the ship’s heat shield had been breached — possibly on the left wing.

The temperature at that point of re-entry: 3,000 degrees. The altitude: 207,135 feet, or 39 miles above Earth.

Government officials said there was no indication of terrorism and the shuttle was well out of the range of missiles when the accident occurred.

Mourning, then fixing

The president and others vowed that the human space program would continue, after a lengthy investigation.

“It’s more than a job;, this is a passion for us,” said Ron Dittemore, NASA’s shuttle program manager. “There’s going to be a period of mourning in this community, then we’re going to fix this problem, and we’re going to get back on the launch pad.”

The shuttle was only 16 minutes from the landing strip at the Kennedy Space Center when NASA lost communication with it.

The last word from Columbia came at 8 a.m. CST from Husband:

“Roger.”

Then, a muffled sound. Then only static.

Residents far below reported hearing a loud bang.

Cherokee County Sheriff James Campbell was at home when he and his wife heard the terrible sound.

“I said it’s probably the space shuttle’s entry back into the atmosphere,” Campbell said. “She said, ‘No come look at the vapor trail.’ It was zigzagging down, and I said, ‘Well something’s wrong.'”

Debris litters 2 states

The sky was scarred with smoke, debris, failure and death.

Debris rained over hundreds of miles of Texas fields and highways, stretching from near Dallas all the way to Louisiana. Residents reported finding metal fragments, piles of ash and what appeared to be a door off the shuttle.

In Hemphill, Texas, near the Louisiana border north of Jasper, debris and human remains were scattered across more than 50 sites, authorities said. Some debris was also found in Newton, Jasper and St. Augustine counties.

Several hundred workers from state, local and federal agencies logged the location of the evidence, tagged it and placed it in bags. Two astronauts were in the area working with NASA.

“Most sites contain debris,” Sabine County (Texas) Sheriff Tom Maddox said. “Only a small number had human remains.”

Authorities throughout the region urged residents not to touch or even approach the debris. It could contain hazardous material, experts said, and it could contain vital clues to the cause of Columbia’s demise.

Seeking a cause

Late Saturday, recovery crews prepared to begin the grim, agonizing search for human remains. And NASA engineers and managers launched the first phase of a painstaking search for the accident’s cause.

Early speculation centered on an explosion caused by a structural defect or the possibility that crucial, heat-protecting tiles on the shuttle’s left wing were damaged when it was struck by a piece of fuel-tank insulation during blastoff.

NASA engineers concluded during the flight that any damage to the wing was minor and posed no safety hazard — an assertion certain to be tested during a probe that NASA Administrator Sean O’Keefe promised would be complete and vigorous.

“At this time, we have no indication that it was caused by anything or anyone on the ground,” O’Keefe said, a comment apparently designed to quell speculation about terrorism.

Flags were lowered to half-staff at the White House, the Capitol, the Kennedy Space Center, the Johnson Space Center near Houston and at countless other locations around the nation.

At the Kennedy Space Center, spouses and children of the astronauts were gathered from the landing strip and taken to a secluded location. A NASA official said they were returning to Houston and “bearing up under the grief.”

Israeli among dead

Before the 16-day scientific flight, Ramon the Israeli spoke evocatively about the symbolic nature of his assignment — and its meaning to his people.

Though a secular Jew, he planned to observe the Sabbath, when possible, and eat kosher food aboard the shuttle. He called it an “act of solidarity with Jewish tradition.”

“I was born in Israel,” said Ramon, “and I’m kind of the proof for the whole Israeli people that whatever we fought for and we’ve been going through in the last century — or maybe in the last 2,000 years — is becoming true.”

Clark, another rookie, echoed his words.

“This is my first flight and I’m very excited,” she said before liftoff. “I can’t wait to look down on our planet from space.”

Chawla, the first native of India to fly in space, was particularly admired by Indian immigrants to America. In a magazine interview after her first flight in 1997, she said:

“You see the continents go by, the thunderstorms shimmering in the clouds, the city lights at night. … Earth is very beautiful. I wish everyone could see it.”

The oldest shuttle in the fleet, Columbia was inaugurated in flight on April 12, 1981. This was its 28th flight in space.

It was supposed to land at the Kennedy Space Center at 9:16 a.m. EST.

It did not.

The U.S. human space flight program began 42 years ago. Until Saturday, there had never been an accident of this magnitude during a landing or an approach.

Space station in question

With the shuttle likely to be grounded for a lengthy period, the fate of the already controversial space station was in serious doubt.

Costing taxpayers $52.7 billion and scheduled to take more than six years, the space station is the most ambitious U.S. space project since the Apollo moon landing. Construction of the station continues and requires dozens of shuttle flights.

The current crew in the station — NASA astronauts Ken Bowersox and Don Pettit and Russian Nikolai Budarin — can return to earth aboard Russian spacecraft.

Questions dog fleet

Even as crews deployed to look for wreckage, NASA engineers and officials began searching for a cause.

The quasi-independent Aerospace Safety Advisory Panel and other experts have been warning NASA for years about the dangers of an aging shuttle fleet and the need for safety upgrades that have been delayed due to lack of funds.

Last year, the panel’s first finding said:

“The current and proposed budgets are not sufficient to improve or even maintain the safety risk level of operating the space shuttle and ISS (The International Space Station).”

The shuttle fleet has endured numerous problems. In recent years, NASA had been concerned about the integrity of fuel injectors installed on some shuttle booster rockets and about wires that connect the shuttles to those rockets.

“(The shuttle’s) perfectly capable of blowing up every time they launch it,” John Pike, director of GlobalSecurity.org, an Alexandria, Va., space and defense policy watchdog group, said. “They bet the agency every time they launch.”

Last year, cracked fuel lines appeared in all four shuttles and forced the three-month suspension of flight. Though small, they could have widened and propelled metal fragments into the engines, with catastrophic results.

Welders were assigned to make repairs, and NASA officials said the problem was solved.

In the recent past, then-NASA Administrator Dan Goldin had been candid about the inherent hazards of space flight.

“When you go into space,” he said, “you risk your life.”