NASA's space shuttle program manager backed away yesterday from a key theory that a piece of insulation breaking away from Columbia's fuel tank during launch damaged the craft's heat shield so badly that it was destroyed in the final minutes of its mission 16 days later.
"Right now it just doesn't make sense that a piece of debris could be the root cause of the loss of the Columbia and its crew. There's got to be another reason," Ron Dittemore said at a news briefing in Houston. "We believe there's something else."
He said investigators are still searching for clues to Saturday's disaster, which killed seven astronauts, including the first Israeli to fly in space.
"We have not narrowed it down to any one conclusion or any particular topic," he said.
Only the day before, Dittemore said NASA's chief suspect was a 2.7-pound piece of flying foam insulation that struck key heat shield tiles on the underside of Columbia's left wing 80 seconds after liftoff. But repeated analyses showed that the foam couldn't have caused fatal damage.
Instead, he said, investigators are concentrating on finding what caused a sudden increase in wind resistance, or drag, recorded by sensors on Columbia's left wing in the flight's final moments. That drag forced the shuttle into a sudden left turn that overwhelmed the automatic flight controls' ability to compensate, he said.
Dittemore also rejected a theory that ice forming on the fuel tank before launch could have critically damaged the tiles, or compounded the damage caused by the foam.
He said inspectors carefully went over the tank before launch and would have delayed it had there been enough ice to endanger the mission.
He added that some of the shuttle's 24,000 silicon-based tiles are damaged on every flight, with an average of more than 100 "hits" or impacts per flight.
"Tiles are an area where we are constantly measuring the number of impacts," Dittemore said.
'Easy to break up'
But he said that the foam is too light, too fragile and would have hit the shuttle with too little force to cause such damage. To reinforce his point, he distributed to reporters a piece of the insulation, which looks like a thick piece of material from an inexpensive picnic cooler.
"It's fragile and it's easy to break up into particles and it's very light weight," he said.
He said that investigators are continuing to try to unscramble the 32 seconds of signals from a secondary transmitter that were picked up after flight controllers lost radio contact.
They also are trolling the waters off Cape Canaveral in Florida for debris from the Jan. 16 launch.
"We're taking a painstaking approach to try to pull out every nugget there, and its going to take us some time," he said.
He said that no high priority or "red tag" debris or key pieces of the shuttle have been recovered yet, which he identified as the left wing structure, the tiles or the Columbia's flight recorders.
In other Columbia developments yesterday:
Crew members' remains arrived at Dover Air Force Base in Delaware, home of the military's largest mortuary, where experts will begin the process of identifying them. Dover officials said they did not know how many sets of remains had been found.
A helicopter hoisted Columbia's nose cone from an East Texas forest, and a recovery team headed to California to inspect material that might be part of the shuttle's wing. NASA officials said they are still looking for "red tag" debris from areas east of Texas that would provide key clues to the early stage of the shuttle's breakup.
A home video of Columbia appears to show a small, bright object falling off the spacecraft as it passed over Arizona. Someone on the ground is heard saying, "Look at the chunk coming off of it. What the heck is that?"
Two Texas men were charged with stealing pieces of the shuttle. Authorities said others have until 5 p.m. tomorrow to turn in Columbia's debris without fear of being prosecuted.
Remains brought in
In Delaware, a C-141 military cargo plane carrying remains from Barksdale Air Force Base in Louisiana touched down just before 2:40 p.m., bearing seven coffin-like cases, symbolizing Columbia's seven crew members. Six cases were draped with American flags, the seventh with the flag of Israel in honor of Israeli astronaut Ilan Ramon.
Remains "will be released to their families as soon as possible," NASA Deputy Administrator Frederick D. Gregory said at a news conference.
After the plane taxied to a stop, Gregory and base commander Col. Scott E. Wuesthoff, accompanied by a chaplain and two honor guards, joined two rabbis who accompanied the flight from Louisiana.
The chaplain, Lt. Col. Karen Stocks, said a prayer before the remains were transferred to four white hearses and taken to the mortuary while a color guard stood at attention.
Workers at the mortuary will use DNA analysis and other techniques to identify the remains.
"It could be days, it could be weeks, it could be months," mortuary director William Zwicharowski said.
He would not comment on the number of remains, and officials disputed reports that some, including Ramon's, already had been identified.
In Jerusalem, Israel Radio reported four sets of remains had been identified, including those of Ramon, an Israeli fighter pilot. Israel's chief army rabbi, Israel Weiss, said that "we received a clear, positive, scientific answer that leaves no room for doubt" about identification of Ramon's body.
Charged with theft
While NASA's Dittemore was warning residents of Texas and Louisiana not to touch shuttle debris because of danger from toxins and explosives, two Texans were arrested yesterday on federal charges that they stole pieces of Columbia that had dropped onto the countryside.
Federal officials in Texas also declared an amnesty period extending until 5 p.m. tomorrow, during which people who have collected shuttle debris can turn it in without fear of prosecution. After that, prosecutions will resume, they said.
Merrie Hipp, 43, of Henderson, was charged with theft of government property. He is accused of stealing a shuttle circuit board Saturday. Bradley Justin Gaudet, 23, of Nacogdoches, was charged in a separate incident with stealing a piece of thermal insulating fabric.
Both pleaded innocent and were freed on their own recognizance.
Their lawyers said both were unnerved by the charges, which carry a maximum of 10 years in prison and a fine of $250,000.
Large search area
Meanwhile, NASA investigators are still searching for key pieces of debris over a huge area that extends westward into New Mexico, Arizona and California. Dittemore said the agency is particularly interested in pieces of the left wing - where Columbia's problems likely originated - as well as pieces of the shuttle's protective tiles and voice and data recorders
Dittemore also urged anyone in those states with photographs or videos of Columbia's breakup to contact NASA and send or upload their pictures to NASA's Web site.
Amateur astronomer Rick Baldridge photographed and videotaped Columbia as it passed over San Jose, south of San Francisco. He said the video showed flares of light that appeared to be parts breaking off the shuttle.
Another amateur astronomer made a digital time-exposure photograph in San Francisco that showed a snake of purplish light corkscrewing through the shuttle's hot glowing trail.
NASA spokeswoman Katherine Watson said the agency has received 1,300 images of Columbia from eyewitnesses.
The Associated Press contributed to this article.