CAPE CANAVERAL, Fla.  —  Again and again and again and again Mission Control called, first on one radio channel and then on another. But from space there was only a silence that stretched on until there was no hope.

Their voices remained calm, professional, despite growing evidence that space shuttle Columbia and its seven astronauts were in very great trouble in their long fall from orbit toward a landing at the Kennedy Space Center.

Observers in California and Texas and Arkansas all reported seeing flashes of light, perhaps from debris burning away, as the winged spacecraft streaked overhead. But the experts in Mission Control at first were seeing only routine data, streaming to Earth as millions of electronic bits. Suddenly, there was a dramatic change in temperature readings. And then silence. No data. No radio voices. No radar tracking. And soon, no hope.

Columbia was traveling at more than 16,400 miles an hour as it approached the California coast in a high-speed descent shortly before 9 a.m. EST.

There was no communications from the astronauts at the time. Typically for a return from space, the spacecraft commander, Rich Husband, and pilot, William McCool, would sit in the control seat at the front windshield, surrounded by elaborate controls including computer screens.

Just behind, at McCool's right shoulder, would be Laurel Blair Salton Clark, a physician. And behind both the pilot and commander was astronaut Kalpana Chawla. In a tight compartment below the cockpit, were the other three astronauts, Michael P. Anderson, David M. Brown, and Ilan Ramon, the first Israeli astronaut.

Public affairs announcer James Hartsfield, speaking to the outside world from a microphone inside Mission Control, calmly ticked through the landing routine, calling off speed and altitude and distance to the landing runway. He told how the spacecraft, flying on autopilot, started the first of a series of banked maneuvers, designed to slow the craft as it entered the increasingly thick atmosphere.

At 8:53 a.m., engineers in Mission Control noticed there was a sudden loss of temperature readings in the hydraulic system in the left wing. Somehow, the temperature sensors were no long sending data.

Three minutes later, signals dropped from the temperature sensors in the left main landing gear.

Mission Control sent a notice to a cockpit electronic screen about the temperature readings.

Milt Heflin, chief flight director, said the crew acknowledged the signal, but it was thought "there was no problem at that time." Such temperature reading losses have been seen before.

Hartsfield continued with routine landing reports, noting that the speeding craft was streaking across the New Mexico-Texas border at an altitude of 40 miles and a speed of 13,200 mph. Columbia, he said, was only 1,400 miles and less than 20 minutes from landing.

There was a muffled blurt on the radio from the crew.

Capsule communicator Charlie Hobaugh broke a long silence by calling to the crew.

"Columbia, Houston," he said, "we see your tire pressure message and we did not copy your last."

"Roger," said Husband. "Uh, buh....."

The communication was cut abruptly, the final word never finished. It was followed by static.

At about the same time, all data signals abruptly stopped. Columbia's computers were no longer talking to Mission Control.

The time was about 9 a.m. EST, said Heflin. "That was when we lost all vehicle data. That's when we began to know that we had a bad day."

Columbia was then moving at more than 18 times the speed of sound and was some 207,000 feet — about 39 miles — above Texas.

Hartsfield calmly said that Mission Control engineers "are continuing to standby to regain communications with the spacecraft."

Hobaugh began a series of plaintive calls, speaking in that professional, no nonsense voice of an aviation veteran.

"Columbia, Houston," he called. "Com (for communications) check."

Silence from space.

"Columbia, Houston," Hobaugh tried again, this time using another radio channel. "UHF (ultra high frequency) com check."

Silence.

"Columbia, Houston," Hobaugh persisted. "UHF com check."

Silence.

Hobaugh tried four times more, but there was only silence.

Hartsfield, still hoping, reported, "Flight controllers are standing by for C-band (radar) tracking data from the Merritt Island tracking station."

Again and again, he told the world that Mission Control was still calling, still looking for Columbia.

But always there was only silence from space.

"There was nothing we could do," a Mission Control official said later. "Just observe."

Within an hour, the flag at the Kennedy Space Center was lowered to half mast.