In the Senate, that forum for speechmaking, they were reduced to silence.

The faces were long and the expressions solemn as the senators filed silently into the chamber. Their chaplain, the Rev. Richard C. Halverson, opened the brief prayer session by saying: "Eternal God, our hearts are smitten, and we are reduced to silence. Let us pray silently." They did.

That same sense of being unable to express the inexpressible affected Americans everywhere yesterday. A day that was to be a herald of success over the state of the union and celebration over American scientific proficiency turned instead into one of national tragedy shared by people from the president down, and all seemed struck by the inadequacy of attempting to put into words what everyone felt.

Once again, it was the technology of the television age that bound together the country and, this time, it was the spectacular technology of the space age that transformed into horror what Americans have come to regard as routinely successful missions into the heavens.

Of all the scenes played repeatedly hour after hour yesterday on television after that shattering fireball explosion over the Atlantic snuffed out the lives of seven Americans aboard the space shuttle Challenger, two were unmatched for emotional impact.

In Concord, N.H., 200 students gathered to watch television at the high school. They brought party horns and hats and confetti in readiness for the space flight of their social studies teacher, 37-year-old Christa McAuliffe.

The sound of cheers, the blaring of horns and the sight of shining young faces after the successful countdown and rocket liftoff at Cape Canaveral swiftly turned to silence. Those stricken looks of stunned students, frozen in mute terror after the sudden explosion, were witnessed by countless millions watching in a televised moment replayed continually throughout the day by the networks.

The other, more intimate, scene showed McAuliffe's mother shedding tears of joy as the shuttle soared into brilliant blue skies over Florida. Then came her tears of immeasurable sorrow.

Another scene was not shared by the general public but had unforgettable power.

This took place in the Senate, where intrusive television cameras are not yet permitted.

Shortly after 2 p.m., after the chaplain opened the brief prayer session, Majority Leader Robert J. Dole (R-Kan.) told fellow senators of the common sense of grief over the personal loss of lives and the collective shock at "the wrenching image of this nation's pride and joy disappearing over the Florida coast."

Then Sen. John Glenn (D-Ohio), the first American in orbit and a presidential candidate in the last election, stood quietly before his desk. He referred to the quarter century that encompassed the U.S. space program and said softly:

"Sometimes triumph is accompanied by tragedy. We hoped to push that day back forever. But it was not to be."

He paused, and his voice seemed to break. After expressing the hope that "the good Lord will grant a measure of comfort and understanding to the families and friends of the seven courageous crew members," he took his seat and stared intently down.

Not a whisper could be heard. The silence was as deep as it is ever likely to be there.

Moments later, the second member of this Senate to have flown in space, Jake Garn (R-Utah), stood.

He had not intended saying anything, he began, adding "it was too difficult to do so." His voice was breaking, too, as he paid tribute to Navy Cmdr. Michael J. Smith, whom he called "my mother hen." Smith, one of his space flight trainers, piloted the fatal flight.

"I simply am incapable of going beyond that," Garn said, "except to say thank you to my colleagues for the time they have given here to pay tribute to my friends." Then he took his seat amid more silence. Not long after, the Senate recessed for the day. Senators and members of the House had planned to assemble later at night for President Reagan's sixth State of the Union address, an occasion that Reagan uses each year to express his sense of buoyant optimism over the nation's future and to pay tribute to its newest heroes.

But this, too, was not to be. The speech was put off until next week. For most of the day the president, like most of the country, was reported to be stunned into silence at what he had witnessed in slow motion, stop-action, on split screen again and again and again over television.

At 5 p.m., he employed the same ubiquitous communications medium to address the nation. "We've never had a tragedy like this," he said.

Then he called the roll -- two women, five men, one of them black, another Oriental, of varying backgrounds and experiences -- who died in a moment that few among the millions who watched will forget. He called them America's newest heroes