Space workers and tourists stood silently Saturday for 73 seconds, the length of the fatal Challenger flight, to remember the seven astronauts who died in the fiery shuttle explosion three years ago.

About 350 peace activists marked the anniversary with a rally outside a Kennedy Space Center gate to protest the launch of Defense Department payloads aboard space shuttles. Four women were arrested for trespassing when they deliberately walked into an off-limits area.Tour buses stopped in their tracks, and engineers, technicians and support crews halted activity as flags at the Kennedy Space Center were lowered to half-staff at 11:38 a.m., the moment Challenger lifted off on Jan. 28, 1986.

"It's an appropriate way to mark the occasion, leaving it to the individuals on how they want to remember it," said Forrest S. McCartney, the center director.

Only about 1,000 of the center's 17,000 employees were at work Saturday, but hundreds of tourists joined them in the tribute.

Meanwhile, a Challenger memorial was dedicated Saturday in downtown Miami.

"It wasn't until we lost Challenger that the American public realized space travel is dangerous," said Rep. Bill Nelson, speaking at the Challenger Seven dedication at Bayfront Park. Nelson, a Florida Democrat, rode on a shuttle mission just two weeks before the Challenger disaster.

The 105-foot white sculpture, shaped like a twisting girder, was designed by Japanese-American artist Isamu Noguchi, who died in December.

Killed in the Challenger tragedy were Dick Scobee, Michael Smith, Judy Resnik, Ellison Onizuka, Ron McNair, Greg Jarvis and schoolteacher Christa McAuliffe.

McNair's brother, Eric McNair of Atlanta, said at a ceremony Saturday in nearby Cocoa Beach that it is important to remember the Challenger explosion "so we will not forget what happened, (thus) lessening the chances of it happening again."

In Concord, N.H., McAuliffe's hometown, Mayor Elizabeth Hager said no special events were made to mark the anniversary.

"I don't think we need to continue to call attention to it," she said. "We were low-key about it just after it happened and I think we can continue to be low-key about it. Clearly we don't have to do anything to remind them (the McAuliffes) of it."

Christa McAuliffe's husband, lawyer Steve McAuliffe, still lives in Concord with his two children.

The accident was blamed on a faulty booster rocket joint that allowed flame to escape and touch off the shuttle's external fuel tank.

The three remaining shuttles were grounded for 32 months while modifications were made. Discovery made the first post-Challenger flight last September and was followed by Atlantis in December.

Discovery is being readied for another flight in late February, although engine turbopump problems could delay that mission.