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“When the first flight goes straight up, it’s hard to chase,” Mr. Carpenter said.

Retired NASA engineer Norm Perry, 77, said Shepard’s successful flight was “a big boost, I mean major,” given that the Soviets had scored two victories with the launch of the world’s first artificial satellite, Sputnik, in 1957, and Gagarin’s flight on April 12, 1961.

The Soviets, he said, “were willing to stick a man in a can and let it go,” while NASA was more cautious. Mr. Perry, who helped build and test the Mercury Redstone that carried Shepard on the suborbital flight, noted that Gagarin’s capsule was automatically controlled, while Shepard and the rest of the Mercury astronauts operated their spacecraft.

Shepard was the first one to pilot a spacecraft, and that’s what he did,” Mr. Perry said.

Shepard later became the fifth man to walk on the moon as commander of Apollo 14. It was barely three weeks after his Mercury shot that President John F. Kennedy announced America would launch a man to the moon and safely return him by decade’s end.

Charlie Duke, the 10th man to walk on the moon, remembered Shepard as “a great boss.” Shepard served for years as the chief of the astronaut corps during the 1960s.

“He was tough but fair,” Mr. Duke said.

In orbit, the two Americans aboard the International Space Station quietly marked the anniversary, along with their four crewmates.

“We’re honoring the occasion in a way, I think, Alan Shepard would really appreciate, by living and working in space,” astronaut Ronald Garan Jr. said. He noted the orbiting lab was “only a dream” 50 years ago.

It was the second day of anniversary festivities.

On Wednesday at the neighboring Kennedy Space Center, the U.S. Postal Service dedicated a first-class Forever stamp to Shepard’s Mercury shot — the first stamp honoring a specific astronaut. And on Saturday, a dozen or so astronauts, including Mr. Carpenter, will ride in open Corvettes in downtown Cocoa Beach, mimicking the parades of the Mercury era.