Rocket engineer Oleg Ivanovsky, right, accompanies Soviet cosmonaut Yuri Gagarin to the Vostok spacecraft before the launch of the first manned space flight on April 12, 1961. (NPO Lavochkin Museum/AP)

Oleg Ivanovsky, a Soviet rocket scientist who played a central role in developing satellites at the dawn of the space age, including the first vehicle to carry a human being in orbit around the Earth, died Sept. 18. He was 92.

His death was announced by Roscosmos, the Russian space agency. The cause and location were not reported.

Mr. Ivanovsky worked for many years as a top engineer at the secret Soviet space facility known as Star City, where he helped design Sputnik, which was launched on Oct. 4, 1957. The unmanned satellite, just 23 inches in diameter, circled the globe for three months and prompted alarm in the United States that the Soviets had taken the lead in engineering, rocketry and the Cold War in general.

A month after the first Sputnik launch, the Soviets sent Sputnik 2 into space, this time with a dog on board. The dog, named Laika, died after a few hours in orbit, apparently from heat exhaustion, “but she gave much to biology,” Mr. Ivanovsky said later.

“We didn’t know if an animal could survive for longer than a few minutes in weightlessness,” he said. “But from the data from Sputnik 2, we could see that she moved, and even ate, after the launch.”

Encouraged that a mammal could survive in space, at least for a short time, Mr. Ivanovsky took a leading role in building a capsule that could carry a Soviet cosmonaut into orbit. A 27-year-old pilot, Yuri Gagarin, was chosen to fly the spacecraft, called Vostok 1.

In 1960, an explosion at the Soviet launch pad in Kazakhstan killed 126 people, and there were other technical setbacks along the way. Mr. Ivanovsky and other engineers estimated the chances of a successful manned flight at no more than 50-50.

Gagarin wrote a farewell letter to his wife in case he would not return from his mission, but he blithely sang a folk song as he climbed into the cockpit on April 12, 1961. His heart rate stayed at a steady 64 beats per minute while he awaited liftoff.

Mr. Ivanovsky accompanied Gagarin to the cockpit.

“There were all kinds of wild fears that a man could lose his mind in zero gravity, lose his ability to make rational decisions,” Mr. Ivanovsky told the Associated Press in 2011.

Concerned that an agitated or deranged cosmonaut might try to fly the Vostok manually, Soviet engineers built in a system requiring Gagarin to enter a three-digit security code before he could assume the controls. The code was inside a sealed envelope, to be opened only in an emergency.

As they walked to the space capsule, Mr. Ivanovsky whispered the code in Gagarin’s ear: 1-2-5. Gagarin smiled and said his flight instructor already had told him the numbers.

Rocket engineer Oleg Ivanovsky, right, accompanies Soviet cosmonaut Yuri Gagarin to the Vostok spacecraft before the launch of the first manned space flight on April 12, 1961. (NPO Lavochkin Museum/AP)

Mr. Ivanovsky helped Gagarin onto the ladder and into the cockpit of Vostok I, patted him on the helmet and secured the hatch to the capsule. But a light indicating that the hatch was properly closed failed to go on.

In the midst of the countdown, Mr. Ivanovsky and two assistants hurried to replace 32 bolts by hand to secure the hatch.

“You should have seen yourself while you were working on the hatch,” an unperturbed Gagarin later told Mr. Ivanovsky. “Your face had all the colors of tarnished metal.”

Gagarin orbited once around the Earth, in a flight that lasted 108 minutes.

Less than a month later, Alan Shepard became the first American in space during a 15-minute suborbital flight. It wasn’t until Feb. 20, 1962, when astronaut John Glenn orbited Earth three times, that the United States began to catch up with the Soviets in the space race.

Oleg Genrikhovich Ivanovsky was born Jan. 18, 1922, in Moscow. Little is known about his early years except that he was drafted into the Soviet military in 1940 and was wounded during World War II.

He began working for a military research center in 1947 and was part of a unit led by Sergei Korolev, an engineer considered the father of the Soviet space program. Mr. Ivanovsky graduated from the Moscow Power Engineering Institute in 1953 and, in 1957, became a chief designer of space vehicles at Star City, outside Moscow.

He participated in unmanned Soviet space launches that examined the moon, Mars and Venus before joining a Russian aerospace firm as a chief engineer in 1965. He retired in 1983. No information about his survivors was available.

Gagarin, who died in a plane crash in 1968 while on a routine military training flight, was hailed as a hero throughout the Soviet Union soon after his historic trip into space. But his arrival back on Soviet soil was hardly auspicious.

Wearing an orange space suit, he leaped out of the spacecraft, as planned, and parachuted into a remote part of rural Russia.

An elderly woman and her granddaughter who were nearby started to flee, fearing that a U.S. spy had landed in the Soviet hinterlands.

“Hey, where are you going?” Gagarin shouted. “I’m one of us!”