Former NASA astronaut Thomas Stafford, a three-star Air Force general known for a historic handshake in space with a Soviet cosmonaut nearly 50 years ago, died Monday in Florida. He was 93.

Stafford was perhaps the most accomplished astronaut of his era who never walked on the Moon. He flew in space four times, helping pilot the first rendezvous with another crewed spacecraft in orbit in 1966 and taking NASA's Apollo lunar landing craft on a final test run before Neil Armstrong and Buzz Aldrin set foot on the Moon in 1969.

By his own account, one of the greatest moments in Stafford's career came in 1975, when he commanded the final Apollo mission—not to the Moon but to low-Earth orbit—and linked up with a Russian Soyuz spacecraft carrying two Soviet cosmonauts. The Apollo-Soyuz Test Project (ASTP) planted the seeds for a decades-long partnership in space between the United States and Russia, culminating in the International Space Station, where US and Russian crews still work together despite a collapse in relations back on Earth.

"Today, General Tom Stafford went to the eternal heavens, which he so courageously explored as a Gemini and Apollo astronaut as well as a peacemaker in the Apollo-Soyuz mission," said NASA Administrator Bill Nelson. "Those of us privileged to know him are very sad but grateful we knew a giant."

According to a report in The New York Times, Stafford's wife, Linda, said he had recently been diagnosed with liver cancer.

No false moves

Stafford was born in Weatherford, Oklahoma, on September 17, 1930. He was a child of the Dust Bowl and dreamed of becoming a pilot from the time he was in grade school. After graduating with honors from the US Naval Academy, Stafford switched services and joined the Air Force, where he trained as a fighter pilot.

He attended the Air Force's Experimental Test Pilot School at Edwards Air Force Base, California, then became a test pilot and instructor. He authored textbooks and flight manuals used by later classes of test pilots, some of whom also went on to become astronauts.

In 1962, NASA selected Stafford as one of nine test pilots for the agency's second class of astronauts. Within two years, NASA assigned Stafford to fly on the first flight of the Gemini spacecraft, the two-man capsule designed to demonstrate spacewalk techniques, rendezvous, and docking, key capabilities to future Apollo flights to the Moon.

But Stafford's commander on Gemini 3, Alan Shepard, was grounded for health reasons. NASA opted to swap them for the mission's backup crew, and Stafford had to wait to fly on Gemini 6 a few months later.

That mission, which was supposed to attempt the first docking between two spacecraft in orbit, didn't get off to an auspicious start. Commander Walter "Wally" Shirra and Stafford were supposed to guide their Gemini spacecraft to a linkup with an unpiloted target vehicle called an Agena, which would launch from Cape Canaveral, Florida, about 90 minutes before Gemini 6, which sat on a different launch pad a couple of miles away.

"We could hear it roar off the pad, and 90 minutes later, when it came across the Cape, we were going to go," Stafford recalled in a 2015 oral history interview with NASA. The launch failed, and the Agena target vehicle did not make it into orbit, so Gemini 6 was grounded.

Within a few weeks, NASA officials devised a new flight plan for Gemini 6. This new mission, called Gemini 6A, would rendezvous with the Gemini 7 spacecraft in December 1965, less than two months after their launch was scuttled due to the Agena failure. This wouldn't achieve the goal of physically docking two spacecraft in orbit, but it would get close, allowing NASA and its crewmen to demonstrate two capsules could fly in formation while orbiting the Earth.

This was an essential capability NASA had to prove in order to meet President John F. Kennedy's goal of landing astronauts on the Moon by the end of the 1960s.

"You got to remember, the whole thing, we’re building this big building at the Cape, with the giant Saturn V (rocket), the lunar module, command module, all based on doing a rendezvous around the Moon," Stafford said. "Nobody had ever done one. That was critical. Everything was based on rendezvous."