One of the Real Space Cowboys has died.

Walter "Wally" M. Schirra Jr. died Thursday morning of a heart attack in a California hospital at the age of 84.

Schirra, known to many by a self-selected nickname, Captain Skyray, was one of the pioneers of space travel and the only astronaut to fly in the Mercury, Gemini, and Apollo missions. He commanded the Gemini 6 mission, carried out the first rendezvous in space, and was portrayed in the movie The Right Stuff2007.

President George W. Bush issued a statement praising Schirra for paving the way for lunar exploration and saying he and his family members are mourning the "loss of an American hero."

NASA introduced Schirra and six others to the public as the world's first astronauts on April 19, 1959. During a press conference that day, and several times afterward, Schirra said he saw space travel as another step in exploration, which he believed was part of human nature and mankind's destiny.

It certainly seemed a natural progression for Schirra, who was born March 12, 1923, to a World War I fighter pilot and barnstormer father and a wing-walking mother in Hackensack, N.J.

Schirra had flown his father's biplane before being appointed to the U.S. Naval Academy in 1942. Soon, he received a bachelor's degree and an assignment on the armored battle cruiser Alaska.

In 1946, he married Josephine Fraser and was assigned to the 7th Fleet in the Pacific. He completed pilot's training at Pensacola Naval Air Station in Florida two years after that and became a naval aviator. He served as a fighter pilot and flew with the U.S. Air Force during the Korean War. He flew 90 combat missions as a jet fighter and served as a test pilot.

He was a member of the Society of Experimental Test Pilots, writing about a Mach 1 airplane called the Skyray when he was invited to the selection process for choosing the world's first voyagers to outer space.

At 36, he was the father of an 18-month-old daughter, Suzanne, and an 8-year-old boy, Walter M. Schirra III. He told reporters that some people he knew questioned his involvement in the new space exploration program, calling it "idiotic." He said he entered the screening process reluctantly and questioned his own intelligence before deciding that it was a professional program, deserving of confidence.

Other times, he joked about being assured that chimpanzees or monkeys would go first.

"I knew I wanted out of there then," he joked at a Naval Aviation symposium in 2002.

Schirra told reporters that he and the other astronauts had envisioned themselves pushing the limits of possibility, thanks in part to exposure to Jules Verne, Buck Rogers, and Flash Gordon. "We were interested in reading these things and obviously we had intentions of following something like this in our lifetimes," he told reporters. "I will readily admit that we didn't think of this, but in flying aircraft, we have been striving to get higher and higher. … As far as going higher, this is just one more step."

When asked about his religious faith, Schirra said that he was an active member of an Episcopal Church. "I think I should like to dwell more on faith in what we call the machine age," he added. "We have faith in the Space Age."

That faith shone through when he tried to name the Apollo 7 mission "The Phoenix," in memory of the crew that perished in the Apollo 1 fire.

After retiring from NASA, Schirra authored and co-authored books, including The Real Space Cowboys. He supported several civic organizations and corporate boards, joined newscaster Walter Cronkite to provide space commentary, and did commercials for Tang, the powdered orange drink astronauts sipped in space.

He also did commercials for Actifed after he and other astronauts popped decongestants to make sure their eardrums didn't burst as they returned from orbit while suffering from common colds.

One legendary story about Schirra's spaceflight comes from his response to someone from command and control asking him if he was a turtle. As a member of a fighter pilot's club, Schirra was obliged to answer "You bet your sweet a-- I am." Knowing his words were broadcast, Schirra switched a microphone off and answered properly before letting the world hear "Roger." Later, when President John F. Kennedy asked him the same question in person, Schirra said he paused before giving the true, longer version response.

Schirra received several military and civilian honors. He loved skiing, hunting, fishing, and yachting. In addition to occasional spaceship travel, the Mercury Seven member was known to travel by horseback from time to time.

Last month, he expressed fondness for his home planet, telling an Associated Press reporter that he had left three times and had found nowhere else to go. He spoke during interviews about the dust clouds he saw over India and China in the first decade of space travel and worried about pollution. He also talked about seeing the world as one home, rather than a globe divided by borders. One month before crossing his own final frontier, he said in an AP interview: "Please take care of Spaceship Earth."