Owen K. Garriott, one of the original scientists selected to explore the cosmos firsthand and the first astronaut to operate an interstellar ham radio station, died on Monday at his home in Huntsville, Ala. He was 88.

His death was confirmed by his son Richard.

Dr. Garriott flew into space twice. In 1973 he was the science pilot of Skylab 3, the record-breaking 59-day mission — more than double the duration of any previous flight — to Skylab, the first United States space station.

He logged nearly 14 hours outside Skylab in three spacewalks, during which physiological and biomedical metrics were monitored to determine the body’s response to long periods spent in reduced gravity.

“We learned the importance of exercise,” Dr. Garriott said. “If you have the appropriate amount of exercise, namely one to two hours a day, then you’re going to come back in essentially as good a condition as when you left.”