San Francisco — Twenty-seven years ago, Franklin Chang-Diaz spent his evenings in a mango tree searching the sky for Sputnik. The Russian satellite never appeared - at least not over his backyard treehouse in San Juan de los Morros, Venezuela.
But it fueled his imagination, and by day he ''flew'' his own space capsule. Simulating a liftoff, he'd lie on a chair flipped on its back inside a box mounted with old radios and electronic scraps.
Today, the National Aeronautics and Space Administration's first Hispanic astronaut laughs and says, ''I'm still doing that kind of simulation.''
Millions of people may admit that when children, they wanted to be astronauts - heroes with wholesome smiles and nerves of steel. But when Dr. Chang-Diaz says , ''I always wanted to be an astronaut,'' you know he's not reading from a NASA press release.
Dr. Chang-Diaz, who was here recently to accept an award from the National Hispanic University (NHU), obviously had the ambition to fulfill his childhood dream.
The son of a Costa Rican ''adventurer'' with a sixth-grade education, Chang-Diaz came to the United States in 1968 after graduating from a Costa Rican high school. He'd written in Spanish to Werner von Braun, father of the US space program, asking how to become an astronaut. A form letter in return said to study science - but study it in the US. So with a one-way plane ticket, $50, and no English skills he came to the US and enrolled in high school - again - in Hartford, Conn.
Within three months he was able to score high enough on English-language college entrance exams to win a scholarship to the University of Connecticut. Subsequently he earned a PhD in nuclear physics from the Massachusetts Institute of Technology. Though turned down the first time he applied to be an astronaut, he was accepted as an astronaut candidate in 1980 and chosen as mission specialist on future space shuttle flight crews.
Hero worship hasn't been heaped on astronauts since the early days of the space program. ''John Glenn orbited the earth three times. Jack Lousma was in space for three months, but nobody knows him,'' observes Chang-Diaz.
But Dr. Chang-Diaz, looking almost half his 33 years with his dark bangs and easy smile, moves through a crowd of Hispanics and revives an old-fashioned and unself-conscious kind of admiration. At this week's NHU convention, where he received the Don Quixote award, there was an endless succession of important Hispanic leaders to shake hands with. He had to be shepherded through mobs of adults and children - seeking autographs, snapshots, or simply a glimpse.
''I have a problem with all this . . . ,'' he said, referring to all the public attention. ''Well, not a problem,'' he added, wanting to be sure his modesty wasn't mistaken for ingratitude. Though his training is based in Houston , he voluntarily makes appearances around the country.
While thoroughly proud of his Hispanic heritage, and fully aware that he is a role model for minority youth, his goal wasn't to be the first Hispanic to fly in the US space program. Franklin Chang-Diaz wanted to be an astronaut, pure and simple.
''It's an honor to be a representative of a group increasingly becoming powerful in the US,'' says the naturalized American citizen. ''But I hope we'll grow out into society and not have to pay attention to race or color. In the meantime, I know it's important to have role models.''
''The reality that he's a Hispanic astronaut kind of makes you feel important . . . like if he can do it, anyone can do it,'' says Juan Leanos, a high school junior who wants to pursue an engineering career.
Marco Antonio Cruz, a 10-year-old from Richmond, Calif., agrees that Chang-Diaz is a role model. ''He makes me anxious to go (into space).''
Indeed, Chang-Diaz himself had a strong role model in his father, who he describes as an adventurer who traveled to Venezuela with his wife and children - losing another son to the rugged jungle conditions where he worked building new roads.
''Neither of my parents was very well educated, but they were enterprising. (His mother, at 54, is now finishing her high school degree and has plans to go to college.) My father was the kind that could easily become an idol, the kind you would say today was a typical Latin macho. He was so full of self-confidence that he could do anything he wanted. I can remember him driving his Willys Jeep with no top, his high boots and khaki shirt and pants and safari hat. He was a real man and I always wanted to be like him. He gave me that sense of adventure.
''But my picture of adventure,'' Chang-Diaz says in perfect English tinged with a soft Spanish accent, ''is more complicated. There can be a lot of intellectual adventure - like doing research in an area no one knows about. You might find something no one else has found.''
That kind of adventure for him takes place in the field of nuclear fusion. And, says Chang-Diaz, this is an area where he thinks he is a positive role model, where he can say with authority, ''que si se puede'' - it can be done.
''It is important for young children to have strong foundations in sciences and math. In the home the kind of structure the parents can provide is the most important thing,'' he says, explaining that though his mother was not well educated, she had an ''inquisitiveness'' and ''a touch of fantasy that made me become interested in space.''
''My parents were very instrumental in having me pursue a career in high technology . . . and I always knew astronauts would have to be more than just pilots (but also scientists),'' says Chang-Diaz, who expects to ride the shuttle into space sometime in 1985.
His long years of work up the educational - and economic - ladder have given him a unique perspective on minority problems. ''I haven't seen any discrimination (against himself) lately. When I was in high school, there were some teachers who just didn't understand. It wasn't really discrimination.
''There's some discrimination still around, but really the more education you have the harder it is for you to be discriminated against. And in high-tech I've noticed that if you can produce it doesn't matter what color you are.''
This perspective lends an interesting twist to his political view, which he offered to one Hispanic conventioneer complaining about loss of funds for financial aid for minority students.
''I realize federal funding for these programs is going down. But the general trend - no matter who's in office - is that there's always been a great deal of opportunity in this country independent of whether Carter is in power or Reagan. It's often hard for people living inside to perceive this, but it's still the land of opportunity,'' he says.
He has not forgotten his roots amid his prosperity - though even that is relative, he adds, noting that PhD astronauts can expect to make $30,000 to $40, 000.
''Sometimes people ask me, 'Why do we have to go up in space?' And I feel we've been there all along on our own spaceship Earth,'' he says, adding that space technology - communications satellites and the like - is very likely to help third world countries progress.