Right off, Peter Strauss looks like Kennedy material. There is the handsome, athletic body (encased in a dark blue suit, tasteful pin-stripe shirt and faultlessly-knotted tie), the jabbing, nervous gestures, the quick boyish grins, together with an occasional gray-green stare fired right down your gullet. The background isn't bad either, what with a summer spent in Europe as a kid, and four preppy years at some place called the Hackley School for Boys. But any similarity ends there, insists 30-year-old Strauss, whose acting star seems neatly poised for blast-off into tthe stratosphere. After all, he only has to play legends, not be them.

Americans, or probably any other people for that matter, just can't resist the idea of myth. As modern myths go, the Kennedys do nicely. They don't gleam like the American cowboy, our greatest legend, but then no one does - not James Dean or Marilyn Monroe or even Elvis. Still there is much myth grist in the Kennedy clan - from Wall Street-dealing old Joe to long-suffering Rose to womanizing Jack to bantam Bobby to magnificently flawed Teddy.

But the Kennedy we probably know least about, the one who got away, is Joseph P. Kennedy Jr., the eldest son and chief servitor of the family dream until he died during World War II over the English Channel when his dynamite-laden PB4Y Liberator blew up just short of the Belgian submarine pens. In a way, Joe Jr. is the most resonant Kennedy myth of all. Strauss add two

On Sept. 18, at 9 p.m., ABC will explore the life of the eldest Kennedy brother in a two-hour, made-for-TV movie entitled "Young Joe, the Forgotten Kennedy." The film, to be telecast as the season-premiere feature on the network's ABC Sunday Night Movie," stars Peter Strauss as the doomed aviator whose driving father wanted him (and only secondly Jack) to be America's first Catholic president. Strauss, heretofore known almost exclusively for his portrayal of Rudy jordache in ABC's "Rich Man, Poor Man" serial, says playing the man who might have been king is the most challenging thing he's ever tried - in or out of acting.

"You see," Strauss said yesterday in an anteroom at WJLA-TV, "we have almost nothing on the man. The world has this hazy image of some boisterous, athletic, good-looking charismatic 29-year-old rich kid dying hauntinly over the English Channel. Strictly one-dimensional. I knew there had to be something more."

The trouble was finding it. All Strauss had to hang his creation of character on, in addition to Charles Cohen's script, he says, were one biography ("The Lost Prince" by Hank Searls), two faded newsreel clips totaling maybe two minutes (one showed Joe Jr. arriving in England with brother Jack and his ambassador father, the other had him arriving at a flying field outside Boston), plus one 30-second clip of his voice (putting a name in nomination at the 1936 National Democratic Convention). That was all.

"I would sit for hours in a screening room studying those clips and that tape," says Strauss, tamping a Marlboro, leaning in close. Suddenly he is smacking his fist. "But I couldn't find the key to the guy, you see. Then out of nowhere I begin noticing the smallest things - like the way his smile seemed just a microsecond too quick, the way he kept moving his hands in and out of his pockets, the way he seemed always to be wanting to look behind him.

"That's when I began to imagine someone who was possibly more lonely - and therefore more interesting - inside than the world ever knew. I mean, here is a man who looks strangely older than his 29 years; who has had one year at Harvard Law School; who has met Churchill and Stalin and the Pope; who has seen Mussolini talk; who has this enormous private reputation as a womanizer - chorus girls, film stars - but who nonetheless seems very lonely, even scared. Someone whose emotional development is not quite at a level with his intelectual development. In other words, somewhere inside this man, there was still very much a boy. A boy who was at war . . . with himself."STStrauss has tacked this last on; he knews it is strong stuff. Will the Kennedy family be offended? "That's too strong a word. I don't know, maybe piqued. I would only hope they recognize that we tried to make the film with a great deal of love for the man - and the myth."