A strange visitor from another planet came to the Smithsonian yesterday, pushing 50, but still leaping buildings in a single bound and fighting a never-ending battle to keep adoring women at a safe distance.

Yes. It was Superman. The guy with powers far beyond those of mortal men, disguised nowadays as Christopher Reeve, mild-mannered actor on a great many metropolitan movie screens.

Reeve, however, doesn't look 50 years old. Then again, neither does Superman, and it's Superman's 50th birthday next July that's the cause for the Smithsonian's yearlong celebration, which officially begins today at the National Museum of American History.

Reeve, changing his clothes in the Madison Hotel instead of a phone booth before going to the museum, was here to honor the Man of Steel at a preview party last night. The latest in a series of Supermen, Reeve has seen the character change over the years, and has given Superman feelings and emotions within those of mortal men.

"Superman is a gentleman," said the star. "I try to downplay the might and bring out the heart, caring and reticence about taking action unless it's really necessary.

"Superman's message is that he's looking down at the world from outer space, and it's like, 'This is a big beautiful marble, and we'd better polish it and take care of it.' "

The Smithsonian has gone to the trouble of finding some of Krypton's lost marbles for its show "Superman: Many Lives, Many Worlds," and despite the sleepy title, it's a super trip down memory lane.

Via comic books, radio, TV and the movies, Superman became a part of American culture (it would be hard to imagine a French Superhomme, a Spanish Superhombre or a British Superoldchap). By that right alone, he belongs in the Smithsonian.

The exhibit covers everything from Superman's comic-book origins to the soon-to-be-released "Superman IV: The Quest for Peace." Among the highlights are a costume from the 1950s television series, Superman's debut in the first issue of Action Comics and clips from the TV show, movies and animated cartoons.

Be forewarned: There are also lowlights, among them a copy of the 1978 "Superman: The Movie" script, a box of Kellogg's Pep cereal with a cartoon on the back, and a "then and now" look at Jimmy Olsen's bow-tie fashions.

"I just happened to keep one of my bow ties," said Jack Larson, the Jimmy of the TV show, also here to help open the exhibit. "It's great to think of my bow tie with Judy Garland's ruby slippers."

And then there are the toys, which, doggone it, are out of reach behind the museum glass. Imagine: a Superman turnover tank (it's a metal windup toy with an attached Superman who, apparently, when wound up, flips the tank over), a Superman battery-operated toothbrush, a Superman wrist watch, a Superman belt, a Superman plate, a Superman puzzle and Superman peanut butter (probably tastes super). And a pair of Clark Kent's glasses, which would enable you to go around incognito, too, if you could only put them on. All these bits of icon arcana have made their way to the hallowed halls of the Smithsonian. "We like to present American history unedited," said exhibit curator Carl Scheele.

The exhibit entrance features a continuous running collection of clips from the TV series, animated cartoons and movies, some of which are more hilarious than others. For example, George Reeves walking around a sound stage with that country-boy-in-the-big-city look on his face in the TV series' first episode can inspire fits of laughter.

Despite his 50-year popularity, Superman's paraphernalia is hard to come by, said Smithsonian historian Ellen Roney Hughes. Most of the objects on display at the exhibit were donated by DC Comics, but, for example, "few tangible objects from the old TV serial can be found," Hughes said. "Many items were routinely discarded or altered or reused on other shows."

But the objects that are on display evoke memories long lost of the Man of Steel, and how he, and we, have changed over the years.

Superman's persona is different from when he was created by two Cleveland teen-agers -- Jerry Siegel and Joe Shuster -- in 1938, and the exhibit reflects the metamorphosis, from action hero ("You've tried to destroy me for the last time, Luthor!") to public service do-gooder ("It's Super Smart to be Thrifty. Buy United States Savings Stamps," a poster blares), to matinee idol ("I'm terribly sorry, Lois, but an emergency has arisen").

What does Superman think of today's Superman? "Basically, Superman is a friend," Reeve said.

He's been a good friend to Reeve, or to Reeve's bank account, but the actor (who in real life really seems to be mild mannered, or at least well behaved) said money is not the cause of his rooting out evil.

"If there's a creative reason to do a 'Superman V' I'll do it," he said. "But I never consider doing a film for my bank account."

And one of the reasons Reeve enjoys the fact that the Superman movies do so well at the box office is that the hero has a lesson to teach.

"There's an audience out there that doesn't want to see another one-man vigilante force," he said. "And the message we're sneaking in is that the whole world is like Smallville."

That would be Superman's boyhood home -- but you knew that.