The new California Science Center is a commanding presence and unifying element at a neglected site.

The California Science Center--which opens Saturday--is a strange fusion of past and future. The massive geometric forms of its new entry plaza gleam like a mock lunar city. Turnthe corner and an elegant Beaux-Arts facade--the only detail restored from the original building--evokes a world of wire-rim glasses and bowler hats.

The 245,000-square-foot, $130-million museum is only the first phase in an ambitious plan that will extend 900 feet along Coliseum Drive. Two new wings will be added later to the science center to create a 600,000-square-foot complex that will include an aquarium and an extension of the existing aerospace halls. For those who regret the loss of the original Museum of Science and Industry, only the brick-and-terra-cotta facade of the original Ahmanson Building remains from the structure that was damaged during the Northridge earthquake.

Despite the building’s clumsy amalgam, this is a powerful work of architecture. Few buildings are able to truly engage their context. This one reshapes it. Designed by Portland, Ore.-based Zimmer Gunsul Frasca Partnership, the Science Center is also the centerpiece of a master plan for a $300-million rehabilitation of Exposition Park. And clearly the architects’ mission was to use the building to give form to that larger urban vision. They succeed on many levels: The Science Center is a generous public work. And the thoughtful organization of its various components gives sudden unity to a park that over time has been severely damaged by ill-conceived additions and general neglect. The building will do much to reassert Exposition Park’s civic presence.

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The Ahmanson Building--opened in 1913 as an exhibition hall--was one of three buildings that frame the 1920s-era sunken rose garden quadrangle that is still Exposition Park’s historic center. Yet the three have rarely functioned as a cohesive whole. The Museum of Natural History, for example, which opened that same year and whose domed facade is the most beautiful of the trio--boarded up its garden entrance during the 1930s, reorienting itself toward the Coliseum. During the 1970s, a second--more brutal--addition was added to the Natural History Museum’s north side, making that museum’s garden entrance virtually obsolete. In fact, although the original entry rotunda has since been restored, it still cannot be entered from the rose garden.

In 1988, when the decision was made to tear down the Museum of Science and Industry, it was the Los Angeles Conservancy that stepped in and convinced museum officials that to do so would destroy the historical character of the park. In the final compromise, only the garden-facing facade has been saved. The rest was demolished to make room for the new science center. (The third building--the former National Guard Armory--has been vacant since 1990. It is now being converted into a school.)

Until the addition of the new science center, Exposition Park had been vaguely divided into two competing parts: sports and culture. The construction of the Los Angeles Memorial Coliseum in 1923 was the first step in the creation of a sprawling sports complex that seemingly had little connection to the more highbrow tone of the old quadrangle. Nearby, a swim stadium was added in 1932; the Los Angeles Sports Arena was added in 1959. By then, any cohesion in the park had been destroyed. Part of the function of the California Science Center building will be to reunite these two zones.

The museum’s new entry faces south, toward the Coliseum. On one side, the cube-like form of a new Imax theater--clad in patterned orange tile--faces the entry to the exhibition galleries. Together these two elements frame the Robert H. Lorsh Family Pavilion, a large, circular plaza partially enclosed under a massive elevated rotunda, 100 feet in diameter and clad in perforated steel. The entire apparatus is perched on concrete columns and supports a long ramp that spirals up the rotunda’s interior and connects the theater to the rest of the museum. The rotunda’s center glitters with thousands of suspended gold and palladium leaf orbs--a playful rendition of the cosmos by artist Larry Kirkland.

The arrangement of these various architectural elements is powerful. The cylinder-shaped rotunda gives the plaza a monumental presence while allowing visitors to slip into it from both ends of Coliseum Drive. But the pavilion also has a functional role: It allows crowds to circulate up to the museum’s second story without crossing back down to the plaza. Visitors will enter the Imax theater from the plaza below and then exit along the rotunda’s internal ramp, enabling them to travel up to the museum’s second floor. From there, they can work their way back down in the opposite direction, though the exhibits. It is a playful urban promenade in miniature. What’s more, it serves to draw the experience of the museum out into the public realm.

That sensitivity to the building’s urban role continues inside. The museum’s Science Court--a towering, wedge-shaped glass atrium lobby--splits the museum in two. The lobby aggressively connects the building’s new southern entry to the rose garden on the north side, both physically and visually. In doing so, it does much to ensure that both become lively urban spaces. Above, exhibition spaces are joined across the lobby via a bridge. From here, visitors can climb onto a high-wire bicycle and ride back and forth over the lobby 43 feet below for a small fee. Within that context, the exhibition spaces themselves become a series of simple black boxes that can be adapted to their various functions with ease.

“World of Life,” for example, becomes a labyrinthine tour through the human body. “Creative World”--a technologically based exhibit more geared to baby architects--displays its own structure more boldly.

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Nonetheless, there are missed opportunities here. Like the exposition pavilions of an earlier era, a science museum could be an ideal place to show off architecture’s newest structural feats. Sir Joseph Paxton’s Crystal Palace--built for the London “Great Exhibition” of 1851--famously outshone any of the exhibits inside it. Zimmer Gunsul Frasca’s design here could have gone further in testing the boundaries of building technology. The building itself could have become a didactic toy, a diagram of the forces of nature, of gravity, of suspension. One could easily imagine a more sinuous, articulated structure supporting the towering atrium space, for example, much like the early works of Spanish architect Santiago Calatrava, which are essentially diagrams of the human muscular system cast in concrete. Or the dramatically exposed mechanical systems of the Pompidou Center in Paris.

Some such efforts were made. Some ducts are exposed; an escalator is encased in plexiglass so that its machinery is visible. But they are easy to overlook; they seem decorative, never wholly integrated into the architecture. Inside the museum, architect and design connoisseurs will also be upset by the administration’s decision to discard Charles and Ray Eames’ 1960 “Mathematica” exhibit, a series of ingenious learning tools on celestial mechanics, the laws of probability and the like. Projects such as that one, by two of the city’s artistic icons, are an important part of Southern California’s cultural legacy and ought to be on display here. Dated or not, such an exhibit embodied what a science museum should be: a place for play as well as discovery.

Even so, there is much to enjoy. Wandering around the building’s exhibits, one is inevitably enveloped in a time when science and technology seemed capable of transforming the world for the better. Giant lunar landing craft suspended from the ceiling. Kids gazing up at the cosmos, longing to be astronauts. These are nostalgic images. Yet they also evoke a more hopeful picture of science and its cultural role. Here, that hopefulness cheerfully spills out into the world around it.

* California Science Center, 700 State Drive, Exposition Park. Daily, 10 a.m. to 5 p.m. Admission is free, except for films screening at the Imax theater, $3.75 to $7.25. Parking, $5. (213) 744-7400.

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More on the New California Science Center

Today in Life & Style: Meet Tess, a 50-foot mostly anatomically correct woman whose transparent body helps demonstrate how the organs keep the body in balance.

Today in Health: In the center’s World of Life section, biological exhibits are designed to educate, enthrall, challenge and amuse individuals of all ages.

Thursday in Calendar Weekend: Lessons learned when students take a sneak peek at the exhibits. Also: Los Angeles finally gets its own 3-D Imax theater.