Chicago's lakefront has a new landmark: a dark semicircle of steel and glass that seems to have descended from the clouds like a flying saucer. The $30 million Sky Pavilion of the Adler Planetarium and Astronomy Museum, which opened Saturday, is the most daring building in years along a shoreline dotted by gleaming white museums based on the temples of antiquity.

What is truly risky about the Sky Pavilion, however, is that it is an addition to one of Chicago's most underappreciated architectural gems - the original Adler, an Art Moderne masterpiece topped by a picturesque, shingled dome.

After the museum proposed an expansion a few years ago, some Chicago architects dubbed the chosen design, a C-shaped glass pavilion by Chicago's Lohan Associates, "the bra" for the way it cupped around the Adler's voluptuous dome. The only responsible course, these critics argued, was to build the addition underground.

Yet now that the Sky Pavilion has been completed, the fears that it would mar the old Adler have turned out to be groundless. The pavilion, which houses a new planetarium theater, exhibit spaces and a 200-seat restaurant with extraordinary skyline views, walks a nervy high-wire act, deftly taking its cues from its landmark predecessor without mimicking it.

In other words, the pavilion is not an instant replay of another spaceshiplike building, Helmut Jahn's 13-year-old James R. Thompson Center, which still seems like a brash intruder butting up against the classical dignity of Chicago's City Hall. This is futuristic architecture of a different sort: Yes, it makes a strong statement. No, it is not an alien that you wish would phone home - and go there too.

The addition also demonstrates a broader lesson: The present doesn't have to parrot the past to respect it. In fact, imitation may not be the greatest sign of flattery. It's far more adventurous -- and fulfilling -- when the architecture of one era plays off against another, as I.M. Pei did in his jewel-like glass pyramid at the Louvre.

True, bad things can happen to good buildings through such an approach; parts of the exterior of the Sky Pavilion, for example, are architecturally undistinguished and fail to honor the exquisite quality of the original Adler.

Yet overall, the pavilion speaks to the virtue of design dialogue. It is both a sensitive expansion and a spectacular addition to the lakefront -- every bit as much an expression of its era as its distinguished predecessor.

Shaped by Dirk Lohan and Al Novickas of Lohan Associates, the Sky Pavilion is markedly different from the Oceanarium addition to the neighboring John G. Shedd Aquarium that the same firm completed in 1991. Though the Oceanarium has a glassy lakeside facade, the white marble walls that flank it strive to be virtually indistinguishable from the original Shedd, which was completed in 1929.

To put things in musical terms, the aim at the Shedd is harmony; at the Adler, it's counterpoint.

But to anyone familiar with the history of the Adler, whose architect was Ernest Grunsfeld Jr., the decision to break decisively with the past was very much in keeping with the old building.

Instead of looking backward to Greek and Roman precedents, as did the architects of lakefront edifices such as the Field Museum, Grunsfeld sculpted an object without precedent: a 12-sided structure of straight, simple lines, clad in a dark, richly textured, granitelike stone.

Adorned at the corners by sculptor Alfonso Iannelli's bronze plaques picturing the constellations, the Adler was topped by a dome that perfectly punctuated the end of a land bridge reaching into Lake Michigan, now called Solidarity Drive.

Over the years, however, insensitive alterations took some of the luster off this diminutive jewel.

A 1972 underground addition, by C.F. Murphy Associates, and a 1980 glass pavilion just to the west of the Adler, by Hammond Beeby and Babka, had the combined effect of creating a bizarre entry sequence.

You did not enter the Adler, as Grunsfeld had intended, by ascending its magnificent, granitelike steps. You reached this shrine to the heavens by descending the stairs (or the elevator) of the glass pavilion and tunneling through a series of dark, dreary underground exhibition halls.

That literally was just the beginning of the Adler's problems.

Exhibition areas were far apart, making it a chore to move between them. And though the planetarium sat along the lake, the building gave visitors little opportunity to look out to the water -- or to the very skies that are the subject of its exhibits.

Meanwhile, the addition of the Oceanarium at the Shedd and blockbuster exhibitions at the already-giant Field made the Adler, way out there at the end of Solidarity Drive, seem like the little museum that couldn't.

Clearly, the addition attempts to change that. Pure and simple, it's a statement building, the kind meant to draw crowds and make contributors open their wallets -- not an easy thing to do with invisible, underground architecture.

Yet the Sky Pavilion is about more than look-at-me imagery. As with Pei's pyramid, the sexy part--the steel and glass pavilion--is merely the tip of the iceberg.

The core of the job is a planning exercise that will radically reshape the Adler when a revamp of the old building is complete in October 1999.

As part of that renovation, the 1980 glass pavilion will be demolished and the building again entered via its steps (or by ground-level doorways that will accommodate school groups and disabled).

Even now the benefits of this reshaping are apparent.

Much of the heretofore underground exhibition space has been moved to the bright and cheery Sky Pavilion, and there are other galleries, easily reachable, directly beneath it. To make room for the pavilion, upper-level offices that once hogged lakefront views have been put downstairs, though their inhabitants have been compensated with skylights.

Space for the pavilion also was made by removing a circular road that used to ring the planetarium; a side benefit is new parkland and a promenade on the building's lake side.

The below-ground exhibition space in the pavilion has a traditional post-and-beam structure and houses "black box" venues such as the planetarium theater.

In contrast, the above-ground portion has an utterly unconventional structure -- an off-kilter "A"-frame that provides a sweeping volume of column-free space as well as stunning vistas of the lake and the skyline.

At once serene and full of motion, the pavilion is one of the finest meldings of space and structure in Chicago since Jahn's masterful United Terminal at O'Hare International Airport was completed in 1988.

What truly makes it of our time--beyond the nervously tilting columns at the edge of the pavilion--is the fact that it would have been impossible to design without a computer that could calculate its innumerable complex angles.

In the pre-computer era, such an unconventional structure would likely have been "over-engineered," with massive and clunky-looking columns and beams, to ensure that it didn't collapse. But with computers, the structural members don't have to do any more heavy lifting than is absolutely necessary. So they can be light and lacy--and, thus, open to sunlight and views.

All these structural gymnastics would have been meaningless, however, if the form that resulted from such attention to function destroyed the jewel-like quality of the original Adler.

For the most part, however, the reverse is true: The new pavilion handsomely complements the old planetarium. For example, the C-shape of the pavilion subtly reinforces the symmetry of the original, creating the equivalent of bookends that set off the domed front.

A more explicit link is made with the reuse of the old planetarium's swirling, granitelike stone to clad the base of the new building. Even here, however, there's a subtle interchange going on between past and present, because the new stone is set horizontally rather than vertically, as in the original.

Seen from afar on Lake Shore Drive, the pavilion strikes up a powerful conversation, not merely echoing the curved profile of the dome, but playing off its stable, symmetrical shape with a dynamic, raking silhouette. From the new lakefront promenade, the pavilion is an extraordinary sight, seeming to hover above a new berm that weds the building to the park around it.

Yet the view is far less pleasing from other vantage points.

As early models made by the architects show, the glass of the pavilion was supposed to be a neutral membrane that revealed the structure supporting it -- transparent in contrast to the opaqueness of the original building.

However, the architects say, exhibition designers wanted the glass to be dark so the exhibits and computers would not be subject to blinding daylight. The museum went along, presumably because it also wanted to keep the pavilion from turning into a super-hot greenhouse with out-of-control air-conditioning bills.

Visually, the result is disappointing, though not disastrous. The structure is concealed rather than revealed, and the end walls of the pavilion are dark and glowering, as though this were Darth Vader's flying saucer.

Worse--and this cannot be blamed on the exhibition designers--those walls have banal detailing, especially in contrast to the exquisite stonework and crystal doors of the original building. They look like a typical suburban office building, hardly what is called for when adding to a building of such distinction.

What saves the pavilion, however, is the intelligence of its plan, the integrity of its structure and the art that infuses its architecture, albeit not in every detail. One of the best moments is a skylit corridor in the pavilion that nudges up alongside the old building. It allows the visitor a previously unobtainable closeup view of Iannelli's masterful bronze plaques of the constellations.

But the relationship between old and new is truly realized in the way the radiating structure of the pavilion, expressed on the exterior by sleek aluminum fins, seems to emanate from the dome of the original like the rays of the sun. Here, the Adler becomes a whole that is more than the sum of its parts, not two buildings merely set alongside each other.

In contrast to the wave of architectural conservatism that came in the wake of Jahn's glitzy Thompson Center, the Sky Pavilion is futuristic architecture that gives one faith in the future. Through both its exhibits and its architecture, it reminds us of the virtue of reaching for the stars. What better way to usher in the millennium?