What's a Nyx? Philadelphia Orchestra audiences find out.

Conductor Esa-Pekka Salonen's once-every-few-years forays into composition consistently leave me dazzled. "This one is the best," is my frequent reaction to each successive piece, and rarely more so than in the 2011 Nyx, which was given a particularly attractive French accent by the Philadelphia Orchestra under principal guest conductor Stéphane Denève.

Maximum use of the orchestra plus density of content are Salonen's hallmarks, especially here. But I particularly love the way the piece answers to no one. Salonen's combination of precision and self-possession suggests he writes exactly what he wants, with no over-the-shoulder glance at the career implications or allegiance to any school of composers. Though this is not music that ingratiates, few current composers are more innately charming.

Inspired by Nyx, the Greek goddess of the night, with all the open-ended personality that implies, the 20-minute symphonic poem is punctuated by arresting horn flourishes and has clarinet solos that lure you further into the music, pose riddles, and throw open the door to catastrophe.

Strings take off on a mad sprint, with detached, ominous bass lines lying in wait below. Themes don't morph so much as they have radical changes of costume. The ending disappoints a bit; I want something more conclusive. But I could swear Salonen had the exotic journey of Rimsky-Korsakov's Scheherazade in mind when he wrote it. 

The ultrafamiliar Grieg Piano Concerto could've been outclassed were it not for pianist Lars Vogt searching for his own personal way into the piece, and in doing so, suggesting that this medium-weight piece is underestimated. Heroic sonorities weren't quite where you'd expect in the first movement, while soft playing ambushed you by arising in also-unexpected places. Discreetly elastic tempos gave fresh shapes to musical gestures you've heard hundreds of times. Passages normally played like an ornamental trill felt like a brief confession. 

The Sibelius Symphony No. 2 was an interpretation in progress. Many compelling moments were apparent, such as the "fugato" passages of the final movement, but other touches left me puzzled, such as the length of the rhetorical pauses in the second movement that sapped momentum and left the music in a less-than-cogent state. I'm sure Denève had his reasons for doing what he did -- he always does -- but they weren't apparent to these ears.

The program is repeated at 8 p.m. Saturday at the Kimmel Center. Information: 215-893-1999 or www.philorch.org