Consider Last Autumn, the new evening-length work he unveils at 8 p.m. Saturday at St. Mark's Church: It's more than two hours long, has 41 movements, and is written for the unconventionally stark instrumentation of French horn and cello.

Though Hersch has been presented by Network for New Music in the past, he's on his own this time, renting the Center City church for rehearsals and performances. The players are a longtime collaborator, cellist Daniel Gaisford, and the composer's hornist brother, Jamie Hersch, a Singapore Symphony Orchestra member who had a new instrument created for the project.

They've been rehearsing for a year. The piece - which Gaisford says is the most challenging in his experience - demands it. How will audiences cope? Will such ambitious music be performed again?

Hersch is preparing an alternative version that replaces horn with saxophone (commissioned by the Washington Performing Arts Society and saxophonist Gary Louie) for a performance next year in New York. So he thinks about practicalities, but not for very long.

The piece is a follow-up to his two-hour piano work The Vanishing Pavilions and is the second part of a massive trilogy.

"And it's meant everything for me," he said at a rehearsal. "I love writing music, and I also recognize that for a lot of these pieces, the chances of them being publicly performed is very, very slim because of their nature. But I feel that I must write them."

No doubt his more mainstream commissions - a new trumpet concerto for the Cleveland Orchestra and a song cycle for baritone Thomas Hampson - will benefit by the artistic stretching represented by Last Autumn. Hersch often moves in the big leagues with concert works that have been performed by orchestras under Mariss Jansons in Pittsburgh, Ignat Solzhenitsyn in Philadelphia, Robert Spano in Brooklyn, and Yuri Temirkanov in Baltimore.

Yet the 38-year-old faculty member at Baltimore's Peabody Conservatory, who lives on the Main Line with his wife and daughter, has the attitude and anxiety of an outsider. He's part of no circle or school of composers. While his contemporaries embrace melodies and harmonic consonance, Hersch's language never hesitates to leap into the abyss - and in ways that, for some listeners, go straight to parts of the soul that few living composers touch.

Some might disagree. His Variations on a Theme by Hugo Wolf, premiered in 2005 by the Chamber Orchestra of Philadelphia, was mildly booed.

That's never easy for a composer - Hersch quotes his mentor, the late George Rochberg, as saying composers need to have "an iron stomach." But seemingly more pronounced are the birth pangs for a piece like Last Autumn, so personal and written so intuitively that Hersch is often at a loss to explain what he's done.

Rehearsals reveal moments of lyricism unusual for Hersch in movements marked "lullaby." The composer admits to singing more in recent years than ever before in his life - thanks to his pre-school-age daughter - but almost resents the suggestion of direct cause and effect. Many of Last Autumn's movements come with poetic inscriptions from German writer W.G. Sebald, but Hersch has only a vague sense of why.

"The air stirs the light" is written over the piece's first movement, suggesting gentle imagery to some - though the music borders on strident.

Cellist Gaisford offers this interpretation: "It's a very bright light."

"Like a tropical storm, rushing into this massive swirling of nature," says Jamie Hersch.

Performances prompt such questions - one reason why hearing his own work sometimes feels like an imposition on the composer. As grateful as he is for the assiduous work that Gaisford and his brother have put into Saturday's performance, Hersch admits, "sometimes it's disquieting."

"The music cannot be more personal. It's very hard to put that out there. People are reacting to the essence of who I am and that's tough. It's tough to relive the feelings that went into it."

His hesitation to reveal the more exact nature of those feelings isn't just a matter of privacy, but a matter of artistic ethics. Hersch believes that art mirrors its beholder; for him to talk about his own real-life experiences would cloud that mirror.

No wonder his music feels confrontational, with no "backstory" to filter it. The silences that Hersch strictly prescribed in the score give listeners time for the haiku-like gestures to sink in. Much of his music lacks typical points of references, leaving listeners further unmoored from what they know. Music with such ambiguous destinations can be frightening.

But the composer is on the same open-ended journey. "I hope that each piece is another step in a long staircase," he says, "and I don't know where it all leads."

Michael Hersch's "Last Autumn" will be performed at 8 p.m. Saturday at St. Mark's Church, 1625 Locust St. Tickets: $10 and $15, at the door.