With every reboot of a classic TV series, the producers face a key question: Reprise the musical theme of the old show or go for an entirely new sound appropriate to a new cast and concept?

Zack Estrin, executive producer of Netflix’s “Lost in Space,” available for streaming on April 13, chose the former, with a twist. “We’re maintaining the core spirit of the show — this great family survival adventure — and along with the rights to the show came this incredible trove of original John Williams compositions,” he says. “We wanted you to feel like you were in a great Steven Spielberg experience of a movie, and what better way to feel that than to have a hint of John Williams?”

The legendary composer, who was a frequent collaborator with “Lost in Space” creator Irwin Allen, wrote two themes for the sci-fi series, which aired on CBS from 1965-68. New series composer Christopher Lennertz incorporates Williams’ second theme into his own musical signature and happily shares billing with the five-time Oscar winner on an end-credits card.

The new “Lost in Space” roughly follows the original outline (shipwrecked family facing hostile conditions on an uncharted planet) but without the campy theatrics that marred the original. The more serious tone appealed to Lennertz (“Supernatural,” “Agent Carter”), a long-ago USC chum of showrunner Estrin.

The challenge for Lennertz was to write and record more than eight hours of music in just 10 weeks. He took two of those weeks just to write themes for the series: a new main theme, a treatment of the Williams theme, plus motifs for the Robinson family, the mysterious Dr. Smith, the robot who befriends young Will Robinson and the Robinson children. It’s an alternately rousing and dramatic score, recorded with the Philharmonia Orchestra at London’s Abbey Road Studios in October and November. Smaller, quieter cues, often featuring piano, synth, ethnic-music soloists or vocals, were recorded separately later.

“It’s unabashedly emotional, very testosterone-driven when it needs to be, but very poignant when it comes to the kids and the family,” Lennertz says. The assignment reminded him of his early days apprenticing with the late Michael Kamen (“Robin Hood: Prince of Thieves”) and Basil Poledouris (“Lonesome Dove”), both well-known for their tuneful, powerful music for films and TV.

“This was an old-school, classic adventure … a show that’s got kids and robots and exploding planets.”

Composer Christopher Lennertz

In that sense, the score for “Lost in Space” is a departure from today’s more popular mood-and-atmosphere works that are largely devoid of melody. “This was an old-school, classic adventure,” says Lennertz, recalling his thank-you note to Estrin for “the ultimate Christmas present: the opportunity to write for a show that’s got kids and robots and exploding planets. I’ve been waiting for this for 46 years.”

Estrin places prime value on the score: “I’ve always felt that music is a piece of casting as important as No. 1, 2 or 3 on your call sheet,” he says. “The music is another Robinson in this family. This was something that Chris always wanted to do. I felt like the boy in him was awakened — to be able to create this thing that so inspired him as a kid, and now here he was, getting to do it.”

POPULAR ON VARIETY