The aliens are coming — and sooner than you think.In a first for the studio, 20th Century Fox is making Ridley Scott’s sci-fi thriller “Prometheus” available for HD download Sept. 18, three weeks before the release of the physical discs. Pic marks the inaugural film in Fox’s strategy of carving out a new digital window for homevid releases. Studio will make all of its films available for HD download about two weeks before the titles hit store shelves. The three-week jump for “Prometheus” window is an exception. The next few pics in Fox’s queue are “Abraham Lincoln: Vampire Hunter,” “Ice Age: Continental Drift” and “Diary of a Wimpy Kid: Dog Days.” And the digital versions will be cheaper: Retailers will offer the digital version of “Prometheus” for less than $15, rather than the $20 they usually offer films through the electronic-sell-through category. On the day of “Prometheus’” launch, the studio will also make 600 of its library titles available through the new service. Those include mainstream movies like “Avatar” and “Rio,” but also less readily available DVD fare like the original 1968 version of “Planet of the Apes” and “French Connection.” The price point for the studio’s library titles may vary slightly from its upcoming releases but will hover around the $15 mark. But Fox will pocket more of its take from the new digital venture, which the studio has dubbed “Digital HD.” Unlike packaged media, digital downloads cost a studio virtually nothing. Studio isn’t the first to make films available for paid download before their hard copy release. Sony and Weinstein Co. have experimented with similar early digital windows. But Fox is throwing significant weight behind its new digital offerings. Studio is committing a major marketing campaign to the initiative, with plans to splash ads for the films across major sports, gaming and music sites including ESPN, Pandora and IGN. Fox hopes to reach 100 million potential consumers within the first week of the digital launch of “Prometheus” through Amazon, iTunes, Vudu, PlayStation, Xbox and Best Buy’s CinemaNow service. The scope shows Fox’s attempt to bring a lightly used digital model to a new mainstream level. It’s also an attempt to drive consumers away from rental and toward the more lucrative electronic sell-through business. “Most people are renting as opposed to buying online,” said one distribution exec. Fox is banking on the rapid growth of broadband penetration around the world. Studio will make digital versions of its films available in more than 50 countries.