NEW YORK — Internet provider Juno Online Services Inc. has devised a way to help underwrite its struggling free service: have subscribers leave their computers running 24-hours-a-day and sell the excess processing power.

        The plan announced Thursday to create a “virtual supercomputer” indicates the hard times being felt by purveyors of free Internet service.

        Juno would harness subscribers' unused microprocessing cycles and hard drive space and sell it to research institutions and corporations that require intensive computation power.

        The practice, known as “distributed computing,” is already done on a volunteer basis by astronomy buffs supporting a project called SETI that is probing for extraterrestrial life.

        In Juno's case, subscribers will download software that, when activated, will discreetly perform computations when the machines are idle. Subscribers' computers will then automatically send information back to Juno's supercomputing clients when they log onto the Internet.

        “It's only going to be running when they're not actively using the computer,” said Charles Ardai, Juno's president and chief executive officer. “The idea is to make it invisible. It shouldn't slow down their machines because it only kicks in when their screen-saver turns on.”

        Mr. Ardai said the company has not secured any deals yet to sell its supercomputing services, but he envisions signing up biotechnology companies as customers.

        At first, the Juno Virtual Supercomputer Network will be tested using volunteers.

        But Mr. Ardai said the company may eventually require participation from subscribers of the free service, who already agree to have a portion of their screens dominated by advertisements.

        “We may well make it mandatory for our free users,” said Mr. Ardai. “If you want the service for free, these are the things we have to do to make the money back.”

        Subscribers of Juno's free Internet service who do not wish to participate in the distributed computing network can sign up with any number of Juno's fee-based online services, he said.

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