BALTIMORE, Maryland (AP) — The main camera on the popular Hubble Space Telescope shut down again over the weekend, the third outage in less than a year, NASA said Monday.

The orbiting observatory entered a protective “safe mode” Saturday morning. An initial investigation determined the backup power supply for the Advanced Camera for Surveys on the 16-year-old telescope had failed.

The ACS, installed in 2002, increased Hubble’s vision greatly and has provided the clearest pictures yet of galaxy formation in the very early universe.

The Hubble was recovered from safe mode Sunday morning, and observations are expected to resume this week using the Hubble’s other instruments. Engineers are also looking into whether the ACS can be switched back over to the primary power supply for operation in a reduced mode, NASA said in a statement.

The ACS had been switched over to the backup in June when its main power supply malfunctioned.

In September, the ACS automatically shut down again as operators were switching between two of its three instruments. Investigators believe debris stuck in a switch caused a voltage drop that shut down the instrument.

In October, NASA Administrator Michael Griffin announced the scheduling of a 2008 space shuttle mission to repair and upgrade the telescope.

Without the mission, batteries and stabilizing gyroscopes would run out of power near the end of the decade, bringing to an end the life of the popular space telescope.

The servicing mission is currently scheduled for September 2008, said Preston Burch, associate director and program manager for the Hubble Space Telescope.