A substantial electrical power failure 200,000 miles from earth has forced Apollo 13 to abort man's third landing and sent the three astronauts into a race back toward earth.

Although Mission Control termed the astronauts chances of getting back "excellent," the oxygen supply for the main capsule cabin was lost in the failure and all emergency plans have been put into operation.

"It was a pretty large bang with a warning light," Captain James Lovell said in his first report back on the power failure. Space experts conjectured that either a meteorite or a fuel-cell explosion might have caused the power failure.

The lunar craft must circle the moon before the astronauts can start back to earth. At the earliest, Apollo 13 will start its return trip at 9:40 p.m. tonight and splash down at 1:20 p.m. Friday afternoon if everything goes right.

The splash down is now scheduled for the Atlantic Ocean. Emergency recovery ships are not stationed in the Atlanticfor Apollo 13 as they were for previous flights. Mission Control said the United States will ask any "ship of opportunity" from any nation to help with the recovery.

Mission control said early this morning that the oxygen supply will hold up if Apollo 13 follows the optimal return path.

Two of the three main power cells were disabled in the sudden power failure. The main oxygen supply for the lunar capsule also blew and the astronauts reported that oxygen was leaking from the craft.

The astronauts immediately shut off the third power cell to conserve energy, but reported difficulty controlling the craft which periodically fell into a pitch and roll, hampering communication with Mission Control in Houston.