In this story:

• Australians to launch search
• Capsule isn't 'leak-proof'
• Related stories and sites

ST. LOUIS (CNN) -- Balloonist Steve Fossett's fourth attempt to circle the globe ended Sunday after his balloon went down in the South Pacific, according to his mission control staff at Washington University.

A satellite picked up an emergency locator beacon from Fossett, giving rescuers a near exact location for their search, said mission control rescue coordinator Joe Ritchie.

"We've got an extremely credible fix on his location," he said. "That's a big deal."

But mission control officials do not know whether Fossett is OK.

Mission control lost contact with Fossett at 10:23 a.m. ET (1423 GMT) Sunday. The Coast Guard in New Orleans then received a signal from Fossett's emergency locator beacon, indicating it had been activated.

A second locator beacon was then activated at 12:02 p.m. ET (1602 GMT). Blount said Fossett may have activated the second beacon himself, but the device is also designed to go off if it makes contact with water.

Fossett was estimated to be about 500 miles from New Caledonia and about the same distance east of the coast of Australia.

RELATED AUDIO

Hear mission recovery director Joe Ritchie give the latest update on Fossett 280K/20 sec. AIFF or WAV sound

Australians to launch search

The Australian Air Force is preparing to send out a C-130 plane to try and rescue Fossett. The plane is expected to take off at 4:30 p.m. EDT (2030 GMT), and an Australian Air Force spokesman said it would take four hours to reach where Fossett is believed to have gone down.

That may be in an area of the Pacific where there are many reefs, and therefore few boats. Seas in the area were relatively calm Sunday, but mission control was concerned because the area is near a coral reef that could be full of sharks.

Bad weather may have contributed to the balloon's emergency situation.

About five hours before the locator sounded, meteorologist Bob Rice cautioned Fossett that he was approaching strong thunderstorms.

"It's hard to say how strong they were," said Rice. "There is a good chance that there might have been a lightning hit on the capsule. But these balloons are very strong. It's just too early to know what happened."

Capsule isn't 'leak-proof'

The location also is out of reach of the nearest air traffic control center in Australia, which means the balloon is not being picked up on radar.

It isn't clear whether Fossett was able to inflate his life raft, or whether he is still in the capsule. The capsule could stay afloat for some time but it is not leak-proof, Blount said. Fossett also has an emergency wet suit on board. Ritchie said the plane will drop survival gear, including a raft, water and first-aid supplies, if Fossett is swimming.

The capsule's condition also depends on how fast it was going when it went down.

Fossett, 54, is making his fourth attempt to be the first balloonist to circle the globe nonstop. He has covered more than 15,200 miles along his track since launching from Mendoza, Argentina, on August 7. The millionaire adventurer has already broken his own record for distance traveled in a balloon.