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Robotic Submarine Comes Up Short in First Deep Dive: Now What?

The unmanned sub had been expected to search for 16 hours but a built-in safety feature ended its first journey early, officials said.

A robotic submarine was set to be launched on a second underwater sweep for wreckage from missing Flight MH370 Tuesday after deep water ended an initial dive, triggering questions about whether the vehicle is capable of completing its mission.

The U.S. Navy-owned Bluefin-21 gathered about six hours of data before returning to the surface of the southern Indian Ocean almost 1,000 miles off Australia's west coast.

The unmanned sub had been expected to search for 16 hours but a built-in safety feature ended the journey after it exceeded its "operation depth limit" of 14,763 feet -- or about 2.8 miles, officials said.

Bluefin-21 can create a three-dimensional sonar map of the area to chart any debris on the sea floor. The initial data collected was analyzed but no objects of interest were found.

Searchers are confident they know the approximate position of wreckage of the Malaysia Airlines jet which vanished on March 8. They are moving ahead with the sub search on the basis of four acoustic signals they believe are from its black box recorders. However, no "pings" have been heard for almost a week.

“Bluefin-21 is planned to redeploy later today when weather conditions permit,” Australia’s Joint Agency Coordination Center said in a release early Tuesday.

If it detects possible wreckage, it will be sent back to photograph it in underwater conditions with extremely low light.

Experts told NBC News that a number of options are available if the Bluefin-21 proves unable to go deep enough.

The director of Blue Water Recoveries, a U.K.-based company that holds the Guinness World Record for discovering the deepest shipwreck, highlighted other craft that may help in the hunt.

“The U.S. Navy has a deep-tow search system that has similar type of sensors called Orion,” David Mearns said. “It’s not an autonomous vehicle like the Bluefin-21, it’s towed with a cable. It has a side-scan sonar system and digital still cameras. The advantage of that system is that it is operated in tandem with the remotely operated vehicle that would do the recovery called Curv 21.”

"It’s not like it will get to 4,500 meters and implode immediately"

Orion can search to a maximum depth of 20,000 feet of seawater, according the U.S. Navy Supervisor of Salvage and Diving.

This is towed behind a vessel at slow speeds, generally from one to three knots depending on the required depth.

However, Stefan Williams, a professor of marine Robotics at the University of Sydney, said he suspected the Bluefin-21 was capable operating at greater depths.

“Any of these sorts of engineered systems will have a fairly conservative safety margin, so it’s not like it will get to 4,500 meters [15,000 feet]and implode immediately, but they will likely be unwilling to push it much further,” he said.

The submarine will take 24 hours to complete each mission: two hours to dive to the bottom, 16 hours to search the sea floor, two hours to return to the surface, and four hours to download the data.

The acoustic search has been focused an area about the size of a medium city - around 230 square miles - and say it could take the underwater robot months to scan and map the whole search zone.

On Monday, officials announced that the air search for signs of the missing Boeing 777 would be called off within 72 hours.

The search for the missing plane is on track to be the most difficult and expensive search and recovery operation in aviation history.

Jay Blackman of NBC News, Reuters and The Associated Press contributed to this report.