Submersible robots explore the ocean's depths

Making the ocean more accessible
Such plans are not without their pitfalls. Reached by e-mail during a joint ocean expedition with Chinese scientists, lead Sentry engineer Dana Yoerger said an autonomous vehicle “only has the limited brainpower we’ve programmed into it. Sometimes with an AUV, an A-minus can be a failing grade.”

Sentry has comfortably passed its initial tests, said Yoerger, but another AUV powered by the ocean has demonstrated both the potential and peril for the new explorers.

A new generation of thermal-powered glider can travel more than nine miles a day and descend about three-fourths of a mile below the surface, using only the ocean’s stored heat, said Benjamin Hodges, a research associate at Woods Hole Oceanographic Institution.

To propel itself through the water, the glider relies on wax that melts and expands at a temperature of about 10 degrees Celsius, or 50 degrees Fahrenheit. As the glider ascends from colder, deeper water to the warmer surface water, the melting wax compresses nitrogen gas in a bottle within the glider to a pressure of 3,000 pounds per square inch, or the amount of pressure typically encountered in hydraulic systems on commercial airplanes. On its next dive, when the glider reaches its maximum depth and needs to ascend, it uses the compressed nitrogen to force oil into an external bladder, increasing the glider’s buoyancy and causing it to rise. Then the cycle repeats.

After completing a four-month mission with no problems, Hodges said in an e-mail, the thermal glider vanished at sea about two months into its second trip.

“We don’t know what happened,” he said, suggesting a ship-strike or entanglement in fishing gear as potential culprits. Another glider is now being built by East Falmouth, Mass.-based Teledyne Webb Research and will likely be tested in the Caribbean.

Despite the snag, researchers say the upper ocean has become far more accessible through sophisticated gliders and other autonomous vehicles that passively drift with the currents.  At Oregon State University, for example, Barth and his colleagues have routinely deployed battery-powered gliders to measure parameters such as the dissolved oxygen content, a critical factor for marine life. As part of the Ocean Observatory Initiative, he said, the collaborators hope to deploy a fleet of six gliders to patrol the waters around the stationary observatories and answer questions like, “What does the neighborhood look like in the region of these very intensive measurements?”

Sentry’s independence and efficiency could aid similar free-ranging explorations, Yoerger said, whether in scouting out hydrothermal vents along mid-ocean ridges, searching for ancient shipwrecks, locating hazardous materials, or inspecting deep sea oil and gas facilities.

Diving into the depths
Nereus, a hybrid submersible still in the testing phase, could build upon that promise with its ability to dive down to nearly seven miles, or the depth of the Mariana Trench east of Japan.

“This is an oceanographer’s dream transformer,” Fornari said, referring to the vehicle’s unique ability to change from an autonomous to remotely operated vehicle. Nereus boasts an ultra-thin ceramic coating that resists the crushing pressure of the deep ocean but doesn’t saddle the vehicle with too much weight.  LED lights will provide illumination and 20 kilometers of monofilament fishing line-like microfiber will transmit data to the surface.

So far, the vehicle has been pressure-tested to an equivalent of what it would encounter in the Mariana Trench and field-tested in depths of 2.5 to 3 miles off the coast of Hawaii. Nereus is being readied to explore the trenches of the western Pacific Ocean in early 2009, and if all goes well, it will move on to the Cayman Trough to search for new vent-fields along the world’s deepest mid-ocean ridge.

Chris German, the chief scientist for the National Deep Submergence Facility at Woods Hole and a fellow member of the joint U.S.-China expedition, noted that the vast majority of the ocean floor has yet to be investigated. Human-occupied vehicles are scarce, he said in an e-mail, and remotely operated vehicles still require a significant investment to deploy to far-flung parts of the world.

“So you really need to know as much as possible about where you are going before you make that kind of commitment,” he said. “That is where we have found AUVs such a great asset.”

Nereus, he said, will not only locate, map and photograph the seafloor but also collect chemical, biological and geological samples.

With the latest class of underwater robots to guide them, German and his colleagues are hoping that such feats — once only dreamed about — will soon become little more than routine aspects of the new era in ocean exploration.

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