Little robots set out at a stately glide to explore the oceans
But the stars of the recent Monterey show were gliders like the Slocums, with no external propulsion at all. When pumps shift oil from an external bladder into an internal void to slightly increase their density, they sink slowly to depths of hundreds or thousands of feet. Then they reverse the process to float to the surface like miniature undersea dirigibles. Delicate wings convert their vertical motion into a forward glide at a stately 1 or 2 knots. On each visit to the surface they phone home (literally, using Iridium satellite phone links), sending position and science data and receiving new orders.
Their endurance is high because they need batteries only for communication, sensors, and buoyancy-adjusting pumps and valves. Developed over the past 10 years largely under Office of Naval Research grants, gliders aren't fast, acknowledges Bellingham. "But if you have a few months, you can cross an ocean." They are named, after all, for Joshua Slocum, a 19th-century New Englander who was the first to sail solo around the world.
The marathon champs of the Monterey exercise were the Scripps Institution of Oceanography's Spray gliders, named for Slocum's boat. While the Slocums, with less expensive batteries, needed to be brought in several times for a change, the five Sprays ran flawlessly for six weeks without a pause or even a visit by boat, traveling many miles every day and diving to depths of more than half a mile. As they and the other AUVs did their thing, data poured in from satellites, airplanes flying over the bay, and instruments moored in its water to verify the accuracy of the AUV readings. Hunched over their computers, researchers knitted the skeins of data into a coherent 3-D picture of bay dynamics and biology.
The exercise was a rousing success, say its leaders, mapping phenomena no one instrument had seen before, including the extent of glowing patches of water caused by luminescent microorganisms. "We got complete coverage across the whole bay. We saw filaments [of glowing water] that extended miles," says Steve Haddock of the Monterey Institute.
Woods Hole scientists now plan to deploy a line of gliders along the eastern seaboard, monitoring currents, upwellings, biological productivity, and other features of the coastal waters. And off Seattle, a hot-pink "Seaglider" built at the University of Washington has been at work since August 21--a record for any AUV--measuring currents along the continental shelf. Its developer, oceanographer Charles Eriksen, expects it to keep going for another four months. Early this month his team launched two more into the stormy, frigid Labrador Sea, where they will spend the winter monitoring currents crucial to global climate. The sea, west of Greenland, is "a place ships just don't go in the winter," he says. "Only gliders make sense."
Run hot and cold. Eventually, gliders may reduce their battery load and boost their endurance even further with "thermal engines" that use the temperature difference between warm surface waters and the frigid depths to alternately thaw and freeze a special hydrocarbon fluid. As the fluid shrinks and expands, it drives a piston, generating energy to run the buoyancy-altering pumps. Webb Research Corp. of Massachusetts, maker of the Slocum gliders, will be testing that clever idea in February off the Bahamas.