About 48 kilometers off the eastern coast of the United States, scientists from Rutgers, the State University of New Jersey, peered over the side of a small research vessel, the Arabella. They had just launched RU27, a 2-meter-long oceanographic probe shaped like a torpedo with wings. Although it sported a bright yellow paint job for good visibility, it was unclear whether anyone would ever see this underwater robot again. Its mission, simply put, was to cross the Atlantic before its batteries gave out.
Unlike other underwater drones, RU27 and its kin are able to travel without the aid of a propeller. Instead, they move up and down through the top 100 to 200 meters of seawater by adjusting their buoyancy while gliding forward using their swept-back wings. With this strategy, they can go a remarkably long way on a remarkably small amount of energy.
When submerged and thus out of radio contact, RU27 steered itself with the aid of sensors that registered depth, heading, and angle from the horizontal. From those inputs, it could ”dead reckon” about where it had glided since its last GPS navigational fix: Every 8 hours the probe broke the surface and briefly stuck its tail in the air, which exposed its GPS antenna as well as the antenna of an Iridium satellite modem. This allowed the vehicle to contact its operators, who were located in New Brunswick, N.J., in the Rutgers Coastal Ocean Observation Lab, or COOL Room.
Modeled after NASA’s command center in Houston, the COOL Room is lined with computer consoles and plasma displays. They indicate, among other things, the locations of various research vessels and remote-controlled gliders working the world’s oceans. Whenever researchers call them up, other nautically relevant charts also dance across the screens, showing, for example, sea-surface temperatures, currents, winds, and cloud cover.
On the day of the launch—27 April 2009—the COOL Room was packed with students, scientists, and technicians, all clinging to the sounds radioed in from the Arabella. When Scott Glenn, leader of the shipboard science team, applauded RU27’s safe passage into the sea, the throng in the control room followed suit. It was one small step for a machine, on what was to be one giant transatlantic leap for mankind.
But mere minutes into the mission, during an initial test dive, it became clear that something was seriously amiss. RU27 was diving too slowly and ascending too quickly. Had somebody forgotten to install one of the extra battery packs, making the glider more buoyant than expected? Unless the cause could be found and rectified, RU27 would have to be plucked from the Atlantic and returned to Rutgers for disassembly and troubleshooting.
A tense half hour passed while the glider completed another test dive. Finally, the Arabella team pinpointed the problem—a hastily mistyped minus sign instead of a plus sign in one of the guidance parameters sent to RU27’s onboard computer. ”It’s nice that it does exactly what it’s told, even if it’s wrong,” Glenn said.
Once that wrinkle was ironed out, researchers in the COOL Room took control of the glider. Tina Haskins, a Rutgers technician who had helped to assemble the glider, surveyed RU27 from the deck of the Arabella while the little yellow submarine drifted close to the ship. ”All right, bid farewell!” she called out. ”This is the last time we’ll see her on the surface for a while.” And with those words still floating in the air, RU27 silently dove beneath the waves.
RU27 is just one of more than 100 gliders doing important oceanographic work all over the world. They are designed and built by a handful of companies and research groups. The maker of RU27, Teledyne Webb Research, of East Falmouth, Mass., is the largest supplier of these unique oceanographic robots. Their up-and-down zigzagging lets scientists survey the upper part of the ocean, which is otherwise largely invisible to them.
Satellites track only the sea surface, and most measurements of conditions at depth come from sensors attached to a few thousand floating oceanographic probes that drift wherever the currents take them. To examine targeted areas, oceanographers can lower instruments from research vessels, but such ships have limitations. They cannot be used to investigate the oceanographic effects of violent storms, for example—at least not intentionally. Also, research vessels rarely remain at sea for longer than a month or so at a time, and they’re extraordinarily expensive to operate.
Remote-controlled gliders suffer from none of these constraints. They are inexpensive and require no crew or support ship (except while they’re being deployed or recovered), making them a cost-effective way to survey large swaths of the upper ocean.
But oceanographic gliders don’t just study the sea: Some, known as thermal gliders, can also use the ocean’s energy as a source of power. They do so by taking advantage of the temperature difference between the surface, which is warmed by the sun, and the ocean’s chilly depths.
To understand how a thermal glider works, imagine for a moment that you’ve filled a garbage bag with tap water and then frozen the entire mass. Next, you attach a good-size lead sinker to it and plop it into the ocean. After bobbing around on the surface for a while, the ice would melt, and the weighted bag would sink. Now, consider what would happen if the seawater surrounding the sinking mass became cold enough to freeze the water in the bag. Ice would re-form, and the bag would float to the surface, lead sinker and all, whereupon the cycle would start all over again. If this contraption had wings, it could also glide forward as it yo-yoed up and down.