Sentry, a new undersea robot built by engineers at Woods Hole Oceanographic Institution (WHOI), completed its first scientific mission last summer, scouting out sites for an undersea observatory network off the coast of Washington state. Sentry is the successor to ABE, the Autonomous Benthic Explorer, a pioneering free-swimming robot, launched in 1995, which revolutionized deep-sea exploration by expanding scientists’ reach into the deep.
Unlike human-occupied submersibles or vehicles connected by cables to surface ships, Sentry and ABE can survey wide swaths of undersea territory on dives lasting a day or more. The autonomous underwater vehicles, or AUVs, are pre-programmed to maintain a designated course but have enough decision-making capacity to avoid collisions with seafloor terrain. They are equipped with sonar to navigate and to map the seafloor and arrays of sensors that can measure water temperature and salinity, for example, or “sniff out” telltale chemicals that signal places and phenomena that scientists want to investigate.
AUVs can “work like bloodhounds,” said WHOI engineer Dana Yoerger, who teamed with WHOI engineers Al Bradley and Barrie Walden to design both ABE and Sentry. “When they find something we’re interested in, they can ‘bark’ and then return from the ‘hunt.’ ” Or, as Sentry did on its premiere mission, it can make high-resolution maps of the seafloor.
Sentry's unconventional shape
“As expected on its maiden voyage, Sentry had some 'teething' problems,” said John Delaney, a University of Washington (UW) oceanographer and the expedition’s chief scientist. Nevertheless, “Sentry has given us a survey with great precision and resolution. These maps will help with the installation of the primary nodes of a networked observatory on the Juan de Fuca tectonic plate,” he said.
"We were very pleased with Sentry’s performance,” Yoerger said. “We had tested the AUV as best we could before we got out here, but the steep terrain of the deep sea is quite a different matter. Our engineering team and our vehicle both performed very well under the stresses of putting a complex system into the ocean for the first time.”
While Sentry mapped, WHOI geologist Dan Fornari and biologist Erich Horgan simultaneously used a towed underwater camera system called TowCam to obtain visuals that can be married with the maps.
Sentry is not shaped like conventional AUVs, which resemble torpedoes. Delaney said it looks like “a flying bar of soap,” albeit one that is 6 feet tall and bright yellow.
“Like a torpedo, Sentry has a streamlined body for low drag,” Yoerger said. “But its shape gives it more stability while cruising through bottom currents.”
The vehicle also has thrusters built into its foils, or wings, which are tiltable to give Sentry more maneuverability to start, stop, change directions, and “ ‘fly’ in different ways,” he said, including hovering like an underwater helicopter for close-up examinations. Traditional torpedo-shaped AUVs tend to keep moving in one direction.
A busy summer ahead
Sentry is designed to dive as deep as 5,000 meters (3.1 miles). It is powered by more than 1,000 lithium-ion batteries—similar to those used in laptop computers, though adapted for extreme pressures—which allow it to dive up to 20 hours.
On this first cruise, Sentry collected as many as 60 million individual soundings of seafloor depths in a single dive. It surveyed about 20.5 square miles (53 square kilometers) of seafloor. Engineers adjusted a balky navigation system and are now investigating the use of new propellers that could improve the vehicle's speed (about 2 miles per hour) and range.
On future missions, Sentry can be equipped with other types of sensors, a digital camera, and a pump and filter system to draw in samples of tiny plankton. Beginning in June, researchers plan to use Sentry on at least three expeditions: to explore deep-sea corals in the Gulf of Mexico; to study undersea oil seeps off Santa Barbara, Calif.; and to map a chain of active underwater volcanoes at the Carlsberg Ridge in the Indian Ocean.
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