Building on previous experience developing tethered robots and autonomous underwater vehicles (AUVs), the team fused the two approaches together to develop a hybrid vehicle that could fly like an aircraft to survey and map broad areas, then be converted quickly into a remotely operated vehicle (ROV) that can hover like a helicopter near the seafloor to conduct experiments or to collect biological or rock samples.
The tethering system presented one of the greatest challenges in developing a cost-effective ROV capable of reaching these depths. Traditional robotic systems use a steel-reinforced cable made of copper to power the vehicle, and optical fibers to enable information to be passed between the ship and the vehicle. If such a cable were used to reach the Mariana Trench, it would snap under its own weight before it reached that depth.
To solve this challenge, the Nereus team adapted fiber-optic technology developed by the Navy's Space and Naval Warfare Systems Center Pacific to carry real-time video and other data between the Nereus and the surface crew. Similar in diameter to a human hair and with a breaking strength of only eight pounds, the tether is composed of glass fiber with a very thin protective jacket of plastic.
Nereus brings approximately 40 kilometers (25 miles) of cable in two canisters the size of large coffee cans that spool out the fiber as needed. By using this very slender tether instead of a large cable, the team was able to decrease the size, weight, complexity and cost of the vehicle.
Another weight-saving advance of the vehicle is its use of ceramic spheres for flotation, rather than the much heavier traditional syntactic foam used on vehicles like the submersible Alvin or the ROV Jason.
Each of Nereus's two hulls contains between 700 and 800 of the 9-centimeter (3.5-inch) hollow spheres that are precisely designed and fabricated to withstand crushing pressures.
WHOI engineers also developed a hydraulically operated, lightweight robotic manipulator arm that could operate under intense pressure.
With its tandem hull design, Nereus weighs nearly 3 tons in air and is about 4.25 meters (14 feet) long and approximately 2.3 meters (nearly 8 feet) wide. It is powered by more than 4,000 lithium-ion batteries. They are similar to those used in laptop computers and cell phones, but have been carefully tested to be used safely and reliably under the intense pressure of the depths.
"These and future discoveries by Nereus will be the result of its versatility and agility--it's like no other deep submergence vehicle," said Tim Shank, a biologist at WHOI who is aboard the expedition. "It allows vast areas to be explored with great effectiveness. Our true achievement is not just getting to the deepest point in the oceans, but unleashing a capability that now enables deep exploration, unencumbered by a heavy tether and surface ship, to investigate some of the richest geological and biological systems on Earth."
On May 31, the team took the vehicle to 10,902 meters, the deepest dive to date. Testing will continue over the next few days and the team will return to port on June 5. On this initial engineering cruise, Nereus's AUV mode was not tested.
On its dive to the Challenger Deep, Nereus spent more than 10 hours on the bottom, sending live video back to the ship through its fiber-optic tether and collecting biological and geological samples with its manipulator arm, and placed a marker on the seafloor signed by those onboard the surface ship.
"The samples collected by the vehicle include sediment from the tectonic plates that meet at the trench and, for the first time, rocks from deep exposures of the Earth's crust close to mantle depths south of the Challenger Deep," said geologist Patty Fryer of the University of Hawaii, also aboard the expedition. We will know the full story once shore-based analyses are completed back in the laboratory this summer. We can integrate them with the new mapping data to tell a story of plate collision in greater detail than ever before accomplished in the world's oceans."