ABOARD THE R/V THOMAS G. THOMPSON—Our operations have been halted for a second day now while we wait for a storm on the other side of New Zealand to spin itself out. With no samples to process, many of us took a day off to catch up on reading or, for some of the undergraduates with us, to study for exams. The Woods Hole Oceanographic Institution's unmanned sub, Nereus, sits lashed to the fantail as we sail through the wind and waves, slowly mapping the seafloor with our sonar or just sitting hove-to, riding out the storm.
The remotely-operated vehicle is one of the key components of this cruise and, once we get it back in the water, it will provide a much bigger picture of the hadal ecosystem than has been seen before. It was built between 2005 and 2009 with support from the National Science Foundation and additional funding from NASA (think inner space) and the National Oceanic and Atmospheric Administration (NOAA) in response to the question posed to ocean scientists asking what they would need to push their research beyond the limitations posed by traditional, tethered underwater vehicles and into the deepest parts of the ocean. As geologist Patty Fryer described the justification for such a tool: “We didn't go halfway to the moon.”
Nereus takes us to another planet right here on Earth.
There are a host of new technologies and new applications for existing technologies that make Nereus possible, but two illustrate how special this vehicle is—and how difficult it is to study the deep ocean. One challenge, of course, is pressure. At the bottom of the Mariana Trench water pressure exceeds 16,000 pounds per square inch (versus 14.7 pounds per square inch in air at sea level). The vehicle has to be built strong enough to withstand such extremes of pressure, but it can't be so heavy that it sinks into the mud on the seafloor. As a result, Nereus is actually built to float at whatever depth it will be working.
This is not an easy balance in something that includes a fair amount of titanium to protect delicate electronics. Normally, syntactic foam, a high-strength mixture of hollow glass spheres embedded in epoxy, is used to offset the weight of the rest of the vehicle. At the time Nereus was being built, however, the only syntactic foams capable of withstanding full ocean pressures were far too expensive. So its designers turned in a different direction.
Ceramics have the advantage of being extremely lightweight, but extremely strong. By packing the frame with hollow ceramic spheres—a strong shape for something under extreme pressure—they were able to achieve a degree of buoyancy that allows the vehicle to work at depth without wasting battery power in driving its thrusters to maintain a desired height over the seafloor.
The other major advance that will enable Nereus to get to the bottom of the Kermadec Trench is its tether. Most ROVs have a heavy, armored tether that provides a data link to the surface and also carries power from the support ship to the vehicle. Beyond about 7,000 meters, however, a standard tether will break under its own weight.
The Japanese addressed this problem when they built a full-ocean-depth ROV by giving it a massively armored cable. But this, in turn, required a winch that was so large they had to build an entirely new ship around it. As a result, the designers of Nereus went in the opposite direction—they built a tether that was lighter to the point of being ephemeral.
Nereus is actually more than just the twin-hulled yellow vehicle in pictures. When it is deployed, the first thing that goes in the water is a grey cylindrical object called the depressor that is connected via an armored cable to the ship. The depressor contains communications gear and other instruments, as well as a detachable end known as the float pack, which is connected to the vehicle. Inside the float pack and depressor are two linked spools of optical fiber.
After the depressor is in the water, Nereus goes in and it descends with the depressor to about 3,000 meters, at which point the float pack separates from the depressor. As Nereus and the float pack continue to descend, the fiber unspools, providing two advantages: a continuous high-bandwidth data connection to the surface and the freedom to maneuver as much as 40 kilometers (25 miles) from the ship. As a result, Nereus is free to transit long distances over the seafloor without requiring its support ship to re-position itself. It also allows high definition video and real-time data to transmit up the tether to controllers in the ship.
I will touch on more of what makes Nereus so unique in later posts. Now, we wait for the weather to clear so that we can make the deceptively short journey to another planet right here on Earth.