Imagine a place where every single inch of your skin is under eight tons of weight. That's not some planet in a galaxy far, far away: it's the bottom of the bottom of the ocean. At its deepest point the Mariana trench is 36,000 feet (11,000m) deep, and water pushes from all sides with 16,000 psi (pounds per square inch) -- over 1000 atmospheres. That spot, called the Challenger Deep, is where James Cameron plans to be traveling very soon. His secretively constructed submersible, the Deepsea Challenger, is designed to test the limits of the oceans and allow him to see, record, and bring samples back from its most remote region.
What does it take to build, operate, and safely retrieve a submersible -- many in the industry reserve the term submarine for military vessels -- from those depths? Cameron and his team haven't been saying much, but analyzing past efforts by Cameron and others, plus what is known publicly, is enough to take you through the tech needed to make Cameron's vision a reality. Dealing with the intense pressure, tricky buoyancy issues, keeping electronics working, and of course creating memorable video from the seabed top the list of challenges.
Pressure
First and foremost of the challenges is the mind-numbing amount of pressure: 16,000 psi. That's like having three entire cars resting on your thumb. And three more on your index finger, and so on. Plenty of materials are strong enough not to be crushed under the pressure, but not very many can be formed into a chamber large enough to fit a human inside with a way to see out.
Classic submersibles, like the Trieste -- the only manned vehicle to have ever reached the Challenger Deep in the Mariana Trench before -- had the tiniest of windows set into massively thick hulls. As Liz Taylor of DOER, which builds undersea craft for researchers says, scientists really want to see. Otherwise, of course, it'd be simpler just to send down an unmanned craft with some cameras. DOER has taken the radical approach for its proposed deep sea vehicle by planning a single, 4-foot in diameter cast sphere of glass. Taylor contends that by having the glass made out of a single piece, it will be strong enough to withstand the pressure. Of course it will still need a hatch opening and some way for all the sub's control systems and camera gear to communicate from inside to outside.
Since the DOER project is still on the drawing board, it's hard to know whether it will work out, but existing designs have taken a more conservative approach to windows. Richard Branson's group has created a "half-dome" of borosilicate (essentially a fancy version of what we all know as Pyrex) that provides an impressive visual range for the single pilot laying in the vehicle. Unfortunately the viewing window on their submersible cracked under a "mere" 2200 psi, casting doubt on how far down the craft will be able to travel without a major rework. As recently as last year, Cameron speculated that Branson was ahead in the X Prize "race to the bottom," but now the tables have turned and Cameron is fast closing in on the prize.
It is likely that Cameron, being brave but not entirely foolish, has taken a more conservative approach, using a smaller porthole that will prove safer in the long run. Even simple portholes are made complicated by the pressure. Instead of a straightforward "plate" like we've all seen on cruise ships or in movies featuring diving bells and shallow water subs, deep sea craft typically use a conical shape, with the outer surface of the window being larger than the inner. That distributes the pressure over a broader area of the supporting hull and makes it possible to build windows that are both safer and more secure.
The design of the pressure vessel, also called a pressure hull, is also a feat of engineering. Pure steel chambers are strong, but traditional steels tend to be much too heavy to be floated easily. Titanium is a better choice, but also heavier than is ideal. Ceramic offers a lightweight and very strong option, but is very brittle and therefore subject to cracking. The Nereus -- an unmanned craft which Woods Hole sent into the Mariana Trench -- had all of the openings in its pressure vessel in the titanium half of its hybrid ceramic-titanium model to avoid further stressing the ceramic piece. Some research has been done into using a ceramic pressure vessel with a glass dome at one end, but it is not clear if any of it is ready for use on the Challenger. Cameron's team has now said that the actual "pilot sphere" is a 43-inch steel unit.
There is another equally deadly, but less obvious, threat from pressure. If a low-pressure area gives way, the resulting force of water at 16,000 psi rushing to fill the space is the equivalent of a large explosion -- in this case actually an implosion. If it were to happen, it is likely the entire vehicle, and its crew, would be destroyed. That is a big reason that deep sea craft are best designed with just a single low-pressure chamber for the crew, while all the other gear is built to simply survive with the pressure -- creating some other design challenges covered later in this article.
Buoyancy
When all the weight for a pressure hull, occupants, batteries, cameras, and other pieces are added together, our deep sea submersible weighs in the tons. It is not known how heavy Cameron's sub is, but Branson's version weighs in at at least 8000 pounds (3628kg). Left to itself, even with the buoyancy of the air inside the pilot's pressure hull, the subs would sink.
A traditional answer to creating the additional buoyancy needed is simply to leave "space." After all, air is much lighter than water, so if the sub was simply filled with air, it would float nicely. Except that there really isn't any way to make the entire sub a pressure hull. And if there was, every square inch of it would have to be completely perfect or the result would be a massive implosion. So instead, extra space inside the hull needs to be packed with something lighter than water that won't give way.
The Trieste, launched in a less-ecologically-aware 1960, used aviation fuel for buoyancy. It is sufficiently lighter than water, but not so compressible as to lose its usefulness under pressure. It is a myth by the way that water, oil and gas don't compress -- at 16,000 psi everything compresses. The Deepsea Challenger itself will compress by about 2.5 inches during the descent, according to sponsor National Geographic(Opens in a new window).
Fortunately, technology has advanced since the day of the Trieste and modern subs often use a clever composite called syntactic foam -- basically a hybrid of glass microspheres in a type of resin -- to serve the same purpose. It can be molded to fit available spaces inside a sub. Now that our sub floats, the next challenge is allowing the crew to adjust its buoyancy to go up and down in the water.
Everyone probably understands the way that submarines submerge and surface, typically by using pumps to empty and fill their ballast tanks. Simple enough. Except that at 16,000 psi with limited power, there is no way to actually pump against the water pressure -- even if you had enough of something to pump. Amazingly, the best answer modern sub designers have come up with is plain old ballast. The Trieste used steel shot in a chute, which was kept in place by an electromagnet. This allowed the crew to release it on demand, while also creating a failsafe -- if the ship or crew were disabled the battery, the electromagnet was designed to discharge causing the sub to surface before the crew ran out of air.
It is likely that Cameron's team is using a similar solution, probably with similar failsafes. Of course this means that each dive will leave a few hundred pounds of metal -- steel plates in this case -- on the bottom of the ocean, but it may be a while before they pollute the Deep the same way oxygen bottles have littered the sides of Mount Everest. It probably also means that they won't want to leave anything important on the bottom, as it'll be hard to go back for it without an entire new dive. From the brief videos available, it doesn't appear that Branson's sub(Opens in a new window), the Deepflight Challenger, has anything quite as extensive, which may make it difficult for it to safely get as deep as Cameron's specially built Deepsea Challenger.
Video
You may have already asked yourself how Cameron -- perhaps one of the greatest movie-makers on the planet -- is going to record his dives if he is stuck in a tiny pressure chamber with thick, oddly shaped walls. Clearly the camera has to be outside. Of course that means the camera needs to be specially designed to operate in 16,000 psi. If you've ever cleaned your own DSLR sensor, nervous about accidentally cracking the filter, you'll realize that video seven miles down takes a very special camera indeed.
Fortunately, this challenge has already been met. Several different designs are available for deep sea cameras built into small high-tech "bottles" which protect them from the pressure while offering a small viewing port over the lens. It is highly unlikely that Cameron will be satisfied with any of the existing offerings, though, so expect for him to innovate with a 3D camera that provides an IMAX-like video experience, all beamed up for the first time from the ultra-deep.
Since there isn't any natural light at those depths for the camera to work with, the Challenger will have to provide its own. Any gas-based lighting solution, like traditional strobes or flashes, would be crushed immediately at depth. It is, however, possible to use LEDs, which are solid-state devices, in conjunction with oil-filled chambers, to make photo lights which can operate at huge pressures. When the unmanned Nereus went to the Challenger Deep, it also featured specialized triggers to conserve power by only lighting the LEDs when the camera shutter was actually open. That might prove more than a little annoying to a human pilot like Cameron, so expect the lights on the Challenger to stay on while he's down there.
Another tradeoff most research subs have made is in the wavelength frequency of the lights and the camera sensors. Blue-green light is more efficient in the deep ocean, so range is improved with the use of blue-green lights and blue-green sensitive sensors. Here too, Cameron is unlikely to compromise -- so expect him to carry full spectrum LEDs and full-spectrum cameras on the Challenger.
Power and electronics
Even wires can't survive in high pressure without additional protection. They need to be carefully encased in oil-filled ceramic tubes that are run between the pressurized "cabin" and other parts of the vessel, including the cameras, thrusters, ballast controls, and robotic arm.
Similarly batteries often contain empty spaces or gas-filled areas. Special oil-filled versions need to be designed to allow for high-pressure operation. Fortunately sub design has come a long way since the lead-acid battery designs used in early submersibles like the Trieste. To avoid damage from curious fish and sharks, like the one which "attacked" the Alvin submersible, or from mud and debris from the bottom, all of the electronics and other mechanisms will also need to be covered with protective plates of some kind.
Testing
National Geographic has released this video of one of Deepsea Challenger's early test dives.
Everyone knows that for a potentially fatal endeavor it makes sense to test and re-test everything. So logically Cameron would drop the Challenger in a pressure chamber and start the pumps. Unfortunately, it is really hard to recreate 16,000 psi -- a lot harder than recreating the vacuum of space or the air passing by a jet airplane. One of the largest test facilities, the high pressure test facility at Penn State University, was used by the Challenger team -- but its largest chamber is only 5 feet wide and about 13 feet long. Not nearly large enough to fit the entire Challenger vehicle. So clearly only pieces or scale models could be tested.
After testing in a tank, it is off to sea trials. Their importance can not be overstated. Branson's Virgin Oceanic Deepflight Challenger cracked its viewport long before it got to the desired depth, resulting in at least a year's delay in their schedule. Cameron's team is being close-lipped about their schedule, but has been conducting active sea trials near Papua New Guinea, and National Geographic reporters are headed to the ocean near Guam for the next runs. The team's successful test dive to 8,000 meters (about 75% of the way) shows that Cameron's group alone, among all the competition, have covered all the bases in this race to the bottom. Cameron's test dive already makes him the record holder for the deepest solo dive ever. Barring some truly unfortunate occurrence he is likely to go where no man has gone since 1960, when Jacques Piccard -- namesake of the even more famous Star Trek captain -- and Navy Lieutenant Don Walsh last visited the Challenger Deep. And certainly Cameron will bring back much better images, videos, and likely samples, with him.
