I'm glad to see new efforts in exploring deep oceans. It's amazing to me how little effort is spent exploring our own planet. We know very little about the deep parts of the ocean.
The world has had the technology to go there since 1960 when a two man crew in the Trieste went to the Challenger Deep. I think the reason new subs don't have a crew is because it's cost prohibitive. Since the median ocean depth is 1/3rd of the depth... most of these subs aren't engineered for extremes. My guess is the next time a crew goes there it'll be funded by some rich explorer looking for a challenge (Richard Branson, etc). Hopefully he'll make some real innovations and share them with the community. In the meanwhile it's good to see a new robotic sub visiting.
Unfortunately the guy who was doing this was Steve Fosset, he commissioned several submarines the likes of which no one had ever seen before but after his death no one has really picked up the ball.
"The water pressure at the bottom of the Mariana Trench is like having the weight of three SUVs on your big toe"
"The fiber optic tether transmits high bandwidth data only, not power...As the vehicle moves through the water, the tether is laid from the vehicle, avoiding any dragging of the tether through the water. This decreases the energy load on the vehicle and the strength requirements for the tether; hence its size and weight can be minimized"
This is amazingly awesome. At the end it talks about how this device connects to the surface using cabling thinner than a human hair. Technology just blows my mind.
but [steel cable] systems would snap under their own weight in the pressure conditions of the Mariana Trench.
Does that make sense? I don't see how pressure can make things weigh more... but I guess 10km of steel cable will have some weight. I'd have thought you'd make the cable have the same net density as water, so it is effectively weightless.
The problem is the weight of the cable. You can reduce the effective weight of the cable in water by, for example, combining polypropylene and steel strands (they experimented with this for some anchor cables in the late 70s early 80s) but the cable has larger diameter with greater drag on the moving robot. Maybe even enough drag to snap the cable anyway. Also, I am not sure how well the experiments turned out, I can see several other possible problems with the combined cable.
Materials act differently under different conditions. Lots of pressure can make some materials very brittle or even softer. Brittle materials plus the weight/stress from the water currents involved could be what they are referring to.
I don't know the specifics of how the steel/iron/other compounds would react to that pressure but that's a guess.
Edit:
Other sources leave out the pressure part, it looks like the weight of the cable would be the issue. I guess it's also why we don't have a space elevator yet.
There is no void (air) inside the cable, and the pressure is even all the way around - the only crushing you can do is move the atoms closer to each other. (Compress actually, not crush, and iron is basically incompressible.)
"moving the atoms closer to each other" is what I was talking about; under some circumstances, that could manifest as a shift to a different crystalline structure. (If you search for information on high-pressure crystalline phases of materials, you'll find a lot of interesting things.) "Iron is incompressible" is often a useful approximation, but no material is actually incompressible (that would entail, among other things, faster-than-light propagation of shock waves and infinite hardness) and many day-to-day materials are less compressible than iron. Glass, for instance.
Most metal objects do actually have voids in them, but you can manufacture metal objects that don't, and it's not clear that having voids in your cable would cause it to be damaged by pressure.
The world has had the technology to go there since 1960 when a two man crew in the Trieste went to the Challenger Deep. I think the reason new subs don't have a crew is because it's cost prohibitive. Since the median ocean depth is 1/3rd of the depth... most of these subs aren't engineered for extremes. My guess is the next time a crew goes there it'll be funded by some rich explorer looking for a challenge (Richard Branson, etc). Hopefully he'll make some real innovations and share them with the community. In the meanwhile it's good to see a new robotic sub visiting.
http://en.wikipedia.org/wiki/Bathyscaphe_Trieste
http://en.wikipedia.org/wiki/Challenger_Deep
EDIT: Added a few sentences.