by Bruce Moomaw
Cameron Park - July 27, 2000 - Clearly, though, to tunnel through not just soil but hard rock, we'll need a much more powerful mole -- and for several years, JPL engineers have been working on the design for such a mole, the "Mars Subsurface Explorer".
As designed by Dr. Brian Wilcox, this would be about a meter long and 3 or 4 cm wide, with a pointed metal tip. Wilcox calls it a "self-hammering nail" -- and it would be faced with the problem of using only a few dozen watts of power from its very thin connecting tether to pound itself at an acceptable rate (several meters per day) through even fairly hard rock.
To do this, it must store up the accumulated energy from a rather small electric motor and then release it as a series of powerful blows with its internal sliding hammer -- but using a spring (as on the tiny Beagle 2 mole) or compressed gas to do this is very inefficient; they end up converting most of the stored energy into heat.
Wilcox therefore developed (and patented) a "rotary hammer" which is far more efficient at utilizing a motor's energy -- and which is so ingenious that I can't resist describing it here.
It consists of a shaft with long, stretched-out screw threads running down it, which is spun at up to 10,000 rpm by a motor that turns on and off every second or so.
A heavy cylindrical tungsten sleeve -- the "hammer" -- rides at the top of the shaft at the start of each cycle. Every time the motor turns off and the shaft starts to slow down, a ratchet mechanism suddenly slams it to a halt -- and the hammer, still spinning at several thousand rpm, whirls itself down those long screw threads and slams into the SSE's nose end with great force (several hundred kilograms!).
Then, as the motor starts up again and the shaft starts to increase its spin rate, the heavy hammer tries to resist spinning so fast -- and thus twists itself back up the screw threads to the top of the shaft just in time for the next shutoff to occur.
During its first preliminary tests, the SSE pounded its way easily through 8 meters of fairly hard rock -- but its designers would like it to detect particularly hard, dense masses or rock ahead and actually steer around them.
Currently the engineers are working on a subsurface sonar system that would make use of the vibrations produced by the pounding hammer itself -- and the plan is to make the SSE two sections connected by a very slightly "bent pipe" joint, which a highly geared motor could swivel around to different compass headings.
If the joint was left in one position, the SSE would very slowly veer off to one side around an obstacle; but normally the joint would be constantly swiveled around to new headings, allowing the SSE to corkscrew itself almost straight downwards.
Any kind of subsurface mole -- whether it uses a jackhammer, incandescent heat, or simply an orthodox rotating drill head to drill downwards -- has another major problem, though.
It can be equipped with miniature instruments that can do quite a bit of on-site analysis of the composition of the rocks and minerals it encounters, including instruments such as a "Raman spectrometer" that can analyze traces of organic compounds.
But if it's looking for subsurface microbes or their remains, any kind of remotely adequate scientific study will of course require sending samples all the way back up to the surface.
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