by Bruce Moomaw
Cameron Park - July 27, 2000 - At least three approaches to this problem are being considered. One version of the SSE would have its thin tether contain not only electric lines, but a very thin capillary line (only a fraction of a millimeter wide), which the lander on the surface could fill with Martian air chilled and compressed so that its CO2 liquefies.
An adequately thin tether would need a surprisingly small amount of this liquid to fill it (if the capillary line was 1/2 mm wide, each kilometer of it would hold less than 1/5 liter of liquid).
The lander would pump liquid CO2 down the line into a small container in the SSE -- and finely ground particles of rock (quite big enough to contain microbes) could be periodically mixed with it, after which the liquid could be squirted back up the line to the surface.
A more ambitious version of the SSE for later Mars missions would actually extrude its own smooth liner to keep its drill hole open. Two liquids would be pumped down to the SSE and mixed to harden into epoxy -- and the walls of the forming epoxy liner itself would be molded to contain the holes for more of the formative liquid to be pumped down through it. Then, small cylindrical rock cores collected by a sample drill onboard the SSE could periodically be shot pneumatically up the shaft back to the surface.
Only 50 kg of material could make a 2-km long hole liner 6 mm wide with a 3-mm hole in its middle -- but obviously this is a design for a rather ambitious unmanned Mars lander.
Finally, there's the "inchworm mole" being separately developed by Honeybee Robotics (the firm which developed the very efficient rock core drill that will be carried on the Athena long-range rover which the U.S. may launch to Mars in 2003).
This gadget would be the ultimate in moles: a completely self-powered, self-contained, tetherless vehicle that would use the electrical power from an onboard RTG to drill down through kilometers of rock, collect samples, and then actually crawl back up to the surface with them.
It would consist of an upper and lower section -- each with a rotating drill head at its tip and braking shoes that could be extended from its sides -- which are connected by a telescopic joint allowing the mole to increase and decrease in length.
While the upper section anchored itself in place with its brakes, the lower section would drill down deeper through the rock and the inchworm would stretch out until its maximum length had been reached -- at which point the lower section would anchor itself with its brakes, and the upper section would release its grip and slide down.
After finishing its explorations and collecting its samples, the mole would then crawl back up to the surface by reversing the procedure: the lower section would anchor itself in place while the upper section drilled upward to its full length, and then the upper section would anchor itself and the lower section would slide upwards to join it.
Honeybee has already developed a non-drilling inchworm that can crawl through narrow underground pipes to inspect them -- but clearly, again, this is a project for the more distant future of unmanned Mars exploration.
Indeed, it's abundantly clear that just drilling deep into Mars (let alone returning samples to the surface) is an extremely difficult task, and one that could be very easily tripped up by even modest errors in our understanding of the physical consistency and structure of the Martian subsurface.
Regardless of what technique we use, the first penetrations underground will have to be shallow ones -- and, in fact, JPL's only near-future goal is a probe that could drill about 200 meters deep and analyze its own samples.
But if MGS' new revelations of possible briny liquid water within only a short distance of the surface are correct, obviously even such a limited probe could produce tremendous scientific dividends.
At this month's Houston conference on "Concepts and Approaches for Mars Exploration", Brian Wilcox expressed hope that the MGS discovery would greatly increase support for such a shallow near-future drilling mission, especially if radar sounding from orbit can accurately locate those spots on Mars where liquid water is closest.
In fact, if liquid water can be located within 10 or 15 meters of the surface anywhere on Mars, returning samples to the surface for more detailed analysis or return to Earth may be immediately practical after all -- using the simpler time-honored techniques of ordinary drilling.
Honeybee Robotics has already developed another kind of rotary core drill which can easily bore down through 5-10 meters of soil or rock, then seal off its tip and pull the entire core sample back up to the surface inside a separate sliding liner within the outer drill-shaft.
The problems that the drills on the Apollo expeditions encountered drilling only 3 meters into the Moon were mostly due to the fact that the eons-long rain of meteorites there had greatly compacted the subsurface soil and rubble -- a problem unlikely to exist on Mars.
NASA would also very much like to have such a 10-meter core drill for another of its highest-priority near-future Solar System missions: a spacecraft that would land on the surface of a comet nucleus and return subsurface samples of material unmodified by the heat of the Sun.
The Honeybee drill in its 5-meter version is only about 15 kg. It contains a rotating vertical cylindrical rack (like the chambers on a Gatling gun) that holds a whole set of short sections of drill shaft and attaches them one after another to the growing buried shaft (and, later, pulls the inner shaft back out of the ground and detaches its sections one at a time to store them and their contained samples).
Such a device would be invaluable on a near-future Mars sample-return mission even without MGS' spectacular new revelations. And, while plainly subsurface drilling -- like acceptably cheap Mars sample return to Earth -- is a technology that will have to be carefully developed one step at a time by flying a whole series of engineering test missions to Mars, MGS has definitely provided a powerful new incentive for starting the development of both those technologies as soon as possible.
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