A standoff sample collection system would be capable of quickly obtaining a sample from environments as varied as comets, asteroids, and permanently shadowed craters on the Moon from vehicles ranging from traditional planetary spacecraft to platforms such as hovering rotorcraft or balloons at Mars, Venus, or Titan. The depth of penetration for this harpoon-based hollow collector design was experimentally determined to be proportional to the momentum of the penetrator, in agreement with earlier work on the penetration of solid projectiles. A release mechanism for the internal, removable sample cartridge was tested, as was an automatic closure system for the sample canister and tether recovery approaches.
By using a charge to power the harpoon into the target, the depth of penetration can be calculated depending on how well the properties of the regolith are known. A mission could potentially carry several harpoons, each carrying charges of different power, or could even consider loading charges for the harpoons incrementally as more information is gathered by the mission after arrival at the target. Powered harpoons could sample terrain that is much too rough to risk approaching with a spacecraft, or that might be completely unknown (such as a permanently shadowed crater).
The harpoon-based sample collector and harpoon test facility were developed to benchmark various harpoon and collector designs. The current design uses dual six-spring steel custom automobile leaf springs coupled to a half-inch steel cable to power the test stand.
The data demonstrates that the penetration depth of the harpoons varied with the momentum. While this is similar to the behavior determined for solid projectiles, the harpoons are hollow, and it was uncertain if the compression of the target material in the gullet of the harpoon would significantly change the behavior of the harpoon. It also means that the depth of penetration can be scaled by varying either the initial velocity or the mass of the harpoon.
While a cylindrical harpoon was initially envisioned, it is much easier to close and seal a rectangular cross-section. Thus, the initial design uses a square cross-section sample canister inside a square cross-section sheath. Making the harpoon sheath round, but keeping the sample canister square in cross-section, will not only allow room within the sheath for such an electronic failsafe mechanism, but will also enable incorporation of other small instruments into the sheath. The two additional features are a three-axis accelerometer to determine the strength of the regolith and a thermocouple to record its temperature. The addition of a battery and a transponder to send the accumulated data to the spacecraft will complete a very robust package. The recovery tether is unique, utilizing a dual, slightly curved, thin metal tether approach to provide minor stiffness in some directions, and dynamics and motion control.
This work was done by Joseph Nuth, Donald Wegel, Lloyd Purves, Edward Amatucci, and Michael Amato of Goddard Space Flight Center. GSC-16869-1