Payload delivery penetrators are bullet-shaped vehicles designed to penetrate a surface and emplace experiments at some depth. The basic technology for these has existed for several decades based largely on military heritage (e.g. Simmons, 1977; Murphy et al., 1981a; Bogdanov et al., 1988), however only in the mid 1990s did proposals for their use in Solar System exploration begin to be adopted for actual flight. In the US, Mars penetrators were studied for several years (and, indeed, field tested) as part of a possible post-Viking mission, while in the Soviet Union planetary penetrator work seems to have started in the mid 1980s.
Impact speeds range from about 60 to 300 ms"1. The resulting impact load experienced by penetrators as they decelerate in geological materials routinely exceeds 500 g, and terrestrial systems in the military field can be rated at 10 000 g or even 100000 g, although the choice of components at these levels is severely limited (being more suited to the relatively simple job of triggering a detonator than making planetary science measurements). Additional impact damping may be included in the form of crushable material (e.g. honeycomb or solid rocket motor casing), sacrificial 'cavitator' spikes protruding ahead of the penetrator's tip (e.g. Luna-Glob high-speed penetrator concept, with speeds exceeding 1.5 km s"1) and gas-filled cavities (e.g. the Mars 96 penetrators).
Masses have ranged from the tiny DS-2 Mars Microprobes at 2.5 kg each (excluding aeroshell) to 62.5 kg each for the Mars 96 penetrators.
Penetrators may consist of a single unit, or a slender forebody and a wider aftbody linked by an umbilical tether, the two parts separating during penetration to leave the aftbody at the surface. Expected forebody penetration depths have ranged from — 0.5 m for the Mars Microprobes (impacting at — 190 m s"1) up to 4-6 m for Mars 96, with 1-3 m expected for the single-body Lunar-A penetrators, which do not need to retain any components at the surface (13 kg each, 140 mm diameter, impacting at —285 ms"1).
Power is usually provided by primary batteries or radioisotope thermoelectric generators (RTGs), although solar arrays have been high-g-tested successfully. The DS-2 Mars Microprobes' nominal lifetime was only a few hours, while the Lunar-A penetrators are expected to have enough power for about a year. Transmission of data back to Earth is usually by means of an omnidirectional antenna and a relay spacecraft.
Experiments flown on (or proposed for) penetrators include the following.
• Thermal sensors (temperature profile, thermal conductivity/diffusivity, heat flow)
• Permittivity/conductivity sensors
• Spectrometers (y-ray, neutron, a/proton/X-ray, X-ray fluorescence, etc.)
• Sample collection for evolved gas analyser/mass spectrometry/spectroscopic analysis
• Penetrators with combined sampling and pyrotechnic return
• Explosive charge
• Meteorological sensors (not applicable to atmosphereless bodies of course!)
Sadly, neither the Mars 96 penetrators nor the DS-2 Mars Microprobes completed their missions - Lunar-A now has the task of demonstrating penetrator technology on another world for the first time, although at the time of writing no launch date has been set. Table 19.1 gives key references for penetrator missions and proposals.
Table 19.1. Penetrator missions, studies and key references
Mars 96 penetrators DS-2 Mars Microprobes Lunar-A penetrators Vesta/Mars-Aster penetrators CRAF/Comet Nucleus Penetrator Luna-Glob high-speed penetrators Luna-Glob large penetrators/
Polar Station BepiColombo Mercury surface element-penetrator option BepiColombo Mercury surface element-hard lander/penetrator option Polar Night lunar penetrators
Surkov and Kremnev, 1998
Smrekar et al., 1999; Smrekar et al., 2001
Mizutani et al., 2001
ESA, 1988; Surkov, 1997
Boynton and Reinert, 1995
Galimov et al., 1999
Surkov et al., 1999; Surkov et al., 2001
Pichkhadze et al., 2002
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