Author's personal copy Available online at www.sciencedirect.com Advances in Space Research 48 (2011) 403–431 www.elsevier.com/locate/asr Planetary penetrators: Their origins, history and future Ralph D. Lorenz ⇑ Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA Received 6 January 2011; received in revised form 19 March 2011; accepted 24 March 2011 Available online 30 March 2011 Abstract Penetrators, which emplace scientific instrumentation by high-speed impact into a planetary surface, have been advocated as an alter- native to soft-landers for some four decades. However, such vehicles have yet to fly successfully. This paper reviews in detail, the origins of penetrators in the military arena, and the various planetary penetrator mission concepts that have been proposed, built and flown. From the very limited data available, penetrator developments alone (without delivery to the planet) have required $30M: extensive analytical instrumentation may easily double this. Because the success of emplacement and operation depends inevitably on uncontrol- lable aspects of the target environment, unattractive failure probabilities for individual vehicles must be tolerated that are higher than the typical ‘3-sigma’ (99.5%) values typical for spacecraft. The two pathways to programmatic success, neither of which are likely in an aus- tere financial environment, are a lucky flight as a ‘piggyback’ mission or technology demonstration, or with a substantial and unprec- edented investment to launch a scientific (e.g. seismic) network mission with a large number of vehicles such that a number of terrain- induced failures can be tolerated. Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Penetrators; Space missions; Landers; Moon; Mars 1. Introduction sloppily interchanged, we would urge rigor in maintaining the distinction. The definition, by requiring a function Penetrators have been proposed as planetary explora- post-impact, also excludes inert impactors such as cannon tion vehicles for four decades, but have yet to fly success- shot, upper stages used to impact the moon, or the comet fully. Penetrators are self-contained vehicles designed to impactor on the Deep Impact mission, whose only function function after traversing some distance in a solid target is to deposit kinetic energy in a target with no post-impact using the kinetic energy of their arrival. This definition operation. (where ‘some distance’ is understood to imply a length Although penetrator advocates often cite extensive mil- scale comparable with or greater than the vehicle length) itary experience with such systems, the history of military excludes self-hammering drills such as the ‘mole’ carried developments and its relationship to the planetary program on Beagle 2, and instruments (‘penetrometers’) to measure has not been laid out in the literature previously. Therefore surface mechanical properties which are part of a larger this paper discusses the military developments that led to vehicle, such as those on Venera landers or the Huygens the advocacy of penetrators as planetary exploration probe (e.g. Lorenz et al., 1994). All of these systems are dis- vehicles. cussed in two proceedings volumes (Ko¨mle et al., 2001; The paper then discusses in detail the various planetary Kargl et al., 2009) following workshops in Graz, Austria penetrator mission concepts (e.g. Fig. 1) that have been in 1999 and 2006, and although the terms ‘penetrometer’ proposed, and the smaller number that have been devel- (an instrument) and ‘penetrator’ (a vehicle) are often oped and/or flown: the timeline of developments is shown in Fig. 2. Finally, the relationship of the probability of fail- ⇑ Tel.: +1 443 778 2903. ure of an individual vehicle’s emplacement and the real or E-mail address: [email protected]. perceived likelihood of programmatic success of the 0273-1177/$36.00 Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2011.03.033 Author's personal copy 404 R.D. Lorenz / Advances in Space Research 48 (2011) 403–431 Fig. 1. A selection of penetrator vehicles shown to the same scale. (A) An ACUSID air-dropped acoustic monitor used in the Vietnam conflict, (B) the original Comet Rendezvous Asteroid Flyby (CRAF) penetrator – note the wide flare, (C) the Mars-96 penetrator (D), NASA proposed 1970’s Mars penetrator, (E) Lunar-A penetrator, and (F) DS-2. Note the remarkably small size of DS-2 compared with the other vehicles. Fig. 2. A schematic of the development history of various penetrator projects. ‘S’ denotes study, while ‘D’ represents more substantial development, including penetration tests, hardware construction and flight. mission overall is discussed, and the (narrowing) scientific the scientific capabilities of instrumentation on penetrators niche for penetrators is noted. A detailed discussion of will be the subject of a separate paper. Author's personal copy R.D. Lorenz / Advances in Space Research 48 (2011) 403–431 405 2. Terrestrial penetrators (‘Panzersprengbombe’, Fleischer, 2003) with more care- fully engineered casings (more slender-nosed, harder steel, The origination of penetrators as planetary exploration and a larger casing-to-explosive ratio) were used by the vehicles is inevitably connected with certain military devel- Luftwaffe. A radio-controlled armor-piercing bomb (the opments, which we will review here to set the planetary PC-1400, or Fritz-X, e.g. Wolf, 1988 or the recent detailed applications in context. There are also a few examples of account by Bollinger (2010)) was also fielded. Since pene- civilian or dual-use instrumentation deployed in this trating weapons typically must be close to their targets, manner. the introduction of guidance is a key development and this and similar radio-controlled weapons subsequently devel- 2.1. Early penetration mechanics oped in the USA Azon and Razon (e.g. Schmitt, 2002) set the stage for future precision-guided weapons, to which Probably the first quantitative assessment of projectile we will momentarily return. penetration into the ground was by Benjamin Robins, Of particular interest in connection with planetary pen- Engineer-General to the East India Company, notably in etrators (and better documented in the English-language his treatise ‘New Principles of Gunnery’ (Robins, 1742), literature), however, are the large unguided weapons that which applies Newton’s (1687) analytic methods to artil- were developed in Britain to address a class of military lery. Robins developed a number of remarkable innova- and industrial targets such as large earthen dams1 such as tions in instrumentation, notably he devised the ballistic the Sorpe dam which could not be practicably breached pendulum to measure the launch velocity of projectiles, by conventional bombing. The British engineer Barnes as well as a whirling-arm apparatus to measure aerody- Wallis proposed (e.g. Murray, 2009) that a large penetrat- namic drag. Even viewed purely as an aerodynamicist, his ing bomb could efficiently couple its explosive energy to the careful investigations were far ahead of his time. He first dam by seismic means (hence ‘earthquake bomb’). In fact recognized the utility of streamlined projectiles and by the mechanism is largely one of explosively creating an careful experimental gunnery (using specially-prepared pre- underground cavity (‘camouflet’) into which the target cision cannonballs, and accurately-measured powder structure subsequently collapses. charges) he was able to detect the effect of spin on projec- This mission required not only a weapon with a large tiles, namely the Robins–Magnus effect, often rather explosive warhead, but also a hardened nose such that unfairly referred to only by the latter, whose experiments the weapon could burrow to depth. In addition to the took place some 150 years later (e.g. Lorenz, 2006). He structural strength required, the explosive filling had to tol- confirmed the dependence of drag on the square of velocity erate the impact loads without detonating prematurely. and even detected the increase the transonic rise in drag Furthermore, the fuzing systems had to function after coefficient (the ‘sound barrier’). impact – often three separate fuzes were installed in order From his experiments, in particular with the ballistic to ensure that at least one would work (although even then, pendulum, Robins’ determined that the penetration of shot at least one bomb failed to explode – one Tallboy bomb into typical solid materials varies as the square of velocity. was found in the mud at the Sorpe dam when the reservoir This essentially means the kinetic energy of delivery is dis- was drained in 1958). sipated by work against a constant strength target (kinetic Compared to other bombs, these weapons (first the Tall- energy per unit area has the same dimensions as strength boy 5443 kg, then the Grand Slam, 10,160 kg – even larger times penetration distance – equivalently the kinetic energy US weapons such as the T-12 were developed later, see per unit volume of ‘tunnel’ swept by the projectile is a mea- Fig. 3) were fairly slender and streamlined – the Tallboy sure of target strength.) was 6.4 m long. This assured both effective penetration of the target (for a constant target strength and impact 2.2. World War 2 ‘earthquake bombs’ energy, following the considerations in Section 2.1, the penetration distance will be maximized by reducing the Despite these rigorous beginnings (Robins work became projectile cross-sectional area) but also maximized