Luna Robotic Sample Return

NASA/GSFC/Arizona State University. Public domain (NASA / US government work).

The problem

A robotic sample return has to land on a surface it cannot inspect beforehand, acquire regolith with a mechanism no one can adjust by hand, seal the material, lift off and survive direct reentry. On the Moon the acquisition step is hard for specific reasons: vacuum, a large thermal swing, one-sixth gravity that limits the weight available on the bit, and soil whose density and strength rise steeply within the first meter [2][3].

The top 10 to 15 cm is loosened by micrometeorite gardening [2]. Bulk density runs from about 1.30 g/cm3 at the surface through 1.52 g/cm3 at 10 cm toward an asymptote of 1.92 g/cm3 [2][3]. The in situ mean is 1.50 g/cm3 over the top 15 cm and 1.66 g/cm3 over the top 60 cm [1], and Apollo relative density rises from 65 percent to 92 percent with depth [8]. Thermal and mechanical limits compound this: the diurnal surface swing reaches 374 K at the maximum and 92 K at the minimum, and a permanently shadowed polar crater averages about 40 K [3]. A fixed lander adds one more constraint: the sample site is whatever the arm or drill can reach.

What has flown

Luna 16 (1970). The first automatic return from another world drilled for 7 minutes to a stop at 35 cm and returned 101 g, with some soil lost during the return [10][11]. Luna 17 carried Lunokhod 1, whose penetrometer made 537 soil measurements over a 10.5 km route [10]; an earlier Apollo-era report counts 327 [7].

Luna 20 (1972). The same drill class reached 25 cm and returned 55 g [11]. Compacted Luna 16 and Luna 20 samples gave internal friction angles of 46 and 32 degrees in Coulomb box tests [5].

Luna 24 (1976). The deepest Soviet penetration: 200 cm of drilling and 170 g returned [11]. The returned core was about 160 cm long [6].

Apollo (1971 to 1972). The Apollo Lunar Surface Drill ran at 280 r/min with 37.8 Hz impact at up to 4.4 J, and Apollo 17 returned 305 cm of core and 110.5 kg [11]. Hand-hammered Apollo 15 to 17 core tubes reached a practical limit near 70 cm after about 50 blows [2]. The Apollo 15 heat flow auger reached 1.5 m against 3.0 m planned [3].

Chang’e-5 (2020). The mission returned 1.731 kg from a 200 cm drilling depth, and its sampling arm repeated positions to better than 1 mm [11]. Another account puts the recovered core at about 1 m against a goal of about 2 m, and records 12 scoops gathering about 1.5 kg [28]. The two figures describe different quantities and the sources do not reconcile them.

Chang’e-6 (2024). The first farside return brought back 1935.3 g [23]. A vision pipeline selected sample points from a binocular point cloud and estimated the sampler pose by point cloud registration [7]. The four-degree-of-freedom arm carried about 30 mm and 2 degrees of absolute error before correction, pose measurement reached 1.1 mm and 0.4 degrees, and scooping depth error was 3.8 mm [7]. The sampling arm reached a 2.8 m radius sector of 120 degrees [7]. The returned farside soil is strongly cohesive: static angle of repose 52.9 degrees, mean particle size 37.3 um [23]. The samples are characterized petrologically in [16].

Lunar PlanetVac. Not a return mission. A pneumatic sampler on the Blue Ghost lander captured 13 g of regolith at Mare Crisium [31]. A later account gives 6.5 cc collected in a five-second sequence with a 1 mm particle separation step [33].

OSIRIS-REx (2020, asteroid Bennu). A non-lunar comparison for gas-driven acquisition: the TAGSAM head carried a 150 g requirement, and ground testing collected as much as about 1.9 kg, with gas-driven collection improving mass by a factor of 4 to 5 over mechanical contact in one comparison [3].

What is being tried now

Rotary-percussive drilling for volatiles (flown to checkout, ground-tested). TRIDENT is a 1 m drill with a 25.4 mm bit, 2 J per blow and up to 972 blows per minute, built as flight units for PRIME-1 and VIPER [32]. The engineering unit drilled nearly 30 m over 337 bites in rock of 4 to 120 MPa below -100 C [32]. The reported flight result is a functional checkout of about 20 minutes that extended the deploy and feed stages to 35 and 100 cm [32]. At Haughton Crater a flight-design drill put 7.80 m of hole in the ground, 2.4 m of it in ice or ice-cemented breccia, and recovered from 5 fault states, but choked in an ice-cemented unit at about 0.26 cm/min [12]. In Antarctica 15 boreholes showed that drill telemetry alone identifies an ice-rock transition near 62 cm, and a percussive spring failed after 80 to 90 m of cumulative drilling [4]. A rover-mounted TRIDENT inside an automated Atacama life-search mission drilled to depth and delivered samples to instruments, and a drill string jammed near 72 to 73 cm in one hole [26].

Thermal and load modeling (ground only). A discrete element model calibrated at -130 C finds an average temperature rise of 2 to 4 C in cryogenic simulant, with 10 to 20 C more at the tool face in vacuum [10]. For auger drills of Chang’e-5 class in dense simulant, weight on bit falls from 72.1 N at 60 r/min to 7.8 N at 200 r/min at 85 mm/min penetration [24].

Pneumatic acquisition (flown once on the Moon). PlanetVac removes the drill reaction problem and has one reported lunar sampling event [31][33]. The Dual-Rasp concept pairs rasp cutters with closed pneumatic sample transfer for an Enceladus lander and was tested in parabolic flight [2].

Vision-guided scooping (flown on the Moon). The Chang’e-6 pipeline is described above [7].

Preserving in situ conditions (proposal and laboratory). All lunar coring to date is regular coring that loses volatiles, vacuum state, bedding and compactness. A proposed method with a polyborosiloxane self-healing seal film reached 99 percent healing efficiency in the laboratory [11].

Mars sample caching and return (flown on Mars, return not flown). The Mars 2020 coring drill takes cores 13 mm in diameter and 76 mm long [27]. Its percussion mechanism operates at 25 to 40 Hz and was life-tested beyond three times the nominal requirement, against a plan of 43 cores and 74 abrasions [8]. The sampling and caching subsystem can return 37 tubes [21]. Curiosity’s drill pressed its stabilizers on the rock with 300 N and drilled holes of 63.94 to 65.86 mm [1]. As of 21 March 2024, Perseverance had filled 21 rock core tubes, and the campaign report records no demonstrated performance for orbiting-sample detection, rendezvous or the transfer arm [18]. The sample transfer arm is a 7 degree-of-freedom, 2.2 m reach design that inserts up to 30 tubes, with 9.35 mm lateral absolute placement error at the orbiting sample container in analysis [11]. No Mars sample has been returned.

Autonomous sampling on ice (ground only). A JPL prototype for the Europa Lander concept ran 58 field tests at glacier sites [5].

The numbers

Mission	Drill depth	Sample returned	Source
Luna 16	35 cm	101 g	[10][11]
Luna 20	25 cm	55 g	[11]
Luna 24	200 cm	170 g, core about 160 cm	[11][6]
Apollo 17	305 cm	110.5 kg	[11]
Chang’e-5	200 cm	1.731 kg	[11]
Chang’e-6	surface scoop	1935.3 g	[23]

System	Status	Figures	Source
TRIDENT	flight checkout, ground-tested	26.5 kg with avionics, 19.5 kg mechanism, 25.4 mm bit, 2 J per blow	[32][4]
Lunar PlanetVac	one lunar event	13 g; 6.5 cc in 5 s	[31][33]
Rock Abrasion Tool	flown on Mars	489 g, 10 W	[12][9]
DAME drill	field trial	3.2 m, under 150 W, 5 of 6 fault types recovered	[9]
MARTE drill	field trial	6.1 m	[9]

Soil properties from the Soviet and Apollo programs:

• Soviet synthesis: porosity 0.8, modal bearing capacity 0.35 to 0.40 kg/cm2, cohesion 0.04 to 0.06 kg/cm2, friction angle 20 to 25 degrees [5].
• Lunokhod 1 cone penetrometer: 8.1 cm at 16.5 kgf on horizontal intercrater ground, 7.8 cm at 19 kgf on a crater rim, 20.5 cm at 23.5 kgf on a crater slope, 5.9 cm at 23 kgf on a stony section [4].
• Fitted depth model: total shear strength 7.4 kPa at 1 m, porosity 40 to 50 percent [2].
• Apollo logarithmic law: density 1.27 plus 0.121 ln(z+1) g/cm3 with z in cm [8].
• Intercrater friction angle and cohesion: 42 degrees and 0.52 kPa over 0 to 15 cm, 54 degrees and 3.0 kPa over 30 to 60 cm [1].

What is not established

• The LPI Luna 24 volume contains no drill description, load, power or retention record; it is geochemistry [6].
• Core length is about 160 cm in most contributions and about 191 cm total subsurface depth in one, and the volume does not reconcile them [6]. It also records a gabbro fragment broken up by drilling and an abundance spike at 149 cm attributed to breccia crushed into the core tube [6].
• Chang’e-5 recovered core length and drilling depth are reported as different figures that the sources do not reconcile [28][11].
• The Soviet soil values are quoted from a projected figure, with several depths interpreted from curve shape, and the Lunokhod and Apollo penetrometers sampled depths too different to compare directly [4].
• Shear strength below about 1 m is extrapolated from a fitted curve, the 3 m stable-wall figure is an analytical scaling, and the data come from equatorial and mid-latitude sites, not polar regolith [2].
• TRIDENT has reported no drilled hole on the Moon [32], and PlanetVac has one reported event [31][33].
• Mars sample return has no returned hardware, and orbiting-sample rendezvous and transfer arm performance are not reported [18].

References