Luna Robotic Sample Return

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

Overview

Three Soviet spacecraft returned lunar samples to Earth with no crew involved: Luna 16 in September 1970, Luna 20 in February 1972 and Luna 24 in August 1976, together returning 326 g of regolith [10]. The mass is small next to the Apollo return, but it was recovered without a crew.

Luna 16 launched on 12 September 1970 and landed at 05:18 UT on 20 September in the northeast of Mare Fecunditatis, at 0.5134 S, 56.3638 E as later refined from Lunar Reconnaissance Orbiter imagery [10]. It returned 101 g. Luna 20 launched 14 February 1972 and landed 21 February at 3.7866 N, 56.6242 E, in highland terrain between Mare Fecunditatis and Mare Crisium and only 1.8 km from the crash site of Luna 18, returning 55 g. Luna 24 launched 9 August 1976 and landed 18 August at 12.7146 N, 62.2129 E in Mare Crisium, 2.32 km from the crashed Luna 23, returning 170.1 g.

Two distinct machines flew. Luna 16 and 20 flew the Ye-8-5 with a shallow drill on an arm, sampling to about 0.3 m. Luna 23 and 24 flew the Ye-8-5M, modified specifically to take a deep core: a new drilling and sampling instrument, the LB09, deletion of the low-altitude Kvant altimeter, a lightened toroidal instrument compartment, and an increase in the return capsule’s soil container diameter from 68 mm to 100 mm, all to reach a design depth of 2.3 m instead of 0.3 m [10].

Specifications

Two vehicle designs, described by the properties of the machines rather than by what any one flight achieved.

Parameter	Ye-8-5 (Luna 16, 20)	Ye-8-5M (Luna 23, 24)
Sampling mechanism	drill on the end of an extended manipulator	LB09 slide-rail deep drill
Design sampling depth	0.3 m	2.3 m
Drilling reaction path	through the manipulator	against the dead weight of the landing module
Sample stowage	encapsulated inside the drill pipe	soft coring bag wound into a sample barrel
Return capsule soil container diameter	68 mm	100 mm
Low-altitude Kvant altimeter	fitted	deleted
Percussion control	not documented	impact motor gated by a spring-set drilling pressure threshold
Return capsule reentry velocity	10.95 km/s, up to 350 g deceleration	as Ye-8-5

Every parameter for both vehicle designs is documented in the mission chronicle [10].

Mission profile

Mission	Landing	Drill depth reached	Sample returned
Luna 16	20 September 1970, 0.5134 S, 56.3638 E, Mare Fecunditatis	35 cm	101 g
Luna 20	21 February 1972, 3.7866 N, 56.6242 E, highlands	25 cm, aborted on motor overheating	55 g
Luna 23	1974, hard landing at 11 m/s	0, drill never activated	0
Luna 24	18 August 1976, 12.7146 N, 62.2129 E, Mare Crisium	about 225 cm along the drill axis, about 2 m true depth	170.1 g, core about 160 cm long

Every landing, depth and sample-mass figure in that profile, and the surface stay under one day in each case (Luna 16 lifted off 26 hours after landing, Luna 20 about a day after, Luna 24 slightly less than a day after), is from the mission chronicle [10]; the coordinates were later refined from Lunar Reconnaissance Orbiter imagery.

The Apollo Lunar Surface Drill used on Apollo 15, 16 and 17 covered a 250 to 300 cm depth range at up to 280 r/min, 37.8 Hz impact frequency and 4.4 J per blow, and its parent program’s deepest and largest return, Apollo 17’s 305 cm and 110.5 kg, dwarfs the entire Luna program’s combined 326 g [11]. Luna 24’s 2.25 m is still the deepest robotic-only penetration and sits just below that crewed figure. Chang’e-5’s later double-tube corer returned 1.731 kg from 200 cm, a single-mission mass Luna never approached but a depth Luna 24 nearly matched [11]. Luna 23 landed at 11 m/s instead of the intended 5 m/s, depressurized its instrument compartment, lost a transmitter, and could never be made to drill; LRO imaging in 2012 showed it lying on its side [10]. For comparison, the crewed Apollo Lunar Module landing gear was rated for a survivable vertical touchdown of only 3 m/s on concrete, a single-use crushable design not meant to be walked away from twice [1], so Luna 23’s 11 m/s landing was well outside what contemporary crewed hardware was built to survive. Luna 24 flew the same design successfully two years later.

Luna 24 remains the last Soviet or Russian spacecraft sent to the Moon [10].

Mobility

None. These are fixed landers. The only articulation is the sampling arm, and the sample site is whatever the arm can reach from the landed position. Luna 20 settled on a slope of 8 to 10 degrees [10]; Luna 24 came to rest 10 m from the rim of a 65 m crater, which determined the composition of the returned core. Both are well inside the range Apollo found survivable: measured lunar highland slopes run from 18 degrees at the shallowest Apollo station to 40 to 45 degrees at crater walls, and soil on crater rims runs looser than intercrater ground by 10 to 12 percentage points of relative density, the same kind of local variation that shaped what the Luna 24 drill actually sampled [3]. Neither lander carried wheels, so none of the corpus’s Apollo Lunar Roving Vehicle traction data, such as its roughly 20 degree slope-traction limit, apply here [8].

Power and energy

No power figures are published. Each lander operated for less than a day on the surface and no lunar night survival was attempted. Luna 20’s drilling was terminated by motor overheating rather than by an energy limit [10].

Thermal

No thermal design values are published, beyond the Ye-8-5M instrument compartment being toroidal and lightened for the deep-drill variant [10].

Compute and avionics

No avionics description is published in detail. Descent used the DA-018 Planeta Doppler landing radar with the Vega altimeter on Luna 16; the low-altitude Kvant altimeter was deleted on the Ye-8-5M. The failure mode on Luna 23 is instructive about the architecture: all altitude measurement stopped at 130 m and the lander continued to a hard but survivable touchdown, meaning terminal descent was open loop below that point [10]. The sampling sequence is commanded from Earth against a 2.6 s round-trip light time, and the imaging system is used to choose where to sample; on Luna 16 that site selection imaging was degraded by poor lighting and sampling proceeded anyway. On Luna 20 the imaging system was used to find a scientifically promising spot, and the sample was collected in stages because soil resistance increased with depth; the staging decision was made on the ground [10].

Luna 24 carried the one onboard control loop in the program: drilling pressure was compared against a set threshold by a spring mechanism, and the impact motor was started whenever the pressure exceeded it, so percussion was applied only against drilling resistance rather than continuously, and an alarm was raised for excessive drilling pressure during the run. Within 15 minutes of landing the lander had deployed its sample arm and begun drilling [10].

Communications

Direct to Earth. Specific band and rate figures are not published, and Luna 24’s percussion control ran onboard against a spring-set threshold rather than over the link. The critical link is not telemetry at all but the physical return: the sample capsule was launched directly from the lunar surface on a direct-ascent trajectory with no mid-course correction on Luna 16, reentering at 10.95 km/s and up to 350 g deceleration before parachuting into Kazakhstan [10].

Payload and instruments

The instrument complement was small and subordinate to the sampling function. Luna 16 and 20 carried a stereo imaging system, the remote sampling arm, and a radiation detector, with Luna 20 adding a radio altimeter [10]. Luna 24 carried a stereo imaging system, the LB09 drill, a radiation detector and a radio altimeter. Neither variant carried a dedicated geotechnical instrument, unlike the contemporaneous Lunokhods with their cone-vane penetrometers; the drill telemetry served instead [6].

The sampling mechanism

On Luna 16 the mechanism is a drill on the end of an extended manipulator. Drilling began at 06:03 UT, 45 minutes after landing, ran for 7 minutes, and stopped when the drill reached a stop at 35 cm depth on excessive load; the arm then withdrew the sample, lifted it in an arc over the top of the spacecraft, and deposited it in a small spherical capsule mounted on the bus, apparently losing some of the material during the transfer, for a recovered mass of 101 g [10].

The auger drill rod carried a coring barrel inside it, and at the target depth both the sample and the drilling tool were transferred to the collection container above the return module [10]. Because the sample stayed encapsulated in the drill pipe, no fidelity storage was possible and the bedding information of the returned material was distorted.

Luna 20 used the same arrangement and stopped at 25 cm after encountering strong resistance and overheating its motor, the sample taken in stages because soil resistance rose with depth [10]. That is what the regolith does: bulk density is about 1.30 g/cm3 at the surface, 1.52 g/cm3 at 10 cm and 1.83 g/cm3 at 100 cm, approaching 1.92 g/cm3 asymptotically below that, because micrometeorite gardening loosens only the top 10 to 15 cm over a layer the same flux has packed tight [2].

Luna 24’s LB09 is a different instrument. The arm pushed the drilling head about 2.25 m into the soil in a rotary drilling mode [10]. Because the drill entered at an angle rather than vertically, the true depth below the surface was about 2 m. The method is a slide-rail deep drill that reacts drilling pressure against the dead weight of the landing module rather than against the arm alone, which raised the achievable pressure substantially. Percussion was gated by the same spring-set pressure threshold used to control the impact motor. Apollo’s own contemporaneous drilling ran into the same soil-strength wall from the other direction: the Apollo 15 heat flow auger seized after only 1.5 m of a planned 3.0 m because cuttings could not clear the joints in high relative-density soil [3], and the hand-hammered thin-walled core tubes used on Apollo 15 through 17 hit a practical ceiling around 70 cm because the blows needed to advance the tube grow with the square of depth [3]. An open borehole is a separate question from advancing one: the Apollo 16 drill hole stayed open unsupported to more than 2 m, back-calculating to a cohesion near 1.3 kPa at a 46.5 degree friction angle, and the Apollo 17 hole to 3 m implies a similar range [3], figures of the same order as LB09’s 2 m open hole at Mare Crisium. Among later planetary drilling programs, the autonomous DAME rig reached 3.2 m in permafrost and breccia on under 150 W with automated recovery from 5 of 6 defined fault types, and the field-deployed MARTE drill reached 6.1 m over a six-week campaign [9], both terrestrial field trials rather than flight hardware. Flown planetary drills have stayed far shallower than LB09: the Mars Exploration Rover Rock Abrasion Tool, at 489 g and 10 W [12], a figure a separate drill-systems review corroborates independently [9], penetrated only a few millimeters into rock. Later concept hardware is little deeper: a 440 g, TRL 6 percussive sampler reached only 19 mm of penetration depth [12], and a 2.7 kg Mini-Corer managed about 20 cm per hour into 100 MPa basalt [12], both sized for centimeters rather than meters; the class LB09 belongs to, judged by weight-on-bit, rotation speed and penetration rate, still awaits a flown successor that matches its depth [12]. A down-hole neutron spectrometer of the same later generation was tested to 15.2 m, but lowered by winch into a pre-drilled hole rather than drilled by the instrument itself [9], a reminder that most of LB09’s depth came from the drill alone. Weight-on-bit for a diamond bit drilling into 280 MPa basalt has been reported starting at 500 N and rising to 950 N as the bit wore [12], loads of the same order LB09 achieved only by bracing against the lander’s dead weight rather than a bit-feed mechanism.

The core was not returned inside the drill tool. A soft coring bag progressively wrapped the column with no slip between bag and soil, and after sampling the bag was wound into a sample barrel in the return module, preserving the in-situ bedding to a degree [10]; Chang’e-5 revived the same soft-bag principle in 2020.

The Luna 24 sample showed a laminated structure, as though laid down in successive deposits, an observation the soft bag made possible by preserving column order. The titanium content and maturity were both unexpected for Mare Crisium, and the explanation, established from LRO imagery in 2012, is that the lander sampled ejecta from the nearby 64 m crater, material brought up from deeper lava flows and only briefly exposed to space; the interpretation was settled 36 years after the flight, by orbital imagery rather than by the sample itself [10]. The core still preserved that laminated structure well enough for depth-resolved geochemistry, though the drilling itself visibly disturbed competent material along the way: one contribution to the Luna 24 conference describes a coarse-grained gabbro fragment broken up by drilling, and another attributes an abundance spike at 149 cm depth to a soil breccia crushed into the core tube [6].

Soil mechanics

The drills doubled as geotechnical instruments. Luna 16 provided the first Soviet in-situ soil mechanics results from a returned sample plus drilling telemetry [4], and the Luna 24 core supported analysis of regolith stratigraphy at Mare Crisium [6]. The Soviet synthesis of lunar soil deformation and destruction draws on the drilling data from these missions alongside the Lunokhod measurements, giving a natural-state porosity of 0.8, a modal bearing capacity of 0.4 kg/cm2, cohesion of 0.04 to 0.06 kg/cm2 and an internal friction angle of 20 to 25 degrees [5]. Laboratory tests on the returned Luna 16 and 20 samples themselves, compacted to 1.6 g/cm3 in a biaxial Coulomb box, gave internal friction angles of 46 and 32 degrees respectively [5], and in a maximally loose state a coefficient of lateral pressure of 0.78 to 0.82 [5]. Confined compression on the same samples showed particle rearrangement completing and further compaction proceeding by particle breakage between 0.5 and 1.0 kg/cm2 of pressure [5], and above a bulk density of 1.6 g/cm3 the deformation mode under a die shifted from compaction to general shear [5]; below that, bearing capacity rose sharply above 1.4 to 1.5 g/cm3 [5]. Along the Lunokhod 1 traverse the same program measured a modal bearing strength of 0.34 kg/cm2 [4] and a modal rotational shear strength of 0.048 kg/cm2 [4], varying with terrain from crater rims (softest) to crater slopes (strongest). The instrument doing the measuring was a cone penetrometer with a 60 degree apex angle, capable of over 20 kgf of penetration effort [4]. On horizontal intercrater ground it reached 8.1 cm depth under a 16.5 kgf load, against only 5.9 cm at similar load on a stony surface section where stones raised resistance sharply [4], and 20.5 cm on a crater slope under 23.5 kgf, the strongest of the units the traverse crossed [4].

The Apollo geotechnical program measured comparable quantities by different methods, and the two do not always agree. At the Apollo 15 Station 8 trench, cohesion came out to 1 kN/m2 against a 50 degree friction angle, with soil porosity of 36.5 percent [7]. More broadly across the pooled Apollo, Luna and Lunokhod record, cohesion falls in a probable 0.1 to 1.0 kN/m2 range and friction angle in 30 to 50 degrees [7], both above the Soviet Luna and Lunokhod figures; the same Apollo soil mechanics experiment put the density-depth profile’s fitted coefficients at rho0 = 1.27 g/cm3 and k = 0.121 g/cm3, and relative density at 65 +/- 3 percent in the top 15 cm rising to 92 +/- 3 percent by 60 cm [8]. Reconciling all the in-situ and laboratory measurements, the Lunar Sourcebook recommends a bulk density of 1.50 g/cm3 in the top 15 cm rising to 1.66 g/cm3 by 60 cm [1], a specific gravity of 3.1 for soil particles [1], and a design bearing pressure of 4.6 kPa for the Apollo Lunar Module footpad [1] against an ultimate strength some three orders of magnitude higher [3]. A fitted depth model built on the pooled record puts total shear strength at 7.4 kPa at 1 m [2], porosity at 40 to 50 percent [2], and the maximum depth a vertical trench or borehole wall stays open at about 3 m [2], figures a designer would need to reconcile against the lower Soviet values before sizing any lunar excavation or foundation; the same model’s bulk density estimate is 1.756 g/cm3 at 0.5 m and 1.869 g/cm3 at 2 m [2], and it sets the shear strength an excavator must overcome at above 2.0 kPa at 1 m and above 4 kPa below 2 m [2]. Depth-resolved intercrater cohesion runs from 0.52 kPa in the top 15 cm, through 0.90 kPa by 30 cm, to 3.0 kPa by 30 to 60 cm, averaging 1.6 kPa over the full 0 to 60 cm column [1]. Friction angle rises the same way, from 42 degrees in the top 15 cm to 46 degrees by 30 cm to 54 degrees by 30 to 60 cm, averaging 49 degrees over the full column [1]. Average lunar soil particle size by weight is 0.04 to 0.13 mm, a well-graded silty sand reworked by a hundred million years or more of micrometeorite gardening [8], and where a wheeled vehicle would matter the interaction stays confined to the upper few centimeters of that column [8], which is moot here since neither Luna lander carried wheels. Individual Apollo core tubes varied around those averages: the Apennine Front tube read 0.80 g/cm3 at the surface rising to 1.35 g/cm3 by 25 to 35 cm depth, while a tube at the edge of Hadley Rille was more than 70 percent denser at the surface, 1.38 g/cm3 [7]. Individual core-tube ranges recorded elsewhere on Apollo range as wide as 1.36 to 1.85 g/cm3 and, where coarse basalt fragments dominated the upper core, as high as 2.29 g/cm3 [3]. A curved shear envelope fitted to reconcile the in-situ and laboratory numbers gives an apparent friction angle that falls from 41 degrees at 5 kPa confining stress to 21 degrees at 100 kPa, which is why a single friction-angle figure taken out of its stress range misleads [3]. Bootprint and boulder-track surveys additionally show relative density varying by about 15 percentage points across lateral scales of only 1 to 2 m [8], the kind of small-scale patchiness a fixed lander cannot survey before choosing where to sample. The Apollo 15 self-recording penetrometer used to gather this kind of data applied at most 111 N through a 0.5 sq in cone [7], and separate laboratory work puts lunar soil particle specific gravity at 2.90 to 3.24 and void ratio around 1.05 [8], both consistent with the Lunar Sourcebook’s recommended values.

Modes of operation

The mission is a fixed sequence: descend, image, drill, transfer, seal, ascend [10]. There is no hibernation state because no lander was intended to survive a lunar night, and no safe mode is documented. The one onboard decision point is the Luna 24 spring-set percussion threshold; every other branch, including the Luna 20 sampling stages, was decided on the ground.

Ground operations

Controlled from the Soviet deep space network, with the mission designed and managed by the Lavochkin bureau. Landing site selection was pre-mission; sample site selection within the arm’s reach was made from the returned stereo imagery. Drilling itself was supervised by load telemetry: Luna 16 halted on excessive load, Luna 20 on motor overheating, and Luna 24 raised an excessive-drilling-pressure alarm while completing its core. On Luna 23, when the drill could not be activated after the hard landing, controllers improvised a limited science program with the immobilized lander and kept contact until 9 November 1974 [10].

Sample distribution was a distinct ground operation with a diplomatic dimension. Luna 16 material was shared with France, the German Democratic Republic and Iraq among others; Luna 20 material was exchanged with NASA for an Apollo 15 sample on 13 April 1972, and shared with France, Czechoslovakia, Great Britain and India; Luna 24 material was exchanged with NASA in December 1976 [10].

Technologies developed

Luna 16 is the first fully automatic recovery of material from the surface of another world. The functional chain it established, land, sample with an arm, transfer to a sealed capsule, ascend directly to Earth, is reproduced by every robotic sample return since, including Chang’e-5. The Ye-8-5M redesign that followed, trading 0.3 m of reach for 2.3 m, went unmatched by any later robotic mission [10].

The Luna 24 soft coring bag, wrapping the column without slip and wound into a barrel for return, is the direct ancestor of the Chang’e-5 dual-pipe soft-bag corer, and it is what distinguishes Luna 24 from the earlier missions, whose samples stayed inside the drill pipe and lost their bedding information. Threshold-gated percussion, applying impact only when drilling pressure exceeds a spring-set limit, is the other transferable element [10].

The Luna 16 transfer arc lost part of the sample and destroyed its stratigraphy, which is why later designs move the sample rather than the arm. Luna 23 shows the drill as the fragile subsystem: the spacecraft survived an 11 m/s touchdown intact enough to talk to Earth for three days and could still not turn its drill [10].

References