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 [1]. 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 [1]. 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 [1].

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)	Source
Sampling mechanism	drill on the end of an extended manipulator	LB09 slide-rail deep drill	[1], [5]
Design sampling depth	0.3 m	2.3 m	[1]
Drilling reaction path	through the manipulator	against the dead weight of the landing module	[5]
Sample stowage	encapsulated inside the drill pipe	soft coring bag wound into a sample barrel
Return capsule soil container diameter	68 mm	100 mm	[1]
Low-altitude Kvant altimeter	fitted	deleted
Percussion control	not documented	impact motor gated by a spring-set drilling pressure threshold	[5]
Return capsule reentry velocity	10.95 km/s, up to 350 g deceleration	as Ye-8-5	[1]

Mission profile

Mission	Landing	Drill depth reached	Sample returned	Source
Luna 16	20 September 1970, 0.5134 S, 56.3638 E, Mare Fecunditatis	35 cm	101 g	[1], [5]
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	[1]
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	[1], [5]

Surface stay was under one day in every case: Luna 16 lifted off 26 hours after landing, Luna 20 about a day after, and Luna 24 slightly less than a day after [1]. Coordinates were 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 [5], so Luna 24’s 2.25 m is the deepest robotic penetration and sits just below the crewed figure. 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 [1]. Luna 24 flew the same design successfully two years later.

Luna 24 remains the last Soviet or Russian spacecraft sent to the Moon [1].

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 [1]; Luna 24 came to rest 10 m from the rim of a 65 m crater, which determined the composition of the returned core.

Power and energy

No power figures are published. Each lander operated for less than one day on the surface: Luna 16 lifted off 26 hours after landing, Luna 20 about a day after, and Luna 24 slightly less than a day after [1]. No lunar night survival was attempted. Luna 20’s drilling was terminated by motor overheating rather than by an energy limit [5].

Thermal

No thermal design values are published, beyond the Ye-8-5M instrument compartment being toroidal and lightened for the deep-drill variant [1].

Compute and avionics

No avionics description is published in detail. The one documented closed loop is Luna 24’s spring-based drilling pressure threshold gating the impact motor [5]. Descent used the DA-018 Planeta Doppler landing radar with the Vega altimeter on Luna 16 [1]; 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.

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 [1]. 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.

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 [5]. The impact motor was turned on frequently during the Luna 24 drilling and an alarm was raised for excessive drilling pressure. Within 15 minutes of landing the lander had deployed its sample arm and begun drilling [1].

Communications

Direct to Earth. Specific band and rate figures are not published. Luna 24’s percussion control ran onboard against a spring-set pressure threshold rather than over the link [5]. 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 [1].

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 [1]. 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 [4].

The sampling mechanism

On Luna 16 the mechanism is a drill on the end of an extended manipulator [1], [5]. 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; Luna 16 stopped drilling 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. The recovered mass was 101 g [1].

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 [5]. 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 [5]; the sample was taken in stages because soil resistance rose with depth [1]. 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 [6].

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 [1], [5]. 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 threshold mechanism described above.

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 [5]; 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 [1], an observation the soft bag made possible by preserving column order [5]. 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 [5].

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 [2]. 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 [3].

Modes of operation

The mission is a fixed sequence: descend, image, drill, transfer, seal, ascend [1]. There is no hibernation state because no lander was intended to survive a lunar night, and no safe mode is documented. The decision points exercised in flight were the Luna 20 staging against rising soil resistance, made on the ground, and the Luna 24 percussion threshold, executed onboard [5].

Ground operations

Controlled from the Soviet deep space network, with the mission designed and managed by the Lavochkin bureau [1]. 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 [5]. 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.

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 [1].

Technologies developed

Luna 16 is the first fully automatic recovery of material from the surface of another world [1]. 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 [5].

The Ye-8-5M is a spacecraft substantially redesigned around a drill: a new instrument, a deleted altimeter, a lightened structure and a wider sample container, trading 0.3 m of reach for 2.3 m [1]. Luna 24 delivered about 2 m of true depth and a 160 cm core [5]. No robotic mission has exceeded it.

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 Luna 16 and 20, whose samples stayed inside the drill pipe and lost their bedding information [5]. Threshold-gated percussion, applying impact only when drilling pressure exceeds a spring-set limit, is the other transferable element.

Three flights returned 326 g. Luna 16’s basalt resembled Apollo 12’s and Luna 20’s anorthosite-rich soil resembled Apollo 16’s [1]. Two of the three landed within about 2 km of a previous Soviet crash site.

The Luna 16 transfer arc lost part of the sample and destroyed its stratigraphy [1], [5], 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.

References