Chang'e-5 Sampling System

The problem

A lunar sample return lander has to take material from the surface and below it, keep it uncontaminated and in order, and seal it in a container that a separate vehicle carries to orbit. All of it happens unattended in vacuum, in dust, at one site chosen months earlier, with light-time and a single lunar day limiting how many commands can be sent. Chang’e-5 was the sample return step of the three-phase Chinese Lunar Exploration Program, which planned to orbit, land, and return [11].

The soil sets the loads. The Apollo and Luna record puts in situ bulk density at 1.50 g/cm3 [15], and the geotechnical record of lunar and Martian soils compiled from Surveyor through recent landers is the basis for off-Earth drilling analysis [16]. Off-Earth drilling adds low gravity that reduces the weight available on the bit, no drilling fluid, and dust intrusion into seals and bearings [16]. Rotary-percussive drilling reduces the weight on bit needed against rotary-only drilling in the same rock, which matters when the lander supplies little reaction mass [17]. Soil strength also varies by site: wheel-track fits give a soil strength coefficient of 3.44 at the Chang’e-4 site against 1.19 at the Chang’e-3 site and 2.45 for Apollo 15 [34].

The Chang’e-5 site was young mare basalt inside the engineering envelope: a mean slope under 8 degrees over most of the candidate area and a solar elevation of 30 to 50 degrees at landing [3]. The regolith there is thick: crater counting over a 1.5 m per pixel orthomosaic put the mean thickness at 7.15 m, ranging 0.74 to 18.00 m [6]. That is well beyond the 1.68 m the drill traveled [12]. The geologic setting was mapped before landing from orbital data, with the young high-titanium mare unit Em4 proposed as first priority [14].

What has flown

Luna 16, 20 and 24, 1970 to 1976. These are the flown precedents for a robotic lunar drill that returns material to Earth [12]. Luna 16 drilled 35 cm for 0.101 kg and Luna 20 reached 25 cm for 0.055 kg, each stopped by excessive load on the motor, and Luna 24 reached 200 cm with 0.170 kg using an impact sampler and a soft core bag [12].

Apollo Lunar Surface Drill, 1971 to 1972. The reference rotary-percussive drill ran at up to 280 r/min with up to 37.8 Hz impact frequency and 4.4 J per blow [12]. Apollo 17 returned 110.5 kg over a 305 cm drilling depth, the largest of the Apollo returns [12].

Chang’e-5. The spacecraft comprised an orbiter, a lander, an ascender and a return capsule. It launched on 24 November 2020, landed on 1 December at 43.1 N, 51.8 W in northern Oceanus Procellarum about 170 km east-northeast of Mons Rumker, and returned 1.731 kg [11], [12]. Two independent mechanisms flew: a rotary-percussive coring drill and a four-joint arm carrying two scoops [11], [12]. Drilling and operations ran about 3.12 h with 1.68 m of drill movement, of which about 1 m was effective lunar soil drilling, and the sealing cover opened in place in 169 s [12]. The surface arm made 12 scoops for about 1.5 kg [11]. The core recovered was about 1 m against a design goal of about 2 m, and the cited reports give no cause for the shortfall [11]. The mass table in [12] lists the drilling depth as 200 cm, the design depth rather than the effective one.

The drill stem is a coaxial pair of pipes. The outer pipe rotates and carries helical blades that discharge cuttings and dissipate heat; the inner pipe stays static relative to the regolith and is thin-walled and hollow to preserve stratigraphic layering [11]. A soft bag lining the inner pipe surrounds the regolith as it enters and is then extracted and convolved into the primary package [11]. The drive combines footage, rotation and percussion coaxially [12]. The auger measured 32 mm in radius with a 14 degree helix angle and the bit had four blades at a 90 degree rake angle, with rotary speed limited below 200 r/min [10]. The scooping arm has a repeat positioning accuracy better than 1 mm [12].

Sampling was operator-in-the-loop. Sample points were confirmed manually with images, and because the arm deforms it cannot reach the target pose in one commanded motion, so an operator fine-tuned the pose after a touch of the surface established terrain height [9].

Chang’e-6, 2024 (far side). The architecture was reused at a site in the southern Apollo basin, within South Pole-Aitken, at 150 to 158 W and 41 to 45 S [9], [13]. Scooping pits from ten planned sample points were imaged at 154.0 W, 41.6 S [9]. The capsule returned with a preliminary 1935.3 g, described as more viscous and clumped than Chang’e-5 material [1]. Far-side soil bulk density is 0.983 g/cm3 [2], and its static angle of repose of 52.9 degrees against 70.4 degrees dynamic is read as strong cohesion [4]. The basalt dates to about 2.8 billion years [7]. The relay link that far-side operations depend on was proven on Chang’e-4, where the Queqiao satellite in a halo orbit about Earth-Moon L2 returned 555 kbit/s from the lander through a directional dish [5].

Other flown samplers. Lunar PlanetVac, a pneumatic sampler, operated on the Blue Ghost lander at Mare Crisium on 3 March 2025 in a five-second acquisition [28]. The first report gave 13 g collected and the second 6.5 cc, and the two are not reconciled [27], [28]. The sampling hood was not flush with the surface, so the yield cannot be attributed to the method [28]. The TRIDENT drill on the PRIME-1 lander completed an activation of about 20 minutes, extending the deploy stage to 35 cm and the feed stage to 100 cm, but the lander came to rest on its side and the drill was buried in thrown regolith, so no lunar regolith was drilled [21]. The NASA press account says the drill drilled into regolith, which conflicts with the flight team’s account [35]. The OSIRIS-REx touch-and-go sampler is the asteroid counterpart: brief contact avoids holding position on a low-gravity surface, and ground tests collected a factor of 4 to 5 more in one gravity than in reduced acceleration [33].

What is being tried now

Vision-guided scooping (flown, Chang’e-6). A binocular camera reconstructs a terrain point cloud, ten sample points are chosen by simulated annealing against slope, relief and obstacle terms with a 400 mm minimum spacing, and a 6D pose estimate by point cloud registration against a sampler template replaces the operator [9]. The arm alone shows about 30 mm and 2 degrees of absolute error. Soil sink under the touching disc of 5 to 20 mm is compensated, and fine-tuning held scooping depth to 3.8 mm average error [9]. All accuracy figures are from an Earth testbed with balloon gravity offload [9].

Drill-soil load modeling (ground). The Chang’e-5 drill tool is modeled as screw conveyance, extruding, bulldozing and in-situ cutting. Conveying torque of 1.4 to 2.2 N.mm is three orders of magnitude below the measured 0.1 to 1 N.m [10]. In dense pozzolana simulant at 85 mm/min and 60 r/min the mean weight on bit was 72.1 N and torque 1.31 N.m, and at 200 r/min the weight on bit fell to 7.8 N [10]. The simulant (35.1 degrees friction, 2.85 kPa cohesion) is denser and more cohesive than CAS-1, the Chinese simulant matched to Apollo 14 regolith [8]. All of this is bench testing.

Vacuum-preserving coring (ground design). A flexible-bit coring platform with a magnetic fluid seal, 850 mm depth capability and up to 10 J percussion is specified to keep the core in lunar vacuum [12]. The figures are design parameters and the paper reports no drilling results.

Volatile-preserving drilling (ground, one flight attempt). Polar prospecting needs the sample to keep its ice. In thermal vacuum at 1e-5 Torr, four of twelve capture attempts in water-doped simulant sealed properly and lost a mean 30 percent of the water, range 22 to 47 percent [18]. A discrete element model puts the temperature rise of cryogenic soil at 2 to 8 C and was validated only at atmospheric pressure [19]. A mass spectrometer viewing the cuttings pile distinguished dry, 2.5 percent and 5 percent water layers by the m/z 18 signal while TRIDENT drilled four 1 m holes in simulant [20]. TRIDENT is a 1 m drill of 26.5 kg with a 25.4 mm bit, 2 J blows and 5.5 N.m average torque [21], [22]. Field campaigns in Haughton crater and Bishop Tuff showed it binding in low-permeability material and exceeding its torque limits [23]. In Antarctica, telemetry identified an ice-rock transition at about 62 cm [24]. These are analog results, and the Moon flight did not drill.

Pneumatic and ultrasonic samplers (ground and one flight). PlanetVac was argued for as a replacement for scoop-and-arm delivery on low-gravity bodies [26] and flew once, as above. The LISTER pneumatic drill and heat flow probe for Blue Ghost was qualified in 1 g chambers [25]. The JPL ultrasonic/sonic driller/corer drills basalt on 2 to 3 W average power with as little as 10 N preload [30]. Auto-Gopher II reached 7.52 m in gypsum at a field site, and piezo hammering raised rate of penetration by 30 percent in limestone [29]. Neither has flown on the Moon.

Remote operation and autonomy (field analog). An end-to-end simulation in the Atacama with the TRIDENT drill and a 4-DOF arm took 5 command cycles for a 10 cm bite collected and dumped, and 15 to 19 to deliver one sample to three instruments [31]. The Curiosity drill feed failure on sol 1536 took 521 sols to recover from with a new drilling method, with no root cause established [32]. It is the flown evidence of what a sampler fault costs a mission.

The numbers

Parameter	Value	Source
Sampling mechanisms	rotary-percussive coring drill, four-joint scooping arm	[11]
Arm repeat positioning accuracy	better than 1 mm	[12]
Auger radius, flight thickness, flight height, helix angle	32 mm, 10.53 mm, 2 mm, 14 degrees	[10]
Bit	four blades, 10 mm blade width, 90 degree rake angle	[10]
Drill rotary speed limit	below 200 r/min	[10]
Landing site	43.1 N, 51.8 W	[11]
Drilling and sampling duration	about 3.12 h	[12]
Drill movement into regolith	1.68 m	[12]
Effective core length	about 1 m, against a 2 m goal	[11], [12]
Surface scoops and mass	12 scoops, about 1.5 kg	[11]
Total returned mass	1.731 kg	[12]
Sealing cover opening time	169 s	[12]
Regolith thickness at the site	mean 7.15 m, range 0.74 to 18.00 m	[6]
Chang’e-6 arm error before vision correction	about 30 mm and 2 degrees (Earth testbed)	[9]
Chang’e-6 scooping depth error after fine-tuning	3.8 mm average (Earth testbed)	[9]
Chang’e-6 pose measurement	1.1 mm and 0.4 degrees in 1.6 s	[9]
Chang’e-6 returned mass	1935.3 g, preliminary	[1]
Chang’e-6 soil bulk density	0.983 g/cm3	[2]
TRIDENT mass, bit, blow energy	26.5 kg, 25.4 mm, 2 J	[21]
Lunar PlanetVac yield	13 g or 6.5 cc, unreconciled	[27], [28]

What is not established

The cited reports give no drill loads, torques, penetration rates, thermal data or telemetry from the Chang’e-5 flight, and the cause of the 1 m recovery against a 2 m goal is not analyzed [11]. Bus power, compute and link parameters for the sampling system are unpublished, and operation timings are mission records without anomaly reports [12].

Every Chang’e-6 accuracy figure is from an Earth testbed whose balloon gravity offload fidelity is not characterized: the 3.8 mm depth error, the pose precision and the 30 mm arm error [9]. The paper reports no on-orbit accuracy, no achieved lunar scooping depths and no retry statistics [9]. The 1935.3 g mass is a press-release figure without uncertainty [1]. The load model was validated in one dense simulant at 1 g [10].

The volatile, ultrasonic, piezo and field-analog results above are ground evidence, and the TRIDENT figures are flight-design values: none of it has drilled lunar regolith [21], [29], [30]. The PlanetVac yield rests on one event with an off-nominal head position [28].

References