Chang'e-5 Sampling System

Overview

Chang’e-5 is the sample return step of the three-phase Chinese Lunar Exploration Program, which planned to orbit, land, and return. The spacecraft comprises four elements, an orbiter, a lander, an ascender and a return capsule. It launched on 24 November 2020 from Wenchang, landed on 1 December 2020 at 43.1 N, 51.8 W in northern Oceanus Procellarum about 170 km east-northeast of Mons Rumker, and the capsule returned to Earth on 17 December 2020, bringing back 1731 g of material including a drill core of about 1 m [11].

Two independent sampling mechanisms were flown: a rotary-percussive drill for a subsurface core, and a four-joint robotic arm carrying scoops for surface material [12]. The drilling and surface primary packages were designed separately to keep the two sample types isolated through physical separation, before both were transferred to the sealed capsule [12]. Luna 16 and 20 drilled with an extended manipulator carrying the sampler and Luna 24 used a slide-rail impact drill [12]. The site was chosen on young mare basalt within the recommended engineering envelope: average slope under 8 degrees over most of the candidate area, and a solar elevation angle at landing of 30 to 50 degrees [3], a window that also constrains the latitude band Chinese landers favor to roughly 30 to 60 degrees [3]. The later Chang’e-6 far-side site turned out to have its own young mare basalts [7]. Two eruption ages were dated there by Pb-Pb isotopes, 4203 million years and 2807 million years [7], the younger one matching the crater-counting age for that terrain [7].

Specifications

Parameter	Value	Source
Sampling mechanisms	rotary-percussive coring drill and a four-joint scooping arm	[11]
Arm joints	4, sampler attached at joint 4	[11]
Arm repeat positioning accuracy	better than 1 mm	[12]
Arm degrees of freedom in sampler attitude	pitch adjustable, roll not adjustable	[9]
Drill maximum single-machine power	greater than 800 W	[12]
Drill rotary speed limit	below 200 r/min	[10]
Auger radius	32 mm	[10]
Auger flight thickness, height, helix angle	10.53 mm, 2 mm, 14 degrees	[10]
Bit	four blades, 10 mm blade width, 90 degree rake angle	[10]
Sampler sensing	binocular work-area camera, close-range sampler camera, touching-disc force sensor	[9]

Mission profile

Parameter	Value	Source
Landing site	43.1 N, 51.8 W, northern Oceanus Procellarum	[11]
Surface operations window	from touchdown on 1 December 2020 to ascent, entirely within one lunar day	[11]
Drilling and coring duration	3.12 h	[12]
Drill travel into regolith	1.68 m	[12]
Effective core length recovered	about 1 m, against a 2 m goal	[11], [12]
Surface sampling duration	15.82 h	[12]
Surface sampling operations	2 contact, 10 non-contact	[12]
Surface sample mass	about 1.5 kg	[11]
Total returned mass	1731 g	[11], [12]
Sealing cover opening time	169 s	[12]

For comparison, the Apollo Lunar Surface Drill used on Apollo 15, 16 and 17 ran to a maximum 280 r/min at up to 37.8 Hz impact frequency and 4.4 J per blow, over a 250 to 300 cm depth range; Luna 16 reached about 35 cm for 101 g and Luna 20 aborted at 25 cm for about 55 g after motor overheating [12].

Mobility

Not mobile. The lander is fixed and the only articulation is the sampling arm. The arm has four joints, with the sampler attached at joint 4, and its motion limits make the valid sampling region a sector on the surface in front of the lander [11].

The sampler can adjust its pitch angle but not its roll [9], so a sideways slope under the scoop cannot be compensated by the arm and directly degrades control of scooping depth. The Chang’e-6 site selection algorithm penalizes slope in the roll direction for this reason [9]. The arm is described by the program as lightweight and small with a large reach and high load, integrating shovelling, digging, suction, receiving and grasping, with repeated positioning accuracy better than 1 mm and several working modes for multi-point sampling [12].

Power and energy

Bus power for the sampling system is not published. Maximum single-machine power in the drilling chain exceeds 800 W [12]. The lander operated between touchdown on 1 December 2020 and ascent entirely within a single lunar day [11], so no night survival was required of the sampling mechanism or its electronics. Total surface mechanism run time was 3.12 h of drilling and 15.82 h of surface sampling [12].

Thermal

No thermal design is published for the sampling system specifically. The drill’s outer pipe carries helical blades whose stated function is both to discharge cuttings and to dissipate heat [11], which is the only thermal design detail published: in vacuum there is no convective path out of a borehole, so the chip flow itself has to carry the drilling heat away.

Compute and avionics

Not published in detail. The sampling system’s sensing is documented: a binocular camera supervising the work area, a close-range camera on the sampler with a narrow field of view used to monitor scooping and inspect collected soil, and a force sensor at the bottom of the sampler’s touching disc that indicates contact with the surface [9]. The imaging suite supporting sampling also included Sampling Monitoring Cameras, Panoramic Cameras and a Far-range Camera [11].

Autonomy

Sample points were confirmed manually, with images as the operators’ main criterion, and each point was analyzed and checked individually for safety and sampling quality [9]. The surface sampling campaign occupied 15.82 h for 12 operations, against 3.12 h for the drilling campaign [12].

Pose fine-tuning was also manual. Because the arm has inherent movement errors, including flexible deformation, the sampler cannot reach the target pose in a single commanded motion. On Chang’e-5 a specialized operator performed the fine-tuning, aided by an added “touching moon” step to confirm the height of the terrain, estimating the sampler height by reference to multiple images and determining the required adjustment [9]. That procedure required complex operator training and carried a high time cost.

The failure modes are asymmetric: too high and the shovel never contacts the soil and collects nothing; too low and it digs in, takes excessive soil and can become stuck, which is a hazard to the mission [9].

Communications

Direct to Earth from the near side at 43.1 N, 51.8 W [11]. No relay satellite was required, unlike a far-side mission, which cannot see Earth directly and depends on a relay spacecraft in a halo orbit around the Earth-Moon L2 point [5]. China’s first such relay, Queqiao, entered that halo orbit on 14 June 2018 to support Chang’e-4’s far-side landing and rover, more than two years before Chang’e-5 flew [5]. Link parameters for the sampling system are not published.

Payload and instruments

Drill

Developed by Beijing Spacecrafts [11]. It comprises a drilling mechanism, a loading device and a coring system. The drill stem is a coaxial pair of pipes: the outer pipe rotates and carries helical blades that discharge cuttings and dissipate heat, while the inner pipe remains static relative to the regolith and is thin-walled and hollow so as to preserve the original stratigraphic layering.

Coring uses a soft sampling bag. The bag is installed on the inner wall of the inner pipe, surrounds the regolith as it enters the hollow pipe, and is then extracted and convolved into the primary package device for drilled samples before transfer to the sealing capsule on top of the ascender [11].

A conventional auger transports material along a rotating flight and destroys stratigraphy. Keeping the inner tube stationary lets the core column slide into the bag without being churned, and convolving the bag afterwards stows a 1 m core in a container much shorter than the core [11]. The design goal was to penetrate and recover about 2 m of regolith; the achieved core was about 1 m over 1.68 m of drill travel [12].

The drive is coaxial footage-rotation-percussion, with maximum single-machine power above 800 W [12]. Rotary speed on Chang’e-5 was limited below 200 r/min against a general upper bound of 300 r/min for planetary regolith augers, and the published drill tool geometry gives an auger radius of 32 mm, flight thickness 10.53 mm, flight height 2 mm and helix angle 14 degrees, with a four-blade bit of 10 mm blade width, 90 degree rake angle, 7.4 mm extruding part height and 75 degree extruding helix angle [10]. The simulant used for these load tests has an internal friction angle of 35.1 degrees and cohesion of 2.85 kPa [10], denser and more cohesive than CAS-1, an earlier Chinese lunar soil simulant matched to Apollo 14 regolith, at 33.3 degrees and 1.0 kPa [8]. CAS-1 was built from low-Ti basaltic scoria from the Jinlongdingzi volcano in northeast China [8].

Drill-soil interaction is modeled in four parts, cuttings screw conveyance, cuttings extruding, cuttings bulldozing and in-situ simulant cutting, from passive earth pressure theory, because the mechanical properties of the cuttings and of the undisturbed simulant differ [10]. The screw conveyance term is small: calculated conveying loads of 1.4 to 2.2 N.mm sit three orders of magnitude below the measured total drilling torque of 0.1 to 1 N.m [10].

Scooping device

The Surface Sampling System was developed by the Hong Kong Polytechnic University and consists of four joints, two samplers and a close-range camera [11]. The sampler assembly carries a touching disc with a force sensor, a front-end shovel, and the close-up camera [9].

Before sampling, images from the sampling monitoring, panoramic, far-range and close-range cameras were used to establish a sampling plan; the arm then scooped surface regolith and rock fragments with the two samplers and placed them into the primary package device for surface samples on top of the lander. The scooping device performed 12 separate sampling operations, 2 of them contact and 10 non-contact, and gathered about 1.5 kg of material from around the lander [12].

Science instruments supporting sampling

Two lander payloads existed largely to inform and contextualize the sampling [11]:

• Lunar Mineralogical Spectrometer (LMS). Acousto-optic tunable filter spectrometer, 480 to 3200 nm across a VIS/NIR module (480 to 950 nm visible, 900 to 1450 nm near infrared) and an IR module (1400 to 2450 nm short infrared, 2400 to 3200 nm middle infrared), spectral resolution 3 to 25 nm, detection range 2 to 5 m, field of view 4.24 by 4.24 degrees, with aluminum and Infragold calibration plates [11]. It observed the surface before and after sampling, targeting hydrated minerals.
• Lunar Regolith Penetrating Radar (LRPR). An ultra-wideband array radar of 12 bow-tie antennas, 1 to 3 GHz, mounted asymmetrically around the drill 90 cm above the ground [11]. It transmits carrier-free pulses of 200 ps full width at half maximum; antennas take turns transmitting while the others receive, giving 132 traces per operating period, each with a 55 ns time window sampled at 18.3 ps. Detection depth is at least 2 m with vertical resolution 5 cm or better [11].

The radar is mounted around the drill so that it measures regolith structure and thickness in the volume the drill is about to enter [11], which the 1.68 m of drill travel then samples directly [12]. A pre-mission crater-counting survey of the landing area, using a 1.5 m per pixel orthomosaic built from 765 orbital images, mapped 958 craters [6]. It put mean regolith thickness at 7.15 m with a standard deviation of 2.59 m, ranging 0.74 to 18.00 m [6], well beyond the radar’s stated 2 m detection depth and the drill’s 1.68 m reach.

Modes of operation

The surface sequence is fixed rather than mode-driven [11]:

1. Unlock the drilling system and scooping system after landing.
2. Drill and core; separate the sample, transfer it to the primary package device for drilled samples, then transport it to the sealing capsule.
3. Scoop surface samples with the arm; transfer them to the primary package device for surface samples on top of the lander for primary packaging; then have the arm carry that device to the sealing capsule on top of the ascender.
4. Seal the capsule, with drilled and surface samples remaining separate inside it.

The sealing packaging device cover opened in 169 s [12].

In step 3 the arm is used twice, as a sampling tool and as a transport mechanism for the whole primary package device, in place of a dedicated transfer mechanism [11]. Sealing is achieved by a sealing ring with metal extrusion before ascent, giving an ultra-low leakage rate [12].

Ground operations

Operated by CNSA through the Lunar Exploration and Space Engineering Center, with the Ground Research Application System in Beijing as the primary sample storage and curation center [11]. During surface operations the ground role was heavy: operators built the sampling plan from returned imagery, confirmed each sample point individually, and performed pose fine-tuning by hand [9].

Ground operations continue long after the flight. The capsule landed in Inner Mongolia on 17 December 2020, was packaged and sealed in a nitrogen-filled transfer box, and moved to the Lunar Sample Laboratory at GRAS [11]. The curation procedure is documented: GRAS receives the sealed package, the sample bags are unsealed, scooped and drilled samples are removed and cataloged separately in two containers, the drilled sample soft bag is cut into sections of 15 cm each, and scooped samples are placed into a square container and classified by type. Permanent storage samples then go to a permanent storage glove box in the long-term storage room while research and backup samples go to a temporary glove box. All tools contacting samples are stainless steel, PTFE, quartz glass or other materials of known composition, and pure nitrogen pressure in the glove boxes is monitored to prevent terrestrial contamination. Storage is duplicated, with a primary facility and a remote disaster-tolerant backup.

Sectioning the drill core at 15 cm preserves depth information only because the soft bag preserved the column order during coring [11].

Technologies developed

The dual-pipe soft-bag corer takes a meter-scale core, preserves its layering, and stows it in a container much shorter than the core: a static inner tube, a thin wall, helical outer flights carrying both chips and drilling heat, and a convolved bag [11]. Luna 16 and 20 returned their samples encapsulated inside the drill pipe, which distorted the bedding information; Luna 24 already used a soft coring bag wound into a sample barrel, and Chang’e-5 extends that principle to a meter-scale core with a static inner tube [12].

Coring and surface sampling are separated into two independent mechanisms whose products stay separate through two-stage packaging, sealing, ascent, rendezvous and return [11], [12]. Chang’e-6 reused the architecture directly at a far-side site, where a direct link like Chang’e-5’s is not possible and a relay satellite is required instead [5], and returned its capsule to Inner Mongolia on 25 June 2024 carrying 1935.3 g of material [1]. That far-side soil has a bulk density of 0.983 g/cm3, lower than the Chang’e-5 samples [2], and a maximum static angle of repose of 52.9 degrees against 70.4 degrees dynamic, both substantially higher than the Chang’e-5 soil simulant, evidence of stronger cohesion in the far-side regolith [4].

The automatic chain of sampling, packaging, sealing, lunar surface takeoff and lunar orbit rendezvous and docking is listed by the program as the set of key technologies the mission was designed to develop [11]. Chang’e-5 is the first robotic sample return via lunar orbit rendezvous rather than direct ascent.

Two-stage packaging with sealing-ring and metal-extrusion sealing achieved an ultra-low leakage rate before ascent [12]. Sample stratification and compactness were nevertheless not preserved in the in-situ sense, and no rock samples were taken, both attributed to limits on drill power and sampling method.

Manual sampling operations were the throughput limit. Individual image-based confirmation of each sample point and operator-driven pose fine-tuning both worked and both cost enough time that the successor mission replaced them with an autonomous vision pipeline [9].

References