Lunokhod 2

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

Overview

Lunokhod 2 was the second and last Soviet lunar rover, delivered to the Moon by the Luna 21 lander. Luna 21 launched on 8 January 1973 on a Proton-K with a Blok D upper stage, entered lunar orbit on 12 January and landed at 22:35 UT on 15 January inside the 55 km crater Le Monnier, on the eastern margin of Mare Serenitatis about 180 km north of the Apollo 17 landing site [9]. The rover drove off the lander at 01:14 UT on 16 January [9], landing at approximately 30.4 E, 25.97 N in the southern part of Le Monnier [4].

The vehicle was a direct evolution of Lunokhod 1, with the same self-propelled chassis and the same sealed, pressurized instrument compartment [4]. It operated for five lunar days and stopped on 10 May 1973, having driven 39.16 km, a distance no vehicle on another world exceeded until Opportunity’s odometer passed it in July 2014 [9].

Specifications

Parameter	Lunokhod 2	Lunokhod 1
Rover mass	836 kg	756 kg
Rover mass, second value	840 kg, 1814 kg with lander	756 kg
Dimensions	170 x 160 x 135 cm	about 2.2 m long, 2.2 m wide
Solar array	250 A-h, gallium arsenide	200 A-h, silicon
Wheel diameter / width	510 mm / 200 mm	510 mm / 200 mm
Track gauge	1600 to 1700 mm	1700 mm wheelbase
Commanded speeds	0.8 and 2.0 km/h	about 0.93 and 2 km/h
Navigation cameras	3 MKTV	2 MKTV
Navigation frame interval	about 6 s	about 20 s

Source: [4], except the second mass value, wheelbase and frame timing [6], and the Lunokhod 1 dimensions and speeds [5].

Mission profile

Parameter	Value
Launch	8 January 1973, Proton-K with Blok D
Landing	22:35 UT, 15 January 1973, crater Le Monnier
Landing point	approximately 30.4 E, 25.97 N
Rover egress	01:14 UT, 16 January 1973
End of operations	10 May 1973
Surface lifetime	139 days, 5 lunar days
Distance driven	39.16 km, 37 km as measured at the time
Average achieved speed	340 m/h, against 140 m/h for Lunokhod 1

Source: [9] for launch, landing and egress timing; [4] for landing point, distance and speed; [6] for surface lifetime.

Luna 21 descended from a 90 by 100 km parking orbit with perilune lowered to 16 km and held for 40 orbits; the braking rocket put the lander into free fall, main thrusters fired 750 m above the surface and shut down at 22 m, secondary thrusters shut down at 1.5 m, and landing was a free fall from that height [6].

Mobility

Eight wheels, four per side, each independently driven with its own motor and brake, on a chassis carried over unchanged from Lunokhod 1. Each wheel is built from three titanium rings 510 mm in diameter with 16 spokes wrapped in a metallic mesh, 200 mm wide, on a track gauge of 1600 to 1700 mm; Lunokhod 1’s wheelbase was 1.7 m [6]. Grousers are fitted across the mesh. Lunokhod 1 itself drove 9.93 km over its mission [9]; a contemporary account gave 9200 m as its closed-loop traverse to date at a May 1971 symposium, six months into that mission [2]. Lunokhod 1’s momentum on the Moon corresponded to an estimated lunar weight in excess of 900 N [2].

Average specific ground pressure at a nominal 30 mm sinkage is 0.05 kg/cm2, about 4.9 kPa [4]. Grousers disturb the soil enough that Lunokhod tracks are identifiable in LRO Narrow Angle Camera frames, while ungrousered Apollo LRV tracks are hard to see outside the descent-stage blast zone at similar wheel loading. Sinkage was normally about 2 cm, rising to more than 20 cm near fresh impact craters [6].

There is no steering mechanism. The vehicle turns by differential wheel speed and can counter-rotate the two sides to turn in place. Turning radius in motion is typically 3 m and turn-in-place radius 0.8 m [4]. Turn-in-place locations are identifiable in LRO images from the disturbed patches they left. Skid steering scrubs the wheels laterally through the soil on every turn.

Two commanded speeds were available, 0.8 km/h and 2.0 km/h; achieved average over the whole mission was 340 m/h. The same chassis design operated on slopes up to 32 degrees on Lunokhod 1 [6]. Commanded speed and achieved speed differ sharply in practice: cycle-by-cycle averages reconstructed for Lunokhod 1 range from 0.01 to 0.17 km/h [5].

For comparison, the contemporary Apollo Lunar Roving Vehicle cruised at 6 to 7 km/h with a 13 km/h maximum, and a generic wheeled lunar vehicle needs 7 to 10 kPa of ground contact pressure capability, with terrain roughness at this scale imposing a parasitic energy cost comparable to a 1.5 degree climb on a smooth, rigid slope; reviews of lunar rover mobility have recommended retaining the capability to climb and descend slopes of about 25 degrees [6].

Terrain worked was not gentle. The traverse crossed transitional mare to highland ground, the Tangled Hills, and the Fossa Recta graben, roughly 19 km long, 400 m wide and 20 to 50 m deep, with slopes up to 20.3 degrees measured on a 5 m baseline and a total elevation range along the route of 144 m. During the fourth lunar day the rover worked to the edge of a tectonic fault in ground littered with blocks 2 to 3 m across [9].

Power and energy

Solar, with battery buffering, and a polonium-210 radioisotope heat source for the night. The solar array is mounted on the underside of a large hinged lid that covers the instrument compartment; the lid opens during the lunar day to expose the array and closes at night.

Lunokhod 2 replaced Lunokhod 1’s silicon cells with gallium arsenide, raising total capacity from 200 A-h to 250 A-h [4]. The rover navigated on battery and stopped periodically to recharge from the array.

Operation was confined to the lunar day. Each of the four lunar nights was spent parked at a fixed stop with the lid closed, the vehicle surviving on the isotope heater; the overnight stop coordinates for all four nights have been recovered from LRO imagery.

Thermal

The top of the instrument compartment is a thermal radiator and the retractable lid over it is the control element: open by day to radiate waste heat while the array underneath collects power, closed at night to cut radiative loss while the polonium-210 source keeps the compartment warm. The compartment is sealed and pressurized [4].

The design has one failure mode and it is what ended the mission. On 20 April the rover drove into a small, steep-sided crater; during the attempt to climb out the open lid contacted the crater wall and scooped soil onto the radiator [4]. With the radiator obscured, waste heat could no longer be rejected. Internal temperature became critical on 10 May and operations stopped, with radio contact lost the following day due to overheating of the vehicle body [4]. An internal Soviet report puts the end of operations at 12:25 UT on 10 May, after temperatures reached 43 to 47 C [9]. The mission was announced as over on 3 June 1973.

Soil on a thermal control surface raises its solar absorptance sharply: 11 percent areal dust coverage doubles the absorptance of a thermal control surface, and fine dust below 34 micrometers at 4.5 percent coverage raised absorptance to 0.245, of which a nitrogen jet recovered only 2 percent [1]. Dust contamination of Apollo 12’s own thermal control surfaces, by comparison, drove a 68 degree Fahrenheit temperature excess on the affected hardware [1]. The lid is simultaneously the radiator cover, the solar array support and the night insulation, so a single terrain contact removed power collection, heat rejection and night survival together, the kind of coupled thermal, dust and terrain constraint common to lunar rover designs generally; later reviews of rover concept studies concluded that modularity, rather than any single subsystem choice, is the single most important factor in developing a viable surface mobility vehicle [13].

Compute and avionics

There is no general-purpose onboard computer. Control is by direct command from Earth, and the onboard autonomy is limited to protective interlocks. Lunokhod 2 added an improved system of automatic locks that halted the vehicle on detecting dangerous slippage on steep slopes or excess voltage in the onboard power system. Detail on the avionics implementation is not available in the open literature consulted here.

Part of the attitude determination hardware failed shortly after landing, and was not recovered; the crew navigated for the rest of the mission from the positions of the Sun and prominent relief features in the camera views [4].

Autonomy

None beyond the protective interlocks above. Direct-command teleoperation of this kind sits at the least autonomous end of the mobility-autonomy spectrum later surveyed across planetary rovers [8]. The rover was driven in near real time from Earth by a five-person crew: commander, driver, navigator, flight engineer and antenna operator [9]. Round-trip light time is about 2.6 s, and the effective control delay including operator lever actuation about 4.1 s [9].

Navigation used the MKTV small-frame television system. Lunokhod 1 carried two cameras at 950 mm above the surface, the eye height of a seated person; Lunokhod 2 added a third mounted above them at standing-person height [4]. The higher camera became the attitude reference once the attitude determination hardware failed, by making the horizon and distant relief usable. Frame interval was cut from roughly 20 s to roughly 6 s, enabled by a higher downlink data rate; chassis and commanded speeds were unchanged between the two vehicles, so frame rate rose by about a factor of three and achieved average speed by a factor of 2.4, from 140 m/h to 340 m/h [4].

Communications

Direct to Earth. The rover carried its own radio system and did not relay through the Luna 21 lander after egress. The program’s own account attributes Lunokhod 2’s shorter navigation frame interval to a higher data transmission rate than Lunokhod 1 [4]; specific band assignments and bit rates are not published.

Payload and instruments

The instrument complement was [9], with technical detail from the 2017 reconstruction [4]:

• An imaging system of three low-resolution navigation television cameras and four high-resolution panoramic photometers. The panoramic cameras are scanning-mirror instruments, one horizontal and one vertical on each side, producing about 3000 lines over 180 degrees by 500 samples over 30 degrees horizontally, and 6000 lines over 360 degrees by 500 samples vertically [4]. Stereo panoramas were obtained by imaging the same scene from two rover positions.
• RIFMA-M, an X-ray fluorescence spectrometer for regolith chemistry, modified from the instrument flown on Lunokhod 1.
• PrOP, a trafficability instrument mounted at the rear of the compartment, measuring the physical and mechanical properties of the regolith. It is a cone-vane penetrometer: a 60 degree cone of 5.0 cm base diameter and 4.4 cm height is pressed into the soil with an effort in excess of 20 kg, then 7.0 cm vanes are rotated through up to 90 degrees, so that a single emplacement yields both bearing strength and rotational shear strength [2].
• SG-70A, a three-component magnetometer on a 1.5 m boom, intended to measure the magnetization of individual surface targets [12]. Its fluxgate sensor covered a range of plus or minus 580 gamma, with a residual absolute error of plus or minus 3 gamma after correcting for the rover’s own magnetic contamination [12].
• AF-3L, an astrophotometer measuring sky luminosity at visible and ultraviolet wavelengths.
• The TL-2 laser retroreflector, a block of 14 prisms, 45 by 20 by 8 cm, in heat-resistant glass under multilayer insulation, with a Rubin-1 photoreceiver [4], [9].
• An X-ray telescope, an RV-2N radiation detector, and the odometer and speedometer.

Across both rovers PrOP made about 100 measurements over more than 47 km of path, penetrating to as much as 100 mm [7]. The combined result is a modal bearing capacity of 0.35 to 0.4 kg/cm2, cohesion of 0.04 to 0.06 kg/cm2, an internal friction angle of 20 to 25 degrees, and modal rotary shear resistance of 0.045 to 0.055 kg/cm2 [7]. Fitted to a Rayleigh distribution, the data give a minimum carrying-power parameter of 0.08 to 0.10 kg/cm2 and a soil nonuniformity parameter of 3.0 cm2/kg [7]. For Lunokhod 1 alone, bearing strength ranged from 0.2 to 1.0 kg/cm2 with terrain, most frequently 0.34 kg/cm2, and rotational shear from 0.02 to 0.09 kg/cm2, most frequently 0.048 kg/cm2 [2]. Individual penetrometer readings on the Lunokhod 1 route varied with terrain: 8.1 cm penetration at a 16.5 kgf cone load on the horizontal intercrater surface, 20.5 cm at 23.5 kgf on a crater slope, and 7.8 cm at 19 kgf on a crater rim [2]. Strength increases with depth, and direct numerical comparison with the Apollo penetrometer values is not valid, because the instruments sampled different depths by different methods. Laboratory testing of the returned Luna 16 and Luna 20 samples put the internal friction angle at 46 and 32 degrees respectively once compacted, and found bearing capacity to rise sharply above a bulk density of 1.4 to 1.5 g/cm3 [7]. A separate account of the Lunokhod 1 route gives a similar bearing strength range of 0.2 to 1.1 kg/cm2 and shear resistance of 0.02 to 0.09 kg/cm2 [5].

LROC Narrow Angle Camera images at 0.5 to 1.7 m/pixel resolved the Luna 21 landing site and let the exact route be followed from the rover’s own wheel tracks, giving the 39.16 km total [4]. The tracks are readable because the grousers on the Lunokhod wheels disturbed the soil strongly, unlike the grouserless Apollo rover wheels at similar wheel loading, whose tracks are hard to see outside the descent stage blast zones. Places where the rover turned in place, within a 0.8 m radius, are detectable in the images [4].

The magnetometer data were used with the Apollo 16 surface magnetometer for electromagnetic sounding of the lunar interior [10], and produced evidence of an electrical conductivity anomaly beneath Mare Serenitatis [11], [12]. About 65 percent of the horizontal magnetic fluctuations fell within a 30 to 60 degree azimuth band, modeled by a nonconducting lithosphere layer about 150 km thick beneath the highlands and 300 km thick beneath Mare Serenitatis [11]; the mean field across Le Monnier Bay itself was 20 to 30 gamma, with craters larger than 50 m carrying local anomalies of 10 to 15 gamma [12]. The comparison Apollo 16 magnetometer at the Descartes highland site recorded a remanent field of 327 gamma [11]. The retroreflector outlived everything else: 1400 ranging measurements were made from the Crimean Astrophysical Observatory’s 2.6 m telescope between 1978 and 1983 at 25 cm accuracy, terminating only with the cancellation of the Soviet lunar program, and ranging continued from other observatories afterwards [4]. The return signal is at least five times weaker than Lunokhod 1’s, possibly from dust obscuration. Reflector positions established to centimeter level define geodetic reference points for the lunar coordinate frame. Lunokhod 1’s own retroreflector, by contrast, could not be recovered at all until its position was picked out of LRO imagery in 2010, to 5 m accuracy [9].

Total imaging return was about 80,000 navigation frames and 86 panoramas, with hundreds of mechanical and chemical surveys of the soil including 25 RIFMA analyses [9].

Modes of operation

The operating cycle is imposed by illumination rather than by onboard state machines. Driving and science occur during the lunar day. At the end of each day the vehicle is parked at a chosen stop, the lid is closed, and it hibernates through the roughly 14 day night on the polonium heater, to be woken when the Sun returns [4]. Four such hibernations were completed. The protective interlocks constitute the only automatic mode transition: on detected slippage or overvoltage the vehicle stops itself.

Per lunar day the traverse distances, remeasured from orbit, were 1299 m, 10034 m, 17622 m, 9136 m and 1014 m, against mission-era odometer values of 1148, 9919, 16533, 8600 and 880 m respectively [4]. The fifth day was cut short by the radiator contamination.

Ground operations

The rover was driven from Simferopol in Crimea by a dedicated crew, working from the navigation camera feed [4]. The crew composition and the general method were inherited from Lunokhod 1: a commander, driver, navigator, flight engineer and antenna operator, driving against imagery that updates every few seconds, which means committing to a heading before seeing its outcome [9]. Contemporary Soviet press reported crew pulse rates of 130 to 135 beats per minute during difficult driving [9].

Route planning used American imagery. Photographs of the surface around the Luna 21 landing site, taken before the Apollo 17 landing, were handed to a Soviet engineer at a planetary exploration conference in Moscow between 29 January and 2 February 1973, after Luna 21 had landed, and were subsequently used by the driver team to navigate [9].

The mission-era route reconstruction was a manual operations map compiled during the mission and held by the Lavochkin museum. The same LRO-based photogrammetric method was applied first to Lunokhod 1’s route in 2013 [3], then to Lunokhod 2’s in 2017, when it yielded 39.16 km against the mission estimate of 37 km and recovered the landing and overnight stop coordinates [4].

References