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 [1]. The rover drove off the lander at 01:14 UT on 16 January. It landed at approximately 30.4 E, 25.97 N in the southern part of Le Monnier [2].

The vehicle was a direct evolution of Lunokhod 1, with the same self-propelled chassis and the same sealed, pressurized instrument compartment [2]. 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 [1].

Specifications

Parameter	Lunokhod 2	Lunokhod 1	Source
Rover mass	836 kg	756 kg	[2]
Rover mass, second value	840 kg	756 kg	[7]
Dimensions	170 x 160 x 135 cm	2.15 m across, about 1.35 m high	[7], [1]
Solar array	250 A-h, gallium arsenide	200 A-h, silicon	[2]
Wheel diameter / width	510 mm / 200 mm	510 mm / 200 mm
Track gauge	1600 to 1700 mm	1700 mm wheelbase	[2], [7]
Commanded speeds	0.8 and 2.0 km/h	0.93 and about 2 km/h	[2]
Navigation cameras	3 MKTV	2 MKTV
Navigation frame interval	about 6 s	about 20 s

Mission profile

Parameter	Value	Source
Launch	8 January 1973, Proton-K with Blok D	[1]
Landing	22:35 UT, 15 January 1973, crater Le Monnier
Landing point	approximately 30.4 E, 25.97 N	[2]
Rover egress	01:14 UT, 16 January 1973	[1]
End of operations	10 May 1973
Surface lifetime	139 days, 5 lunar days	[7]
Distance driven	39.16 km, 37 km as measured at the time	[2]
Average achieved speed	340 m/h, against 140 m/h for Lunokhod 1

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 [7].

Mobility

Eight wheels, four per side, each independently driven with its own motor and brake, on a chassis carried over unchanged from Lunokhod 1 [2]. 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. Wheelbase is 1.7 m [7]. Grousers are fitted across the mesh.

Average specific ground pressure at a nominal 30 mm sinkage is 0.05 kg/cm2, about 4.9 kPa [2]. 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. Regolith bulk density in the first 30 cm is 1.53 to 1.63 g/cm3, friction angle 30 to 50 degrees and cohesion up to 1 kPa [9].

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 [2]. 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 [2]. Achieved average over the whole mission was 340 m/h. The same chassis operated on slopes up to 32 degrees on Lunokhod 1 [7].

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 [2]. 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 [1].

Power and energy

Solar, with battery buffering, and a polonium-210 radioisotope heat source for the night [2]. 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 [2]. Solar flux at the Moon is 1367 W/m2 mean over a 1315 to 1421 W/m2 range [11]. The rover navigated on battery and stopped periodically to recharge from the array [7].

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 [7]; the overnight stop coordinates for all four nights have been recovered from LRO imagery [2]. Lunar surface temperature before sunrise is a mean 96 K at the equator and a mean 89 K at 45 degrees latitude [11].

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 [2], [7]. The compartment is sealed and pressurized, so heat moves internally by gas convection rather than through conduction paths. Regolith conductivity is low enough that the ground is not a usable heat sink: the contact term for Apollo 11 mare regolith at 1640 kg/m3 is 1.868e-3 W/(m.K), and for Apollo 16 highland regolith at 1500 kg/m3 it is 4.84e-4 W/(m.K) [11].

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 [2]. 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 [2]. An internal Soviet report puts the end of operations at 12:25 UT on 10 May, after temperatures reached 43 to 47 C [1]. 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 [8]. 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.

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 [2]. 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 [2].

Autonomy

None beyond the protective interlocks above. The rover was driven in near real time from Earth by a five-person crew [7], [9]. Round-trip light time is about 2.6 s, and the effective control delay including operator lever actuation about 4.1 s [1].

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 [2], [7]. 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 [2].

Chassis, commanded speeds and control latency were unchanged between the two vehicles; 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 [2].

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 [2]; specific band assignments and bit rates are not published.

Payload and instruments

The instrument complement was [1]:

• 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 [2]. 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 [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 [4].
• SG-70A, a three-component magnetometer on a 1.5 m boom, intended to measure the magnetization of individual surface targets [2].
• AF-3L, an astrophotometer measuring sky luminosity at visible and ultraviolet wavelengths.
• The TL-2 laser retroreflector, a French-supplied block of 14 prisms, 45 by 20 by 8 cm, in heat-resistant glass under multilayer insulation, with a Rubin-1 photoreceiver [2].
• 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 [3]. 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, modal rotary shear resistance of 0.045 to 0.055 kg/cm2, and natural porosity of 0.8 to 0.9. 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 [4]. Strength increases with depth, crater rims are softer and slopes stronger than the intercrater plains, which agrees with the Apollo observations. Direct numerical comparison with the Apollo penetrometer values is not valid, because the instruments sampled different depths by different methods.

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 [2]. 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 [2].

The magnetometer data were used with the Apollo 16 surface magnetometer for electromagnetic sounding of the lunar interior, and produced evidence of an electrical conductivity anomaly beneath Mare Serenitatis [5], [6]. 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 [2]. 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.

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 [1].

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 [2]. 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 [2]. 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 [2]. 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. Contemporary Soviet press reported crew pulse rates of 130 to 135 beats per minute during difficult driving [1].

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 [1].

The mission-era route reconstruction was a manual operations map compiled during the mission and held by the Lavochkin museum [2]. The definitive route was only established in 2017, by following the wheel tracks in LRO Narrow Angle Camera imagery and photogrammetric stereo processing to a DEM and orthomosaic, which yielded 39.16 km against the mission estimate of 37 km and recovered the landing and overnight stop coordinates.

Technologies developed

The eight-wheel independently driven skid-steered chassis, three titanium rings per wheel with 16 mesh-wrapped spokes and grousers, carries no steering actuators, provides eight independent drive failure domains, turns in place within 0.8 m, and operated on slopes to 32 degrees [2], [7], [9]. It was reused unchanged between the two flight vehicles.

Frame rate rather than vehicle capability bounded teleoperated productivity: an unchanged chassis at unchanged commanded speeds, with one camera added and the navigation frame interval cut from about 20 s to about 6 s, returned 340 m/h against 140 m/h [2].

The retroreflector is the only part of the mission still returning data. Ranging from the Crimean Astrophysical Observatory 2.6 m telescope between 1978 and 1983 produced 1400 measurements at 25 cm accuracy, and reflector coordinates established to centimeter level define geodetic reference points for the lunar coordinate systems in current use [2].

The lid couples three functions to one mechanism and one exposed surface: solar array support, thermal radiator cover and night insulation. A single contact with a crater wall on 20 April 1973 deposited soil on the radiator and ended the mission on 10 May [2], [1]. Dust on a thermal control surface is not recoverable by gas jet: fine dust below 34 micrometers at 4.5 percent areal coverage raised absorptance to 0.245, of which a nitrogen jet recovered 2 percent [8].

References