Pragyan

ISRO.

Overview

Pragyan is a six-wheeled solar-powered rover built by ISRO and carried inside the Vikram lander on Chandrayaan-3. It carries two elemental-composition instruments, an Alpha Particle X-ray Spectrometer and a Laser Induced Breakdown Spectroscope, and communicates only with the lander, which relays to the Indian Deep Space Network [5]. Two flight units were built to the same configuration. The first was lost during the Chandrayaan-2 landing attempt that followed lander separation on 2 September 2019: descent was reported as normal to an altitude of 2.1 km, after which communication from the lander to the ground stations was lost [9]. The second landed at 69.367621 S, 32.348126 E on 23 August 2023 and operated for one lunar day [3].

Specifications

Parameter	Value	Source
Mass	26 kg	[5], [6]
Dimensions	917 x 750 x 397 mm	[6]
Wheels	six, grousered	[3]
Wheel center spacing, front to rear	680 mm
Track width, outer wheel faces	750 mm
In-place turn circle diameter	1012 mm
Mobility primitives	forward, reverse, in-place rotation
Electrical generation	50 W	[5], [6]
Power source	solar	[10]
Payload count	2	[6]
Communications	rover to lander only	[5]

The Chandrayaan-2 unit was specified at 27 kg and 50 W with a 500 m design range [10]. The Chandrayaan-3 unit is published at 26 kg and 50 W [5], [6]. No rated drive speed, gradeability or obstacle height is published for either.

Mission profile

Parameter	Value	Source
Launch	LVM3-M4, 14 July 2023	[7]
Landing	23 August 2023, 12:34 UTC	[3]
Ramp-down onto the surface	24 August 2023	[7]
Landing site	69.367621 S, 32.348126 E	[5]
Prime landing ellipse	4 km x 2.4 km
Surface mission life	one lunar day, about 14 Earth days	[5], [6]
Distance driven	about 101 m	[3]
Sleep mode entered	4 September 2023	[7]
Recovery attempts ended	22 September 2023

Vikram carried a mass of 1749.86 kg including the rover, and generated 738 W at winter solstice [6]. The rover was accommodated inside the lander and deployed down a primary and secondary ramp mechanism [5].

The landing site is low-relief terrain. A 0.30 m per pixel digital elevation model covering 2.18 x 2.24 km around the lander gives a mean slope of 3.61 degrees with a standard deviation of 2.93 degrees and a 95th percentile of 9.25 degrees; within a 250 m buffer of the lander the mean falls to 3.10 degrees and the 95th percentile to 7.24 degrees, with 13.7 m of local relief [4]. The rover is resolved in that model as a discrete topographic feature 8.3 m from the lander at 69.5079 S, 32.3331 E.

Landing displaced about 2.06 t of epiregolith over an area of 108.4 square meters around the landing site [8].

Mobility

Mobility is six wheels fitted with grousers that dig into the regolith [3]. The published mobility primitives are forward drive, reverse drive and in-place rotation about the rover center. The distance between the centers of the front and rear wheels is 680 mm and the distance across the outer faces of the left and right wheels is 750 mm, so an in-place rotation sweeps a circle of radius 506 mm and disturbs a circular patch of regolith about 1012 mm across [3].

That signature is what allowed the traverse to be reconstructed from orbit. Rover tracks are not directly visible in Chandrayaan-2 Orbiter High Resolution Camera imagery at about 25 cm nadir resolution, but the in-place turn patches raise the local gray count by 15 to 20 counts in a difference image built from October 2021 and April 2024 acquisitions taken at similar illumination, 9.1 and 10.8 degrees solar elevation respectively [3]. Eight in-place turn locations, carrying mobility-ids M-14 through M-34, were matched to the uplinked mobility commands with positional accuracy within 1 m.

Several of those rotations were near 180 degrees and were commanded for solar power generation rather than for traverse [3]. Total distance driven was about 101 m over the mission, against the 500 m design range published for the Chandrayaan-2 unit [10].

Power and energy

Electrical generation is 50 W from a single solar panel [5], [6], [10]. No battery capacity is published on the ISRO pages or in the retrieved literature. Rover attitude was managed for power: in-place rotations of approximately 180 degrees were commanded specifically to improve solar generation [3]. The rover was not designed to survive lunar night; the mission life is one lunar day, and after the rover was placed in sleep mode on 4 September 2023 attempts to re-establish communication around sunrise on 22 September 2023 did not succeed [7].

Thermal

No rover thermal design values are published. The instrument-level requirements are published. LIBS is qualified for an operating range of -20 to +55 degrees C and a storage range of -40 to +70 degrees C [2]. The APXS silicon drift detector must be held near -35 degrees C, which is achieved by an integral thermoelectric cooler with an active Peltier controller that maintains the setpoint for ambient temperatures below +30 degrees C and reaches a stable operating temperature about two minutes after switch-on; the detector is coupled to the metallic portion of the payload assembly and to a radiator plate to drain the heat [1].

Compute and avionics

A common rover FPGA-based electronics unit services the science payloads [1]. For APXS the rover FPGA generates the peak-detector control signals on an event trigger, reads the 12-bit serial ADC output, and assembles packets of about 1 KB holding individual X-ray events with header data. LIBS takes its telecommands over a 16-bit serial data interface from the rover [2]. Neither the flight processor part nor the radiation tolerance approach is published.

Autonomy

Operation was command-driven rather than autonomous. Mobility was uplinked to the rover through the lander as discrete numbered mobility commands, the mobility-ids that were later correlated against orbital imagery of the disturbed regolith [3]. No onboard hazard detection or autonomous path selection is published, and the ISRO program pages attribute hazard detection and avoidance to the lander, through the Lander Hazard Detection and Avoidance Camera and its processing algorithm, not to the rover [5].

Communications

The rover communicates only with the lander [5], [10]. The lander carries the link to the Indian Deep Space Network on X-band, with the Chandrayaan-2 orbiter held as a contingency path. No rover link frequency or data rate is published.

Payload and instruments

Instrument	Function	Source
APXS	X-ray fluorescence elemental composition, excited by a 244Cm source emitting both X-rays and alpha particles	[1]
LIBS	Laser-induced plasma emission spectroscopy over 220 to 800 nm for Mg, Al, Si, K, Ca, Ti, Fe and volatiles	[2], [6]

APXS uses alpha-particle as well as X-ray excitation, which raises sensitivity to low-Z elements by more than two orders of magnitude relative to X-ray-only excitation [1]. The detector is a silicon drift detector of 30 square millimeters active area and 450 micrometer thickness behind an 8 micrometer beryllium window. The instrument sits on a deployment mechanism mounted to the rover bottom chassis that steers it from a stowed 0 degree position to a deployed 90 degree position; deployed, the detector stands about 55 mm above the surface and sees a circular field of view about 12 cm across [1]. Stowing it during drives restores ground clearance and protects the sensor head from dust. A calibration target plate fixed to the rover chassis covers the front face of the stowed instrument, giving in-flight gain and offset monitoring. The source capsule has a 6 mm active diameter inside an 8 mm disc and a 10 mm encapsulation, shielded by a 2 mm gold cup inside a 1 mm stainless steel outer layer [1]. A rover hold-down restrains the mechanism through launch and is released during roll-out from the lander ramp.

LIBS parameters, all from [2]:

Parameter	Value
Laser	diode-pumped solid state, 1540 nm
Pulse energy	3 to 4 mJ
Pulse duration	7 ns
Beam diameter	0.8 mm
Instrument to surface distance	205 mm plus or minus 8 mm
Spot size at that distance	about 100 micrometers
Angular field of view	plus or minus 3 degrees
Focusing lens	N-BK7, 200 mm focal length
Collection optics	two-fold lens-mirror, 37 mm entrance pupil, optical gain 0.65
Spectrograph	flat-field, f/2.2, aberration-corrected concave holographic grating
Slit	50 micrometers by 500 micrometers
Detector	UV-enhanced linear CCD, 2048 pixels, 14 by 200 micrometers
Spectral range	220 to 800 nm, resolution better than 1 nm
Mass	about 1.1 kg
Steady state power	1.19 W
Peak power	10.42 W for 20 shots at 5 Hz
Envelope	180 x 150 x 80 mm
Structure	Mg-alloy base, Ti-alloy opto-mechanical mounts

Modes of operation

Published modes are drive, science measurement at a mobility stop, and sleep. LIBS and APXS measurements are taken at mobility stops, which is how the local compositional heterogeneity around the landing site is sampled [2]. APXS is deployed to 90 degrees for a measurement and stowed for each drive [1]. The rover was placed in sleep mode on 4 September 2023 ahead of lunar night [7].

Ground operations

Operations were run by ISRO through ISTRAC, Bengaluru, with the deep-space link at IDSN [5], [7]. Commanding was open-loop from the ground: numbered mobility commands were uplinked through the lander, and the ground record of those commands was later used as the reference against which orbital change-detection was validated [3]. Post-mission reconstruction of the traverse was performed at the Space Applications Centre from OHRC imagery.

Technologies developed

Chandrayaan-3 flew the Chandrayaan-2 rover configuration essentially unchanged; the program’s design response to the 2019 loss was concentrated in the lander, which gained a revised configuration, four throttleable 800 N engines, a Laser Doppler Velocimeter, a Ka-band altimeter and the Lander Hazard Detection and Avoidance Camera [5]. From the rover, the outputs that outlived the mission are the two miniature composition instruments and their in-situ results: LIBS returned the first unambiguous in-situ detection of sulfur on the lunar surface on 28 August 2023, and APXS detected minor elements on 30 August 2023 [8]. The LIBS design point, a 1.1 kg instrument drawing 1.19 W in steady state and firing 3 to 4 mJ pulses at 205 mm standoff, is an order of magnitude below the mass and power of standoff LIBS instruments flown on Mars [2].

Chandrayaan-2 hardware remains the operational asset behind the reconstruction work: the OHRC on the Chandrayaan-2 orbiter, at about 25 cm nadir resolution, resolved both the lander and the rover [3], [4].