Orwell Astronomical Society (Ipswich)
Lunokhod 2 Explores The Moon, 1973
On 08 January 1973, at 9.55am Moscow time, the USSR launched the probe Luna 21 towards the Moon. Throughout the journey, only one fault occurred. A tracking ship in the Atlantic Ocean received signals that one of the landing legs failed to open; however this later turned out to be a false alarm. Five days1 after launch, the craft went into orbit around the Moon and, at 1.31am on 16 January, it landed. Despite its weight, the probe touched down with a velocity of a mere 2 m/s, only 20% that of a parachute landing on Earth! On the day of its landing, at 4.14am, ground controllers used remote control to drive the lunar rover Lunokhod 2 off Luna 21 and onto the lunar surface.
Lunokhod 2's landing site was inside the crater Le Monnier, at coordinates 30° 27' E, 25° 51' N, five kilometres distant from the mountains which define the southern boundary of the feature. The landing site was 180 km north of the location in the Taurus-Littrow mountains where Apollo 17 had landed one month earlier. Le Monnier lies in a smooth mare region on the eastern edge of Mare Serenitatus, in contrast to the mountainous region where Apollo 17 landed.
Mare Serenitatus is a very interesting region. It was formed some 3,000 million years ago by a comet or asteroid which collided with the Moon creating a basin and causing a lava flow to start. The lava flow solidified in the basin to form a discus shape with a radius of approximately 250 km, several kilometres thick in the middle and thinner at the edges. The lava formed a very high density mascon (mass concentration) which caused Apollo 15 to dip 75 m in its orbit about the Moon. Scientists eagerly anticipated comparing data from Lunokhod 2 and Apollo 17; it was anticipated that such a comparison could give clues to the origin of Mare Serenitatus. The USSR performed an analogous comparison of data from Lunas 16 and 20 which both returned soil samples from the eastern part of Mare Fecunditatus (Luna 16 from the smooth Mare itself and Luna 20 from the mountainous region on its eastern edge).
Radio Moscow gave some details about Lunokhod 2. The vehicle had eight wheels, in the hub of each of which was a motor, a brake, a temperature gauge and a load gauge. Lunokhod 2 could turn while stationary or while moving either backwards or forwards. It was one of a second generation of moonprobes introduced in the early 1970s starting with Luna 15. The vehicle could negotiate crevasses up to 0.5 m wide, medium sized boulders, deep dust and craters. The Lunokhods were powered by batteries continuously recharged by solar sells and therefore could remain operational for a long time. Lunokhod 1 (delivered to the Moon by Luna 17) roved over the lunar surface during 1970-71. After a few months its moving parts began wearing out, and after 11 months its radioactive heating failed and it froze and ceased to function.
Ground controllers had scheduled a period of inactivity for some 50 hours after disembarking Lunokhod 2 from Luna 21. Following the inactive period, on 17 January ground controllers held a two hour communication session with the rover. The angle of the Sun made it difficult for the five-man crew at ground control to estimate distances from the TV pictures returned by Lunokhod 2. At one point during the session, ground controllers brought Lunokhod close to Luna 21 in order to obtain pictures which would show how the craft had been affected by its journey through space; they almost drove the rover into the landing platform, executing a turn only four metres away from it.
Lunokhod 2 started its scientific programme as planned on 18 January when ground controllers held a third communications session with the vehicle, lasting two hours. Ground controllers instructed it to move to a region of the lunar surface undisturbed by the descent motor of Luna 21. The journey involved passing through a narrow gap between two craters. Lunokhod 2 then undertook an analysis of the soil, finding it to be identical with that analysed by Lunokhod 1 in Mare Imbrium in 1971.
Ground controllers held the next communications session with Lunokhod 2 on the night of 19-20 January. The session lasted six hours, during which time they commanded the vehicle to move away from its flat landing site towards a mountainous region. The vehicle drove for a distance of 1150 m, moving to 1050 m from the landing site. Ground controllers steered it around natural obstacles and drove at various speeds in a south-easterly direction. The white peaks of the Taurus mountains became visible over the horizon. Shortly after setting out on this journey, Lunokhod 2 had to negotiate a crater 15 m in diameter. During similar manoeuvres with Lunokhod 1, ground controllers held keen debates about how to approach the obstacle. However, building on the experience obtained with the earlier rover, the ground controllers of Lunokhod 2 approached the crater without need of a prolonged debate and in the space of 30 minutes the vehicle covered a distance of nearly 30 metres. Again building on previous experience, ground controllers drove Lunokhod 2 at a maximum speed twice that of Lunokhod 1.
On 20 and 21 January, ground controllers held two more communication sessions with Lunokhod 2, during which they turned on scientific instruments and took panoramic TV pictures to select a suitable parking space to "hibernate" during the two-week long lunar night. They selected a location over a kilometre away from the landing site, on the edge of a small, deep crater with an outcrop of bedrock on its edge, and parked the vehicle there in preparation for lunar nightfall. They intended to study the bedrock later in the hope that it would aid understanding of the chemical composition of material of the transitional zone in the "coastline" of Mare Serenetatis.
By the end of its first working lunar day, Lunokhod 2 had travelled over a kilometre, returned many television pictures of the lunar surface, conducted astrophysical observations by means of a new visible and ultraviolet light sensing device, conducted observations of the Milky Way, and used detectors of X-rays and solar corpuscular radiation. All systems had functioned well.
The first lunar night of Lunokhod 2 began with sunset over Mare Serenitatus on 24 January. Ground controllers powered down the vehicle, as without sunlight it cannot charge its batteries. However, it is likely that Lunokhod 2 maintained its instrument compartment at a temperature and pressure similar to that on Earth. On 29 January, using sensors attached to a magnetometer arm, Lunokhod 2 measured the external temperature as -183° C; this was the first measurement of the temperature of lunar night. The measurement is interesting, since the wheels of Lunokhod 2 were never colder than -128° C. The vehicle's systems stood up well to the cold.
The second lunar day for Lunokhod 2 began on 09 February. At 3.00pm (Moscow time) ground controllers interrogated the rover as to its condition, and found that a reserve of power remained. The initial communications session lasted 11 hours. Lunokhod 2 first measured the magnetic field associated with the rock of the crater where it was parked. Ground controllers then commanded it to take a journey across the lunar surface spanning 364 metres and involving the execution of 120 turns. During the journey, Lunokhod 2 measured the magnetic field in areas up to 40 m distant from the crater at which it had parked, and captured many panoramic pictures of the surface. The rover passed many boulders and craters which, although relatively small, were difficult to negotiate and, on several occasions, it sank in loose rock up to the hubs of its wheels. During the journey, the crew of ground controllers was changed twice.
On 10 and 11 February, Lunokhod 2 undertook two more journeys of exploration, covering 2 km and 1.6 km respectively. Lunokhod 2 undertook observations of a crater some 30 m diameter and then travelled in a south-easterly direction, heading towards the Taurus-Littrow mountains. On the journey, it again took measurements of the physical and mechanical properties of lunar rock, measured the strength of the magnetic field and took more panoramic pictures. Sections of the route abounded in small craters and rocks which considerably hindered progress.
On 14 February, in an area almost 5 km distant from the landing site and at an altitude some 300 m higher, in the foothills of the Taurus Mountains, Lunokhod 2 discovered an unusual piece of moonrock, which had been ejected from the interior of the Moon by an impact which resulted in the formation of a large crater. The rock was approximately one metre in length and proved to be an extremely strong monolith: a pressure of 100 atmospheres applied to it left only slight marks. It had a smooth surface whereas rocks lying nearby were pockmarked with tiny craterlets, indicating that it was much younger than most of the rocks in the vicinity (the majority of which are tens and even hundreds of millions of years old).
The astrophotometer aboard Lunokhod 2 found that the lunar sky was 10-15 times brighter after sunset than the Earth's sky. Andrei Severny, Director of the Crimean Astrophysical Observatory, which designed the instrument, said that the measurement provided clear evidence of a dust atmosphere on the Moon, with dust particles playing the role of gas molecules. He explained: The dust atmosphere may well produce the strong glow, since its particles disperse the light from the Earth and the Sun.
On 16 February, during a communication session, Lunokhod 2 covered more than 2.5 km, travelling in a southerly direction. It entered an area surrounded by gently sloping hills some 100-150 m high. The vehicle reached a point 5.7 km distant from its landing site, in an area undoubtedly much older than the basalt of Mare Serenitatus. In the days after 16 February, Lunokhod 2 entered the Taurus Mountains proper. In subsequent communications sessions, it continued to move in a southerly direction, at times traversing slopes as steep as 25° with skidding reaching 80%. However, the vehicle had little difficulty traversing such terrain.
Originally, when Lunokhod 2 left its landing site in Le Monnier, it found itself on a wide flat plain with the only mountains visible situated a few kilometres to the south. The mountains 55 km to the north of the landing site were below the horizon. Towards the end of the second lunar day, Lunokhod 2 ascended some high slopes until it reached the top of a peak 400 metres above the landing site. At the summit, ground controllers stopped the rover to take panoramic TV pictures: the cameras could see all the way across the flat plain of Le Monnier to the mountains 55 km to the north and to the mountainous area 25 km to the south. Lunokhod 2 started compiling a panoramic stereoscopic view from the peak on 20 February and finished the following day. Ground controllers were able to use the apparent positions of the Earth2 and the Sun in the sky to determine the exact location and bearing of the vehicle. Further analysis of the panoramic, stereoscopic view will provide information about the formation of the crater Le Monnier and how it became a bay at the edge of Mare Serenitatus.
On 21 February, Lunokhod 2 descended the opposite slope of the mountain, still travelling southwards and, as expected, came across a crater about 2 km in diameter and 200 m in depth. This crater came into existence some 3,000 million years ago, about the same time as Le Monnier. Lunokhod 2 obtained a series of photographs of the bottom of the crater in two communications sessions on 21 and 22 February. Detailed analysis of the images will assist with interpretation of a number of peculiarities concerning the formation of the crater. Ground controllers then commanded Lunokhod 2 to return to the crest of the crater and orientate itself east-west in readiness to meet the onset of the dawn of its third lunar day. They checked its systems and closed the lid on the solar battery to protect it from the rigours of the lunar night, which descended on 24 February.
By 22 February, Lunokhod 2 had travelled a total of 11.1 km. While this may seem a modest total, it is worth remembering that Lunokhod 1, its predecessor, broke down after 11 months of operation having covered a total of 10.5 km. The fact that Lunokhod 2 has travelled a greater distance in only two months reflects the increased skill and experience of the ground controllers.
After the passage of the lunar night, the Sun rose on Lunokhod 2 on 09 March. Ground controllers reactivated the vehicle on 11 March, held a communications session with it and let it recharge its batteries in the sunlight. They then commanded the vehicle to move some 30 m, taking TV pictures as it travelled. On 12 March, Lunokhod 2 reached an interesting lunar crater. Ground controllers announced that it would spend some time studying the crater under sunrise angles of illumination to complement the observations made at the close of the previous lunar day under sunset angle of illumination.
Lunar midday occurred on 16 March. Close to midday, shadows were very short and it was difficult to discern relief with the TV cameras. At the start of the lunar afternoon, with the Sun moving westwards to sunset, Lunokhod 2 started another long traverse. On 17 March, it travelled 2230 metres and, on 18 March, a further 3130 metres. By 19 March, the vehicle had covered a total of 17.7 km since landing on the Moon.
On 23 March, ground controllers parked Lunokhod 2 two kilometres from a large tectonic feature south of the crater Le Monnier. The feature was a fault up to 100 metres wide and 16 km long. Ground controllers then shut down Lunokhod 2 for the third time, in preparation for lunar sunset which occurred on 24 March.
After local dawn on 09 April, ground controllers drove Lunokhod 2 to within 200 m of the fault feature then waited until the Sun rose to a suitable elevation and spent the rest of the lunar day examining the feature. They found that local terrain around the feature included a large area of exposed bedrock, with loose regolith covering the rest of the region to a depth of 5-30 cm. By 22 April Lunokhod 2 had covered a distance of 36.2 km; ground controllers then shut it down for the lunar night until the start of its fifth lunar day.
The Sun rose over Lunokhod 2 for the fifth time on 07 May. In a communications session on 08 May, ground controllers reactivated the vehicle after its fourth lunar night and opened its solar panel. On 09 May, Lunokhod 2 resumed movement, left the tectonic feature that it had studied during the previous lunar day and set off in a north-easterly direction towards the nearest shore cusp of the Taurus Massif. The fifth hibernation period of Lunokhod 2 began with lunar sunset on 22 May.
Unfortunately, all attempts to re-establish communication with Lunokhod 2 on 03 June, following lunar sunrise, failed, and eventually ground controllers were forced to concede that the vehicle had been lost. At the time of writing (July 1973) ground controllers have not yet determined the cause of the failure.
The 11 month lifespan of Lunokhod 2 was much longer than any of the Apollo missions which lasted for only a matter of days. The Lunokhods, of course, have their limitations when compared with a manned mission to the Moon. The argument over whether men or robots are best at lunar exploration is invalid: both have advantages and disadvantages, therefore both should be used. Unfortunately there are unlikely to be further manned Moon missions for many years!
According to US Intelligence, a Russian Proton-type rocket carrying a third Lunokhod was launched towards the end of April 1973 but crashed into the Pacific Ocean.
Thirty-seven years after the demise of Lunokhod 2, on 15 March 2010, NASA released images of Le Monnier captured by the Lunar Reconnaissance Orbiter (LRO). Within two days, Sasha Basilevsky identifed tracks created by the rover and its final resting place. See right-hand image below: the small white arrows indicate the rover's tracks, the large white arrow its final resting place. It is thought that on 09 May, the rover's open lid touched a crater wall and became covered with dust. When the lid was closed, the dust (a very good insulator) was dumped onto radiators, preventing the vehicle from disposing of excess heat. The following day, controllers saw the internal temperature rise, eventually leading to failure on 11 May, when communications were lost. The black arrow indicates the crater where the Lunokhod is thought to have acquired the dust.
Whereas American moonprobes, manned or unmanned, usually took three days to reach the Moon, the USSR used a minimum energy trajectory which took five days, enabling a greater payload to be transported for a given amount of fuel.
The Earth appeared as a thin crescent in the lunar sky.