Yutu (Chinese: 玉兔; pinyin: Yùtù; literally: "Jade Rabbit") is an unmanned lunar rover that formed part of the Chinese Chang'e 3 mission to the Moon. It was launched at 17:30 UTC on 1 December 2013, and reached the Moon's surface on 14 December 2013. The mission marks the first soft landing on the Moon since 1976 and the first rover to operate there since the Soviet Lunokhod 2 ceased operations on 11 May 1973.
The rover encountered operational difficulties toward the end of the second lunar day after surviving and recovering successfully from the first 14-day lunar night (about a month on the Moon), and was unable to move after the end of the second lunar night, though it continued to gather useful information for some months afterward. In October 2015, Yutu set the record for the longest operational period for a rover on the Moon. On 31 July 2016, Yutu ceased to operate after a total of 31 months, well beyond its original expected lifespan of three months.
The Yutu lunar rover was developed by Shanghai Aerospace System Engineering Institute (SASEI) and Beijing Institute of Spacecraft System Engineering (BISSE). The development of the six-wheeled rover began in 2002 and was completed in May 2010. The rover deployed from the lander and explored the lunar surface independently. The rover's name was selected in an online poll, and is a reference to the pet rabbit of Chang'e, the goddess of the Moon in Chinese mythology.
The official mission objective was to achieve China's first soft-landing and roving exploration on the Moon, as well as to demonstrate and develop key technologies for future missions.
The scientific objectives of Chang'e-3 mainly included lunar surface topography and geological survey, lunar surface material composition and resource survey, Sun-Earth-Moon space environment detection, and lunar-based astronomical observation. Chang'e 3 performed the first direct measurement of the structure and depth of the lunar soil down to a depth of 30 m (98 ft), and investigated the lunar crust structure down to several hundred meters deep.
Unlike NASA and ESA, the China National Space Administration reveals little about its missions to the public, so detailed information about Chang'e 3 is limited. Aspects of Yutu's design and several of its experiments may have been based on NASA's Mars Exploration Rovers. Its wheel design is believed to have been considerably influenced by what was used on the Russian Lunokhod 1 rover.
The Yutu rover has a mass of 140 kg (310 lb), with a payload capacity of 20 kg (44 lb). It is smaller than the Mars Exploration Rovers, Spirit and Opportunity, and carries similar instruments: panoramic cameras, an infrared spectrometer and an alpha particle X-ray spectrometer (APXS). Yutu is also equipped with a robotic arm to position its APXS near a target sample. In addition, the rover could transmit live video, and has automatic sensors to prevent it from colliding with other objects.
Yutu was designed to explore an area of 3 square kilometres (1.2 sq mi) during its three-month mission, with a maximum travelling distance of 10 km (6.2 mi). Energy was provided by two solar panels, allowing the rover to operate through lunar days. During the 14-day lunar nights, the rover went into sleep mode, during which heating was provided by radioisotope heater units (RHU) using plutonium-238 and two-phase fluid loops.
The rover carried a ground-penetrating radar (GPR) on its underside, allowing for the first direct measurement of the structure and depth of the lunar soil down to a depth of 30 m (98 ft), and investigation of the lunar crust structure down to several hundred metres deep.
The rover carried an alpha particle X-ray spectrometer (APXS) and an infrared spectrometer, intended to analyze the chemical element composition of lunar samples. The APXS was the only payload on the robotic arm.
There were two panoramic cameras and two navigation cameras on the rover's mast, which stands ~1.5 m (4.9 ft) above the lunar surface, as well as two hazard avoidance cameras installed on the lower front portion of the rover. Each camera pair was used to capture stereoscopic images, or for range imaging by triangulation.
Chang'e 3 landed on 14 December 2013 and deployed the Yutu rover 7 hours 24 minutes later.
The planned landing site was announced to be Sinus Iridum. However, the lander descended on Mare Imbrium, about 40 km (25 mi) south of the 6 km (3.7 mi) diameter Laplace F crater, at 44.1214°N, 19.5116°W (2640 m elevation)
The rover was successfully deployed from the lander, and made contact with the lunar surface on 14 December, 20:35 UTC. On 17 December it was announced that all of the scientific instruments apart from the spectrometers had been successfully activated, and that both the lander and rover were "functioning as hoped, despite the unexpectedly rigorous conditions of the lunar environment". From 16 December to 20 December the rover did not move, having been partially powered down. Direct solar radiation had raised the temperature on the sunlit side of the rover to over 100 °C, while the shaded side simultaneously fell below 0 °C.
By 22 December Yutu had completed its first tasks: to photograph the lander from several different angles, following a roughly semi-circular route from north to south of the lander, while at the same time being photographed and filmed by the lander. A number of these images have been released, including a stereoview of the lander and videos of the rover in motion. The lander and rover then commenced their respective science missions.
In addition to successfully deploying its robotic arm, Yutu completed checks on 23 December to ensure that it was prepared for the coming lunar night, and moved about 40 metres south of the lander. The lander was also tested the following day. The lander entered sleep mode first, at around 11 am China Standard Time on 25 December, followed by the rover at 5:23 am on 26 December. Both had to withstand the extreme cold of the two-week-long lunar nights.
On 11 January 2014, following the lunar night, the rover and lander were taken out of sleep mode, and on 16 January the rover completed its first examination of the lunar soil. On 25 January 2014, near the end of the second lunar day, China's state media announced that the rover had undergone a "mechanical control abnormality" and stated that the problem was caused by the "complicated lunar surface environment". The Planetary Society reported that the rover was not responding properly to commands from Earth, so it "could not prepare for the oncoming night properly." Specifically, the rover suffered a control circuit malfunction in its driving unit, which prevented it from entering normal dormancy and folding its mast and solar panels.
Command Control was expecting the rover to contact Earth on 12 February 2014 had it endured its second lunar night. Since it did not transmit any signals, it was declared permanently inoperative. On 13 February, it re-established communication with Command Control. China's lunar program spokesman Pei Zhaoyu declared that although Yutu was able to communicate, "it still suffers a mechanical control abnormality."
The rover entered its third hibernation period on 22 February. It was still unable to move and serious technical troubles persisted that hampered science operations. Chinese space scientists eventually ascertained that the control circuit had failed, and this prevented Yutu from entering normal dormancy as planned, but stated that the ground penetrating radar, panoramic and infrared imaging equipment were functioning normally.
While amateur observers were unable to detect transmissions from the lander, Chinese officials reported that the craft was still operating its UV Camera and Telescope as it entered its 14th lunar night on 14 January 2015. On 18 April 2014, Wang Jianyu, deputy secretary general of the Chinese Society of Space Research stated that the failure was not mechanical, but electrical, and they were looking to bypass it. He also explained, "The temperature on the Moon is considerably lower than our previous estimation, adding that "certain components may be suffering from "frostbite". During 15 April, the Chang'e 3 mission, including its Yutu rover, witnessed a total eclipse of the Sun by the Earth from surface of the Moon.
Yutu was unable to move its solar panels back to the insulating position during lunar nights, exposing the internals to the nightly cold. With each night, some capability was lost, but it exceeded its expected three-month life. The scientific instruments may have worked, but subsequent science data became very limited as the NIR spectrometer and the ground-penetrating radar were limited to always making the same observation. Mission Control planned to keep on using the Yutu until it completely stopped working, as it would provide valuable data on the endurance of its components.
The rover was still transmitting every Lunar day as of December 2015. By the end of October 2015, Yutu had set the record for the longest operational period of a rover on the Moon, though most of its time was spent immobile.
The rover's ground penetrating radar found evidence of at least nine distinct rock layers, indicating that the area had surprisingly complex geological processes and is compositionally distinct from the Apollo and Luna landing sites.