Tiangong / Shenzhou: China's Human Spaceflight Program / Tianzhou Cargo Spaceship
History of China's Manned Space Program:
Shenzhou is China's first manned spacecraft, developed for China's manned spaceflight program to fulfil the missions of achieving manned orbital flight, developing EVA and rendezvous docking techniques, and transporting crews to and from the space station. The spacecraft was modelled after the Russian Soyuz-TM but slightly larger in size and has been developed using Chinese technology. The first unmanned test flight took place in 1999 and the first manned mission was launched in 2003. The Shenzhou spacecraft is expected to remain in service into the 2020s as the crew transportation vehicle for China's manned space station. 1)
China embarked on a manned spaceflight program in 1992, with the goal of constructing a manned space station in LEO (Low Earth Orbit) by 2022. Phase-I of the program aimed to develop a manned spacecraft vehicle that can carry astronauts to orbit and bring them back to Earth safely. The newly developed Shenzhou spacecraft made its first unmanned test flight on November 20, 1999, and sent China's first astronaut Yang Liwei to orbit in October 2003.
After an initial study phase, the Chinese space industry settled on a three-module manned capsule design modelled after the Russian Soyuz-TM. The spacecraft, named Shenzhou ("Devine Vessel"), features a forward orbital module, a re-entry module that can accommodate three astronauts, and an aft service module. The vehicle has a mass of just under 8,000 kg and could be launched atop a man-rated version of the CZ-2E orbital launcher.
The primary contract for developing the Shenzhou spacecraft was awarded to the Beijing-based CAST (China Academy of Space Technology), with SAST (Shanghai Academy of Spaceflight Technology) sharing some development tasks, including the spacecraft's service module, power system, propulsion, and docking system. The spacecraft's environment control and life support system was developed by Beijing Institute of Space Medicine and Engineering (507 Institute). The Beijing-based CALT (China Academy of Launch Vehicle Technology) was tasked with the development of the man-rated spacecraft launcher designated CZ-2F.
China has two space agencies: CNSA (China National Space Administration), a civilian organization, which is part of the civilian Ministry of Industry and Information Technology; CNSA deals primarily with robotic missions including the robotic lunar missions. — Human spaceflight missions reside under CMSA (China Manned Space Agency), which is a branch of the PLA (People's Liberation Army) that controls the Shenzhou crew vehicles and the Tiangong space station development. CMSA was created in 1993.
Table 1: Overview of the initial Shenzhou mission chronology
Three-step program: The overall development strategy of China's manned space program is the "Three-step" development strategy: 2)
• The first step is to launch a manned spaceship, set up primarily integrated experimental manned spacecraft engineering, and carry out space application experiments.
• The second step is to make technology breakthroughs in EVA (Extravehicular Activities) as well as space RVD (Rendezvous and Docking) of manned spaceships and spacecrafts, launch a space lab, and provide solution for space application of a certain scale with man-tending on a short-term basis.
• The third step is to establish a space station, and provide a solution for space application of large scale with man-tending on a long-term basis.
The objective of the Shenzhou-1 spaceship, launched on November 20, 1999, was to verify the manned space technology. It marked the beginning of the first step in "Three-step" development strategy. Then in 2001 and 2002, China launched three unmanned spaceships, Shenzhou-2, Shenzhou-3 and Shenzhou-4, to verify more technologies in support of manned spaceflight.
On October 15, 2003, China launched the first manned spaceship Shenzhou-5, and China's first astronaut Yang Liwei was sent into space and returned safely. In 2005, China launched the Shenzhou-6 spaceship with two astronauts aboard to further verify the human space transportation technology. At this point the first step of the "Three-step" development strategy was completed.
RVD (Rendezvous and Docking) Missions: China developed the Shenzhou-8, Shenzhou-9 and Shenzhou-10 spaceships and the Tiangong-1 space lab; it completed three times a rendezvous and docking mission.
From 2011 to 2013, China launched Tiangong-1 space lab and the Shenzhou-8, Shenzhou-9, and Shenzhou-10 spaceships, these missions verified RVD technology successfully. At this point the first stage of the second step in "Three-step" development strategy was completed. Next, China will carry out the flight mission of the second stage in the second step.
Table 2: Overview of Shenzhou RVD missions
The Shenzhou spaceship is a modular assembly consisting of a forward cylindrical orbital module, an aerodynamic reentry module, and an aft cylindrical service module with a pair of solar panel wings. A launch escape tower is attached to the front-end of the assembly and is jettisoned 2 minutes after the lift-off. The crew stays inside the habitable reentry module during launch and reentry and controls the spacecraft from there. The orbital module, which is also habitable, provides additional habitable space for the crew during orbital flight (Ref. 1).
The spacecraft assembly is nearly 9 m in total length (excluding the launch escape tower), 2.5 m in diameter, with a gross launch mass of 8,130 kg and an orbital mass of 7,800 kg. It consists of 13 subsystems: space frame, GNC (Guidance Navigation and Control ), data management, telemetry and communications, thermal control, propulsion, power, mission payload, ECLSS (Environment Control and Life Support System ), crew, instruments and lightening, emergency rescue, and reentry and landing. It is fitted with a total of 52 engines, including four main engines and 48 small-thrust control vectors (16 on the orbital module, 8 on the reentry module, and 24 on the service module).
Figure 1: Illustration of the Shenzhou spaceship (image credit: CAST)
1) Orbital module: The orbital module at the front of the spacecraft assembly is used to carry key equipment including a space toilet, and also provides additional habitable space for the crew to live and conduct scientific experiments during orbital flight. The module is 2.80 m in length, 2.25 m in diameter, with an internal volume of 8 m3 and an orbital mass of 1,500 kg. The module is connected to the reentry module via a 65 cm diameter cylindrical hatch, which is sealed off during ascent, rendezvous docking, and reentry. A large cylindrical hatch located on the side the module allows the crew to enter the spacecraft before launch, and can also be used for the astronauts to exit and reenter the spacecraft during EVA. There is a third, smaller cylindrical hatch on the side of the orbital module in some early Shenzhou missions, possibly used as a window for Earth observation payloads.
• For the early solo flight variant of the Shenzhou vehicle, the orbital module was fitted with its own independent altitude control, propulsion, telemetry and communication systems, as well as a second pair of smaller solar panel wings and 16 thrusters (in four groups of 4 thrusters). This design allowed the module to remain flying in orbit in an automated mode for another six months after being jettisoned from the reentry module at the end of the manned mission.
• Chinese spacecraft engineers originally envisioned to use the jettisoned orbital module as the target for the next Shenzhou spacecraft to practice orbital rendezvous and docking, but this idea was later abandoned. Instead, after the jettison the orbital module served as an orbital bus to carry scientific experiment or Earth-observation packages, with additional mission payload attached externally to the front of the module.
• The Shenzhou 7 mission in 2008 featured a specially modified orbital module which doubled as an airlock and storage space for two EVA spacesuits. The spacewalking astronauts exited and returned to the spacecraft vehicle via the large cylindrical hatch on the side of the module, while the hatch connecting to the re-entry module remained sealed off throughout the EVA. This orbital module design lacked the automated orbital flight ability and was simply discarded and de-orbited at the end of mission.
• From the Shenzhou 8 mission onwards, the Shenzhou spacecraft has been built in the crew transport configuration, which features an APAS-style docking port and an optical and radar-based automated rendezvous and docking system at the front end of its orbital module. Designed by SAST and believed to have been derived from the Russian APAS-89, the docking system features a single androgynous docking port, radio beacons, transponders, communication antenna, UHF radar, laser rangefinder, and electrooptical tracking system. The inside diameter of the docking port tunnel is 0.8 m. During a rendezvous docking, the Shenzhou vehicle chases and closes in the space station though a V-bar approach from behind.
2) Reentry module: The reentry module provides a fully pressurized and habitable living space for the crew during the ascent and reentry phase of the flight. The module accommodates Soyuz-style molded seats for up to three crew members, flight instrument panel, control sticker, periscope, and the communications system, with an internal volume of 8 m3. There are two windows for the crew to observe the outside.
• The module is fitted with eight (in four pairs) 5 N control engines, including 2 pitch/yaw thrusters, 2 translation thrusters, and 4 roll thrusters to maintain its flight status during reentry.
• During the descent stage of the flight, the re-entry module first makes an unpowered (but controlled) ballistic descent through the atmosphere, with its heat shield protected blunt end pointing forward. The module has a spherical aerodynamic design, with its center of mass deliberately and precisely offset from its axis of symmetry to achieve an angle of attack during the free fall. This allows the yield of a small lift to reduce g-force from 8-9 g for a purely ballistic trajectory to 4-5 g during the hypersonic free-fall, as well as greatly reducing the peak reentry heat. In an emergency situation, the module can also reentry using a purely ballistic trajectory, which would increase the g-force to 8-9 g. If necessary, the module can splashdown on water and then remain afloat.
• The reentry module is fitted with five parachutes: a pilot chute (4.25 m2 surface area), a second pilot chute (surface area: 0.7 m2), a drogue chute (surface area: 24 m2), a main parachute (surface area: 1,200 m2), and a backup parachute (surface area: 760 m2). The parachutes are deployed from the altitude of 10,000 m.
• The 280 kg heat shield is jettisoned before landing so that the four landing rockets at the bottom of the module could fire to allow a soft-landing.
3) Service module: The inhabitable service module is larger than that of the Soyuz-TM. It accommodates the navigation, communications, flight control, thermal control, propulsion systems, as well as batteries, oxygen tanks, and propellant tanks. It has a pair of adjustable solar panel wings 17 m in span to obtain maximum solar insulation regardless of the spacecraft's flight status.
• The propulsion system consists of four high-thrust main engines and 24 smaller-thrust control vectors, plus four 230 liter propellant tanks containing a total of 1,000 kg N2O4/MMH liquid propellant. The four main engines, each rated at 2.5 kN, are located at the base of the spacecraft's service module. There are eight (in four pairs) 150 N pitch/yaw thrusters, eight (in four pairs) 5 N pitch/yaw thrusters, and eight (in four pairs) 5 N roll / translation thrusters.
4) Launch escape assembly: The launch escape assembly incorporates the launch escape tower, the orbital module, the descent module, the upper portion of the payload fairing and four foldable grid aerodynamic flaps. In case of an anomaly during the launch, the assembly with the crew can be pulled away from the remainder of the launch vehicle within seconds, by rockets mounted on the launch escape tower above the payload fairing. The whole assembly is 15.1 m in length and 3.8 m in diameter, and has a total mass of 11,260 kg. It is powered by the solid fuel rocket motors mounted on the launch escape tower and payload fairing. The two-piece payload fairing is also equipped with rocket motors for high-altitude escape.
• The launch escape tower is 8.35 m in length and fitted with six rocket motors: four main escape motors, a pitch motor and a separation motor with eight nuzzles. The main four main escape motors are mounted symmetrically on the lower part of the launch escape tower at an angle of 30° to the axis of the launch vehicle. Above them are eight smaller separation motors. The pitch motors are mounted at the top of the tower.
5) ECLSS (Environmental Control and Life Support System): The ECLSS ensures a habitable and safe environment for the onboard crew throughout the flight mission. It consists of the environment control system, oxygen and other gases storage tanks, water supply and processing system, astronaut waste disposal and collection system (‘space toilet'), fire/smoke detectors, and fire suppression system. During the flight mission, every day a typical crew member consumes 0.83 kg oxygen and 1.8 liter of water, and produces 0.9 kg carbon dioxide. The Shenzhou vehicle can create and maintain an inside atmosphere similar to that on Earth, with conventional air (nitrogen/oxygen). A liquid-circulating temperature control unit maintains the temperature inside the habitable modules between 17-25°C. Air dampness is maintained between 30-70%. An air ventilation and purification system detects and absorbs dusts, carbon dioxide and carbon monoxide.
• The Shenzhou orbital module is fitted with a space toilet, which uses a vacuum system to collect human waste of the onboard crew. A food heater can provide the crew with hot meals during the flight. The crew is required to wear the intra-vehicular activity pressure suits during launch, docking, undocking and descent. The suit, which was based on the Russian Sokol design, can protect the crew members in the event of hull breach or pressure loss.
Figure 2: Photo of the Shenzhou-8 spaceship acquired during its rendezvous with Tiangong-1 on Sept. 29, 2011 (image credit: CMSA, CAST)
Figure 3: Photo of the Shenzhou-10 crew and their spacecraft after the landing on June 26, 2013 (image credit: CMSA, CAST)
Tiangong-1 Space Lab
The Chinese man-tended space lab Tiangong-1 is also known as "Target Vehicle". The objective is to serve as a ‘target vehicle' for the perfection of orbital rendezvous and docking techniques, as well as to demonstrate short-term orbital living. A total of three expeditions, including an unmanned and two manned missions, were made to the Tiangong-1 station between November 2011 and March 2013. Chinese official writings described the Space Lab as a temporarily-manned Earth-orbiting spaceship, which can be visited and tended by astronauts on a short-term basis and fly in automated mode on a 300-400 km LEO (Low Earth Orbit) between visits. 3)
Spacecraft design: The Tiangong-1 Space Lab is 10.4 m in length and 3.35 m in diameter, with an orbital mass of 8,506 kg. The module consists of two cylinder-shape sections: a habitable Experimental Compartment with an internal space of 14.4 m3 (2.0 x 1.8 x 4.0 m), and an inhabitable Service Compartment that houses propulsion, power, life support, and communication systems. A pair of solar wings each with 4 solar panels are attached to the Service Compartment. Visiting astronauts can enter the Experimental Compartment via the 0.8 m diameter hatch of the docking port on the front end of the compartment.
Each solar array consists of four panels and is about 3.1 x 10 m in dimension. The solar arrays provide an average power of 2.5 kW with peaks of up to 6 kW. Sun sensors are used to automatically rotate the arrays for proper Sun exposure. With its arrays deployed, Tiangong-1 has a span of 23 m. Silver-zinc batteries are also located in the aft section and provide power when the vehicle passes through an eclipse. Similar to Shenzhou vehicles, Tiangong-1 operates on a 28 V power bus. 4)
The main propulsion system features two high-expansion-ratio main engines using Monomethylhydrazine and Nitrogen Tetroxide as propellants. Eight vernier jets at the base of the module are used for fine-maneuvers. Four sets of two Reaction Control Engines are mounted on the external base of the Service Module and are used for pitch and yaw attitude control maneuvers. Roll control is also accomplished with small thrusters located on the SM. It is presumed that the propellant tanks are identical with those of Shenzhou. Propellants are stored in four 230 liter tanks capable of holding 1,000 kg of hypergolic propellants. Tank pressurization is accomplished via two small 20 liter high-pressure gas tanks.
The Tiangong-1 module is connected to the visiting Shenzhou spacecraft via an androgynous docking mechanism developed by SAST (Shanghai Academy of Spaceflight Technology). The system is believed to be similar to the Russian APAS-75, consisting of a docking port, radio beacons, transponders, communication antenna, UHF radar, laser rangefinder, and electrooptical tracking system. The inside diameter of the docking port is about 0.8 m.
The Shenzhou spacecraft, carrying the visiting astronauts, will act as the ‘chasing' spacecraft, while the Space Lab will act passively as the ‘target'. In order to dock successfully, the two spacecraft will need to have a relative velocity of less than 0.2 m/s and lateral deviation of fewer than 18 cm. The rendezvous docking can be controlled manually by the astronauts onboard the Shenzhou spacecraft, under the remote control from ground, or automatically by onboard computer.
Figure 4: Artist's rendition of the deployed Tiangong-1 spaceship (image credit: CAST, Ref. 2)
Launch: The Tiangong-1 Space Lab was launched on September 29, 2011 aboard a Long March 2F vehicle from JSLC (Jiuquan Satellite Launch Center) in China.
Orbit: Near circular orbit, altitude of 380-400 km, inclination = 42.77º.
While flying unpiloted between docking missions, Tiangong-1 can serve as a platform for a range of Earth observation and scientific research missions using its onboard mission payloads. Its mission payloads included:
• HSI (Hyperspectral Imager): HSI was developed by the CIOMP (Changchun Institute of Optics, Fine Mechanics, and Physics) and SITP (Shanghai Institute of Technical Physics). The instrument is carried in the non-habitable section of the spacecraft's Experimental Compartment. The imager can collect image data simultaneously in multiple narrow, adjacent spectral bands to provide a wealth image data of the Earth surface.
• Material science: The spacecraft also carries mission payload for crystal growth experiments. Images and video data of the experiments are transmitted to ground via the downlink.
• Space environment exploration: The spacecraft carries instruments to detect and analyze solar energetic particles, atmospheric chemistry and physics, and ionospheric disturbances.
Figure 5: Tiangong 1 docking port (image credit: CMSA)
Figure 6: Inside view of Tiangong-1 (image credit: CMSA)
Status of the Tiangong-1 mission:
• Tiangong-1 was predicted to reenter the Earth's atmosphere on 2 April 2018 at 00:30 UTC ± 1.7 hours. The reentry has been confirmed as 02 April 2018 at 00:16 UTC. Reentry occurred in the Pacific Ocean. 5) — This prediction was performed by The Aerospace Corporation on 2018 April 1.
Figure 7: This map shows the predicted location of Tiangong-1. The ground tracks indicate the uncertainty in the reentry prediction (yellow = before prediction, green = after prediction). Actual reentry location is shown in red (image credit: The Aerospace Corporation)
Figure 8: Altitude: past data future prediction, updated on 1 April 2018 (image credit: The Aerospace Corporation)
• March 30, 2018: Every week, on average, a substantial, inert satellite drops into our atmosphere and burns up. Monitoring these reentries and warning European civil authorities has become routine work for ESA's space debris experts. 6)
- Each year, about 100 tons of defunct satellites, uncontrolled spacecraft, spent upper stages and discarded items like instrument covers are dragged down by Earth's upper atmosphere, ending their lives in flaming arcs across the sky.
- Some of these objects are big and chunky, and pieces of them survive the fiery reentry to reach the surface. Our planet, however, is a big place, mostly covered by water, and much of what falls down is never seen by anyone, sinking to the bottom of some ocean, or landing far from human habitation.
- While still in orbit, these and many other objects are tracked by a US military radar network, which shares the data with ESA, since Europe has no such capability of its own.
Informing European and international partners:
- It's the task of ESA's Space Debris team to look at these data and issue updates to ESA Member States and partner civil authorities around the globe.
Figure 9: TIRA (Tracking and Imaging Radar) is operated by Germany's FHR (Fraunhofer Institute for High Frequency Physics and Radar Techniques), and is located at Wachtberg, Germany (Fraunhofer FHR)
- The TIRA system primarily serves as the central experimental facility for the development and investigation of radar techniques for the detection and reconnaissance of objects in space. TIRA also provides valuable support for space missions: space agencies from all over the world use the special capabilities of the Fraunhofer scientists and their system.
- The radar is protected by a radome (white cover). The radome has a diameter of 47 meters and is therefore the largest of its kind worldwide. The building has an overall height of approximately 56 meters and can be seen as a white 'golf ball' from a great distance.
- The radome accommodates an antenna with a diameter of 34 meters. It can be turned 360° in azimuth (horizontal) and 90° in elevation (vertical). The movable part weighs 240 tons and can be turned at a speed of 24° per second (in azimuth), i.e. a full rotation takes 15 seconds.
- As the name implies, the TIRA system comprises a tracking radar and an imaging radar. The narrow-band, fully coherent, high power tracking radar has a transmission frequency in L-band (1,333 GHz) and the wide-band imaging radar has a transmission frequency in Ku-band (16.7 GHz) and is currently equipped with a high target resolution.
- The space observation radar TIRA is a unique system that offers space agencies all over the world (outside the USA) the possibility to measure orbits with high precision or produce a high-resolution image of objects such as satellites. The system is therefore used to gain precise measurements of space debris, prevent evasive maneuvers for operative satellites or create an image of an object that has gone out of control. This includes technical faults or the uncontrolled re-entry of satellites into the Earth's atmosphere.
- In recent years, Fraunhofer researchers supported the scientific community with measurement data and images of the German X-ray satellite ROSAT and the Russian space probe Phobos-Grunt as these reentered the atmosphere.
- The radar data of space objects and internally developed techniques are used to determine characteristic features of the objects, e.g. orbital elements, intrinsic motion parameters, orbital lifetime, target shape and size, ballistic coefficient, mass and material properties.
• Update 29 March 2018: The current estimated reentry window runs from midday on 31 March to the early afternoon of 1 April (in UTC time); this is highly variable. 7)
- Reentry will take place anywhere between 43ºN and 43ºS. Areas above or below these latitudes can be excluded. At no time will a precise time/location prediction from ESA be possible. This forecast was updated approximately weekly through to mid-March, and is now being updated every 1~2 days.
Figure 10: Tiangong-1 reentry window forecast as of 29 March (image credit: ESA)
Figure 11: Tiangong-1 altitude decay forecast as of 29 March (image credit: ESA)
• March 29, 2018: In the next few days, an unoccupied Chinese space station, Tiangong-1, is expected to reenter the atmosphere following the end of its operational life. Most of the craft should burn up. ESA is hosting a campaign to follow the reentry, conducted by the Inter Agency Space Debris Coordination Committee (IADC). 8)
- The 13 space agencies/organizations of IADC are using this event to conduct their annual reentry test campaign, during which participants will pool their predictions of the time window, as well as their respective tracking datasets obtained from radar and other sources. The aim is to cross-verify, cross-analyze and improve the prediction accuracy for all members.
Figure 12: These radar images (the image is a composite of two separate images) were acquired last week by the Tracking and Imaging Radar system – one of the world's most capable – operated by Germany's Fraunhofer FHR research institute at Wachtberg, near Bonn, when the craft was at an altitude of about 270 km (image credit: Fraunhofer FHR. Used by permission)
- Data and images from the radar are being pooled as part of the IADC campaign.
- The spacecraft is 12 m long with a diameter of 3.3 m and had a launch mass of 8506 kg. It has been unoccupied since 2013 and there has been no contact with it since 2016.
- The craft is now at about 200 km altitude, down from 300 km in January, in an orbit that will most likely decay sometime between the morning of 31 March and the early morning of 2 April.
- Owing to wide variations in atmospheric dynamics and the break-up process, among other factors, the date, time and geographic footprint of the reentry can only be forecast with large uncertainties.
- In the history of spaceflight, no casualties from falling space debris have ever been confirmed.
• January 11, 2018: The image of Figure 13 shows China's space station Tiangong-1 – the name means ‘heavenly palace' – and was captured by French astrophotographer Alain Figer on 27 November 2017. It was taken from a ski area in the Hautes-Alpes region of southeast France as the station passed overhead near dusk. - The station is seen at lower right as a white streak, resulting from the exposure of several seconds, just above the summit of the snowy peak of Eyssina (2837 m altitude). Several artefacts in the original have been removed. 9)
Figure 13: Photo of French astrophotographer Alain Figer of the Tiangong-1 station (seen at lower right as a small white streak), captured on 27 Nov. 2017 (image credit: A. Figer, used by permission)
- Tiangong-1 is 12 m long with a diameter of 3.3 m and had a launch mass of 8506 kg. It has been unoccupied since 2013 and there has been no contact with it since 2016.
- The craft is now at about 280 km altitude in an orbit that will inevitably decay some time in March–April 2018, when it is expected to mostly burn up in the atmosphere. "Owing to the geometry of the orbit, we can already exclude the possibility that any fragments will fall over any spot further north than 43ºN or further south than 43ºS," says Holger Krag, head of ESA's Space Debris Office.
- "The date, time and geographic footprint can only be predicted with large uncertainties. Even shortly before reentry, only a very large time and geographical window can be estimated."
- The station's mass and construction materials mean there is a possibility that some portions of it will survive and reach the ground.
- In the history of spaceflight, no casualties from falling space debris have ever been confirmed.
- ESA is hosting a test campaign to follow the reentry, which will be conducted by the Inter Agency Space Debris Coordination Committee, a grouping of the world's top space agencies including ESA, NASA and CNSA (China National Space Administration).
• January 3, 2018: Though the Tiangong-1 station's mission was originally meant to end in 2013, CNSA (China National Space Agency) extended its service to 2016. By September of 2017, the Agency acknowledged that they had lost control of the station and indicated that it would fall to Earth later in the year. According to the latest updates from satellite trackers, Tiangong-1 is likely to be reentering our atmosphere in March of 2018. 10)
- Given the fact that the station measures 10 m x 3.35 m, with a mass of 8,506 kg and was built from very durable construction materials, there are naturally concerns that some of it might survive reentry and reach the surface. But before anyone starts worrying about space debris falling on their heads, there are a few things that need to be addressed.
- For starters, in the history of space flight, there has not been a single confirmed death caused by falling space debris. Thanks to the development of modern tracking and early warning systems, we are also more prepared than at any time in our history for the threat of falling debris. Statistically speaking, you are more likely to be hit by falling airplane debris or eaten by a shark.
- Second, the CNSA has emphasized that the reentry is very unlikely to pose a threat to commercial aviation or cause any impact damage on the surface. As Wu Ping – the deputy director of the manned space engineering office – indicated at a press conference back on September 14th, 2017: "Based on our calculation and analysis, most parts of the space lab will burn up during falling."
- In addition, The Aerospace Corporation, which is currently monitoring the reentry of Tiangong-1, recently released the results of their comprehensive analysis. Similar to what Wu stated, they indicated that most of the station will burn up on reentry, though they acknowledged that there is a chance that small bits of debris could survive and reach the surface. This debris would likely fall within a region that is centered along the orbital path of the station (i.e. around the equator).
- To illustrate the zones of highest risk, they produced a map (shown below) which indicates where the debris would be most likely to land. Whereas the blue areas (that make up one-third of the Earth's surface) indicate zones of zero probability, the green area indicates a zone of lower probability. The yellow areas, meanwhile, indicates zones that have a higher probability, which extend a few degrees south of 42.7° N and north of 42.7° S latitude, respectively.
Figure 14: Illustration of the predicted reentry region for Tiangong-1 (image credit: The Aerospace Corporation)
- Last, but not least, the European Space Agency's IADC (Inter Agency Space Debris Coordination Committee) will be monitoring the reentry. In fact, the IADC – which is made up of space debris and other experts from NASA, ESA, JAXA, ISRO, KARI, Roscosmos and CNSA (China National Space Administration) – will be using this opportunity to conduct a test campaign.
- During this campaign, participants will combine their predictions of the reentry's time window, which are based on respective tracking datasets obtained from radar and other sources. Ultimately, the purpose of the campaign is to improve prediction accuracy for all member states and space agencies. And so far, their predictions also indicate that there is little cause for concern.
• November 6, 2017: ESA experts will host an international campaign to monitor the reentry of a spacecraft expected early next year. Early next year, an uncrewed Chinese space station, Tiangong-1, is expected to reenter the atmosphere following the end of its operational life, during which most of the craft should burn up. 11)
- ESA will host a test campaign to follow the reentry, which will be conducted by the IADC (Inter Agency Space Debris Coordination Committee). IADC comprises space debris and other experts from 13 space agencies/organizations, including NASA, ESA, European national space agencies, JAXA, ISRO, KARI, Roscosmos and the CNSA (China National Space Administration).
- IADC members will use this event to conduct their annual reentry test campaign, during which participants will pool their predictions of the time window, as well as their respective tracking datasets obtained from radar and other sources. The aim is to cross-verify, cross-analyze and improve the prediction accuracy for all members.
- ESA will act as host and administrator for the campaign, as it has done for the twenty previous IADC test campaigns since 1998. A special case for ESA was the campaign in 2013 during the uncontrolled reentry of ESA's own GOCE satellite.
- The Tiangong-1 spacecraft (Heavenly Palace) is 12 m long with a diameter of 3.3 m, it had a launch mass of 8506 kg. It has been unoccupied since 2013 and there has been no contact with the spacecraft since 2016.
Figure 15: Illustration of the Tiangong-1 space station (image credit: CMSE/China Manned Space Engineering Office)
- The craft is now at about 300 km altitude in an orbit that will inevitably decay sometime between January and March 2018, when it will make an uncontrolled reentry.
- "Owing to the geometry of the station's orbit, we can already exclude the possibility that any fragments will fall over any spot further north than 43ºN or further south than 43ºS," says Holger Krag, Head of ESA's Space Debris Office. "This means that reentry may take place over any spot on Earth between these latitudes, which includes several European countries, for example. - The date, time and geographic footprint of the reentry can only be predicted with large uncertainties. Even shortly before reentry, only a very large time and geographical window can be estimated."
- Owing to the station's mass and construction materials, there is a possibility that some portions of it will survive and reach the surface.
- In the history of spaceflight, no casualties due to falling space debris have ever been confirmed.
- ESA's Space Debris Office, based at ESOC (European Space Operations Center) in Darmstadt, Germany, will concurrently conduct an international expert workshop in the week of 28 February, focusing on reentry predictions and atmospheric break-up studies, enabling experts to share their latest findings and research in these and related topics.
- Separate from the IADC campaign, ESA will regularly update ESA Member State civil authorities with detailed information on the reentry, as it does during all such events.
• September 30, 2016: It appears that the Tiangong-1 mission had ended unexpectedly due to a dysfunctional battery charger, a source close to the Chinese space industry disclosed. While all eyes are now fixed on the recently launched Tiangong-2 space lab module and the upcoming expedition missions, its predecessor Tiangong-1 is on an uncontrolled course to come crashing down to Earth in late 2017. 12)
- According to the original mission description document published by CMSA (China Manned Space Agency), at the end of its mission the Tiangong-1 module would perform a controlled destructive reentry into the South Pacific Ocean. The mission description document did not give an exact date for the space module's controlled reentry. After the ending of the last expedition mission Shenzhou 10 in June 2013, the space module was put into a sleep mode to continue flying in orbit to allow the ground control to collect data on the longevity of its key components.
- During the press conference for the Tiangong-2 launch on 14 September 2016, the spokeswoman of the CMSA, Wu Ping, confirmed that Tiangong-1 was intact and operating on a 370 km orbit, with an orbital depletion rate of about 100 m daily. The space module is expected to burn up during an uncontrolled atmospheric re-entry sometime in late 2017.
Figure 16: Computer rendering of the Tiangong-1 space lab docked with a visiting Shenzhou spaceship in orbit (image credit: CMSA)
• On 21 March 2016, CMSA (China Manned Space Agency) announced that the ground mission control had lost all telemetry and communications with Tiangong-1, leaving no ability to safely control its descent. — On Sept. 14, 2016, CMSA confirmed that Tiangong-1 had descended to a 370 km orbit, and was losing altitude at a rate of 100 m per day. The space module is expected to burn up during an uncontrolled atmospheric reentry sometime in the second half of 2017 (Ref. 3). 13)
• From October 2013, the Tiangong-1 space lab turned into orbit extended mission stage. After completion of all tasks, Tiangong-1 formally terminated data service in March, 2016 (Ref. 2).
• From June 11-26, 2013, Shenzhou-10 spaceship and the Tiangong-1 space lab completed rendezvous and docking flight mission. It is the first application flight mission of a Chinese spaceship. Shenzhou-10 carried 3 astronauts and achieved orbit residence of 36 man-days, which is the longest astronaut orbit residence of China until now. Shenzhou-10 carried out fly-around technology verification, and further verified the RVD and combination management technology again.
- Tiangong-1 was designed for an operational life span of two years. After the departure of the last crew in June 2013, the space module was put into a sleep mode to continue flying in orbit, in order to allow the ground control to collect data on the longevity of key components before the module is commanded to gradually reenter the atmosphere.
• On 29 September 2011, Tiangong 1 was launched atop the CZ-2F launch vehicle. About 9 and a half minutes into the flight, Tiangong 1 was separated from the launched rocket and entered its initial parking orbit. After two orbit elevation maneuvers, Tiangong-1 entered a 360 km near circular orbit, where it was tested remotely by the ground control.
Tiangong-2 Space Lab
In 2016, five years after the launch of the first Space Lab module Tiangong 1, the China Manned Space Program (CMSP, Project 921) is ready to launch a second module. Two identical modules were originally built for the Tiangong-1 mission. After the successful launch of Tiangong-1, its backup was modified into an improved module added with upgraded systems and new capabilities, most notably the ability to be resupplied and refuelled by a cargo vehicle named Tianzhou. 14)
To ensure the ‘new' space module was still qualified for orbital flight after spending five years inside the spacecraft hangar at CAST, engineers had to carefully assess the conditions of nearly 300 components and parts on the module, either putting them through a life-extension process or replacing those that no longer met the requirements.
Unlike its predecessor Tiangong 1, which was mainly intended as a target vehicle for perfecting orbital rendezvous docking, Tiangong 2's main objective was to "verify key technologies including cargo transportation, on-orbit propellant resupply, and medium-term living quaters for astronauts", as well as "conducting space science and application experiments on a relatively large scale", according to the program's official statement.
The flight missions of the Tiangong-2 space lab, the Shenzhou-11 spaceship and Tianzhou-1 cargo ship together constitute the second stage of the second step planned by the "Three-step" development strategy. China will launch the Tiangong-2 space lab in the third quarter of 2016. This will be followed by the Shenzhou-11 spaceship carrying 2 astronauts into space and dock with Tiangong-2 in 2016.According to plans, the astronauts will stay in the combined station of Shenzhou-11 and Tiangong-2 for 30 days. Shenzhou-11 is developed on the basis of Shenzhou-10, and makes some improvement in orbit control, RVD measurement, reentry and life support, to satisfy the requirement of the Tiangong-2 mission (Ref. 2).
The Tiangong 2 module is almost identical to its predecessor in size and appearance, consisting of two cylinder-shaped sections: a habitable Experiment Compartment serving as the main living quarters and laboratory for the crew; and an inhabitable Service Compartment that houses propulsion, power, life support, and communications systems. A pair of solar wings each with 4 solar panels are attached to the Service Compartment. The space module is 10.4 m in length and 3.35 m in diameter, with an orbital mass of about 8600 kg.
1) Experiment Compartment: The front experiment compartment is 5 m in length and 3.35 m in diameter, with a habitable internal volume of 14.4 m3 (2.0 x 1.8 x 4.0 m). The front end of the compartment, which provides a small free space for the crew to live and conduct experiments, is surrounded by heat pipes designed to conduct heat from internal systems to an external radiator. Inside the compartment is an instrument panel for flight controls and communications, a foldable table for eating and conducting experiments, and two sleep stations. There is a window on either side of the compartment allowing for outside observations by the crew. The remaining part of the compartment is packed with equipment and experiments.
• The module is connected to the visiting Shenzhou and Tianzhou spacecraft via an androgynous docking mechanism developed by SAST (Shanghai Academy of Spaceflight Technology). The system is believed to have derived from the Russian APAS-75, consisting of a docking port, radio beacons, transponders, communication antenna, UHF radar, laser rangefinder, and an electrooptical tracking system. Visiting astronauts enter the Experimental Compartment via the hatch on the 0.8 m diameter docking port. The docking port on Tiangong-2 has been modified in order to support in-orbit refuelling/resupply operations.
2) Transition section: Behind the experiment compartment is a 1.1 m-long transition section, tapered from 3.35 m diameter of the experiment compartment to the 2.25 m diameter of the aft service compartment. The section houses the nitrogen and oxygen tanks used for environmental control, and the water tank. The gases are stored in steel alloy spheres at a pressure of 21 Mpa.
3) Service Compartment: The aft service compartment is about 3.3 m in length and 2.5 m in diameter, and has been derived from the Shenzhou service module. The Space Lab is fitted with a different propulsion system to that of Shenzhou, with a 490 N dual-chamber high-expansion-ration main engine, four sets of two small aft-firing engines at the base to provide vernier thrust for fine maneuvers, four sets of two small engines mounted around the external base of the module for pitch/yaw control, and four roll control thrusters. The unified propulsion system feeds both attitude control and main engines from four 230 liter propellant tanks loaded with up to 1,000 kg of N2O4/MMH propellants. The engines are pressure-fed using six 20 liter titanium cold gas tanks pressurized to 23 Mpa. This gas is used to force propellant at 2 Mpa using diaphragms within the propellant tanks.
4) Solar wings: Two four-panel solar wings, with a total span of about 23 m, deploy from the sides of the Service Compartment. These can be rotated to obtain maximum solar insulation regardless of spacecraft attitude. Each wing, about 3.1 m x 10 m, provides about double the electrical power of the Shenzhou system (total about 7 kW peak, 2.5 kW average). The back surface filled silicon solar cells of the arrays have an efficiency of 14.8% on Shenzhou. Sun sensors between the panels measure the sunlight incidence angle which allows the panels to be automatically commanded to an optimum angle. Silver-zinc batteries in the service module provide emergency power in case of failure of the solar arrays. The spacecraft's power bus operates at 28 V.
5) Robotic arm: The module is fitted with a robotic arm, developed by CAST (China Academy of Space Technology). The 10 m long robotic arm is designed to help the assembly and maintenance of the space station, move equipment and supplies around the station, and support astronauts in EVA.
6) Banxing -2 companion microsatellite: The microsatellite, designed and developed by SAST, will be launched on the piggyback of Tiangong-2 and then released in orbit to demonstrate relevant technologies. Just like its predecessor Banxing-1 launched by the Shenzhou-7 mission in 2008, Banxing 2 will probably also carry an onboard camera to capture images of the mothership in orbit.
Figure 17: Photo of the Tiangong-2 space lab module inside the Spacecraft ATI Center at CAST (image credit: CAST, Ref. 14)
Figure 18: Photo of the Tiangong-2 Experiment Compartment and front hatch connecting to the docking port (image credit: CAST
Figure 19: Photo of the Tiangong-2 main engine (image credit: CAST)
Launch: The Tiangong-2 orbital module (mass of 8.6 tons) was launched on September 15, 2016 (14:04:09 UTC) on a Long March-2F vehicle from JSLC ( Jiuquan Satellite Launch Center), located in China's Gansu Province (Gobi Desert). 15)
Orbit: Near circular orbit, altitude of 380-393 km, inclination = 42.79º, period of 92 minutes.
• January 17, 2019: An international consortium of scientists studying GRBs (Gamma-Ray Bursts) as part of the POLAR (GRB polarimeter) experiment has revealed that high-energy photon emissions from black holes are neither completely chaotic nor completely organized, but a mixture. 16)
- GRBs are short and intense bursts of gamma-rays, which suddenly appear from deep space but randomly in direction and time. They are the brightest explosions in the universe since the Big Bang. Although discovered more than 50 years ago, the nature of GRBs is still poorly understood, especially what powers the explosions. Studying GRBs is important to understand how massive stars end their lives, black hole formation and the most relativistic jets in the universe.
- The current study is based on high-precision polarization measurements for prompt emissions of five GRBs recorded by POLAR. The results show that within short time slices, GRBs are found to oscillate in the same direction, but the oscillation direction changes with time.
Figure 20: Illustration. Left: Merger of two neutron stars (or a black hole and a neutron star) to produce gravitational waves; Right: Gamma-ray burst (image credit: Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, CAS)
- POLAR was launched on board the Chinese space laboratory Tiangong-2 on Sept. 15, 2016. A total of 55 GRBs were detected and confirmed within about six months. The five GRBs currently reported are part of this group and represent the largest ever high-precision sample of GRB prompt emissions.
- The analysis of the five GRBs in the current study shows that the average polarization degree (~10%) is not as high as some GRB models predicted. In addition, a new finding of the evolution of the intrapulse polarization angle provides us with a new insight into the microcosmic world of GRB physics.
- Measuring the prompt emission polarization of GRBs is important because it may provide information about relativistic outflow geometry and magnetic field structure. Such measurement is quite difficult, however - one reason few successful measurements were made before POLAR.
• January 14, 2019: The orderly chaos of black holes. During the formation of a black hole a bright burst of very energetic light in the form of gamma-rays is produced, these events are called gamma-ray bursts. The physics behind this phenomenon includes many of the least understood fields within physics today: general gravity, extreme temperatures and acceleration of particles far beyond the energy of the most powerful particle accelerators on Earth. In order to analyze these gamma-ray bursts, researchers from the University of Geneva (UNIGE), in collaboration with the Paul Scherrer Institute (PSI) of Villigen, Switzerland, the Institute of High Energy Physics in Beijing and the National Center for Nuclear Research of Swierk in Poland, have built the POLAR instrument, sent in 2016 to the Chinese Tiangong-2 space laboratory, to analyze gamma-ray bursts. Contrary to the theories developed, the first results of POLAR reveal that the high energy photons coming from gamma-ray bursts are neither completely chaotic, nor completely organized, but a mixture of the two: within short time slices, the photons are found to oscillate in the same direction, but the oscillation direction changes with time. These unexpected results are reported in a recent issue of the journal Nature Astronomy. 17) 18)
- When two neutron stars collide or a super massive star collapses into itself, a black hole is created. This birth is accompanied by a bright burst of gamma-rays — very energetic light such as that emitted by radioactive sources — called a GRB (Gamma-Ray Burst).
Is black hole birth environment organized or chaotic?
- How and where the gamma-rays are produced is still a mystery, two different schools of thought on their origin exist. The first predicts that photons from GRBs are polarized, meaning the majority of them oscillate in the same direction. If this were the case, the source of the photons would likely be a strong and well organized magnetic field formed during the violent aftermath of the black hole production. A second theory suggests that the photons are not polarized, implying a more chaotic emission environment. But how to check this?
- "Our international teams have built together the first powerful and dedicated detector, called POLAR, capable of measuring the polarization of gamma-rays from GRBs. This instrument allows us to learn more about their source," said Xin Wu, professor in the Department of Nuclear and Particle Physics of the Faculty of Sciences of UNIGE. Its operating system is rather simple. It is a square of 50 x 50 cm2 consisting of 1600 scintillator bars in which the gamma-rays collide with the atoms that make up these bars. When a photon collides in a bar we can measure it, afterwards it can produce a second photon which can cause a second visible collision. "If the photons are polarized, we observe a directional dependency between the impact positions of the photons, continues Nicolas Produit, researcher at the Department of Astronomy of the Faculty of Sciences of UNIGE. On the contrary, if there is no polarization, the second photon resulting from the first collision will leave in a fully random direction."
Order within chaos
- In six months, POLAR has detected 55 gamma-ray bursts and scientist analyzed the polarization of gamma-rays from the 5 brightest ones. The results are surprising to say the least. "When we analyze the polarization of a gamma-ray burst as a whole, we see at most a very weak polarization, which seems to clearly favor several theories," says Merlin Kole, a researcher at the Department of Nuclear and Particle Physics of the Faculty of Sciences of UNIGE and one of the main authors of the paper. Faced with this first result, the scientists looked in more detail at a very powerful 9 second long gamma-ray burst and cut it into time slices, each of 2 seconds long. "There, we discovered with surprise that, on the contrary, the photons are polarized in each slice, but the oscillation direction is different in each slice!," Xin Wu enthuses. It is this changing direction which makes the full GRB appear as very chaotic and unpolarized. "The results show that as the explosion takes place, something happens which causes the photons to be emitted with a different polarization direction, what this could be we really don't know," continues Merlin Kole.
- These first results confront the theorists with new elements and requires them to produce more detailed predictions. "We now want to build POLAR-2, which is bigger and more precise. With that we can dig deeper into these chaotic processes, to finally discover the source of the gamma-rays and unravel the mysteries of these highly energetic physical processes," explains Nicolas Produit.
Figure 21: The dedicated Gamma-ray Burst Polarimetry experiment POLAR on top of China's Tiangong-2 spacelab launched on September 15, 2016. The glowing green light mimics the scintillating light when a gamma-ray photon hits one of the 1600 specially made scintillation bars. The artwork is based on a picture taken by a camera located several meters behind (image credit: POLAR, University of Geneva)
• September 2017: The LSS (Large-Scale Spacecraft) Tiangong-2, intended as a testbed for key technologies, and to carry out large space science and applications experiments, is analyzed for POD (Precise Orbit Determination) support. The mean orbital altitude of LSS is about 390 km with an inclination of ~43º, when the spacecraft is in a three-axis Earth-pointing stabilization attitude mode, which is a normal attitude mode. The LSS precise orbital states are being used by several scientific payloads onboard for variable purposes; in particular, high-accuracy altimetry of land and sea surfaces must be supported by a precise position of the spacecraft. Radial orbit errors affect altimeter-derived topographic height directly, so it is a major component in the overall measurement errors estimation. 19)
- After processing the roughly 1-month measurement data generated by a dual-frequency GNSS receiver onboard, the precise orbital states are determined using a reduced dynamic orbit determination method. The orbital accuracy has been verified by a residuals method and orbits overlap verification. In addition, the orbital accuracy has also been verified independently by ground-based satellite laser ranging. So far, we have concluded that the large-scale spacecraft's orbit determination acquires a3D orbit accuracy of 3.6 cm (1σ) , showing a promising prospect to carry out a variety of scientific experiments that need accurate spacecraft's position. Meanwhile it demonstrates that large-scale spacecraft in low earth orbit could achieve cm-level accuracy orbit.
• Nov. 21, 2016: The two astronauts who completed China's longest-ever manned space mission returned to Earth safely Nov. 18, 2016. Zhang Youxia, commander-in-chief of China's manned space program, announced that the Tiangong-2 and Shenzhou-11 manned flight mission, which lasted over a month, was a "complete success." The completion of the Tiangong-2 and Shenzhou-11 mission "marked a major breakthrough" in China's manned space program. 20)
- Tiangong-2 will remain operative in orbit following Shenzhou-11's return to Earth and will wait to dock with Tianzhou-1, China's first cargo spacecraft. Tianzhou-1 will be launched in the first half of 2017 to verify refueling technology, a key technology for any space station.
• On November 18, 2016, China's Shenzhou-11 spacecraft returned to Earth bringing home two astronauts from China's longest-ever orbital mission, in a milestone for its vaulting ambitions. Shenzhou-11 detached from the Tiangong-2 space lab at 04:41 GMT. The capsule touched down at 05:59 GMT. China's state broadcaster CCTV showed the return capsule's separation from the Tiangong-2 space lab 393 km above the Earth, and its descent through the atmosphere to its landing on the grassland of Inner Mongolia. 21)
- The two astronauts, Jing Haipeng and Chen Dong, spent the 33-day mission orbiting the earth carrying out experiments including cultivating silkworms, growing lettuce, and testing brain activity.
- According to CAS (Chinese Academy of Sciences), the samples from space material and plant growth experiments carried out on China's space lab Tiangong-2 are in good condition and have been delivered to scientists for further research. The material and plant samples were retrieved after the successful landing of the Shenzhou-11 spacecraft's reentry module on Nov. 18. 22)
- According to CAS, 12 out of 18 material samples sent to space via Tiangong-2 in September, including semiconductor, nano and thin film materials, were taken back for study, while the other six will remain in space to test their physical and chemical features in zero gravity for future development of material processing techniques.
Figure 22: China's spacecraft recovery team works around the Shenzhou-11 landing capsule after its touchdown in Inner Mongolia (image credit: Xinhua)
• On October 23, 2016: The two crew members aboard China's Tiangong-2 space lab released the small companion satellite BX-2 (Banxing-2), also referred to as "selfie stick" . The microsatellite (~47 kg) is equipped with high-resolution cameras to capture views of the Tiangong-2/Shenzhou complex and perform a rendezvous exercise with the orbiting laboratory. 23)
- Fitted with a 25 Mpixel camera and an ammonia-based propulsion system, the Banxing-2 satellite is expected to loiter around Tiangong- 2 and Shenzhou-11, and eventually return to the vicinity of the complex to take pictures from above with Earth in the background, according to Chinese state media reports.
- The first batch of photos from Banxing 2's departure are looking up at the mini-space station complex, with the blackness of space as a backdrop (Figure 24). The black-and-white visible image released by Chinese space officials this week shows the Tiangong-2/Shenzhou-11 complex linked together, forming a structure more than18 m long and 4 m wide.
Figure 23: Artist's concept of the Banxing-2 microsatellite (image credit: China Manned Space Engineering Office)
Figure 24: This view of the Tiangong-2/Shenzhou-11 complex was photographed by the BX-2 microsatellite on Oct. 23 and downlinked to Earth on Oct. 25 by BX-2 (image credit: CCTV)
• On October 20, 2016 at 14:21 UTC, under the command of mission control, the Tiangong-2/Shenzhou-11 complex carried out an orbital maneuver burn to lower its altitude by about 4 km, to 380 km. The complex also made a 180° turn in the azimuth direction, so that the Tiangong 2 module could resume its ‘normal' flying position, with the docked Shenzhou 11 vehicle at front and the space laboratory's engine thrusters towards back. 24)
- According to the Chinese state media, the two astronauts established a living and working routine in space, with an 8-hour working day, for six days a week. The time onboard the space laboratory has been set to the China Standard Time (Beijing time, UTC+8), in order to match the time zone at the mission control center in Beijing.
Figure 25: Animation of the Tiangong-2/Shenzhou-11 complex performing the orbital maneuver as shown in the Chenese media [image credit: CCTV (Chinese Central Television) and CNTV (China Network Television)]
• October 19, 2016: The Shenzhou-11 spaceship docked automatically with the Tiangong-2 space lab at 19:24 UTC on October 18. Shenzhou-11, China's sixth manned spacecraft, will undertake the longest-ever space mission in the country. The two astronauts will spend a total of 33 days in space. 25)
Figure 26: Photo taken on Oct. 19, 2016 shows the screen at the Beijing Aerospace Control Center showing astronauts Jing Haipeng (R) and Chen Dong celebrating on the success of the automated docking between Shenzhou-11 manned spacecraft and the space lab Tiangong-2 (image credit: Xinhua)
• The Shenzhou-11 spaceship was launched on October 16,2016 (23.30:00 UTC) on a Long March-2F vehicle from JSLC (Jiuquan Satellite Launch Center). Onboard are two astronauts, the objective is to rendezvous and dock with the Tiangong-2 Space lab and gain experience from a 30-day residence (allowing for a full mission duration of 33 days), and to test its life-support systems. 26)
• Sept. 20, 2016: China's Tiangong-2 space lab arrived in its operational orbit after completing a pair of orbit-raising maneuvers following its successful launch on Sept. 15. 27)
Tiangong-2 will be outfitted with a combination of internal and external payloads. While most of the internal experiments require a crew to perform them, the external payloads can be used as an Earth-imaging capability to add to China's vast fleet of Earth observers from a unique non-synchronous orbit.
Tiangong-2 carries a total of 14 mission and experiment packages, including:
• The world's first-ever in-space CACS (Cold Atomic fountain Clock in Space)
• Space-Earth quantum key distribution and laser communications experiment
• A Gamma ray detector, POLAR
• Liquid bridge thermocapillary convection experiment
• Space material experiment
• Space plant growth experiment
• Multi-angle wide-spectral imager
• Multispectral limb imaging spectrometer
• Stereoscopic microwave altimeter.
Note: The instruments will be described (in more detail) as the information becomes available.
CACS (Cold Atomic Clock in Space)
According to Zhen Xu, a scientist involved with the atomic clock project at SIOM (Shanghai Institute of Optics and Fine Mechanics), CACS is the world's first cold atomic clock to operate in space, it will have military and civilian applications. 28) 29)
CACS, several thousand times more accurate than the clocks used in GPS satellites, will start its journey on the Tiangong-2 space lab. Cold atomic clocks are the most accurate clocks in the world. Low-frequency lasers lower their internal temperatures to -273ºC , and slow down the movement of atoms inside. Slow-moving atoms decrease the likelihood of counting errors, and result in a more accurate counting of time.
"The frequency of the atom will not change. It is the same wherever it is. Unlike in mechanical clocks and electric clocks, atomic clocks aren't drastically affected by their surrounding environment. We are going to operate the most accurate cold atomic clock in space. It is the first time ever, not only for our country, but also for the world," Liu Liang, chief designer of the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, said.
Rubidium atoms count time inside China's cold atomic clock. Atoms are usually affected by gravity, but the low level of gravity in space will weaken the earth's gravitational pull and increase the accuracy of China's cold atomic clock. "Atoms usually fall because of gravity, making it difficult to keep track of time for a long time. But up in space, we don't have that problem," Liu said.
Figure 27: Photo of the CACS instrument in its dewar, developed at SIOM of CAS (Chinese Academy of Sciences), image credit: SIOM
POLAR (Gamma Ray Burst Polarimeter)
The astroparticle detector POLAR was developed in a collaboration between China, Switzerland and Poland. Nicolas Produit of the University of Geneva is the PI (Principal Investigator) of POLAR. 30) 31)
The international collaboration consists of the following institutes: DPNC (Département de Physique Nucléaire et Corpusculaire) and ISDC (Integral Science Data Center) of the University of Geneva, PSI (Paul Scherrer Institute) ,Villigen, Switzerland, NCBJ (Narodowe Centrum Badań Jądrowych,) Świerk ,Poland, and the IHEP (Institute of High Energy Physics) of CAS (Chinese Academy of Sciences), Beijing. The objective is to measure the polarization of photons from GRBs (Gamma Ray Bursts), to shed light on the astrophysical mechanism behind this extremely violent cosmic phenomenon.
POLAR is a novel compact spaceborne detector conceived for a precise measurement of hard X-ray polarization and optimized for the detection of the prompt emission of GRB photons in the energy range 50 -500 keV. GRBs are sudden flashes of gamma-rays that appear randomly in the sky and outshine for a few seconds in all other gamma-ray sources. They are produced at cosmological distances, and are considered as the brightest events in the universe after the Big Bang. In the past 40 years many instruments have performed extensive studies of GRBs, but their creation mechanism and their progenitors are still uncertain. Several theories have been elaborated to explain their origin: The fireball, the electromagnetic, and the cannonball models are at present the most commonly accepted. All of them relate the emission of the GRB to the creation of a black hole, differing in the physical processes involved in the g-ray generation, and also in the level of linear polarization of the emitted photons. The direction and the level of polarization of high-energy photons emitted by astrophysics sources such as GRBs are therefore very good observable candidates for the understanding of the corresponding emission mechanisms, source geometry and strength of magnetic fields at work. 32)
POLAR consists of a target of 40 x 40 plastic scintillator bars, each of dimension 6 x 6 x 200 mm3 , wrapped in a highly reflective foil, organized in 25 independent modular units, with 64 bars each. Each unit is read-out by a flat panel multi anode photomultiplier tube (MAPMT; H8500, Hamamatsu), mechanically coupled to the bottom of the scintillator bars via a thin transparent optical pad, and enclosed in a 1 mm carbon fiber socket. The electrical signals coming from the MAPMT are first processed by an ASICs and FPGA at the front-end electronics, then sent to the POLAR central computer, where the trigger decision is taken considering the outputs of all modular units. This modular design provides a good mechanical stability and facilitates the interchange of modules during the testing phase of the detector. The bars in each modular unit are kept together with two aligning plastic frames located at the top and at the bottom of the carbon fiber sockets to provide resistance to vibrations and to reduce the optical crosstalk between adjacent channels.
Figure 28 shows the structure of POLAR detector (left) and a POLAR single module (right). The whole target, together with the central computer, the power supplies and the rest of the electronics, is further enclosed in a carbon fiber box that enhances the mechanical stability and acts as a shield against low energy charged particles.
Figure 28: Left: The POLAR OBox, Right: A single module of POLAR (image credit: IHEP)
The box-like POLAR instrument will be fitted outside Tiangong-2, with its back to the Earth so that it can catch the signals of explosions billions of light years away.
Tianzhou Cargo Spaceship:
The objective of the cargo spaceship is to supply propellant and cargo for the space station or space lab, and carry waste material to be burned in the atmosphere on spacecraft reentry. Tianzhou is a space cargo transportation and supply spacecraft with high cargo ratio developed independently in China. It consists of cargo cabin and propellant cabin (Figure 29). According to the different cargo transportation requirements of space station, the cargo cabin is designed with three configurations, which are: pressured configuration, semi-pressured configuration, and open configuration. Tianzhou has completed the ground verification of propellant refueling and cargo loading technologies now, and turned into the formal prototype development stage.
Figure 29: Artist's rendition of the deployed Tianzhou-1 automated cargo resupply vehicle, derived from the Tiangong-1 space lab (image credit: CAST)
Just like the space stations built by Russia and the United States, China's future space station will also require constant supply of consumable materials including food, water, and propellants from Earth. To support this role, Chinese designers (CAST) developed a cargo resupply ship named Tianzhou ("Heavenly Vessel"), based on the design of the Tiangong-1 space laboratory module. 33)
The Tianzhou cargo ship is 10.6 m long and has a maximum diameter of 3.35 meters. Its maximum takeoff mass is 13.5 tons, enabling it to carry over 6 tons of supplies.
The launch mass of Tianzhou-1 is expected to be around 13,000 kg with a payload of around 6,000 kg. The spacecraft vehicle will be launched atop the newly developed CZ-7 launcher rocket. Launch, rendezvous and docking shall be fully autonomous, with mission control and crew used in override or monitoring roles. Once the resupply mission is completed, the cargo ship will perform a controlled descent to be burned up in the upper atmosphere.
Orbit: Near circular orbit, altitude of about 385 km, inclination = 42.79º, period of 92 minutes.
• Silkroad-1, a CubeSat of CNSA for Earth observation.
Experiments of Tianzhou-1 on orbit:
• June 21,2017: China's Tianzhou-1 cargo spacecraft completed its second docking with Tiangong-2 space lab at 2:55 p.m. Monday (19 June 2017), after flying around the space lab. 36)
- Tianzhou-1 separated from Tiangong-2 on Monday morning and remained at distance of five kilometers behind the space lab for about 90 minutes.
- Then, it was commanded to fly around Tiangong-2 from behind to a distance of five kilometers in front of the space lab. During the flight, both Tianzhou-1 and Tiangong-2 turned in a semicircle.
- The experiments tested docking technology at different directions, which is of great importance to building a space station, according to the China Manned Space Engineering Office.
• May 01, 2017: Following a successful space mission by its Tianzhou-1 cargo spacecraft, China is gearing up for multiple manned missions into space in three years time, according to the head of the country's manned space program. 37)
- Between 2019 and 2022, China seeks to build a massive, 60-ton space station reminiscent of the International Space Station, China Daily reported. Tianzhou-1's recent flight "was the last flight mission of the country's manned space program before construction of a permanent space station," director of China's manned space program Wang Zhaoyao said on April 28.
- Chinese scientists believe the accomplishment "shows that China's manned space program has entered the space station era," Wang said. Most of the "key" technological developments and flight products have finished testing, he noted. "Chinese astronauts are preparing for the space station era," the director explained, adding that astronauts "are expected to stay in space for three to six months," while future missions could last longer.
• On April 28, 2017, China's Tianzhou-1 cargo spacecraft and Tiangong-2 space lab completed their first in-orbit refueling , another success of the Tianzhou-1 mission. Mastering the technique of refueling in space will help the country to build a permanent space station. 38)
- The in-orbit refueling, under control of technicians on Earth, takes about five days, as the propellant is transmitted from the cargo spacecraft to the space lab.
- A second refueling in space will be conducted after the cargo ship's second docking with the space lab in June, which aims to test the ability of the cargo ship to dock with the space station from different directions.
• On April 22, 2017, Tianzhou-1 docked with the Tiangong-2 space lab (also known as "Heavenly Palace 2"). It made first contact with the space lab at 04:16 GMT on Saturday and docking was completed at 08:23 GMT on April 23, 2017. 39)
- Tianzhou-1 will complete the first propellant refueling test of China. In the construction and operation mission of the China space station, future Tianzhou cargo spaceships will be launched to service the Tiangong-2 space lab as needed. — In addition, each voyage is a precious opportunity to conduct space experiments.
- Tianzhou-1 also conduct experiments in space, including one on non-Newtonian gravitation, and will dock two more times with Tiangong-2 before falling back to Earth, Xinhua said.
• Scientists will test a medicine to treat bone loss during the maiden voyage of China's first cargo spacecraft Tianzhou-1. The medicine has been specially developed for astronauts, but they hope it will benefit ordinary people too. 40)
- Chinese scientists will use the micro-gravity environment to test the effect of 3-hydroxybutyric acid (3HB) in preventing osteoporosis, said research leader Chen Guoqiang ,who is also director of the Center for Synthetic and Systems Biology at Tsinghua University. Normally, the solid structure of bone tissue is stimulated and maintained by gravity and physical exercise. But the micro-gravity environment in space eases the load on bones, causing rapid bone loss and osteoporosis, Chen said. "One day of bone loss in space is equivalent to a year on Earth," he said. — "We hope to test the effect of the medicine in a real space micro-gravity environment," Chen said.
- Since Tianzhou-1 cannot carry animals, scientists will compare the osteoblast cell samples treated and not treated with 3HB. Microscope images of the samples will be transmitted to Earth.
Outlook of a future China's Space Station
The overall objectives of China's Space Station Program are to establish a space station which has a basic configuration composed of a Core module, an Experiment module I and an Experiment module II, with the capability to reliably fly on orbit for a long time. With the help of other systems, China's space station can support astronauts to live safely in the station's environment and work effectively in support of space science experiments and space technology tests. At present, China's space station has completed design work on the basis of the achievement of some key technologies (Ref. 2).
The three modules of China's space station will be launched by the CZ-5B launch vehicle, and the space station is expected to be established around 2020. The Core module will be launched first, followed by the Experiment modules I and II. They will be launched and attached to the core module after rendezvous , docking and reposition to establish the basic configuration of China space station. The design life of China's space station is more than 10 years; the life-span can be extended through maintenance and repair on orbit. The orbit of China's space station is a LEO circular orbit with the altitude of 340 ~450 km.
Figure 30: Illustration of the future China Space Station configuration (image credit: CAST)
Core module: The Core module is the management and control center of the space station. It can support the docking and assembly of the Experiment module, manned spaceship, cargo ship and so on, and support astronaut long-term residence and cargo supply. The core module is equipped with big robot arm, and has the function of airlock. Space medicine and biology experiments can be carried out in the Core module.
Figure 31: Illustration of the Core module (image credit: CAST)
Experiment module: The main task of the Experiment module I is to support space experiments in and out of the cabin, in addition it backs up some platform functions of the Core module. The astronaut consumables, backup equipment and supply cargo are stored in the Experiment module I. The main airlock is installed in the Experiment module I to support EVA. The Experiment module I is also equipped with a small robot arm.
Figure 32: Illustration of the Experiment module I (image credit: CAST)
The main task of Experiment module II is also to support space experiments in and out of the cabin. It is equipped with a special cargo airlock to transfer payloads from a facility in and out of the cabin with the help of astronaut and robot arm.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).