An image simulating the docking of space shuttle Shenzhou-11 with space lab Tiangong-2 on October 19, 2016 (XINHUA)
After blazing through the atmosphere, the debris of the retired Tiangong-2 space laboratory splashed into the south Pacific on the evening of July 19, staging a glorious homecoming.
The space lab, launched on September 15, 2016, had hurtled in orbit for more than 1,000 days before commencing its controlled deorbiting, which is a crucial step since a spacecraft out of control may pose a danger to other spacecraft in orbit and to humans on the ground, said Zhu Congpeng, chief designer of the space lab.
"To let Tiangong-2 retire by choice is to ensure the absolute safety of its termination," Zhu told the media.
This is the second time that China has completed controlled deorbiting of a spacecraft. The first was carried out in 2017 for Tianzhou-1, a space shuttle that ferried supplies and fuels to Tiangong-2.
Before its deorbiting, Tiangong-2 and its remaining payloads still functioned normally and the propellant it carried was still sufficient to sustain it in orbit for several more years, said Zhu.
But it had completed its mission, and as time went by, it would have become less reliable, and once it became defunct, it would be out of control and gradually fall into Earth's atmosphere. At that point, some materials with high melting points may not combust completely, and the resulting debris may injure people on the ground, he explained.
It also took a lot of resources to keep monitoring and testing the spacecraft; now. After the deorbiting, these resources can be released and invested in other missions, he said.
To guarantee a safe re-entry into the atmosphere, a flight control team from the China Academy of Space Technology carried out a number of simulations and drafted a detailed flight control plan, according to Zhu. During the deorbit, they adjusted the spacecraft's speed and angle so that its debris eventually fell into an uninhabited area.
The deorbiting prevented the spacecraft from becoming a hazard in space, Zhu said, noting that currently, Earth is surrounded by more than 20,000 pieces of space debris with diameters larger than 10 cm, and such debris can destroy any spacecraft it collides with.
It will take a decade for the debris in low Earth orbit 400 km from Earth to descend naturally into the atmosphere and burn up, and it will take those in higher orbits even longer, Zhu said.
Two children observe a model of the cold atomic clock carried on board the Tiangong-2 space lab during an exhibition on May 16, 2018 (XINHUA)
While it was in space, the lab served as a platform for conducting more than 60 space science experiments and technological tests, according to Xinhua News Agency.
In 2016, it accommodated two Chinese astronauts for a month, marking the longest stay in space by Chinese astronauts. Jing Haipeng and Chen Dong arrived at the space lab in October of that year on board Shenzhou-11 during China's sixth manned space mission. They lived and conducted scientific and technical experiments from the lab, which were captured by video cameras and broadcasted online.
While in space, the duo raised six silkworms in a transparent cylindrical container, where the creatures grew and spun cocoons. They also cultivated lettuce, thinning the seedlings and injecting air into their roots. The lettuce flourished, was harvested and brought back to Earth for further study. In addition, they tested a brain-computer interface, an experiment allowing astronauts to operate computers with their minds.
Tiangong-2 was also a platform where a number of other cutting-edge science experiments were conducted including Earth-space quantum key distribution, a cold atomic clock, and gamma-ray burst (GRB) observation.
Quantum key distribution is a secure communication method using quantum science to encrypt and decrypt messages.
A cold atomic clock is said to be the most accurate timekeeper in the universe. While a mechanical watch loses almost one second a day and a quartz watch about one second every 10 days, a cold atomic clock won't lose a single second in 30 million to 300 million years in space.
In the field of GRB monitoring, a device called POLAR detected 55 GRBs, the strongest explosions in the universe, according to Xinhua. It was jointly developed by Chinese and European scientists to investigate the nature of GRBs as well as the origin and evolution of the universe.
The space lab also had imaging equipment to observe the land, oceans and atmosphere of Earth and test remote-sensing capabilities.
On April 22, 2017, the Tianzhou-1 cargo spacecraft accomplished its first automated docking with the space lab, and carried out a refueling a few days later. Then the cargo spacecraft made a second docking with the space lab from a different direction followed by a third docking using fast-docking technology, which shortened the normally two-day process to only six hours.
Tiangong-2's successful mission marks the fruition of the second stage of China's manned space program. Started in 1992, it is being carried out in three stages. The goal of the first stage was to send an astronaut into space, which was achieved in 2003. The second stage involved sending manned spaceships to space labs where astronauts can live temporarily and conduct experiments. The goal of the third stage is to build the permanent China Space Station (CSS).
During the first two stages, China has launched into space 11 manned spaceships carrying 11 astronauts in total, a cargo spacecraft and two space labs, according to official statistics. China's first space lab, Tiangong-1, was sent into orbit in September 2011 and three astronauts lived there for 15 days.
China plans to put the CSS in place around 2022, which will hold many devices for experiments, said Zhou Jianping, chief designer of China's manned space program.
According to China's Manned Space Agency (CMSA), the CSS will consist of a core module and two experiment modules, and has a designed life span of 10 years.
China is prepared to open the CSS to scientists from around the world who are interested in conducting their own experiments aboard the space station.
At the end of May 2018, the UN Office for Outer Space Affairs (UNOOSA) and CMSA announced the opportunity for other countries to conduct space experiments on board the CSS, welcoming applications from all UN members.
In June, the outcome of the selection process was announced. According to the UNOOSA, nine projects involving 23 institutions from 17 UN members were selected. The projects, to be conducted by governmental organizations, private sectors and international associations, involve such research areas as space life science, biotechnology, microgravity fluid physics, microgravity combustion, astronomy and space technologies.
One of them is the POLAR-2 project, which will focus on answering questions regarding the nature of GRBs, building on the investigations conducted on Tiangong-2.
Also selected is the spectroscopic investigations of nebular gas project, whose aim is to map the sky using an ultraviolet long-slit spectrograph. It will be implemented by the Indian Institute of Astrophysics and the Institute of Astronomy of the Russian Academy of Sciences.
Another selected project will study the effects of spaceflight conditions on both healthy and cancerous tissue, and will be carried out by three universities and a research center from Europe.
A related project will examine the effect of microgravity on the growth and biofilm production of disease-causing bacteria. It will be jointly conducted by the Mars Society-Peru Chapter and the Mars Society-Spain Chapter.
In addition to carrying scientific payloads from the international community, China also welcomes overseas astronauts to work with Chinese astronauts on the CSS. Moreover, foreign spacecraft using a Chinese docking mechanism can be docked with the CSS, Zhou said.
Copyedited by Rebeca Toledo
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