Where does astronauts get water outside the earth?

There are three urine collection buckets in front of the urinals in the men's restroom of the research building of the 206th Institute of the Second Academy of China Aerospace Science and Industry Corporation. There is a note on it - "Please cooperate with the scientific experiment." Although preservatives have been added to the buckets, the volatile smell still wafts out from time to time, making people cover their noses and hold their breath.

On May 10, the Tianzhou-4 cargo spacecraft took off smoothly. Products such as the calcium removal assembly, urine filter assembly, and lubricating water bag assembly developed by the 206th Institute of the Second Academy were on board the ship. These products are consumables of the space station's environmental control and life support urine treatment subsystem and are used to ensure the Shenzhou 14 astronaut crew's six-month in-orbit life.

After the Tianhe core module was launched, the urine treatment system, which was hailed as the most technically difficult and the most complex system, performed well in orbit. By the time the Shenzhou 13 astronaut crew successfully completed their on-orbit mission, a total of 1,700 liters of urine had been processed and more than 1,400 liters of distilled water had been produced, providing the astronauts with essential recycled water for their on-orbit life.

Water comes and goes

The space station is a small world built by humans outside the earth. The transportation connections established by spacecraft are very limited. Every material for life in the sky must be transported to the sky one by one and in trips, like ants moving house. Humans have created an ecological environment similar to that of the earth outside the earth, and tried their best to retain every drop of water.

When astronauts are in orbit for less than a month, the demand for water and other materials is relatively small and can be fully provided by the ground. China's space program has entered the space station era and will carry out at least ten years of in-orbit operations. It will establish a physical and chemical regeneration life support system to achieve the recycling of oxygen, water and other materials, which can greatly reduce the ground supply required for long-term space activities.

Where does the water in the space station come from? Cargo spacecraft regularly send water bags up to provide astronauts with "primitive supplies" for survival in space.

How to use water? It is estimated that each astronaut needs to drink 2.5 liters of water every day. In addition, the oxygen in the space station also needs to be supplied through electrolysis of water to meet the astronauts' oxygen consumption.

How is water circulation achieved? Astronauts excrete water in the form of urine, sweat, and water vapor through body metabolism, of which urine accounts for the vast majority.

The water metabolized by the human body is purified by the water treatment and urine treatment subsystems in the space station, realizing the recycling of water resources in the space station. At present, the water recovery rate in the space station is better than 95%, and the water utilization rate is better than 83%, which greatly reduces the amount of water supply for cargo spacecraft.

"The peak power of the urine treatment system is 300 watts, which is lower than the power of most household electric kettles," said Director Cui of the Space Electromechanical Research Laboratory of the 206th Institute.

Embracing the next decade

The urine processing subsystem can process urine collected over three days in 10 to 12 hours, a hard-won achievement. When tackling key technical problems, the team carried out special research work to solve technical problems in urine processing, and the problem of calcium crystal precipitation in urine was one of the problems.

In a weightless environment, the body's calcium loss increases. When the calcium in the urine exceeds its solubility, it will crystallize and clog heat exchangers, valves, filters and other products. This is fatal to a system based on the principle of distillation and concentration.

In order to inhibit calcium crystallization in the urine treatment system, the team worked together and consulted with many parties. Finally, they got inspiration from boiler water softening. Under the guidance of material experts from Fudan University in Shanghai and Nankai University in Tianjin, they selected two materials for removing calcium and selected the best one after comparing various indicators. Today, the urine treatment system of the space station can extract 5 liters of distilled water from 6 liters of urine, and the concentrated urine residue still does not form calcium crystal precipitation.

"Scientific research is not always smooth sailing." Director Cui said that regenerative environmental control and life support is a new path opened up by the 206 Institute in the field of space engineering. Over the past ten years, the team has cut through mountains and built bridges across rivers, and has mastered key technologies one after another. Young people continue to join the team, keeping the team energetic and finally completing this "long-distance race".

The development of the urine treatment system implements the aerospace system engineering thinking. During the development process, the system does not insist on the performance of individual indicators and single-machine products, but optimizes the matching of each single machine, component and system to optimize the system and maximize its efficiency.

The urine treatment subsystem needs to operate on orbit for more than 10 years, so at the beginning of the development, the on-orbit replaceable unit (ORU) was divided according to the product's life and reliability indicators for astronauts to repair and replace on orbit. The consumables that need to be replaced are regularly replenished by cargo spacecraft. Director Cui introduced: "It's like car maintenance, replacing engine oil, filter elements, batteries, tires, etc. according to the mileage."

The Wentian Experimental Module to be launched later will also be equipped with a urine processing subsystem, and the space station complex will enter the "double kidney" era. Director Cui said: "Ten years of hard work will finally lead to the success of the project. All the setbacks, pains and efforts we have encountered before are worth it.