Liu Yang, China's first female astronaut, has touched the hearts of millions with a letter to her two children that was made public after she set off into space for the second time earlier this month.

"You're my strongest armor and softest spots. I didn't allow you to see me off at the launch site because I was afraid that I would burst into tears," Liu wrote in the letter to her daughter, 8, and son, 6, just days before being launched into space for the second time on June 5 on the Shenzhou XIV mission.

Liu, 43, became China's first female astronaut in 2012, when she spent 13 days in space during the Shenzhou IX mission. During the Shenzhou XIV mission, she will stay at the Tiangong space station for six months, working with two male colleagues to complete the construction of the station.

"I promise you that I will build a big house in space and fill it with the dreams of many. I also promise you I will take many beautiful pictures so you can share them with others. I will also take your wishes to space. You have my word," she said in the letter.

The letter was first posted on June 6 on the Sina Weibo account of Our Space, which mainly publishes content on China's space missions. The authenticity of the letter was later confirmed by space administration authorities.

Many people said that Liu's words allowed them to see that a national hero is also an ordinary mother, and they wished for her safe return.

"Liu fully expressed her love for her children in the letter. It brings tears to my eyes," commented one netizen on Sina Weibo, China's Twitter-like microblogging platform.

Liu joined the People's Liberation Army Air Force in 1997 and has notched 1,680 hours of safe flight time. She was selected as a member of China's second generation of astronauts in May 2010.

"I feel more confident and relaxed," Liu told the media ahead of the launch of the Shenzhou XIV mission.

"When I was about to return to the orbital module during the Shenzhou IX mission, I stopped and turned around to salute the experimental space station module that I stayed in. At that moment, I told myself that I would come back to space in the near future. I didn't expect the 'near future' to be a decade long," Liu said.

A number of complicated tasks await the Shenzhou XIV crew, including extravehicular activities, mechanical arm tests and experiments in lab modules.

In the letter, she encourages her children not to be afraid of failure while pursuing their dreams.

Liu added that she felt sorry that she could not be there for her son on his first day at primary school.

"Although I cannot be with you, I won't love you any less," she wrote in the letter.

"My babies, if you miss me, just look up to the starry skies. Every time you see the stars twinkle, it's because I'm saying'I love you'."