Most people will remember the excitement of receiving their first letter from a pen pal. The beauty of receiving those letters lay in the anticipation, the waiting and of course, the desire to know what the letter would contain.

When she received her letter, Chen Man-lin, a 15-year-old student from Hong Kong, was practically over the moon because her pen pal was in orbit aboard the Tiangong Space Station when he wrote his inspiring reply.

A Form 3 student (or ninth-grader) at Pui Kiu Middle School in North Point, Chen learned in June about the national letter-writing campaign to the astronauts on Tiangong, which was titled "Sending My Dream to Tiangong".

She drafted an outline letter the same evening, detailing every moment of her own planned journey toward contributing to national aerospace development, and what she knew about astronauts and the national aerospace development program.

Chen's decision to write the letter was more than just an impulse. Her interest in space had begun in 2013 when as a 6-year-old, she watched the live broadcast from orbit of a science lecture.

Over 60 million primary and middle school students nationwide watched astronaut Wang Yaping's lecture and the images of floating balls of water and spinning tops, experiments she performed in space to demonstrate the concepts of weight and mass in a zero-gravity environment, captured the imaginations of Chen and many other young viewers.

Since then, Chen has closely followed news and developments in the sector and poured that passion for space into her letter, as well as some of her frustrations and doubts over her studies and her choices in life.

The three-page reply from Shenzhou XIV astronaut Chen Dong, arrived about a month later.

As one of the 30,000 students from all over the country to take part in the campaign, Chen was the only one in Hong Kong chosen to receive a reply.

Her school threw a party to celebrate the arrival of the letter on July 19, which was hand-delivered to the student by Hong Kong's Secretary for Innovation, Technology and Industry, Sun Dong.

Along with his reply, Chen Dong also sent a video recorded on the space station, which the school's teachers and students gathered to watch together.

Seeking to encourage the Form 3 student, Chen Dong recalled how he fumbled along the way to becoming an astronaut, and shared his tips for overcoming the drowsiness he'd felt when studying astronomy, physics, aerospace technology and other disciplines necessary to achieve his goals.

He also spoke of how he had been tested to his limits during hypergravity training, a technique that involves wearing weights for several hours every day.

"During training, every inch of me was subject to incredible pressures. It sometimes felt almost like my organs were being displaced," he said.

This intense training continued for six years, during which the astronaut all but cut himself off from the outside world, until he finally qualified for a mission.

It was his dreams that kept him going whenever he felt lost or exhausted, he wrote.

On Oct 17, 2016, during his first space mission, Chen Dong was stunned by the sight of the entire universe unfolding before his eyes.

"I will be honest with you, life is like the sky, and all its different kinds of weather. Not every day is sunny, there will also be rainy, snowy and stormy days, and the sky will continuously change color. Don't focus on stumbling blocks, or you will never become your best. Every day offers a valuable lesson, and they help make you who you are. Please cherish each and every one, and you will be rewarded with a great surprise," he wrote.

"Every night, before I went to sleep, I would look out of the space station's windows to catch a glimpse of our country. The gleaming lights of every household proved to me that it was a thriving place, and that is all I have ever fought for," he added.