Four photos of Mars taken by the Tianwen 1 probe were unveiled in China last month, but the excitement generated by humanity's next great step in space exploration was shared around the world.

Just as eye-catching as images of the Martian surface itself was the Chinese national flag on the Zhurong rover's landing platform.

After the historic landing on May 15, the flag was slowly unrolled. Its red color stands out in the photo, marking China's pride in becoming the second country after the United States to successfully land on the red planet. The earliest attempts at a landing, made by the Russians in 1971, ended in failure.

Few people are likely aware that state-of-the-art technology was needed to make unrolling the flag possible.

According to He Yanchun, deputy general manager of the Surface Engineering Department of the Lanzhou Institute of Physics in Lanzhou, Gansu province, a subsidiary of the China Aerospace Science and Technology Corp and producer of the flag device, due to the location and limitations of space, the 360 millimeter long and 240 mm wide flag was made out of shape memory composite to be able to take the form of a scroll during the travel period to Mars before being unrolled.

When the probe landed, the flag gradually unfolded after being lightly heated. "Unlike folding or sticking up the flag, the device was a modern adaptation of traditional Chinese scroll paintings, and it created a good effect," He said.

Shape memory material is able to "memorize" its original shape and automatically recover that shape once prompted. The material also functions in extreme conditions such as low temperatures and high radiation, experts said.

With no previous examples to learn from, the research team conducted hundreds of trials to overcome significant challenges. One was ensuring the flag remained tightly rolled up during the monthslong flight to Mars, despite exposure to the harsh environment of space and drastic changes of temperature. To do that, the research team used extra materials to tighten the locking force, said Zhao Yinzhong, a designer of the department.

Electric heating bands were installed in both ends of the scroll. Once heated, the shape memory composite material straightened, allowing the flag to fall under the force of gravity.

"The heating and unrolling process was complicated and required precise design. For one thing, it was affected by the extreme and variable Martian environment, characterized by temperature differences, low air pressure and microgravity. For another, the weight and power consumption of the flag device needed to be adapted to the overall design of the probe, which posed a great challenge in terms of design and production," Zhao said.

"Our team conducted hundreds of experiments covering all possible conditions and collected a great deal of data before finally figuring out the perfect voltage and heating time. People working in the aerospace science and technology sector must be able to 'crack hard nuts' despite difficulties."

According to the institute, the flag device weighed less than 200 grams in order not to add too much weight to the probe. The unrolling of the flag barely resulted in any vibration or shock to other components.

Another national flag was pasted on the rover itself, with a length of 96 mm and width of 64 mm, using the same technology used on the Chang'e 3 and Chang'e 4 lunar probes.

As Zhurong continues to explore key scientific questions, such as potential locations of water and ice and volcanic activity on Mars, the flag will "wait" for good news of potentially historic findings, at the landing platform.