China's Shenzhou-21 mission will carry four specially trained mice to the country's space station, the first time mammals will join Chinese astronauts in orbit.

The "space mice," two males and two females, were selected from 300 candidates after more than 60 days of intensive training to test their fitness, balance and adaptability.

With 95 percent genetic similarity to humans, the mice will help scientists "understand how the space environment affects long-term human spaceflight."