A relay satellite is a type of communication satellite that provides data transmission, observation and control services for other spacecraft. So the launch of Queqiao, Magpie Bridge, on Monday is an important step in China's bid to land the first spacecraft on the far side of the moon.

Queqiao will serve to establish a reliable long-distance data transmission link between China's Chang'e-4 lunar probe and controllers on Earth.

China has already sent a series of relay satellites into geosynchronous orbit 36,000 kilometers away from the earth for its manned spacecraft. Queqiao is similar to those but it is now on its way to the Earth-moon second Lagrange Point, a gravitationally stable sweet spot about 455,000 kms from the Earth. The satellite will "park" there and wait for China's robotic Chang'e 4 lander-rover spacecraft, which is expected to launch in November or December.

Chinese experts designed several antennas for the relay satellite, including one shaped like an umbrella with a diameter of 5 meters, which opened in space after the satellite separated from the carrier rocket.