China's fourth lunar mission, Chang’e 4, is expected to begin on May 21 with the launch of a Long March 4C rocket from the Xichang Satellite Launch Centre in the southwest of China. The launch will carry a spacecraft named Queqiao, which will serve as a communication relay satellite between Earth and the lunar farside. The name Queqiao means "magpie bridge" in Chinese and comes from a Chinese folk tale, a love story about a flock of magpies that form a bridge crossing the Milky Way once a year to reunite lovers known as the Cowherd and the Weaver Girl, as well as their children.

How to pronounce Queqiao and Longjian

Chang’e 4 is planned to send both a lander and a rover to explore the far side of the Moon. If successful, Chang’e 4 will be the first-ever soft-landing mission on the lunar farside. However, huge technical challenges exist. Since the lunar farside always faces away from the Earth, direct communication between any farside in-situ explorers and the Earth is blocked.

This is because the Moon is a synchronously rotating satellite of our planet Earth, which means it rotates exactly once for each time it circles Earth. The match of spin and orbit means the moon is rotationally locked to the Earth, so we can only directly see one hemisphere of the moon (aka, the nearside) from the Earth. (Because the Moon's orbit is slightly elliptical, we actually get to peek slightly over the east and west edges of the Moon every orbit, but most of the farside remains mysterious.) Synchronous rotation is nothing unique between Earth and the Moon. Nearly all the regular satellites of the outer solar system planets, and even some exoplanets are in synchronous rotation states with their parental planet or star.

How do we have any data on the lunar farside? Orbiters circling the Moon gather data autonomously and send data back when they're in view of Earth. Humans first saw the lunar farside through a camera on the Soviet Luna 3 spacecraft in 1959. However, important scientific measurements for the lunar farside that require real-time contact with the Earth cannot be conducted directly. For example, gravity measurements by Doppler tracking, communication between a lander/rover and the Earth, Lunar Laser Ranging (LLR) measurements, and other experiments aren't possible over the farside. The first direct gravity measurement for the lunar farside was not conducted until 2007, when Japan's Kaguya m ission used a 4-way relay satellite for Doppler tracking. To enable communication from the future first lander and rover on the lunar farside, we will need a communication bridge, Queqiao.

The Queqiao satellite is 425 kilograms in weight and has a planned lifetime of more than 5 years. Both the solar panels and lithium batteries provide its energy. It will perform relay communication using a 4.2-meter diameter umbrella antenna, talking with the lander/rover at X-band and communicate with the Earth ground station at S-band frequencies. Meanwhile, the lander will also receive UHF transmissions from the rover, decode them, and send the decoded data on to the relay satellite.