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If China’s lunar exploration program goes according to plan, scientists could send a robotic lander to pick up samples from the far side of the moon and return them to Earth by the early 2020s, a top Chinese space official said.

China is developing a pair of identical sample return craft to fly to the moon to bring back the first lunar samples since 1976.

The first of the sampling probes, Chang’e 5, will launch by the end of next year and attempt a landing at an unspecified location on the near side of the moon. Chinese engineers are simultaneously building parts for a backup mission named Chang’e 6, according to Wu Yanhua, vice administrator of the China National Space Administration.

If next year’s mission successfully brings lunar soil samples back to Earth, the Chang’e 6 mission could be directed to a destination on the far side of the moon, Wu said this week at the International Astronautical Congress in Guadalajara, Mexico.

“Based on the success of the Chang’e 5 sample return mission, the Chang’e 6, which is a redundancy, we’ll decide on its next step, whether it’s to be on the near or the far side of the moon for a sample return mission,” Wu said Monday.

No mission has ever landed on the far side of the moon before, let alone return samples.

China has built robotic lunar missions in pairs to have a backup spacecraft nearly ready for launch in case the first one fails.

The country’s first lunar probe, Chang’e 1, went into orbit around the moon in 2007. The nearly identical Chang’e 2 spacecraft launched in 2010 to survey the moon from orbit, before escaping into solar system and conducting China’s first flyby of an asteroid.

China’s most recent lunar mission was Chang’e 3, which touched down in the moon’s Mare Imbrium region in December 2013 with a stationary landing platform and a mobile rover. Its sister craft, Chang’e 4, is supposed to launch in 2018, aiming for the first controlled landing on the far side of the moon.

All human and robotic lunar landings to date have explored the face of the moon visible from Earth, but scientists are eager to gain close-up observations of the lunar far side, the moon’s more rugged and heavily cratered hemisphere.

Wu said the Chang’e 4 launch will be preceded by liftoff of a specialized signals relay satellite to be positioned at the Earth-moon L2 Lagrange point, a gravitationally stable location where the communications platform will constantly loiter over the far side of the moon.

The telecom relay craft is required to bounce commands and scientific data between Chang’e 4 and ground stations on Earth once the probe lands.

Wu said China intends to make the Chang’e 4 relay satellite available to other users who are planning robotic and human expeditions to the moon or nearby.

The communications satellite “will be used not only for the Chang’e 4 mission, but also for supporting future manned and unmanned lunar exploration missions to the far side, and cis-lunar activities,” Wu said.

The overture is another sign of growing openness in China’s space missions, at least by the Chinese program’s historic standards.

Four science instruments provided by research institutions in Germany, Sweden, Italy and the Netherlands are also slated to fly on the Chang’e 4 mission, Wu said.

Chang’e 4’s science suite will collect data on the moon’s seismic activity, survey underground geologic layers with a ground-penetrating radar, and observe the universe with a VLF receiver, exploiting the pristine radio environment on the lunar far side for astronomical research.

Radio astronomy missions deployed closer to Earth must contend with radio interference from human-made sources, but those emissions never reach the surface of the far side of the moon.

“Mineralogical and geochemical surveys on the far side to study the formation and evolution of lunar crust, and observations at low radio frequencies to track the signals of the universe’s ‘dark ages’ are priorities,” wrote Wang Qiong and Liu Jizhong, scientists from the China National Space Administration’s Lunar Exploration and Science Engineering Center, in a paper published in the journal Acta Astronautica in June.

A likely destination for Chang’e 4 is the Apollo crater, located near the edge of the moon’s South Pole-Aitken basin, the largest and oldest lunar impact site. Scientists believe the huge basin, which extends up to 5 miles (8 kilometers) deep and stretches almost 1,500 miles (2,500 kilometers) across, exposes parts of the deep lunar crust, and perhaps even material from the ancient molten lunar mantle.

Scientists are eager for a mission to land there, and a sample return probe to bring back rocks from the South Pole-Aitken basin ranks among the highest planetary science priorities for NASA. Such a mission is a candidate to be the next project selected by NASA’s New Frontiers program, a line of billion-dollar interplanetary spacecraft that includes the New Horizons Pluto flyby probe, the Juno orbiter at Jupiter, and the OSIRIS-REx asteroid sample return mission launched earlier this month.

In his remarks in Mexico on Monday, Wu did not disclose when Chang’e 6 could be ready for launch, but China’s other sister missions — Chang’e 1 and 2, and Chang’e 3 and 4 — launched or are due to launch three to five years apart.

With Chang’e 5 scheduled to land on the near side of the moon in late 2017, a Chang’e 6 sample return mission to the far side — if given the green light — could follow a few years later.

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Follow Stephen Clark on Twitter: @StephenClark1.