THE WOODLANDS, Texas — A leading official in China’s lunar and Mars exploration program was reportedly denied a visa by the United States to attend a scientific conference here this week.

Guobin Yu, vice director of the Lunar and Space Exploration Engineering Center of China, was scheduled to speak at a symposium here March 19 to discuss China’s plans to explore the moon and Mars.

However, Jim Head, a professor of geological sciences at Brown University and organizer of the “Microsymposium 58” meeting held before the start of the Lunar and Planetary Science Conference, said that he was informed by Yu just before the start of the event that the U.S. embassy in Beijing had denied his request for a visa to attend the conference.

Head said that the denial came after Chinese government officials approved Yu’s plans to speak at the conference “Approval for this visit came from the highest levels of the Chinese space program and, in fact, from the Chinese government,” he said.

He said that he was unaware of the reason the U.S. government gave for denying a visa to Yu, of if any reason was provided at all. The embassy did not respond to a March 19 email seeking information about the denied visa. That denial did not extend to other Chinese scientists: Head said more than 20 people from Chinese universities, academies and scientific institutes were at the symposium.

As part of the Chinese government approval process, Head said that Yu was authorized to provide “important” updates about the China’s plans to send missions to the moon and Mars, but he did not elaborate on the substance of those updates.

Head, who has visited China to meet with Chinese scientists, offered his own overview of their exploration efforts. “Their program is extremely robust,” he said, with the next major milestone being China’s first lunar sample return mission, Chang’e-5, scheduled for launch late this year.

A second lunar sample return mission, Chang’e-6, is planned for around 2020, according to Head. That may be followed by missions to the lunar polar regions in advance of human missions later in the 2020s or 2030s.

China’s first full-fledged Mars mission is scheduled for launch in 2020, flying both an orbiter and a lander, the latter equipped with a rover. Head said that China is also studying a Mars sample return mission for around 2030.

The lunar exploration program in particular, which included two orbiter missions and one lander, has helped build interest in planetary science studies in China. “There’s not historically been a major lunar and planetary science community in China, but in the last decade or so it’s been growing,” he said.

Despite the visa denial and other obstacles to cooperation, such as provisions in NASA appropriations bills in recent years that restrict bilateral cooperation with China, Head said he was hopeful for greater cooperation in the future. He noted that he’s worked with scientists in Russia and the former Soviet Union for 45 years, overcoming technology transfer and other obstacles even during the Cold War. “There’s much to be gained from cooperation and collaboration in the scientific community,” he said.