On 15 June 2013, the Shenzhou 10 crew completed China's first orbital maintenance operation, replacing Tiangong-1's interior cladding. Additional maintenance work was conducted on the space station's seal rings. On 20 June 2013, Wang Yaping delivered a remote video lecture from orbit to students across China, demonstrating physics in microgravity with her colleagues. On 24 June 2013, CPC general secretary Xi Jinping contacted the astronauts via remote video link to congratulate them. After a series of successful docking tests, Shenzhou 10 undocked and returned safely to Earth on 26 June 2013. With a duration of 15 days, Shenzhou 10 was China's longest crewed space mission, until Shenzhou 11's 30-day mission to Tiangong-2 in 2016.SCIO briefing on China's Tiangong-2 and Shenzhou 11 manned space mission China.org.cn 19 November 2016
