By the mid-18th century, the Qing had successfully put outer regions such as Inner and Outer Mongolia, Tibet and Xinjiang under its control. Imperial commissioners and garrisons were sent to Mongolia and Tibet to oversee their affairs. These territories were also under supervision of a central government institution called Lifan Yuan. Qinghai was also put under direct control of the Qing court. Xinjiang, also known as Chinese Turkestan, was subdivided into the regions north and south of the Tian Shan mountains, also known today as Dzungaria and Tarim Basin respectively, but the post of Ili General was established in 1762 to exercise unified military and administrative jurisdiction over both regions. Dzungaria was fully opened to Han migration by the Qianlong Emperor from the beginning. Han migrants were at first forbidden from permanently settling in the Tarim Basin but were the ban was lifted after the invasion by Jahangir Khoja in the 1820s. Likewise, Manchuria was also governed by military generals until its division into provinces, though some areas of Xinjiang and Northeast China were lost to the Russian Empire in the mid-19th century. Manchuria was originally separated from China proper by the Inner Willow Palisade, a ditch and embankment planted with willows intended to restrict the movement of the Han Chinese, as the area was off-limits to civilian Han Chinese until the government started colonizing the area, especially since the 1860s.

Answer this question, if possible (if impossible, reply "unanswerable"): When did Tibet become part of Qing China?
mid-18th century