卫星(wèi xīng) - satellite, moon of a planet in Mandarin

卫星 (wèi xīng) covers both meanings of the English word: a natural moon orbiting a planet, and a machine put into orbit by people. 月亮是地球的卫星 is the standard schoolbook line, and 发射卫星 is to launch a satellite.

The measure word is 颗, the one used for small round things and for stars generally, so 一颗卫星. When the context could go either way and you need to be explicit about a man made craft, say 人造卫星. The word also prefixes freely: 卫星电话, 卫星电视, 卫星导航.

The trap is the character 卫 rather than the meaning. It is the same 卫 as in 卫生 (hygiene) and 保卫 (to defend), and its sense of guarding is what makes the compound work, so do not read it as a phonetic. Note too that Chinese calls the moon 月亮 in everyday speech and only reaches for 卫星 when classifying it astronomically.