<Ul> <Li> Top: 9P / Tempel (impactor collision: Deep Impact), 67P / Churyumov--Gerasimenko (Rosetta) </Li> <Li> Middle: 17P / Holmes and its blue ionized tail, and 81P / Wild (Wild 2) visited by Stardust </Li> <Li> Bottom: Hale--Bopp seen from Earth in 1997, and C / 2011 W3 (Lovejoy) imaged from Earth orbit </Li> </Ul> <Li> Top: 9P / Tempel (impactor collision: Deep Impact), 67P / Churyumov--Gerasimenko (Rosetta) </Li> <Li> Middle: 17P / Holmes and its blue ionized tail, and 81P / Wild (Wild 2) visited by Stardust </Li> <Li> Bottom: Hale--Bopp seen from Earth in 1997, and C / 2011 W3 (Lovejoy) imaged from Earth orbit </Li>

How many comets are in the milky way