Abstract　 　　     

Comet C/2012 S1 (ISON) was discovered in September 2012 when it was about 6 AU from the Sun, and was soon entitled "Comet of the Century" because its peak brightness was then expected to outshine the full Moon. The uniqueness of C/ISON is its combination of an Oort cloud comet and a sungrazing comet with a perihelion distance of merely 1.7 solar radii from the Sun's surface. Oort cloud comets that enter the inner solar system for the first time after their formation contain important clues about the physical and chemical conditions of the solar system formation. Sungrazing orbit means that the tremendous heat from the Sun near the perihelion will vaporize almost every compositional constituents, enabling easy measurements. C/ISON already disintegrated before or during its perihelion passage on November 28, 2013, leaving only dust on or near its original orbit. Its peak brightness was orders of magnitude lower than what we expected for a "Comet of the Century". However, its uniqueness in cometary science has drawn a huge number of spontaneous and organized observations from both professionals and amateurs, especially under the multi-dimensional observing campaign organized by NASA utilizing almost all possible platforms on the ground and in space. In terms of cometary research and planetary science, C/ISON did not disappoint us. Its many puzzling properties need to be fully understood through a thorough analysis of the wealth of data. I will discuss the uniqueness of C/ISON, its significance in studying comets, and some preliminary results.