  In its 16 years of scientific measurements, the Spitzer Space Telescope
performed a number of ground breaking and key infrared measurements of Solar
System objects near and far. Targets ranged from the smallest planetesimals to
the giant planets, and have helped us reform our understanding of these objects
while also laying the groundwork for future infrared space-based observations
like those to be undertaken by the James Webb Space Telescope in the 2020s. In
this first Paper, we describe how the Spitzer Space Telescope advanced our
knowledge of Solar System formation and evolution via observations of small
outer Solar System planetesimals, i.e., Comets, Centaurs, and Kuiper Belt
Objects (KBOs). Relics from the early formation era of our Solar System, these
objects hold important information about the processes that created them. The
key Spitzer observations can be grouped into 3 broad classes: characterization
of new Solar System objects (comets D/ISON 2012 S1, C/2016 R2, 1I/`Oumuamua);
large population surveys of known object sizes (comets, Centaurs, and KBOs);
and compositional studies via spectral measurements of body surfaces and
emitted materials (comets, Centaurs, and KBOs).
