  We propose a scientific program to complete a census of planets,
characterizing their masses, orbital properties, and dynamical histories using
continued observations of the Kepler field of view with the Kepler spacecraft
in a two reaction wheel mission. Even with a significantly reduced photometric
precision, extending time-domain observations of this field is uniquely capable
of pursuing several critical science goals: 1) measuring the architectures of
planetary systems by identifying non-transiting planets interleaved among known
transiting planets, 2) establishing the mass-radius relationship for planets in
the important transition region between small, gas-rich sub-Neptune planets and
large, rocky super-Earths, and 3) uncovering dynamical evidence of the
formation and evolution of the inner regions of planetary systems. To meet
these objectives, the unique multi-object observing capabilities of Kepler will
be used in a set of concurrent campaigns with specific motivations. These
campaigns focus largely on the ability to interpret Transit Timing Variations
(TTVs) that result from dynamical interactions among planets in a system and
include: 1) observations of systems that exhibit large TTVs and are
particularly rich in dynamical information, 2) observations of systems where
additional transit times will yield mass measurements of the constituent
planets, 3) observations of systems where the TTV signal evolves over very long
timescales, and 4) observations of systems with long-period planet candidates
where additional transits will remove orbital period ambiguities caused by gaps
in the original Kepler data.
