  We study the development of activity in the incoming long-period comet C/2017
K2 over the heliocentric distance range 9 < r_H < 16 AU. The comet continues to
be characterized by a coma of sub-millimeter and larger particles ejected at
low velocity. In a fixed co-moving volume around the nucleus we find that the
scattering cross-section of the coma is related to the heliocentric distance by
a power law with heliocentric index $s = 1.14\pm0.05$. This dependence is
significantly weaker than the inverse square variation of the insolation as a
result of two effects. These are, first, the heliocentric dependence of the
dust velocity and, second, a lag effect due to very slow-moving particles
ejected long before the observations were taken. A Monte Carlo model of the
photometry shows that dust production beginning at r_H ~ 35 AU is needed to
match the measured heliocentric index, with only a slight dependence on the
particle size distribution. Mass loss rates in dust at 10 AU are of order 1000
kg/s, while loss rates in gas may be much smaller, depending on the unknown
dust to gas ratio. Consequently, the ratio of the non-gravitational
acceleration to the local solar gravity may, depending on the nucleus size,
attain values comparable to values found in short-period comets at much smaller
distances. Non-gravitational acceleration in C/2017 K2 and similarly distant
comets, while presently unmeasured, may limit the accuracy with which we can
infer the properties of the Oort cloud from the orbits of long-period comets.
