  Simultaneous space- and ground-based microlensing surveys, such as K2's
Campaign 9 (K2C9) and $WFIRST$, facilitate measuring the masses and distances
of free-floating planet (FFP) candidates. FFPs are identified as single-lens
events with a short timescale, of-order 1 day. Measuring the mass of the
lensing object requires determining the finite size of the source star $\rho$,
as well as the microlens parallax $\pi_{\rm E}$. A planet that is bound to but
widely separated from a host star can produce a light curve similar to that of
an FFP. This tension can be resolved with high-resolution imaging of the
microlensing target to search for the lens flux $F_l$ from a possible host
star. Here we investigate the accessible parameter space for each of these
components --- $\pi_{\rm E}$, $\rho$, and $F_l$ --- considering different
satellites for a range of FFP masses, Galactic distances, and source star
properties. We find that at the beginning of K2C9, when its projected
separation from the Earth (as viewed from the center of its survey field) is
$\lesssim$0.2 AU, it will be able to measure $\pi_{\rm E}$ for Jupiter-mass FFP
candidates at distances larger than $\sim$2 kpc and to Earth-mass lenses at
$\sim$8 kpc. At the end of its campaign, when $D_\perp = 0.81$ AU, it is
sensitive to planetary-mass lenses for distances $\gtrsim$3.5 kpc, and even
then only to those with mass $\gtrsim$M$_{\rm Jup}$. From lens flux constraints
we find that it will be possible to exclude all stellar-mass host stars (down
to the deuterium-burning limit) for events within $\sim$2 kpc. Together these
indicate that the ability to characterize FFPs detected during K2C9 is
optimized for events occurring toward the beginning of the campaign. $WFIRST$,
on the other hand, will be able to detect and characterize FFPs with masses at
least as low as super-Earths throughout the Galaxy during its entire
microlensing survey.
