  Planet 9 is an hypothetical object in the outer Solar system, which is as yet
undiscovered. It has been speculated that it may be a terrestrial planet or
gas/ice giant, or perhaps even a primordial black hole (or dark matter
condensate). State-of-the-art models indicate that the semimajor axis of Planet
9 is $\sim 400$ AU. If the location of Planet 9 were to be confirmed and
pinpointed in the future, this object constitutes an interesting target for a
future space mission to characterize it further. In this paper, we describe
various mission architectures for reaching Planet 9 based on a combination of
chemical propulsion and flyby maneuvers, as well as more advanced options (with
a $\sim 100$ kg spacecraft payload) such as nuclear thermal propulsion (NTP)
and laser sails. The ensuing mission duration for solid chemical propellant
ranges from 45 years to 75 years, depending on the distance from the Sun for
the Solar Oberth maneuver. NTP can achieve flight times of about 40 years with
only a Jupiter Oberth maneuver whereas, in contrast, laser sails might engender
timescales as little as 7 years. We conclude that Planet 9 is close to the
transition point where chemical propulsion approaches its performance limits,
and alternative advanced propulsion systems (e.g., NTP and laser sails)
apparently become more attractive.
