  Asteroid 3200 Phaethon resembles a comet in some ways, including a
highly-eccentric orbit (e=0.89) and a strong associated meteor shower (the
Geminids). Yet this object has never been observed to exhibit any cometary
activity, i.e., gas production. We observed 3200 Phaethon with the Caltech
Submillimeter Observatory on two occasions, once while it was near its closest
approach to Earth as it neared perihelion, and another while it was further
from Earth post-perihelion. Observations of the J=2-1 and J=3-2 rotational
transitions of 12CO, typically strong lines in comets and indicative of gas
production, yielded no detection. Upper limits on the 12CO production of 1.8e28
molecules/s and 7.6e28 molecules/s for Phaethon were determined on these two
occasions.
