  Here we present high cadence photometry taken by the Acquisition Camera on
Gemini South, of a close passage by the $\sim540$ km radius Kuiper Belt Object,
(50000) Quaoar, of a r'=20.2 background star. Observations before and after the
event show that the apparent impact parameter of the event was
$0.019\pm0.004$", corresponding to a close approach of $580\pm120$ km to the
centre of Quaoar. No signatures of occultation by either Quaoar's limb or its
potential atmosphere are detectable in the relative photometry of Quaoar and
the target star, which were unresolved during closest approach. From this
photometry we are able to put constraints on any potential atmosphere Quaoar
might have. Using a Markov chain Monte Carlo and likelihood approach, we place
pressure upper limits on sublimation supported, isothermal atmospheres of pure
N$_2$, CO, and CH$_4$. For N$_2$ and CO, the upper limit surface pressures are
1 and 0.7 $\mu{bar}$ respectively. The surface temperature required for such
low sublimation pressures is $\sim33$ K, much lower than Quaoar's mean
temperature of $\sim44$ K measured by others. We conclude that Quaoar cannot
have an isothermal N$_2$ or CO atmosphere. We cannot eliminate the possibility
of a CH$_4$ atmosphere, but place upper surface pressure and mean temperature
limits of $\sim138$ nbar and $\sim44$ K respectively.

Earth and Planetary Astrophysics