Spitzer observations of `Oumuamua and `Oumuamua's density and shape

1I/`Oumuamua is the first confirmed interstellar body in our Solar System. Here we report on 4.5 micron observations of `Oumuamua made with the Spitzer Space Telescope on 2017 November 21-22. We did not detect the object and place an upper limit on its flux. This implies an effective spherical diameter upper limit and albedo lower limit, with a range of values that depends on what value of the thermal beaming parameter is used. We also place upper limits on the amount of dust, CO, and CO2 that are emitted from this object that are lower than previous results; we are unable to constrain the production of other gas species. We also present our estimate of `Oumuamua's density and aspect ratio (elongation). There is no description of the suite of `Oumuamua's physical properties that corresponds to any population of bodies in our Solar System. Our results extend the mystery about `Oumuamua's origin and evolution.