Context : Comet 8P/Tuttle is a Nearly Isotropic Comet ( NIC ) , whose physical properties are poorly known and could be different from those of Ecliptic Comets ( EC ) owing to their different origin .
Two independent observations have shown that 8P/Tuttle has a bilobate nucleus .

Aims : Our goal is to determine the physical properties of the nucleus ( size , shape , thermal inertia , albedo ) and coma ( water and dust ) of 8P/Tuttle .

Methods : We observed the inner coma of 8P/Tuttle with the infrared spectrograph ( IRS ) and the infrared camera ( MIPS ) of the Spitzer Space Telescope ( SST ) .
We obtained one spectrum ( 5 – 40 \mu m ) on 2 November 2007 and a set of 19 images at 24 \mu m on 22 – 23 June 2008 sampling the nucleus rotational period .
The data were interpreted using thermal models for the nucleus and the dust coma , and considering two possible shape models of the nucleus derived from respectively Hubble Space Telescope visible and Arecibo radar observations .

Results : We favor a nucleus shape model composed of two contact spheres with respective radii of 2.7 \pm 0.1 km and 1.1 \pm 0.1 km and a pole orientation with RA = 285 \pm 12 ^ { \circ } and DEC = +20 \pm 5 ^ { \circ } .
The nucleus has a thermal inertia lying in the range 0 – 100 J K ^ { -1 } m ^ { -2 } s ^ { -1 / 2 } and a R -band geometric albedo of 0.042 \pm 0.008 .
The water production rate amounts to 1.1 \pm 0.2 \times 10 ^ { 28 } molecules s ^ { -1 } at 1.6 AU from the Sun pre-perihelion , which corresponds to an active fraction of \approx 9 % .
At the same distance , the \epsilon f \rho quantity amounts to 310 \pm 34 cm at 1.6 AU , and reaches 325 \pm 36 cm at 2.2 AU post-perihelion .
The dust grain temperature is estimated to 258 \pm 10 K , which is 37 K larger than the thermal equilibrium temperature at 1.6 AU .
This indicates that the dust grains contributing to the thermal infrared flux have a typical size of \approx 10 \mu m. The dust spectrum exhibits broad emissions around 10 \mu m ( 1.5- \sigma confidence level ) and 18 \mu m ( 5- \sigma confidence level ) that we attribute to amorphous pyroxene .

Conclusions :