The 3-carbon ring molecule C3H2 is classified chemically as a carbene and is highly reactive in the laboratory. It was the first cyclic molecular species detected in interstellar molecular clouds. Immediately following its identification at short radio wavelengths, it was discovered to be nearly ubiquitous in the interstellar medium (Matthews and Irvine 1985), and it has subsequently also been found in the envelope of evolved carbon-rich stars (evolved stars are those which have completed their main hydrogen fusion stage and are thus nearing the end of their energy-producing lifetimes). The linear isomer of C3H2 is also detected in molecular clouds, but its abundance is typically an order of magnitude lower than that of the cyclic isomer (Cernicharo et al. 1991). In diffuse clouds, the abundance ratio between the linear and the cyclic isomers increases by a factor 10 with respect to that observed in dense molecular clouds (Cernicharo et al. 1999). Both...