Acetylene is produced in great abundance in the solar system wherever carbonaceous solids are exposed to energetic radiation. It has been the subject of several studies, revealing a cubic-orthorhombic displacive transition at 148 K. However, there has been no systematic study of structural trends over the full temperature range of stability. We propose to measure diffraction data from 4.2 K to 180 K suitable for full structure refinement, allowing us to determine very precisely the unit-cell dimensions (better than 1 part in 10^5), and to observe in detail the evolution of the orthorhombic strain below the transition to the cubic high-temperature phase. This work will provide complementary neutron powder refinements of both phases of acetylene, and form the basis for a future crystallographic analysis of the acetylene dimer, solid diacetylene (C4H2).