The influence of environmental conditions on solid fuel ignition is of particular interest in spacecraft fire safety because of the large difference in environments between a spacecraft and earth (low gravity, low gas flow velocities, low pressure, elevated oxygen concentration). Considering that fire safety is essential when dealing with spacecraft vehicles, where space is confined and egress is difficult or almost impossible, low gravity fire initiation has a prominent importance. In addition to microgravity, low cabin pressure may further decrease the convective heat losses from the solid, leading to a faster heating of the materials and therefore raising the fire hazard on board. A numerical model developed with the CFD code Fire Dynamics Simulator (FDS) was used to analyze the effect of reduced gravity and ambient pressure on the transport processes taking place in the piloted ignition of an externally irradiated solid fuel. The model simultaneously solves the gas phase and solid phase conservation equations, using a one-step second order Arrhenius reaction rate for the gas phase kinetics and a one-step global Arrhenius reaction rate for the solid phase decomposition. The transition from an incipient premixed reaction at the pilot to the establishment of a self-sustained diffusion flame anchored on the solid fuel surface is analyzed and described in detail and compared for several cases of reduced pressure and gravity. The influence of these parameters on the ignition delay time and the mass flux at ignition is also calculated and compared to experiments at 1g for a range of sub-atmospheric pressures. The results show that reduced pressure and reduced gravity have similar effects on the piloted ignition of a solid fuel in low velocity flows, indicating that heating and pyrolysis of the solid are the primary mechanisms in the process. The results of this work may guide in the selection of materials in future space exploration vehicles and indicate when microgravity testing may be substituted by reduced ambient pressure conditions to analyze their ignition properties.