2025ApJS..281...10C

We describe the SPHEREx Sky Simulator (henceforth the Simulator), a software tool designed to model science data for NASA's SPHEREx mission that will carry out a series of all-sky spectrophotometric surveys at ∼6″ spatial resolution in 102 spectral channels spanning 0.75-5 μm. The Simulator software implements models for astrophysical emission, instrument characteristics, and survey strategy to generate realistic infrared sky scenes as they will be observed by SPHEREx. The simulated data include a variety of realistic noise and systematic effects that are estimated using up-to-date astrophysical measurements and information from prelaunch instrument characterization campaigns. Through the preflight mission phases, the Simulator has been critical in predicting the impact of various effects on SPHEREx science performance and has played an important role in guiding the development of the SPHEREx data analysis pipeline. In this paper, we describe the Simulator architecture, preflight instrument, and sky models, and summarize high-level predictions from the Simulator, including a prelaunch prediction for the 5σ point source sensitivity of SPHEREx, which we estimate to be m_AB 18.5-19 from 0.75 to 3.8 μm and m_AB 16.6-18 from 3.8 to 5 μm, with the sensitivity limited by the zodiacal light background at all wavelengths. In the future, on-orbit data will be used to improve the Simulator, which will form the basis of a variety of forward-modeling tools that will be used to model myriad instrumental and astrophysical processes to characterize their systematic effects on our final data products and analyses.