The erosive effect of ultraviolet radiation and atomic oxygen at the lower earth orbit space environment of the International Space Station on Epon 862 based epoxy composites with 12 and 100 nm silica particles was investigated. Although exposure to ultraviolet radiation had a small effect on surface erosion, restricted to 2 µm of the top surface, concurrent exposure to ultraviolet radiation and atomic oxygen resulted in significant erosion. Atomic oxygen erosion of nanocomposites with 1–5 wt% silica particles resulted in a carpet-like residual surface layer whose thickness and morphology were dependent on the size and concentration of the embedded silica particles. The eroded surface of the control epoxy had high surface roughness in the form of 10–40 µm long conical protrusions. With the addition of silica particles, the residual surface layer became fibrous and rich in silica particles, and its density increased with the weight fraction and size of the silica particles. The large and uneven erosion depth of samples exposed to atomic oxygen and ultraviolet radiation resulted in a surface damage layer with average thickness between 5 and 100 µm with significantly reduced mechanical properties compared to the surface of the as-fabricated nanocomposites. The erosion yield of the control epoxy due to atomic oxygen was 4.36 × 10−24 cm3/atom and the addition of silica nanoparticles reduced it significantly to 1.78 × 10−25 cm3/atom. In particular, silica nanoparticles of diameter 12 and 100 nm and weight fraction 5% reduced the erosion yield of the control epoxy by 90% and 96%, respectively.