Complex sentence: The Sun gains a very small mass through the impact of asteroids and comets; however the Sun already holds 99.86% of the Solar System's total mass, so these impacts cannot offset the mass lost by radiation and ejection.
Split into simpler sentences: The Sun gains a very small mass through the impact of asteroids and comets. However, the Sun already contains 99.86% of the Solar System's total mass, so these impacts cannot offset the mass lost by radiation and ejection.