What acts as the biggest competitor to seeing the Milky Way from the ISS, overriding even the Sun?

The Earth itself acting as a massive, illuminated object

Counterintuitively, the most significant obstacle to viewing the faint structures of the Milky Way from the ISS is not a source external to the Earth system, but the Earth itself when illuminated by the Sun. While terrestrial observers contend with light pollution or natural airglow, an astronaut in orbit faces the planet's surface scattering sunlight across its own atmosphere and limb. This creates a bright arc that occupies half of the visual field, effectively washing out fainter celestial objects. Therefore, the biggest challenge becomes avoiding the glare produced by the sunlit Earth, demanding a specific directional viewing angle rather than just waiting for nighttime.