The classic rainbow, the primary arc caused by sunlight refracting through dispersed water droplets, cannot typically be seen from orbital space. A rainbow is not a physical object but an optical illusion that requires a precise geometric setup involving the Sun, water particles, and an observer. This necessary alignment is almost always absent when viewing Earth from an orbiting spacecraft.
The Physics of Rainbow Formation
The appearance of a rainbow depends on three specific components: direct sunlight, a collection of water droplets, and the observer’s position relative to the light source. White sunlight enters a spherical raindrop, and the light is slowed and bent, a process known as refraction. This initial bending separates the white light into its constituent colors because each wavelength bends at a slightly different angle.
After entering the droplet, the light reflects off the back interior surface before being refracted a second time as it exits toward the observer. This double-refraction and single-reflection sequence concentrates the separated colors of light at a very specific angle. The most intense light of the primary rainbow is returned to the viewer along a cone approximately 42 degrees wide relative to the anti-solar point.
The anti-solar point is the spot directly opposite the Sun, meaning the sun must always be at the observer’s back for a rainbow to be visible. The circular shape of the rainbow results from the symmetry of this 42-degree cone of light. All water droplets positioned along the edge of this cone contribute light to the visible arc.
Why Space Breaks the Viewing Geometry
The fundamental barrier to seeing a typical rainbow from space is the disruption of this required viewing geometry. For an observer on the ground, the sun is at their back, and the water-filled rain clouds are in front of them, allowing the 42-degree cone of light to reach their eyes. In orbital space, such as on the International Space Station, the observer is looking down at the Earth’s atmosphere.
The vast majority of the water vapor and liquid droplets necessary for rainbow formation are concentrated in the lower troposphere, far below the observer. To see a rainbow, the astronaut would need the sun at their back and the water droplets in front, forming the required 42-degree angle with the incoming sunlight. When looking down, the observer’s line of sight is generally perpendicular to the plane where the rainbow-forming droplets exist. This geometric alignment, which requires the observer to be positioned between the light source and the water particles, becomes impossible from orbit.
High-Altitude Atmospheric Phenomena
While the classic rainbow is geometrically impossible to see from orbital space, astronauts and high-altitude pilots can observe other related optical effects. One such phenomenon is the full-circle rainbow, which is physically possible from an aircraft or high mountain. When viewed from above, the ground no longer blocks the lower half of the rainbow, revealing its true circular shape.
Another optical effect visible from high altitudes is the glory, often seen centered around the shadow of an aircraft or observer on clouds below. Unlike the rainbow, which is caused by a single internal reflection, the glory is a much smaller series of concentric, colored rings caused by the complex backscattering of sunlight by very tiny cloud droplets. The light rays are scattered back directly toward the source, making the glory appear around the anti-solar point. Astronauts have reported seeing circular glories, confirming that these distinct phenomena can be observed from space when looking down at the cloud tops.