Radiation Pressure: Problem Solving

Radiation pressure equals the solar radiation intensity divided by the propagation speed of electromagnetic waves. For a satellite 1.5 × 10¹¹ meters from the sun, the solar intensity is 1.3 × 10³ watts per square meter. Dividing by the speed of light yields a pressure of 4.3 × 10⁻⁶ newtons per square meter on panels that completely absorb the incident sunlight.

Radiation pressure arises from momentum transfer when an electromagnetic wave is absorbed by a surface. The rate at which momentum transfers to the absorbing surface perpendicular to the wave's propagation direction equals the force on that surface. Dividing this force by the absorbing area gives the average radiation pressure, directly linking momentum and radiation pressure.

When a perfectly reflecting surface absorbs radiation along the normal direction, the momentum direction reverses upon reflection, doubling the radiation pressure compared to complete absorption. This doubling occurs because the momentum change is twice as large when the wave bounces back rather than being absorbed, creating stronger force on reflective surfaces.

Solar radiation intensity equals the sun's average power output divided by the area of a sphere at the satellite's distance. With the sun radiating 3.8 × 10²⁶ watts and the satellite at 1.5 × 10¹¹ meters away, the intensity is 1.3 × 10³ watts per square meter. This intensity value is essential for determining radiation pressure on solar collecting panels.

Comet dust tails are displaced by radiation pressure from sunlight. Although this pressure is small, it is strong enough to push electrically neutral dust particles away from the comet's path. Unlike the ion tail, which responds to the solar wind, the dust tail is affected solely by the radiation pressure produced by the sun's light.

In massive, bright stars, radiation pressure inside the star becomes extremely high and significantly increases internal gas pressure. This elevated pressure counteracts the gravitational force pulling inward, preventing the star from collapsing under its own gravity. Radiation pressure thus plays a critical role in maintaining stellar stability and structure.

Radiation force equals radiation pressure multiplied by the panel area. With a pressure of 4.3 × 10⁻⁶ newtons per square meter and a 20-square-meter panel, the resulting force is 8.6 × 10⁻⁵ newtons. Though small, this force is significant enough to affect satellite trajectories and orbital dynamics over time.