21 January 2021

One of the puzzling laws of nature is that the basic units of light — photons — are both waves and particles and though they have no mass they exert pressure on the objects they hit. On Earth this pressure is so tiny that it goes unnoticed but in outer space it is the main force other than gravity and has a large cumulative effect over time. It is called radiation pressure or the force of light.

Spacecraft engineers design for radiation pressure because it affects how a spacecraft moves. For instance, when light is reflected off an object in outer space it picks up momentum from the force of light and moves away from it. This diagram shows the change in momentum where light (dark blue color) is reflected.

Just over seven years ago during the Move An Asteroid Competition MIT suggested using radiation pressure to protect Earth from dangerous asteroids. How? Paint the asteroids white to reflect more light. Check out the plan in this vintage article: Paintballs To The Rescue.

(images from Wikimedia Commons; click on the captions to see the originals)