solseil

Solseil is a term used in Danish, Norwegian, and other Germanic languages to denote a solar sail, a concept in spacecraft propulsion that uses the pressure of sunlight on a large, reflective membrane to generate thrust. The word derives from sol (sun) and seil or segel (sail).

A solar sail consists of a very thin, highly reflective film deployed on lightweight booms. Photons from the Sun reflect off the surface, imparting momentum and producing continuous thrust without onboard propellant. At roughly 1 astronomical unit from the Sun, the radiation pressure is about 9 micro-newtons per square meter; the resulting acceleration depends on the sail’s areal density (mass per unit area). With areal densities on the order of 5–20 g/m^2, accelerations are typically in the tenths to a few millimeters per second squared, enabling gradual but sustained velocity change over days, weeks, or years.

Historically, the idea dates to early discussions of light-based propulsion, with experimental demonstrations accumulating in the late 20th and early 21st centuries. Notable missions include JAXA’s IKAROS (2010), the Planetary Society’s LightSail demonstrations (2015, 2019), and ongoing research into large-scale, deep-space missions. Breakthrough Starshot outlines the use of very small, lightweight sails driven by high-power lasers for interstellar propulsion, though this remains exploratory.

Solseil technology faces challenges in deployment reliability, material aging, debris protection, and precise attitude control. Nevertheless, solar sails offer a propellant-free means of long-duration propulsion for satellite repositioning, deorbiting, and deep-space exploration.

See also: solar sail.