Radiation Pressure
Solar sail spacecraft. Builds on Traveling EM Waves.
NASA engineers propose a spacecraft with a huge, ultra-thin mirror as a ‘solar sail.’ The sail has no fuel, no engine, no propellant. It just unfurls a giant reflective sheet facing the Sun, millions of kilometers from any planet.
Can sunlight alone actually push this spacecraft and accelerate it through space?
Yes! Light carries momentum even though it has no mass. Each photon hitting the sail transfers momentum p = E/c. A reflective sail gets DOUBLE the push compared to an absorbing one, because the photon’s momentum reverses direction. The force is tiny but continuous — and in the vacuum of space with no friction, the spacecraft accelerates endlessly!
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Watch photons bounce off the solar sail and push it forward. The continuous stream of light creates a small but relentless force that accelerates the craft.
P = 2I/c (reflection)
Reflection doubles the momentum transfer compared to absorption. A perfect mirror gets twice the push of a black surface!
Comets have two tails: one from the solar wind (ions) and one from radiation pressure pushing dust away from the Sun. The dust tail always points away from the Sun.
Japan’s IKAROS (2010) was the first spacecraft to demonstrate solar sailing. The Planetary Society’s LightSail 2 raised its orbit using only sunlight.
Focused laser beams can trap and move individual cells and molecules. Arthur Ashkin won the 2018 Nobel Prize for this application of radiation pressure.
Inside stars, radiation pressure from nuclear fusion fights against gravitational collapse. This balance determines a star’s size and lifespan.
“A beam of light has no mass, yet it pushes. This tiny force shaped comet tails, may propel humanity to the stars, and holds up the Sun itself.”
Solar Sail Lab
Design your own solar sail spacecraft. Adjust the sail area, reflectivity, and light intensity. Watch the force build up and see the trajectory unfold over time. How long does it take to reach Mars?
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Start with a 100 m² sail at Earth’s distance from the Sun (intensity ~1361 W/m²). Note the tiny force. Now increase sail area to 10,000 m² (100m × 100m) — this is realistic for future missions. Compare a perfectly reflective sail vs a black absorbing sail. Toggle between them and watch the force double!
Radiation pressure is incredibly small by everyday standards (about 9 μN/m² at Earth). But in the frictionless vacuum of space, this force acts continuously, building velocity day after day. Solar sails could reach speeds impossible for chemical rockets.