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Solar Sailing

Short answer: Light has no mass but does carry momentum. When photons bounce off a mirror-like sail, they push it twice: once when they arrive and again when they reflect.

The force (solar radiation pressure) is tiny, so sails have to be huge and spacecraft light. In the vacuum of space, though, the push never stops. Japan's IKAROS first flew a solar sail between planets in 2010. The Planetary Society's LightSail 2 showed a CubeSat could raise its orbit with sunlight, and NASA's ACS3 (launched April 23, 2024) is testing lightweight carbon-fiber booms for an 80 m² sail. By tilting the sail, a craft can speed up, slow down or 'tack' to change its orbit.

VideoWrite-up5 sources

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What should you know about Solar Sailing?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. Photons have no mass but carry momentum, so a mirror-like sail gets pushed twice: once when light hits it and again when it bounces off.

    DD-039-F01 · Write-up · Exhibit A (Massless but pushy)

  2. LightSail 2's sunlight push was only about 0.058 mm/s², but after a month that adds up to about 550 km/h.

    DD-039-F02 · Write-up · Exhibit C (Tiny but relentless)

Video transcript: Solar Sailing: This Spacecraft Is Pushed by Sunlight Alone

DID YOU KNOW? Solar sails are a method of spacecraft propulsion that use large, reflective sheets to harness the momentum of photons streaming from the sun. Although each photon has no mass, its tiny push of momentum accumulates over vast areas and time, gradually accelerating the spacecraft without the need for fuel. NOW YOU KNOW!

Burned-in captions, read frame by frame (see transcript.md) · 17.8s Reel ·

Video coming soonSolar Sailing · Instagram Reel
Solar Sailing: This Spacecraft Is Pushed by Sunlight Alone Instagram Reel · · 17.8s

The write-up

This spacecraft is pushed by sunlight alone. Photons have no mass, but they carry momentum. A big mirror adds it up.

  1. Exhibit A

    Massless but pushy

    Photons have no mass, but they carry momentum. When they hit a mirror-like sail they push it once on arrival and again as they bounce off.

  2. Exhibit B

    Light, not wind

    The push comes from solar radiation pressure, not the solar wind. The solar wind's charged particles push with less than one percent as much force, roughly a thousandth.

  3. Exhibit C

    Tiny but relentless

    LightSail 2's 32 m² sail gave it an acceleration of about 0.058 mm/s². That's tiny, but over a month it adds up to about 550 km/h (341 mph).

  4. Exhibit D

    Steering by tilting

    By angling the sail, a spacecraft can tack like a sailboat, or turn it against its direction of travel to slow down and lower its orbit. ACS3 tests both raising and lowering its orbit.

  5. Exhibit E

    Pioneers

    JAXA's 14-m IKAROS, launched in 2010, was the state of the art in solar sailing. LightSail 2 later showed that small spacecraft can change their orbits with sunlight.

  6. Exhibit F

    ACS3

    NASA's ACS3 launched on April 23, 2024. Four carbon-fiber composite booms unroll from a CubeSat and stretch a sail about 9 m (30 ft) per side, roughly 80 m² in total.

Questions people ask about Solar Sailing

What pushes a solar sail?

Photons carry momentum. The solar wind's push is less than one percent as strong.

Source: Write-up · Exhibit A (Massless but pushy) · What is solar sailing? (The Planetary Society); How do photons propel solar sails? (Astronomy, 2020)

Why does a reflective sail get a bigger push than a black one?

Light pushes once on arrival and again on reflection.

Source: Write-up · Exhibit A (Massless but pushy) · What is solar sailing? (The Planetary Society); How do photons propel solar sails? (Astronomy, 2020)

Which agency flew the IKAROS solar sail in 2010?

JAXA's 14-m IKAROS was the state of the art in solar sailing.

Source: Write-up · Exhibit E (Pioneers) · Wilkie et al., Composite Solar Sail (ACS3) Technology overview (NASA Technical Reports Server, 2021); ACS3, NASA's Advanced Composite Solar Sail System (The Planetary Society)

How can a solar sail slow down?

Tilting the sail lets it tack like a sailboat, including lowering its orbit.

Source: Write-up · Exhibit D (Steering by tilting) · What is solar sailing? (The Planetary Society); Wilkie et al., Composite Solar Sail (ACS3) Technology overview (NASA Technical Reports Server, 2021)

What are ACS3's sail booms made of?

Carbon-fiber-reinforced polymer booms are light, stiff and resist warping.

Source: Write-up · Exhibit F (ACS3) · Advanced Composite Solar Sail System (ACS3) (NASA); ACS3, NASA's Advanced Composite Solar Sail System (The Planetary Society)

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. What pushes a solar sail?

    1. The solar wind
    2. The momentum of sunlight
    3. Magnetic fields
    4. A small rocket
    Show the answer

    Answer: B. The momentum of sunlight Photons carry momentum. The solar wind's push is less than one percent as strong. Source: Write-up · Exhibit A (Massless but pushy) · What is solar sailing? (The Planetary Society); How do photons propel solar sails? (Astronomy, 2020)

  2. Why does a reflective sail get a bigger push than a black one?

    1. It's lighter
    2. Photons push again when they bounce off
    3. It's hotter
    4. It's magnetic
    Show the answer

    Answer: B. Photons push again when they bounce off Light pushes once on arrival and again on reflection. Source: Write-up · Exhibit A (Massless but pushy) · What is solar sailing? (The Planetary Society); How do photons propel solar sails? (Astronomy, 2020)

  3. Which agency flew the IKAROS solar sail in 2010?

    1. NASA
    2. ESA
    3. JAXA
    4. ISRO
    Show the answer

    Answer: C. JAXA JAXA's 14-m IKAROS was the state of the art in solar sailing. Source: Write-up · Exhibit E (Pioneers) · Wilkie et al., Composite Solar Sail (ACS3) Technology overview (NASA Technical Reports Server, 2021); ACS3, NASA's Advanced Composite Solar Sail System (The Planetary Society)

  4. How can a solar sail slow down?

    1. It can't
    2. By angling the sail against its direction of travel
    3. By folding up
    4. By firing thrusters
    Show the answer

    Answer: B. By angling the sail against its direction of travel Tilting the sail lets it tack like a sailboat, including lowering its orbit. Source: Write-up · Exhibit D (Steering by tilting) · What is solar sailing? (The Planetary Society); Wilkie et al., Composite Solar Sail (ACS3) Technology overview (NASA Technical Reports Server, 2021)

  5. What are ACS3's sail booms made of?

    1. Steel
    2. Aluminum
    3. Carbon-fiber composite
    4. Wood
    Show the answer

    Answer: C. Carbon-fiber composite Carbon-fiber-reinforced polymer booms are light, stiff and resist warping. Source: Write-up · Exhibit F (ACS3) · Advanced Composite Solar Sail System (ACS3) (NASA); ACS3, NASA's Advanced Composite Solar Sail System (The Planetary Society)

Cite this case file

Easterling, Marcus. "Solar Sailing." The Detail Department (DetailEnjoyer), case file DD-039, last updated Sep 27, 2025. https://detailenjoyer.com/case/solar-sailing/

Permalink: https://detailenjoyer.com/case/solar-sailing/ · Last updated

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