# Solar Sailing (DD-039)

> 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.

- Department: Space (also Tech)
- First published: 2025-09-27
- Last updated: 2025-09-27
- Permalink: https://detailenjoyer.com/case/solar-sailing/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## On the timeline

- **2010**: Japan's IKAROS flies a solar sail between planets
- **April 23, 2024**: NASA launches ACS3 to test lightweight carbon-fiber sail booms

Full timeline: https://detailenjoyer.com/timeline/?case=solar-sailing

## Quick facts

- **DD-039-F01**: 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. _(Write-up · Exhibit A (Massless but pushy))_
- **DD-039-F02**: LightSail 2's sunlight push was only about 0.058 mm/s², but after a month that adds up to about 550 km/h. _(Write-up · Exhibit C (Tiny but relentless))_

## Write-up

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

1. **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. **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. **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. **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. **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. **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.

## Related case files

- [Solar Prominence (DD-038)](https://detailenjoyer.com/case/solar-prominence/): What else the sun throws at spacecraft.
- [Stellar Birth (DD-037)](https://detailenjoyer.com/case/stellar-birth/): Where starlight comes from.
- [Gravity, or Lunavity? (DD-007)](https://detailenjoyer.com/case/lunavity/): Another trick for fighting gravity.
- [Photon Film (DD-067)](https://detailenjoyer.com/case/photon-film/): More light at work: a camera fast enough to rebuild a laser pulse in flight.
- [Alcubierre Drive (DD-060)](https://detailenjoyer.com/case/alcubierre-drive/): The speculative alternative: a warp drive Einstein's equations allow but no one can build.
- [Airborne Wind Turbines (DD-050)](https://detailenjoyer.com/case/airborne-wind-turbines/): Another craft that puts a natural force high above the ground to work.
- [Self-Made Modular (DD-061)](https://detailenjoyer.com/case/self-made-modular/): More technology built to work in space.
- [Praise the Sun! (DD-475)](https://detailenjoyer.com/case/praise-the-sun/): I linked more on sunlight in space
- [Spacewalker (DD-652)](https://detailenjoyer.com/case/spacewalker/): Another way to move a spacecraft without a rocket engine

## Reel vs. record

- **Confirmed**: 'each photon has no mass' but carries momentum, and reflective sails turn that into thrust. [Planetary Society; Astronomy]
- **Incomplete**: 'gradually accelerating the spacecraft.' Sails can also slow a craft down and lower its orbit by angling the sail against its direction of travel, and ACS3 tests both. [Planetary Society; NASA NTRS]
- **Caveat**: 'without the need for fuel' is true for propulsion (no rocket propellant), but the push weakens with distance from the sun. [NASA ACS3; Astronomy]
- **Clarification**: the push comes from sunlight (radiation pressure), not the solar wind, which pushes with less than one percent as much force. [Planetary Society; NASA NTRS]
- **Footage note**: the animation shows a four-boom diamond sail unfolding from a small satellite over Earth. That's the layout of NASA's ACS3, but the Reel credits only 'galaxies' and doesn't name the mission. [frames; NASA ACS3]

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

- Duration: 17.8 s · Published: 2025-09-27
- File: https://detailenjoyer.com/assets/videos/DPHXVL9kRoZ.mp4

### Transcript

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!

## Questions & answers

**Q: What pushes a solar sail?**  
A: 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))_

**Q: Why does a reflective sail get a bigger push than a black one?**  
A: 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))_

**Q: Which agency flew the IKAROS solar sail in 2010?**  
A: 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))_

**Q: How can a solar sail slow down?**  
A: 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))_

**Q: What are ACS3's sail booms made of?**  
A: 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))_

## Sources

- [What is solar sailing? (The Planetary Society)](https://www.planetary.org/articles/what-is-solar-sailing)
- [Advanced Composite Solar Sail System (ACS3) (NASA)](https://www.nasa.gov/smallspacecraft/what-is-acs3/)
- [ACS3, NASA's Advanced Composite Solar Sail System (The Planetary Society)](https://www.planetary.org/space-missions/acs3)
- [Wilkie et al., Composite Solar Sail (ACS3) Technology overview (NASA Technical Reports Server, 2021)](https://ntrs.nasa.gov/api/citations/20205011395/downloads/Wilkie_SciTech_2021_ACS3_overview_20201209_rev_0_for_review.pdf)
- [How do photons propel solar sails? (Astronomy, 2020)](https://www.astronomy.com/science/how-do-photons-propel-solar-sails/)

## 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/