How do solar sails work, and have they flown?

What a solar sail is and why it matters

This article covers solar sails, large reflective sheets meant to change a spacecraft's speed or orbit using the pressure of sunlight rather than fuel. The Japan Aerospace Exploration Agency (JAXA) calls the effect photon acceleration, and in July 2010 it reported confirming it with its IKAROS spacecraft (JAXA, 2010).

The appeal is that a sail spacecraft would not need to carry propellant for every manoeuvre. NASA says a sail can in principle operate indefinitely, limited by how long its materials and electronics last in space (NASA, ACS3). That is a design argument, not a flight result. The push is small: JAXA gave IKAROS's thrust from sunlight as 1.12 millinewtons (JAXA, 2010), and NASA notes that because solar radiation pressure is small, a sail must be large to produce useful thrust (NASA, ACS3).

How it works

JAXA defines photon acceleration as accelerating an object by the pressure of sunlight on it. In its analysis, the push depends on the sail's effective area and on how much light it reflects like a mirror (JAXA, 2010). JAXA estimated these two values for IKAROS in flight and reports that light pressure deforms the sail in orbit, so the estimates scatter around the design values (JAXA, 2010).

The sail also has to unfold correctly in space. IKAROS used the spin of the spacecraft body to deploy and hold its membrane taut, in two stages, without supporting booms (JAXA project). NASA's ACS3 instead uses four booms that unroll from a small spacecraft and span the diagonals of a square sail (NASA, ACS3).

What has been demonstrated

IKAROS (JAXA), confirmed in flight in 2010. IKAROS launched on 21 May 2010 (Japan Standard Time) (JAXA, 2010). JAXA's project page gives its sail as a square 14 metres per side, made of polyimide resin about 0.0075 mm thick, on a spacecraft of about 310 kg at liftoff (JAXA project).

In a press release dated 9 July 2010, JAXA said it had confirmed photon acceleration while determining the spacecraft's orbit after sail deployment. Doppler measurements of the spacecraft's velocity departed from calculated values that left out photon acceleration, starting after the second-stage deployment in June 2010 (JAXA, 2010). JAXA gave the thrust as 1.12 millinewtons, which it said was the expected value. That figure is an estimate derived from orbit determination rather than a direct force measurement (JAXA, 2010). Ground data were lost for about one hour around 9:36 UT while the second-stage deployment was under way (JAXA, 2010).

JAXA's most recent update on the project page, dated 29 May 2015, says IKAROS had entered hibernation mode for the fifth time, as expected (JAXA project).

Ground tests of sail material. A June 2025 master's thesis from Cal Poly by Hannah Rost tested single-sided aluminised Mylar film against atomic oxygen, which is present in low Earth orbit. It ran 49 atomic oxygen tests, and two tests that added vacuum ultraviolet light, and saw no additional deterioration from the ultraviolet at the exposure levels tested (Cal Poly thesis). The thesis concerns drag sails used to bring satellites down from low orbit, not sails for propulsion, and these are ground tests, not flight data.

What is only reported

This section covers agency statements and proposals that published flight results do not yet confirm.

ACS3 (NASA). NASA's mission page says ACS3 launched on 23 April 2024 on a Rocket Lab Electron rocket. Its square sail is about 9 metres per side, held by four booms about 7 metres long made of a flexible polymer reinforced with carbon fibre (NASA, ACS3). NASA says the booms are 75% lighter, and designed to distort 100 times less with heat, than previously flown metallic deployable booms (NASA, ACS3). These are NASA's own figures. One of the mission's stated objectives is to characterise the sail's thrust as the spacecraft gradually changes orbit, but the page does not report a measured thrust or orbit change. When we checked it on 23 October 2026, the page described the sail as currently deployed, with onboard cameras still imaging its shape (NASA, ACS3).

NEA Scout (NASA). NEA Scout launched on 16 November 2022 as a secondary payload on Artemis I. NASA's page says it "was to be propelled by a solar sail" to photograph a near-Earth asteroid, but that after launch the project team could not communicate with the spacecraft (NASA, NEA Scout). Its sail propulsion was never demonstrated.

Breakthrough Starshot. Breakthrough Initiatives proposes gram-scale spacecraft with sails a metre or so across and a few hundred atoms thick, pushed by a ground-based laser array (a "light beamer") about a kilometre across. The concept page says the proposed light propulsion system is on a scale "significantly exceeding any currently operational analog" (Breakthrough Initiatives). This is a proposal, not a test.

What is speculative

None of the sources we opened shows a sail reaching interstellar speeds. Starshot's aim of sending craft to Alpha Centauri is a long-range goal: the page gives no dates and says only that the research and engineering phase is expected to last a number of years (Breakthrough Initiatives). NASA lists possible future uses for larger composite-boom sails, such as space weather early warning satellites, near-Earth asteroid reconnaissance and communications relays (NASA, ACS3). These are possibilities, not planned missions in the sources we read.

Limits and open problems

• Thrust. The IKAROS estimate of 1.12 millinewtons is very small, so any change in speed or orbit builds slowly (JAXA, 2010).
• Shape. Light pressure deforms the sail in orbit, so its real performance drifts from design values (JAXA, 2010).
• Deployment and operations. IKAROS lost ground data for about an hour during deployment (JAXA, 2010). NEA Scout's team could not communicate with the spacecraft after launch (NASA, NEA Scout).
• Mass. NASA's figure of 75% lighter booms for ACS3 is the agency's own statement, not an independent measurement (NASA, ACS3).
• Materials. The Cal Poly thesis notes that atomic oxygen in low Earth orbit degrades thin sail membranes (Cal Poly thesis). We did not find long-term in-orbit data on membrane wear in the sources we opened.
• Control. JAXA's project page says the IKAROS membrane carries an attitude control device (JAXA project), but the sources we opened give no data on how well sails can be steered in practice.

What to watch next

• A published thrust or orbit-change result from ACS3. NASA's page lists this as a mission objective but, as of 23 October 2026, reports no result (NASA, ACS3).
• Any published flight measurements of sail thrust or membrane condition. These would move today's reported claims into the demonstrated column.