$Diffractive solar sail concept art$ A diffractive solar sail, or diffractive lightsail, is a type of

solar sail Solar sails (also known as lightsails, light sails, and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large surfaces. A number of spaceflight missions to test solar propulsion and navigati ...

which relies on

diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...

instead of reflection for its

propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...

. Current diffractive sail designs use thin

metamaterial A metamaterial (from the Greek word μετά ''meta'', meaning "beyond" or "after", and the Latin word ''materia'', meaning "matter" or "material") is a type of material engineered to have a property, typically rarely observed in naturally occu ...

films, containing micrometer-size

grating A grating is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings usually consist of a single set of elongated elements, but can consist of two sets, in which case the second set is usually perpendicu ...

s based on polarization or subwavelength refractive structures, causing

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

to spread out (i.e. diffract) and thereby exert

radiation pressure Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of ...

when it passes through them.

History

The idea of using diffraction for a solar sail was first proposed in 2017 by researchers at the

Rochester Institute of Technology The Rochester Institute of Technology (RIT) is a private university, private research university in Henrietta, New York, a suburb of Rochester, New York, Rochester. It was founded in 1829. It is one of only two institute of technology, institut ...

. This was enabled in part by advances in material design and fabrication (particularly of gratings), and

optoelectronic Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radia ...

control. In 2019 a diffractive solar sail project from the Rochester Institute of Technology suggested a

solar Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. "solar panels") ** Solar calendar, a calendar whose dates indicate t ...

polar orbit A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of abo ...

mission with diffractive sails that could reach a higher solar inclination angle and smaller orbital radius than one with reflective sails, reaching

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

's NIAC phase II. In 2022 the NIAC project reached phase III and gained US$2 million of support from NASA, with involvement of researchers from both

Johns Hopkins University The Johns Hopkins University (often abbreviated as Johns Hopkins, Hopkins, or JHU) is a private university, private research university in Baltimore, Maryland, United States. Founded in 1876 based on the European research institution model, J ...

and the Rochester Institute of Technology.

Advantages over reflective sails

Reflective solar sail designs tend to consist of large, thin reflective sheets. By the

law of reflection Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray of light emerges from the reflecting surface at the same angle to the surf ...

, the forces acting on them will always be normal to the sheet surface; the sheets must therefore be tilted during navigation, which poses structure and control challenges, and reduces the power reaching the sail. These in turn can lower reliability, increase mass, and reduce acceleration. Furthermore, reflective sails tend to absorb a reasonable proportion of the light hitting them, causing them to heat up; this can cause structural problems, particularly when the sail is repeatedly heated and then allowed to cool. Also, each

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

hitting the sail is used once, i.e. it's either reflected or absorbed. On the other hand, in a diffractive sail the grating can redirect light even when the sheet directly faces the sun, allowing much more efficient control with maximum power hitting the sail. The diffractive film can be designed to allow for

control of the gratings, thereby reducing mass and increasing reliability relative to

mechanical Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations o ...

control. Since the film is translucent, most of the light just passes through the sail, reducing overall heating. Photons can be reused: either by passing through a second diffraction grating for more thrust, or by going to a

solar cell A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.

to provide

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

References

{{spacecraft propulsion Spacecraft attitude control Spacecraft propulsion Spacecraft components Interstellar travel Optoelectronics Photonics