Breakthrough Starshot’s four-meter sails are the latest (and best funded) concept in a long series of beamed propulsion ideas. As Jim Benford explains below, the idea of beaming to a sail goes back over fifty years, with numerous papers and the beginnings of laboratory work in the intervening decades. What follows is the first cut at a timeline of this work, one that Jim intends to supplement and re-publish here with full references. Keeping in mind the scope of the timeline as Jim explains it, feel free to suggest any missing references in the comments. Discover Magazine, by the way, has just published a look at the Benfords’ work on beamed sails called “Riding on a Beam of Light,” now available for subscribers online.

by James Benford


From recent media pieces following the announcement of Breakthrough: Starshot, I gather that the press is not aware of how much has been done by the propulsion community over the last decades in the areas of photon beam-driven sail system concepts, to include experiments and simulations of sail beam-driven flight, sail stability (‘beam-riding’), and dynamics (such as beam-driven spin of sails for stability).

This is a brief history of the deep origins, significant events in the development of beam-driven sails for interstellar travel purposes. It excludes, for example, solar sails, launch to orbit, and mass beams. For those interested in mass beams, my thoughts on the subject, with references, appear in several earlier articles here [see, for example, Sails Driven by Diverging Neutral Particle Beams] as well as in a JBIS review article by Nordley and Crowl (“Mass Beam Propulsion, An Overview”, JBIS 68, pgs. 153-166, 2015).

First mention of beam-driven propulsion for interstellar purposes in the refereed literature was by Bob Forward, in Missiles and Rockets in 1962. More people perhaps read the Marx paper in Nature four years later, but Marx didn’t pursue his idea further, at least not in the literature.

But Bob did continue to work on it and built the foundations of the subject. This is described in his unfinished autobiography, Fast Forward Fifty Years.

My thoughts on the timeline of significant events, meaning ideas and actions that advanced the field and influenced following events, including recent milestones, follow below.


Image: Beamed propulsion leaves propellant behind, a key advantage. Coupled with very small probes, it could provide a path for flyby missions to the nearest stars. Credit: Adrian Mann.

The Visionary Era


  • 1962: First mention of beam-driven propulsion for interstellar purposes in the refereed literature, by Robert L. Forward, in “Pluto – Gateway to the Stars,” in Missiles and Rockets.
  • 1966: G. Marx publishes the concept for Interstellar Vehicle Propelled by Laser Beam’ in Nature. He considered only x-rays to drive the sail, because he flinched from large apertures for the antenna and sail.
  • 1967: Robert Forward and Eugene Mallove publish the first Interstellar Bibliography.

Image: Interstellar propulsion theorist Robert Forward.

  • 1974: Authors Larry Niven and Jerry Pournelle feature an interstellar laser-pushed lightsail in their science fiction novel The Mote in God’s Eye, basing their design on unpublished calculations by Robert Forward.
  • 1975: Richard Dickinson and Robert Brown demonstrate efficient transmission of 34 kW over 1.5 km.
  • 1976: Robert Forward proposes a roadmap for interstellar exploration with a launch to Alpha Centauri in 1995.
  • 1980: Bruce Murray, director of JPL, convenes a workshop to make a concept for an interstellar probe. Members include Freeman Dyson, Robert Forward, Robert Bussard, William Fowler and Robert Leighton. Dyson and Forward quantify Forward’s 1962 concept, including a means of returning a payload. Murray is disappointed by the large size of the sail and beamer, as well as the enormous power required. The Workshop papers are never published. Forward continues work on his concept.
  • 1982: Robert Forward proposes laser-driven return star missions in his science fiction novel Flight of the Dragonfly. The final expanded version, Rocheworld, with technical appendix, appears in 1990.
  • 1984: Forward presents “Roundtrip Interstellar Travel Using Laser-Pushed Sails,” the first detailed analysis showing the feasibility of the lightsail concept.


Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The ‘paralens’ in the image is a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus.

  • 1985: Forward, encouraged by conversation with Freeman Dyson, proposes microwave beam-driven interstellar flybys in “Starwisp: An Ultra-Light Interstellar Probe.” He also introduces the ‘smart sail’ concept whereby the sail contains microcircuits which capture images and diagnostic data, returns data to Earth.
  • 1986: Forward conducts first study of dielectric film sails.
  • 1989: Geoffrey Landis analyzes the laser driven lightsail, concluding that the performance is driven by the temperature limits of the sail, and proposes refractory dielectric films as sail material.

First Steps

  • 1999: NASA Advanced Concepts Office, largely driven by JPL, proposes a roadmap for interstellar exploration with a launch to Alpha Centauri in 2028. Limited funding of initial studies and experiments follow.


Image: Plan for the development of sails for interstellar flight, 1999.

  • 1999: Tim Knowles of ELSI invents the carbon micro-truss, a strong, very low areal mass density graphite sheet that doesn’t melt; it sublimes at very high temperature.
  • 1999: Geoffrey Landis and James Benford propose using the new carbon micro-truss, strong structures of very low mass per square meter. They show that, with this material, a laboratory experiment can demonstrate microwave beam-powered sailing.
  • December 20, 1999: Leik Myrabo-led team measures deflection of a pendulum moved by laser beam hitting sails of carbon micro-truss, giving acceleration of 0.15 gravities.
  • Feb 2000: Geoff Landis reexamines use of dielectric films for highly reflective sails, points out drawbacks to multiple layers.
  • March 2000: Gurkirpal Singh of JPL, in a dynamics simulation, shows that conical sails can ride a beam stably.


  • April 5, 2000: James Benford-led team achieves first flight, at JPL. Sails of carbon micro-truss, driven by microwave beam, are accelerated up to 13 gravities; sails are recovered intact. They discover the highest accelerations are due to desorption of deeply embedded chemicals when the sail reaches high temperatures. Experiments are described in “Microwave Beam-Driven Sail Flight Experiments.”
  • June 12, 2000: James Benford-led team spins a suspended micro-truss sail with circularly polarized microwave beam. Basic physics of spin-driven effects are demonstrated.
  • December 8, 2000: Leik Myrabo team, using a carbon-dioxide laser, flies molyebdenum-coated carbon sails at 1 gravity above liftoff, giving velocities up to 3 m/sec.

Image: First flight of beam-driven sail. Carbon sail lifts off truncated rectangular waveguide under 10 kW microwave power. (4 frames, 30 ms interval, 1st at top).

  • July 2000: Geoff Landis, in “Starwisp Revisited,” substantially reduces mass and power required for microwave-driven mesh sail.
  • 2002: Stable beam-riding of a conical sail on a microwave beam demonstrated experimentally by the Benford brothers, Gregory and James, at UC Irvine. Physics of spin-driven effects on various sail shapes are explored in both experiment and theory.

stable video still

Image: Geometry of 3-D sail simulation used to access sail stability. Beam is injected from horn at bottom.

  • 2003: Chaouki Abdallah and team at UNM, with a 3D dynamics simulation, confirm that the conical (umbrella) sail is the most passively stable shape, while other shapes, such as inverse conical, are not stable. They quantify the stable parameter range for beam shape, angle of the cone and location and mass ratio if sail and payload. They show that active dynamic feedback control can stabilize most shapes.

Screenshot from 2016-06-22 15:05:29

Image: Graphic user interface for sail stability simulation, Chaouki Abdallah 2003. Geometry
of sail shown above, power distribution of beam shown below.

Becoming Real

  • May 21, 2010: First deployment of a functional solar sail in space, “IKAROS” (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) is demonstrated by the Japanese Space Agency JAXA.
  • 2011: “100 Year Starship”, a joint project between NASA (Pete Worden) and DARPA (David Neyland), convenes a colloquium in Orlando to discuss interstellar travel, stimulating interest in practical starflight.
  • 2012: Greg Matloff proposes graphene for sail material that can sustain high accelerations.
  • 2013: Starship Century Symposium held at UC San Diego May 20, date of the publication of the book Starship Century. A second Starship Century Symposium is held at the Royal Astronomical Society in London. In the book James Benford predicts “the first starship will be a sail – a sailship – driven by a beam of photons.”
  • 2013: James Benford creates a cost-optimization equation for beam-driven propulsion systems.
  • April 2015: Philip Lubin introduces concept for small ultra-light ‘spacecraft on a wafer’ using multi-layer dielectric on metalized plastic film. In “A Roadmap to Interstellar Flight,” Lubin draws on Forward’s laser-sail work and extends it into much smaller sail designs.
  • August 2013: 1st Interstellar Congress held in Dallas, Texas by Icarus Interstellar. Eric Malroy introduces concepts for the use of nanomaterials in sails.
  • March 2016: Yuri Milner concludes the only credible way to achieve interstellar travel in the near-term is beam-driven sails, in consultation with Avi Loeb, Pete Worden and Pete Klupar.
  • April 12, 2016: Yuri Milner, Stephen Hawking and Pete Worden announce the beginning of Breakthrough Starshot, a project to launch beam-driven sails to Alpha Centauri within 20 years. Freeman Dyson is present.
  • 2016: Kevin Parkin creates an engineering inference engine for optimizing beamed sail systems, including cost minimization.
  • April 15, 2016: Breakthrough: Discuss, a 2 day symposium, brings together the Breakthrough Starshot team, as well as that of Breakthrough: Listen, a SETI project begun it 2015.

The notional timeline for Starshot is said to be 10 years of R&D, 10 years for construction of the System, 21 years transit time for the first sailcraft. At 0.2 c, it would enter the Alpha Centauri double-star system in 2057, a century after the first Earth satellite.

“Make no mistake — interstellar travel will always be difficult and expensive, but it can no longer be considered impossible.”

-Robert Forward, 1985

We owe it to the future to record our past. What I have described is journalism, but journalism is only the first draft of history. Doubtless there are significant events I have forgotten or never knew. Please suggest additions and modifications. A revised timeline will appear later, including references.

I thank those who I interviewed over the years about history: Robert Forward, Freeman Dyson, Geoff Landis, Kevin Parkin, Gregory Benford, Leik Myrabo, Pete Klupar, Avi Loeb and Gregory Matloff.