By Bill D’Zio

July 19, 2019

The romantic days of high adventure and ships harnessing the wind may be over, but one organization thinks a new kind of sail can propel future spacecraft.  The Planetary Society has championed solar sailing for decades.

Unlike chemical rockets that need to bring along heavy propellants and  provide short, powerful bursts of thrust, solar sails provide continuous, very slight thrust.  Since the solar sail provides this trust over a longer time, the spacecraft can potentially reach higher speeds over time. The technology is still maturing, so giant spacecraft with solar sails and swashbuckling spacefarers may not be any time in the near future but moving small satellites with solar sails may be.

To advance this technology, The Planetary Societies’ LightSail® solar sail project is crowdfunded.  The LightSail 2 spacecraft aims to become the first spacecraft in Earth orbit propelled solely by sunlight.  It is also now in space thanks to the SpaceX Falcon Heavy launch on launched 25 June 2019.  A SpaceX Falcon Heavy launched LightSail 2 as part of the U.S. Air Force’s STP-2 mission from Kennedy Space Center in Florida.

PALLAS-1 Engine

Credit:Planetary Society

After being deployed, the goal is to raise LightSail 2’s orbit by a measurable amount, showing that solar sailing is a viable means of propulsion and an alternative to chemical, nuclear and solar-electric propulsion.  For CubeSats, small, low mass options are crucial to lower the cost of space exploration. CubeSats are ideal candidates as these standardized spacecraft can leverage this technology and be replicated quite easily.

Innovative solar sail technology, following on prior efforts.

Lightsail 2, isn’t the first solar sailing spacecraft.  In 2005 the Planetary Society launched the world’s first solar sailing spacecraft, the Cosmos 1, but was unfortunately lost due to a rocket failure. Fast forward to 2015, the LightSail 1 spacecraft was successful in a test flight. 

For the launch, LightSail 2 was enclosed within a small satellite built by Georgia Tech students the Prox-1. Prox-1 and LightSail 2 will be delivered into a roughly circular, 720-kilometer orbit with an inclination of 24 degrees.  

If all goes according to plan, LightSail 2 will deploy from Prox-1 on 2 July 2019.  After being deployed from the Prox-1 satellite, LightSail 2 will spend at least 1 week undergoing a shakedown.  During this shakedown, Lightsail 2 will complete system health and status checks to make sure that everything is going according to plan.  A few days later, it will open its hinged solar arrays and start to collect energy from the sun. A day or so later, the sails will start to deploy.  Over roughly 3 minutes, 4 booms made of high strength cobalt-alloy will extend like tape measures to pull the spacecraft’s 4 triangular sails from storage.

The deployed solar sails will be ready to start harnessing the power of the sun.  Lightsail 2 will begin swinging its solar sail into and away from the Sun’s rays as it orbits the Earth, giving the spacecraft enough thrust to modify its orbit.  Initially, the spacecraft will start in a circular orbit, where the major axis and the minor axis are equal.  Over the course of a month, the orbit will elongate, causing the minor axis to reduce and major axis to increase.

At a certain point, the atmospheric drag overcomes the forces of solar sailing, ending the primary mission. The Lightsail 2 spacecraft will remain in orbit roughly a year before entering the atmosphere and burning up.  Future spacecraft may have the ability to continuously fly the spacecraft higher.

The Planetary Society has not disclosed plans for a follow-on mission but is already helping advance solar sail technology through a Space Act Agreement with NASA. The Near-Earth Asteroid “NEA” Scout, a CubeSat, is planned for the first flight of the Space Launch System to lunar orbit. NEA Scout will use its solar sail to leave the Moon and visit a near-Earth asteroid.

Solar sail technology may have gotten off to a slow start, but appear to be gaining momentum in the international space travel realm.  To support this project and other worthwhile projects by the Planetary Society, please visit and become a member for as little as $50 per year.

Lightsail 2 Credit:Planetary Society website

The IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun)   Credit: JAXA website

So How does Solar Sails fit commerical space and space exploration?

Without being restricted by heavy engines, solar sails could help drastically reduce the cost of space travel.  Long distances may take some time due to the slow accleration, therefore human flight with solar sails isn’t on the near term.  Beyond satellites, transporting cargo and goods that are not time sensitve could be a viable option, but a very large sail would be required.

In 2010, Japan’s Ikaros become the first successful solar power flight using sunlight propulsion while on an interplantary mission. The IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) mission objective was to verify that a spacecraft can fly only by solar powered sail, and that thin film solar cells can generate power. 

So the IKAROS was similar to Lightsail, but different.  A Solar Sail is a space ship that gathers sunlight as propulsion like the lightsail and lightsail 2 spacecraft. On the other hand, a Solar “Power” Sail gets electricity from thin film solar cells on the kite section as well as the acceleration by solar radiation.  By combining an ion-propulsion engine with a high specific impulse driven by solar cells, the net result is a “hybrid” engine that is combined with photon acceleration to realize fuel-effective and flexible missions. 

  According to the JAXA webiste, “This will be the world’s first solar powered sailcraft employing both photon propulsion and thin film solar power generationduring its interplanetary cruise.” Although not quite the same, Ikaros was more of a hybrid rather than a pure solar sail.

The Planetary Society and JAXA aren’t the only ones working on innovative solar powered options.  NASA’s Solar Electric Propulsion (SEP) project a key focus on the mission to Mars.   NASA is developing critical technologies to enable cost-effective new trips to Mars and other inner solar system destiations such as asteroids and Venus.  NASA amies to support a variety of commercial spaceflight activities.

The SEP would draw electric power from on-board solar arrays, and then electrically propelled system will use 10 times less propellant than a traditional rocket.   The reduced fuel mass will deliver the thrust  capable of propelling robotic and crewed missions beyond low-Earth orbit. 

In the near term, alternate types of technology like NASA’s Solar Electric Propulsion may be more viable for manned missions than Solar Sails.