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LightSail FAQs

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LightSail® is a citizen-funded project to test solar sailing technology for CubeSats. Solar sailing uses reflective sails to harness the momentum of sunlight for propulsion. CubeSats are small, standardized satellites that hitch rides to space with larger payloads.

CubeSats have revolutionized the space industry, thanks in part to the continuing trend of technology miniaturization. They use off-the-shelf hardware and can be manufactured rapidly, which makes them an economical alternative to large satellites.

CubeSats can test new technologies that aren't ready full-fledged missions, or fly single science instruments to measure very specific things like Arctic sea ice loss. Educators use CubeSats to give students real-world space experience at a fraction of the cost of large missions. Companies like Planet deploy swarms of CubeSats to blanket the Earth with daily imagery, while other groups envision using CubeSats to deploy a global broadband Internet constellation.

One disadvantage to CubeSats is that they typically lack propulsion, which limits their range of applications. LightSail will demonstrate the viability of using solar sailing for CubeSats.

LightSail 2 artist concept with Earth behind

Josh Spradling / The Planetary Society

LightSail 2 artist concept with Earth behind
Artist's concept of LightSail 2 above Earth.

Project History

The idea of solar sailing dates back to the 1600s, but the story of LightSail begins in the late 1970s, when NASA considered flying a giant solar sail to Halley's comet, and Planetary Society co-founder Carl Sagan promoted the concept during an appearance on The Tonight Show with Johnny Carson. The project was ultimately cancelled.

The Planetary Society was founded in 1980. Scientific collaboration between the Society and Russia led to the creation of Cosmos 1, a solar sail spacecraft launched aboard a repurposed ICBM. Both the partial test (2001) and full flight (2005) of Cosmos 1 failed due to problems with the Russian rocket.

Cosmos 1

The Planetary Society

Cosmos 1
Cosmos 1, The Planetary Society's first solar sail, was lost due to a Russian rocket failure in 2005.

The first successful solar sail finally took flight in 2010, when Japan's IKAROS spacecraft was deployed from a Venus-bound probe named Akatsuki.

NASA has investigated using solar sails to de-orbit CubeSats with atmospheric drag. The first attempt, Nanosail-D, was lost during a Falcon 1 rocket failure in 2008. A follow-on mission named Nanosail-D2 was successful in 2010.

The Planetary Society's LightSail program, initiated in 2009, aimed to construct a CubeSat similar to Nanosail-D that would demonstrate true solar sailing. LightSail 1 was awarded a slot aboard an Atlas V launch in 2015, but the target orbit was not high enough for solar sailing thrust to overcome atmospheric drag. We accepted the free ride anyway, and flew LightSail 1 to test the spacecraft's sail deployment mechanism. The mission was a success, and we downloaded a selfie of the spacecraft's sails in space.

LightSail 2 will launch aboard SpaceX's Falcon Heavy rocket. The mission will attempt the second-ever controlled solar sail flight, and the first in Earth orbit.

LightSail 1 updated solar sail selfie

The Planetary Society

LightSail 1 updated solar sail selfie
This image was captured by a camera aboard LightSail 1 on June 8, 2015, shortly after solar sail deployment. It was color-corrected by Dan Slater to remove the camera's artificial purplish tint based on ground test images, and is a closer approximation to what the human eye would see.

Launch date

LightSail 2's current launch date is no earlier than 24 June 2019. Rocket launch schedules are notoriously unpredictable, so we advise anyone traveling to Florida for the launch to make sure their travel plans are changeable and/or refundable.

LightSail 2 Mission Details

LightSail 2 will be enclosed within Prox-1, a Georgia Tech student-built spacecraft the size of a small washing machine. Both spacecraft will be attached to the upper stage of SpaceX's Falcon Heavy rocket, which is launching a payload for the U.S. Air Force called STP-2.

Prox-1 will detach from the Falcon Heavy into a circular, 720-kilometer-high orbit. A week later, it will deploy LightSail 2.

LightSail 2 and Prox-1 artist concept with Earth behind

Josh Spradling / The Planetary Society

LightSail 2 and Prox-1 artist concept with Earth behind
Prox-1 deploys the LightSail 2 spacecraft in Earth orbit.

After a checkout period of a few days, LightSail 2 opens its hinged solar arrays. About a day later, it unrolls four, tape measure-like sail booms, which pull the spacecraft's 4 triangular sails from storage. The sail deployment sequence takes roughly 2 minutes.

LightSail 2 will then begin swinging its solar sails into and away from the Sun's rays as it circles the Earth, giving the spacecraft enough thrust to raise its orbit (technically, the orbit semi-major axis) by several hundred meters per day. This portion of the mission will last 1 month.

LightSail 2 orbital raising

Josh Spradling / The Planetary Society

LightSail 2 orbital raising
LightSail 2 makes 90-degree turns to gradually raise the semi-major axis of its orbit by several hundred meters per day.

LightSail 2's attitude control system does not have the precision to maintain a circular orbit. Therefore, as one side of the spacecraft's orbit rises, the other side will dip lower, until atmospheric drag overcomes the forces of solar sailing, ending the primary mission. The spacecraft will remain in orbit up to a year before succumbing to destructive reentry.

Though LightSail 2 cannot raise its orbit indefinitely, this would be possible by angling the sail more precisely during each orbit.

How is LightSail 2 different than LightSail 1?

Appearance-wise, the spacecraft are nearly identical. LightSail 2 has a momentum wheel allowing it to rotate its sails toward the Sun's rays each orbit, and a cluster of small mirrors have been added to one end of the CubeSat to allow for ground-based laser ranging. LightSail 2's software has also been completely overhauled to correct bugs that were discovered during the first mission, and improve the quality of data the team will receive on the ground.

Will Prox-1 rendezvous with or image LightSail 2?

No. Though Prox-1's mission hardware was integrated at the time it was delivered to the U.S. Air Force for launch, the software required further development, and the mission scope was changed.

Won't space debris and micrometeorites punch holes in LightSail 2's solar sails?

The sails use a "rip-stop" design with seams every few inches. A puncture from a micrometeorite or space debris shouldn't spread very far. LightSail 2 can withstand small impacts without suffering any major mission impact.

Spacecraft Specifications

LightSail 2, sails stowed
3-Unit CubeSat
Size comparison: Loaf of bread
Dimensions: 11.3 x 11.3 x 34 cm (4.45 x 4.45 x 13.4 in)
Weight: 5 kg (11 lbs)

LightSail 2, sails deployed
Size comparison: Boxing ring
Sail material: Mylar
Sail thickness: 4.5 microns (1/5000th of an inch)
Sail layout: Four triangular sails forming a square connected with cobalt steel booms (booms unwind like tape measures)
Sail boom length: 4 m (13 ft)
Sail width: 5.6 m (18.4 ft)
Total sail area: 32 sq. m (344 sq. ft)

Prox-1 measurements:
Size comparison: Small washing machine
Dimensions: 61 x 56 x 30 cm (24 x 22 x 12 in)

LightSail 2 sail deployment test success, 23 May 2016

Jason Davis / The Planetary Society

LightSail 2 sail deployment test success, 23 May 2016
The Planetary Society's LightSail 2 spacecraft sits on its deployment table following a successful day-in-the-life test at Cal Poly San Luis Obispo on May 23, 2016.

I have a specific question about a piece of LightSail hardware.

Check out our academic resources page for schematics, models, and a parts listing of all the spacecraft's major components.

I have a specific question about LightSail's software. Is it open source?

The software is Linux-based and authored under the guidance of Cal Poly San Luis Obsipo. There is no open-source version of LightSail's specific software, but you can download all available Cal Poly CubeSat software on their GitHub page.

Funding and Partners

Project cost: $7 million from 2009 through March 2019
Funding was provided by Planetary Society members, private citizens and a KickStarter campaign that raised $1.24 million in 2015.

Design and construction: Stellar Exploration, Inc.
Lead contractor for integration and testing: Ecliptic Enterprises Corporation
LightSail testing facilities: Cal Poly San Luis Obispo
Ground stations: Cal Poly San Luis Obispo and Georgia Tech
Contractors to Ecliptic: Boreal Space, Aquila Space, NXTRAC

Program manager:  Dr. Bruce Betts, The Planetary Society
Project manager and mission manager: Dr. Dave Spencer, Purdue University

See and track LightSail 2

Naked-eye observing

LightSail 2 will fly in a 24-degree inclination orbit, which will keep it much closer to the equator than LightSail 1. We estimate this will restrict viewing to latitudes less than 42 degrees north or south (at 42 degrees, LightSail 2 will never rise higher than 10 degrees from the horizon).

LightSail 2 estimated viewing range

NASA / The Planetary Society

LightSail 2 estimated viewing range
LightSail 2 will fly in a 24-degree inclination orbit, which will keep it closer to the equator than LightSail 1. We estimate this will restrict viewing to latitudes less than 42 degrees north or south (at 42 degrees, LightSail 2 will never rise higher than 10 degrees from the horizon).

We'll have a dashboard at where you can enter your location and get predictions for visible passes, but here are some general predictions by continent:

North America: Locations north of Chicago and New York will not be able to see LightSail 2.
South America: LightSail 2 should be visible everywhere except for southern Argentina and Chile.
Europe: LightSail 2 will not be visible in the U.K., Germany or France, but more southern spots like Madrid and Rome will be able to see it.
Asia: Most of Japan and China will be able to see LightSail 2, but Russia is too far north.
Africa: LightSail 2 will be visible anywhere in Africa.
Australia: LightSail 2 will be visible in all of Australia, and in northern parts of New Zealand.

Morse code beacons

If you are in a location where you can see LightSail 2, you might also be able to hear it. Every 45 seconds, the spacecraft transmits its call sign, WM9XPA, in morse code. Radio enthusiasts tuning in to the spacecraft's 437.025 MHz signal should hear this pattern:

LightSail 2 Morse code beacon, WAV format
LightSail 2 Morse code beacon, M4R format

(Note that you can also download those files to use as a ringtone for your phone!)

In Morse code, WM9XPA looks like this: .-- -- ----. -..- .--. .-

And if you capture the Morse code beacon using software-defined radio, it looks like this:

LightSail 2 Morse code beacon

The Planetary Society

LightSail 2 Morse code beacon

Data packets

Radio enthusiasts can download LightSail 2's beacon structure for help with decoding packets. Here are some additional useful parameters:

WM9XPA | 437.025 MHz  |  AX.25  |  FSK  |  9600 bps

Inclination: 24 degrees
Altitude: 720 km, circular

Solar Sailing

Light is made of packets of energy called photons. While photons have no mass, they have momentum. Solar sails capture this momentum with sheets of large, reflective material such as Mylar. As photons bounce off the sail, most of their momentum is transferred, pushing the sail forward.

The resulting acceleration is small, but continuous. Unlike chemical rockets that provide short bursts of thrust, solar sails thrust continuously and can reach higher speeds over time. Sunlight is free and unlimited, whereas rocket propellant must be carried into orbit and be stored onboard a spacecraft. Solar sailing is considered one possible means of interstellar space travel.

How can solar sailing be used for interstellar travel? Won't the sun's rays have less effect as the sail moves farther from the Sun?

Yes, but only acceleration, not speed, will decrease.

At Mars, the mean energy flux from the Sun is just 43 percent of what it is at Earth. At Jupiter, it's just 4 percent. So roughly speaking, beyond Mars's orbit, a solar sail quickly becomes ineffective. For missions destined to leave the solar system, theorists have proposed either building up speed in the inner solar system, using supplemental lasers, or both.

How big a payload could a solar sail carry?

The key metric for solar sail performance is area-to-mass ratio, or A/m. Basically, the push your spacecraft gets from sunlight depends on the relationship between sail area and vehicle mass. The higher your A/m ratio, the better.

IKAROS, the Japanese solar sail launched in 2010, has an A/m ratio of 1.3. LightSail 2, which weighs less than 5 kilograms, performs slightly better, with a ratio of 7.

For large payloads, you need large sails. In the late 1970s, NASA designed two solar sails that would have visited Halley's comet with 1-metric-ton payloads, placing them on the order of the New Horizons probe that visited Pluto.

One design, a square sail, measured 850 meters per side, giving it an area of 0.7 square kilometers. A second design, called the heliogyro, resembled two ceiling fans stacked on top of one another, and had 8-meter-wide blades that were 6.2 kilometers long apiece.

Could a solar sail carry humans?

Yes, a solar sail could, in theory, carry any type of payload.

Does a solar sail fly on the solar wind?

No. Solar wind is made up of ionized particles ejected by the sun. These particles move slower than light and create a force less than one percent as strong light pressure. Solar sails fly solely on photons.

How fast is LightSail 2?

Assuming perfect reflectivity, the sun exerts a force of 2.91x10⁻⁴ N/m² on LightSail's 32-square-meter sails. The resulting acceleration is 0.058 mm/s². That's about as much force as a paperclip exerts on the palm of your hand.

In one month of constant sunlight, LightSail's speed would increase by 549 kilometers per hour, roughly the speed of a jet airliner at cruising speed.

In 16 months of constant sunlight, LightSail's speed would increase by 8,556 kilometers per hour, fast enough to escape the moon's gravity well.

How can light have momentum if photons have no mass? Isn't the equation for momentum p=mv?

Yes, but p=mv only works for non-relativistic masses.

For objects traveling near the speed of light, the universal equation for momentum is E²=(pc)² + (mc²)²

This allows photons to have momentum, and that momentum can be transferred to another object like a solar sail.

What's Next

LightSail 2 is a technology demonstration mission. One of our biggest goals is to help pave the way for more advanced solar sailing CubeSat technologies. We currently don't have plans for a follow-on mission.

NASA's NEA Scout mission will hitch a ride to lunar orbit aboard the first Space Launch System flight, and use a solar sail to visit a near-Earth asteroid. The Planetary Society shares LightSail data with NEA Scout engineers and program managers through a NASA Space Act Agreement to help inform their mission.

NEA Scout artist's concept


NEA Scout artist's concept

In 2016, the group Breakthrough Initiatives announced a bold initiative to send a fleet of laser-powered solar sails to our nearest star, Alpha Centauri. The plan builds on The Planetary Society's long history of solar sail projects. Other groups are experimenting with using solar sails to dispose of defunct satellites. While this technique does not necessarily rely on sunlight for propulsion, it builds on the concept of packaging large sails into small areas for deployment in orbit.

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