Purpose
LightSail is a citizen-funded project by The Planetary Society, the world’s largest non-profit space advocacy group. The project's goal is to demonstrate solar sailing, an innovative method of propulsion using the sun’s energy, as a viable propulsion for CubeSats.
CubeSats are small, standardized satellites that hitch rides to space with larger payloads. They have made low-cost space missions a reality for universities and research groups. However, providing propulsion for these spacecraft has been a major challenge that has limited their usefulness so far. LightSail shows how a relatively large solar sail can be bundled into a small size, deployed on orbit and used for propulsion.
Mission
The LightSail 1 flight, launched in May 2015, was a successful shakedown cruise designed to test the spacecraft's solar sail deployment mechanism. LightSail 1, hitching a free ride to orbit aboard a United Launch Alliance Atlas V rocket, did not fly high enough for the push from solar sailing to overcome Earth's atmospheric drag.
The second mission, in 2017, will mark the first controlled, Earth-orbit solar sail flight and fly aboard the first operational launch of SpaceX’s Falcon Heavy rocket. LightSail 2 will team up with Prox-1, a small spacecraft designed by Georgia Tech to demonstrate automated rendezvous and inspection techniques.
FAQ: Won't space debris and micrometeorites punch holes in LightSail's solar sails?
The sail uses a "rip-stop" design with seams every few inches. A puncture from a micrometeorite or space debris shouldn’t spread very far. LightSail can withstand a couple small hits without any major mission impact.
Solar Sailing
Light is made of packets of energy called photons. While photons have no mass, they have momentum. Solar sails capture light momentum with large, lightweight mirrored surfaces—sails. As light reflects off a sail, most of its momentum is transferred, pushing on the sail.
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. Solar sailing is considered one possible means of interstellar space travel.
FAQ: How fast is LightSail?
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.
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.
FAQ: How do you calculate LightSail's acceleration?
The calculation for radiation pressure by reflection, measured in N/m², is:
Preflect = (2E/c)cos2α
E = energy flux, W/m²
c = speed of light, m/s²
α = angle between the sail surface and photon stream
The mean energy flux at Earth’s orbit around the sun is 1,366 W/m².
When LightSail is perpendicular to the sun’s rays, α is zero.
Preflect = 9.1x10⁻⁶ N/m²
This shows us how much pressure the sun applies to a square meter of sail in Earth orbit. To calculate how much force is applied to LightSail, we multiply by 32 square meters.
Plightsail = 2.91x10⁻⁴ N/m²
Finally, we can use Newton’s second law of motion, F = ma, to calculate how much LightSail will accelerate.
a = F/m
F = 2.91x10⁻⁴ N/m²
m = 5 kg
a = 5.8x10⁻⁵ m/s²
FAQ: How could solar sailing be used for travel beyond Earth orbit? Won't the sun's rays have less of an effect as the sail moves farther from the sun?
Yes. But only acceleration--not speed--will decrease. As mentioned above, the mean energy flux for Earth’s orbit around the sun is 1,366 W/m². At Mars, irradiance 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 thrust in the inner solar system first, or using supplemental lasers, or both.
FAQ: How can light have momentum, if photons have no mass? Isn't the equation for momentum p=mv?
Yes, but in the roughest sense, 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.
Specifications
Spacecraft (sails undeployed)
Size comparison: Loaf of bread
Type: 3-Unit CubeSat
Dimensions: 10 x 10 x 30 cm (4 x 4 x 12 in)
Weight: Less than 5 kg (11 lbs)
Sails (deployed)
Size comparison: Boxing ring
Material: Mylar
Thickness: 4.5 microns (1/5000 of an inch)
Layout: Four triangular sails forming a square, connected with four tape-measure-like booms
Boom length: 4 m (13 ft)
LightSail width: 5.6 m (18.4 ft)
Total sail area: 32 sq. m (344 sq. ft)
FAQ: What hardware and software does LightSail use?
LightSail runs on an Intrepid System Board, manufactured by Tyvak Nano-Satellite Systems, Inc. You can learn about the Intrepid platform here. LightSail's software is Linux-based.
FAQ: How similar is the hardware and software between the two LightSail spacecraft?
From a hardware standpoint, the two spacecraft are nearly identical, with two major exceptions: We added a corner cube array to the LightSail 2 spacecraft to assist with laser ranging, and the LightSail 2 spacecraft also has a momentum wheel to help it tack during solar sail maneuvers.
For the LightSail 2 mission, the software has been significantly overhauled. The value of the LightSail 1 test mission was discovering bugs and areas ripe for improvement. From the contents of our beacon packets to the algorithms we'll use for solar sailing, virtually no area has been untouched. We also implemented a rigorous testing program to make sure the software is ready for flight.
Funding and Partners
Project cost: $5.45 million
Funding sources: Planetary Society members, private citizens
Part of the project's cost came from a highly successful KickStarter campaign, which raised $1.24 million in mid-2015.
Design and construction:
Stellar Exploration, Inc.
Lead contractor for integration and testing:
Ecliptic Enterprises Corporation
Ground stations:
Cal Poly San Luis Obispo and Georgia Tech
LightSail test mission integration and testing:
Cal Poly San Luis Obispo
Contractors to Ecliptic:
Boreal Space, Aquila Space
