Goal to understand how Solar Sails work.

1. Goal to understand how Solar Sails work. Objectives: To learn about what solar sails are To learn about how solar sails work To understand the limitations and capabilities of solar sails

2. Solar sails • Attach a spacecraft to a large sail. • Like a sailboat except instead of normal wind it deflects sunlight and the solar wind

3. NanoSail-D • 10 square meters • Oddly enough this one is using solar sails to slow down the spacecraft to decrease its orbit • It sets the sail to plow ahead so that it runs into light particles called photons • This creates something called the Pointing-Robertson Effect • Could be used on future satellites to get rid of them after they are no longer of use

4. How it works for future spacecraft • Instead you turn the sail so that sunlight hits it directly • The sunlight pushes the sail. • For light: Energy = momentum * speed of light • Light looses momentum, solar sail gains momentum (momentum is conserved) • A 10 square meter sail would effectively provide at Earth orbit a 14 kilowatt engine (28 kilowatts if the light is perfectly reflected)

5. Solar Wind • Gives you another source of push at low velocities • Solar wind at the Earth goes about 520 km/s • This would give an effective thrust power of 0.05 Watts for a 10 square meter sail. • However it is not about power, it is about momentum (light still has 600 times more effect though)

6. Result • Suppose you attached this 10 square meter solar sail to a 100 kg science instrument. • The Newton thrust would accelerate the spacecraft at a rate of 9*10-7 m/s2 • Yes this is tiny • It would take about 1 year to speed your craft us by about 30 m/s. • However, if you made the sail 1000 square meters (30 feet by 30 feet) and the total craft only went up to 150 kg then you would accelerate by up to 2 km/s in the first year

7. Now we play what if • Lets suppose that a square meter of solar sail had a mass of 1 kg but a cost of only $25k. • First, lets make a $1billion space shuttle. • We need to get it into orbit, there goes half our money. • $500 mil to make a solar sail.

8. Set up • Mass of shuttle is 60,000 kg. • $500 mil gives us 20,000 square meters of solar sail, mass of 10,000 kg. • Total mass 70,000 kg. • In 1 year you speed up by 85 m/s.

9. But • Lets use $10 billion. • $2 billion for the conventional rocket to get the shuttle into orbit • $8 billion gives us about 320,000 square meters of solar sail • Total mass 220,000 kg • Gives us 430 m/s in a year.

10. Asteroid impact prevention • 1 km asteroid lets say it has a mass of 4 trillion kg. • No way a solar sail can help, right? • Well… • Imagine a 10 km by 10 km solar sail. • Mass of sail is 50 million kg • Cost $2.5 trillion • Acceleration would be 0.7 mm/s per year

11. Sounds bad, but • Suppose we get a 10 year warning. • This would push the asteroid a distance of 12000 km which is almost the diameter of the earth. • The asteroid misses the earth.

12. IKAROS • Japanese mission to Venus • Sped up craft by 100 m/s in 6 month span • Was first to successfully use solar sails as primary propulsion • 400 square meters

13. Advantages • Accelerates forever • Steer able • Requires ZERO fuel • Requires no energy except to keep your sails pointed in the right direction • In fact, can be used to generate energy

14. Problems • Has to be lightweight • Acceleration is really slow • Sunlight drops off as radius squared, so too does the acceleration • Works great really close to the sun but…

15. Result • NASA seems to have discontinued research on solar sails

16. Conclusion • Solar sails have limited uses • Require no fuel and can be used virtually forever • Best used close to a star or used for minor course corrections • Good to use on satellites to get them out of orbit • Requires patience