#### Transcript Goal to understand how the solar system works.

```Goal to understand how Solar
Sails work.
Objectives:
1) To learn about what solar sails are
2) To learn about how solar sails work
3) To understand the limitations and
capabilities of solar sails
Solar sails
• Attach a spacecraft to a large sail.
• Like a sailboat except instead of normal
wind it deflects sunlight and the solar wind
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
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)
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.
momentum (light still has 600 times more effect
though)
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
• 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
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.
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.
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.
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
• 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.
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
•
•
•
•
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
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…
Result
• NASA seems to have discontinued
research on solar sails
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
```