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Starships
Lecture Twenty-Eight, Apr. 11, 2003
Pluto
• In 1991, the United
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States Postal Service
issued a series of stamps
commemorating
American space
exploration. The stamp
for Pluto consisted of a
painting of a grayish disk
with an apology of sorts
underneath: "Not Yet
Explored."
Spacecraft have visited
other planets.
Pluto’s discovery
• Clyde W. Tombaugh spent years searching for a
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hypothesized ninth planet to explain small perturbations
in the orbits of Uranus and Neptune, and then on Feb.
18, 1930, he found Pluto. Note, the perturbation
calculations were wrong, and Pluto, turned out to be
smaller than Earth's Moon — too small to account for the
perturbations, in any case.
In June 1978, James W. Christy noticed that in one
image, Pluto's point of light seemed to have a bump on
it. He went back and looked at other photographs and
Pluto seemed to have a bump in many of those images,
too, and the bump seemed to be moving around. This
was discovery of moon Charon.
Is Pluto a Planet?
• It is very small. Much smaller then several moons.
• It has an odd orbit that is tilted out of the plane by 17
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degrees and is very eccentric. So eccentric that it
crosses the orbit of Neptune.
Pluto discovered 72 years ago. In recent time we have
found many other minor planets even farther from the
sun. Some of these are quite large (although none yet
as large as Pluto).
Many astronomers think we will soon find an object as
large or larger than Pluto, far from sun in Kuiper belt.
Pluto is a planet because have called it so for 70 years!
If Pluto were to be discovered today it might be
classified as a minor planet, just one of many in the
Kuiper belt.
Pluto and its moon Charon are a
“double planet”
Pluto
• Has an atmosphere and mysterious bright
and dark marking with large contrast.
• Charon probably formed in a large collision
like earth’s moon.
• Spectra suggest nitrogen, methane and
carbon monoxide frost on Pluto, and water
frost on Charon.
New Horizons Mission to Pluto
• Planned for 2006 launch
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and 10 year journey to
Pluto.
Need very small
spacecraft that can be
accelerated to high
speed.
Uses nuclear power and
miniature instruments.
Plan is to flyby Pluto and
explore one or more
other Kuiper belt objects.
Interstellar Flight
• Can we search beyond our solar system for life
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near other stars?
Can we go to another star?
What can we learn about life near other stars by
using sensitive instruments (without leaving the
solar system).
Can we leave our Milky Way galaxy and search
for life in other galaxies? (Probably no.)
A Light Year
• Light travels 186,000 miles per second.
• In one year light travels 6 £ 1012 miles.
• Thus a light year is 6 trillion miles!
• The nearest star is 4.3 light years away.
• Interstellar journeys need to cover many
trillions of miles!
Where would you like to go today?
Andromeda Galaxy with hundreds of billions of stars
Interstellar flight is hard
• Need to go fast given huge distances.
• It takes a very big and efficient rocket to go fast.
• Suppose you have a 1 ton payload and a 9 ton
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rocket can boost the payload to a speed v.
How big a rocket does it take to go 2v?
Answer 100 tons total and a 1000 ton rocket is
needed to go 3v!
2 stage rocket
• A 10 ton system (9 ton
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rocket plus 1 ton
payload) can go a speed
v.
Then a 90 ton booster
rocket can boost this 10
ton payload to a speed v.
After booster runs out of
fuel fire 9 ton 2nd stage to
accelerate from v to 2v.
1 ton payload
9 ton 2nd stage
rocket
90 ton
booster
rocket
Giant Saturn 5
Moon Rocket
and Apollo
spacecraft is
363 feet tall.
Rockets are gigantic
• Because first stage must accelerate both
payload and all the weight of the upper
stages and their fuel.
• A rocket spends most of its energy
accelerating the fuel rather then
accelerating the payload.
Options for Interstellar flight
• Use a fuel with much more energy per
pound compared to conventional chemical
fuels such as hydrogen and oxygen.
• Use some scheme where it is not
necessary to carry the fuel.
• Be prepared for a very long trip!
Slow Boats
• Can a mission of 1,000 or 10,000 years succeed?
• Automated probes to explore another solar
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system.
Generation ships (or space colonizes). How
large would they have to be and with how many
people? How to accelerate this large mass?
Suspended animation (send people in
hibernation).
Send frozen fertilized human eggs and
specialized machines.
More Powerful Rockets
• Nuclear power such as project Orion.
• Antimatter rockets.
• Laser acceleration: Place a large mirror on
the starship and bounce a powerful
ground based laser off of it to accelerate
the ship. Don’t need to accelerate the fuel
on earth! Problem, how to stop?
Nuclear Explosive Power
• Explode a whole series of small nuclear
weapons behind a pusher plate.
• Very high temperatures reached in nuclear
explosions accelerate debris to very high
speeds.
• This makes an incredibly powerful rocket if
your spacecraft survives.
Antimatter Rockets
• Each particle has a corresponding antiparticle
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with the same mass but opposite charge. The
anti-electron is called a positron. A hydrogen
atom has an electron orbiting a proton.
An antihydrogen atom has a positron orbiting an
antiproton. This is the simplest atom of
antimatter.
When antimatter and matter meet they
annihilate into gamma rays releasing giant
amounts of energy.
Making antimatter
• Accelerators can make antiparticles by colliding
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high energy particle beams.
We can collect these antiparticles using
magnetic fields and other means and accelerate
them to produce beams of antiparticles. SLAC in
California collides a positron beam with an
electron beam.
To date we have only been able to make a few
atoms of antihydrogen by combining positrons
and antiprotons.
Antimatter rockets
• Antimatter is perhaps the ultimate fuel
because it can release the most energy
per mass.
• However very very hard to make the
antimatter, store it and annihilate it in
such a way that all the gamma rays go
backwards so the rocket will recoil
forward.
Relativistic Time Dilation
• If a rocket has enough energy to go a significant
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fraction of the speed of light strange things
happen.
Time seems to slow down on the rocket.
Trocket = [1-(v/c)2]1/2 Tearth
Here v is speed of rocket and c is speed of light.
Example v=0.9c: Trocket=0.43 Tearth
Passangers age only 4 years during a 10 year
trip at 0.9 c.
’39 by Brian May of Queen
In the year of '39 assembled here the volunteers
In the days when the lands were few
Here the ship sailed out into the blue and sunny morn
Sweetest sign ever seen
And the night followed day
And the story tellers say
That the score brave souls inside
For many a lonely day sailed across the milky seas
Ne'er looked back, never feared, never cried
Don't you hear my call though you're many years away
Don't you hear me calling you
Write your letters in the sand
For the day I take your hand
In the land that our grandchildren knew
Aah, aah, aah, aah
In the year of '39 came a ship in from the blue
The volunteers came home that day
And they bring good news of a world so newly born
Though their hearts so heavily weigh
For the earth is old and grey, little darling we'll away
But my love this cannot be
Oh so many years have gone though I'm older but a year
Your mother's eyes from your eyes cry to me
Don't you hear my call though you're many years away
Don't you hear me calling you
Write your letters in the sand for the day I take your hand
In the land that our grandchildren knew
Don't you hear my call though you're many years away
Don't you hear me calling you
All your letters in the sand cannot heal me like your hand
For my life
Still ahead
Pity me
Starships
• Are very very hard to build because the
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distances are so great.
It might cost 100 billion $ to send people to
Mars. This is comparable to the space station or
the Apollo moon landings.
A starship might cost our full gross national
product???
Only for a far future time when worlds economy
is larger.
Justifying the Cost
• Maybe starships are too expensive just to
explore other stars.
• Colonization of new worlds could justify
great cost. Or avoiding disaster on earth.
• For now work hard to explore other stars
with sensitive instruments from within our
solar system.