Interstellar Travel
ASTR 1420
Lecture 25
Sections 13.1 & 13.2
Four spacecrafts flying away from the Sun
Voyagers : 1977
Pioneers : 1972 & 1973
Voyager 1 reached the
end of Solar System
in 2010
Pioneer 10 (& 11)
• Launched on March 1972 (moving away from the Sun at 12.2 km/sec)
now at ~28 light minutes away!
Scenes from Earth from the Pioneer Plaque
will take ~120,000 years to the nearest star
(if it were aimed directly at it).
Rockets = Newton’s 3rd Law
• For every action, there is an equal and opposite re-action!
Space Travel
with Saturn XXX?
 the largest rocket ever
built (Saturn V; used in
the Apollo mission)
• If we build a larger
version of this rocket,
can our descendants
travel among the stars?
No!
Saturn V rocket
Chemical Rockets limited by Mass ratio
• Need to accelerate fuel also!  Mass ratio
o mass ratio = weight of a rocket with fuel / without
Current technology
o To escape from Earth: mass ratio = 39
o Best single-stage rocket: mass ratio < 15
• Multi-staged rockets
o Necessary, and used, to leave Earth, or even for
intercontinental ballistic missiles
o Interstellar travel: impractical, hundreds to
thousands of stages required
o Most powerful rocket engine with 100 stages
achieve 0.001c
takes 4,000 yrs to the
nearest star!
Space Elevator
• SpaceElevator Competition ($1M USD) “Elevator:2010”
Nuclear Rockets
• Method
o Advantage: higher energy/mass
ratio of nuclear reactions
o Disadvantage: difficult to controll
especially fusion
o Maximum speed: ~ 0.1c,
• Project Rover
o Fission rocket
o Achieve speeds 23 times those of
chemical rockets
o Application: manned mission to
Mars, since abandoned
Nuclear Thermal Rocket Engine
Project Orion
Explode H bombs behind the spaceship and let
the shock waves propel the spaceship
Too expensive, also violates “ban on nuclear
explosions in space”
Project Daedalus
UK plan to reach Barnard’s star (5.9 Lyrs away)
Use pellets of 2H and 3He, ignited by
an electron beam from the spacecraft
Solar Sailing
Solarwind only reaches 0.003c
need to use sunlight
Planetary Society - Cosmos 1
June 21, 2005, launched on Volna rocket from
Russian sub. Failed to reach orbit
To make a interstellar trip, it requires a
prolonged propulsion from the Earth
 Enormous energy consumption and large
focusing mirror array (hundred km across).
How to stop? A return trip?
Solar Sail feasible?
• 10-ton payload, sail 1000 km x 1000 km in size. v∞ is then only 0.04 c.
• It would take roughly 75 years to reach the nearest star (3 Ly away; ignoring
deceleration & stopping)
• Oops! The SAIL ALSO has mass!
 A 1000 km x 1000 km. A gold-leaf sail 1 atom thick (a real sail would have
to be much thicker) would have a mass of 170 tons (it effectively becomes
the payload), and so the top speed is actually 0.009 c. Now it takes over
300 years to get anywhere!
Antimatter rocket
• Antimatter rocket
o Exists: all matter has antimatter
o Matterantimatter annihilation: all mass converted into light! (100% change
versus < 0.7% mass-to-energy conversion in nuclear reactions)
o Problem: controlled storage
• Edward Purcell (1952 Novel Prize for nuclear magnetic resonance ):
Imaginary antimatter rocketship with 100% engine efficiency…
o maximum speed of 0.99c  still requires 14 time more mass in fuel than the
payload.
o if we want to stop at the designation  14 times more fuel to stop
 14 times 14 = 196 times more fuel
o for a round trip at 0.99c speed  196x196 more fuel  ~40,000 than the mass
of the payload!!
Fuel to accelerate & decelerate is the main issue!!
Interstellar Ramjets : collect fuel during flight!
• Ramjets
o Collect Hydrogen from the interstellar medium and fuse it
o Need a scoop that is hundreds of kilometer wide
In space, the density of Hydrogen is so low
 Size of the spaceship ≈ the size of worlds
 Use a magnetic funnel for fuel collector? (energy to create the magnetic field?)
Artist rendering of the
imaginary interstellar
ramjet rocketship
Furthermore, Speed Limit!!
• Einstein’s special theory of relativity:
For anything with a mass
 impossible to travel faster than the speed of light
• Even at the speed of light, the nearest star is α Centauri at 4.4 Lyrs away.
 fastest round trip takes still 8.8 years!
 trip across the Galaxy takes 100,000 years!
• Could it be that Einstein’s theory is wrong and that we will someday find a
way to break the cosmic speed limit?
No!
 possible that a more comprehensive theory in the future may replace
Einstein’s relativity theory, but such one will be inclusive of many verified
results including the speed of light barrier.
One advatange : Travel Time gets shorter at high-speed!
Time dilation
time is different for high-speed
travelers than for people stay at home
Tship = TEarth
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HAZARD of interstellar flight
• A spacecraft hit by an 1-mm dust grain (mass of 0.012 grams) while
traveling at 0.1c
 collision energy = (E=1/2 mv2) of 5.4x109 J.
 Same as an 1-ton object hitting at Mach 9.5 (7,000 mi/hr)!!
Unless there is a way to screen out all
interstellar dust, the spacecraft will be
easily destroyed!!
Need for shields (and it requires additional Energy!)
High-speed interstellar travel is impractical!
• Difficulties of high-speed interstellar travel
o
o
o
o
Fuel issue (for accelerate and decelerate)
Speed limit
Travel time (round-trip possible?)
Space hazard
Interstellar Arks
OK, fast traveling is challenging.
How about slow traveling over long time?
• Hibernation of crews
o How do we put people to sleep?  hibernation gene from animal?
o How do we wake them up?
• Long life (make human’s life longer!)
o Pure speculation at the moment
o Robotic missions would be simpler
• Multi-generational trip:
o Perseverance in the mission and/or infighting
o Loss of expertise
o Moral issue (1st generation=volunteers, later generations?)
Energy use of an Interstellar Ark
• Speed for interstellar travel
o Escape velocity from Earth: 11 km/sec
o Travel velocity, say 0.001c = 300 km/sec, dominates energy requirement
• Kinetic energy
o
o
o
o
o
Mass: say 108 kg (105 ton) for 5,000 people
= 9 x 1018 Joules = 2.5 x 1012 kW hr
1% of the world annual energy consumption
250 billion dollars (@ $0.10/kW hr)
1/5 of US GDP
o Add cost of provisions, energy efficiency, deceleration!
Ion Engine
•
•
•
•
Continuous firing…but weak thrust!
Need to be free from other ions… (i.e., in space)
Much more efficient!
Already used by NASA (1998, Deep Space 1) and
ESA (2004, SMART-1, lunar orbiter).
How about a short-cut? Hyperspace and Wormholes
• Hyperspace
o General theory of relativity (1916): space is warped by gravity
• Wormholes
o
o
o
o
o
Rotating black holes connect to another flat space
Other flat space may connect to ours somewhere, but may be not
We will know only after we go through the wormhole
Stellar black holes: have too strong a tidal force, which would rip us apart
Massive black holes: only known in galactic nuclei, have to get there
In summary…
Important Concepts
Important Terms
• Difficulties of high speed space
travel
• Difficulties of low speed space
travel
• Speed limit
• Fuel and rocket
• Time dilation
• Mass ratio
Chapter/sections covered in this lecture : 13.1 & 13.2
Terra-forming and Colonization : next class!