Interstellar Travel
ASTR 1420
Lecture 24
Sections 13.1 & 13.2
Four spacecrafts flying away from the Sun
Pioneer 10
• Launched on March 1972 (moving away from the Sun at 12.2 km/sec)
now at ~28 light minutes away!
Scenes from Earth
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!
Engine (and rocket) being pushed forward
Exhaust Flow pushed backward
Tyranny of Distance
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!
Chemical Rockets are 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
Even with the most powerful hypothetical rocket
engine with 100 stages
achieve 0.001c
takes 4,000 yrs to the
nearest star!
Space Elevator
Can reduce (by x100) the cost of sending material to space!
• SpaceElevator Competition ($1M USD)
How about different types of rockets?
Nuclear Rockets
Nuclear Thermal Rocket Engine
o Advantage: higher energy/mass
ratio of nuclear reactions
o Disadvantage: difficult controlled
use, especially fusion
o Maximum speed: ~ 0.1c,
Dec 1, 1967
• Project Rover (1955-1972)
o Fission rocket
o Achieved speeds 23 times those
of chemical rockets
o Application: manned mission to
Mars, since abandoned
1st ground experimental nuclear 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)
Construction in the
Earth orbit
Use pellets of 2H and 3He, ignited by
an electron beam from the spacecraft
Ion Engine
•
•
•
•
Using heavy noble gas (e.g., Xe, Ar, etc.) as fuel gas.
Ionize them by shooting electrons on them
Ions are accelerated by specially designed electric acceleration grid.
accumulated electrons are dumped outside.
Ion Engine
•
•
•
•
Continuous firing…but weak thrust!
Need to be free from other ions…
In vacuum. Much more efficient!
Already used by NASA (1998, Deep Space 1)
and ESA (2004, SMART-1, lunar orbiter).
Picture of the Deep Space 1’s
ion engine.
weak thrust but continuous firing for long, long time!
Solar Sailing
Solar wind only reaches
0.003c, need to use
sunlight
 decreasing sunlight
with distance causes a
challenge.
Planetary Society - Cosmos 1
June 21, 2005, launched on Volna rocket from
a Russian submarine. Failed to reach orbit.
privately funded project.
Solar Sail feasible?
• 10-ton payload, sail 1000 km x 1000 km in size  maximum speed is then
only 0.04 c.
• It would take roughly 75 years to get 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 0.009 c. Now it takes over 300 years to get
anywhere!
• Continuous powering up the sail from Earth
by using a focused laser is too expensive.
Return trip? How to stop?
Interstellar Ramjets
• Accelerating fuel is a problem. Then, what about collecting fuel on the way?
• Ramjets
o Collect Hydrogen from the interstellar medium and fuse it (magnetic funnel for
fuel collector).
o Need a scoop that is hundreds of kilometer wide
 Spaceship the size of worlds
Antimatter rocket (hypothetical)
• Antimatter
o Exists: all matter has antimatter
o Matterantimatter annihilation: all mass released as  rays (100% efficiency!)
(E=mc2 , only <0.7% of mass change to E in nuclear reactions)
o Problem: controlled storage
• Edward Purcell : Imaginary antimatter rocketship with 100% engine
efficiency…
o maximum speed of 0.99c  but 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
 14×14 = 196 times more fuel
o for a round trip at 0.99c speed  196x196 more fuel  ~40,000 than the mass
of the payload!!
Even with a hypothetical antimatter rocket, interstellar travel is very difficult!
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 light-years
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?
Highly Unlikely!
 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.

Travel Time at high speed
• Time dilation
time is different for highspeed travelers than for
people stay at home
time runs slower at high
speed!
Tship  TEarth
v 2
1  
c 
HAZARD of interstellar flight
• A spacecraft traveling at 0.1c hit by an 1-mm
grain (mass of 0.012 grams)
 Same energy as a 1-ton object hitting at
Mach9.5 (7,000 mi/hr)!!
Unless there is a way to
screen out all interstellar
dust, the spacecraft will
be easily destroyed!
Additional Energy
requirement!!
imaginary spaceship with a plasma shield
Interstellar Arks
OK, fast traveling is challenging.
How about slow traveling over long time?
• Hibernation
o How do we put people to sleep?  hibernation gene?
o How do we wake them up?
• Long life:
o Pure speculation
o Robotic mission would be simpler
• Multi-generational:
o Perseverance in the mission and/or infighting
o Loss of expertise
Human hibernation??
• A Swedish man, Peter Skyllberg age 44, survived in a snow-trapped car for
2 months!
• Experts believe that he might have been in a kind of human hibernation.
Feb, 2012
2 months
at -30°C
Simulated long-term space journey
• Nov 11, 2011 -- Six members of
Russian Mars500 simulator stepping
out of the windowless capsule after
520 days.
• simulated the confinement, stress,
weightlessness, and fatigue of
interplanetary travel.
• long confinement put the team
members under stress as they grew
increasingly tired of each other's
company.
Energy use of an Interstellar Ark
• Velocity for interstellar travel
o Escape velocity from Earth: 11 km/s
o Travel velocity, say 0.001c = 300 km/s, 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!
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
• Need to accelerate fuel also
• Advantage of in-orbit launch
• Various rocket types and their pros
and cons
• Speed limit and time dilation
• Difficulties of high speed space
travel
• Difficulties of low speed space
travel
• Mass ratio
Chapter/sections covered in this lecture : 13.1 & 13.2
Terra-forming and Colonization : next class!