Transcript Geller Slides on Space Travel - Department of Physics and Astronomy

Some Words on CETI and Some
Space Travel Basics
ASTR 390
Spring 2010
Dr. H. Geller
What I Will Cover Today
Some words about CETI
Space Travel
Space Environment
Spaceflight Projects
Spaceflight Operations
A Cartoon about CETI
What does a telescope do?
Collect electromagnetic waves
Collecting ability proportional to the
square of the diameter of the objective
Resolve electromagnetic sources
Related to the atmosphere, wavelength
and curvature of the objective
 Magnify surfaces of planets and the Moon
Magnification only of Moon, Sun and planets
Looking
Beyond
the Eyes
Optical Telescopes
Reflector
Refractor
Different Views of Sun
Sun in Hydrogen-alpha
Sun in X-ray
Radio
Astronomy
Basics
A Little More Detail
Jansky’s Original Radiotelescope
Grote Reber’s Telescope
170 foot
Diameter
Radiotelescope at
Green Bank,
WV
The 100meter Green
Bank
Telescope
Even Bigger than you Think
Jupiter in
Radio
Saturn in
Radio
3C296
Radio/Optical
Composite
Smoothing Data
Visualizing the Data
Colorizing the Data
Dealing With Noise
Worldwide Noise Sources
The Space Environment
Solar System
Reference System
Gravity and Mechanics
Trajectories
Planetary Orbits
Electromagnetics
Solar System Considerations
Distance
From Sun
Energy,
temperature,
condensation of
matter
Hostile
Environment
Radiation (gamma
ray)
Radiation (x-ray)
Radiation (UV)
Coordinate Reference Systems
Geographic
Celestial
Precession
Gravity and Mechanics
Orbits
Kepler
Newton
Orbital Transfers
Planets and Gravity
Flight Project Considerations
Mission Inception
Experiments
Spacecraft Classification
Telecom
Onboard Systems
Science Instruments
Navigation
Mission Inception
Instruments
Telecommunications
Onboard Systems
Operations Considerations
Launch
Cruise
Encounter
Extended Operations
Deep Space Network
Launch Vehicles
Cruise Portion of Mission
Encounter Portion of Mission
Need for Deep Space Network
Interstellar
Spaceflight
Considerations
THE PHYSICS AND MATH OF
SPACE TRAVEL
For a spacecraft accelerating at a rate a, the velocity v reached and
distance x traveled in a given interval of time t is:
v(t) 
at


2
at
1
c

2
c 2 
at
x(t) 
 1
 1 c
a 

c = speed of light
Accelerating at 1g = 9.8 m/s2:
Crew Duration (yr)
1
10
20
40
Earth Duration (yr)
1
24
270
36,000
Range (pc)
0.02
3 - nearest stars
42
5,400 - center of Galaxy
iClicker Question
What does the letter “c” stand for in
the equations shown?
A Speed of sound
B Speed of light
C A constant of unknown value
D A generic constant
E Speed of time
Considerations for Interstellar Travel
Three considerations for interstellar travel
1. Imagination - not a problem today
2. Technology - constantly improving
3. Laws of Nature - may provide ultimate limits
Unless there is a MAJOR revolution in technology rockets are all we have.
Rocket engines most efficient when v~vexhaust. Going faster
makes them less efficient.
Rockets must accelerate not only the payload but also all the
fuel they carry!
For a final velocity Vf, a ratio of initial mass (payload plus fuel)
to final mass (ditto) M, and exhaust velocity W, then:
1  M 2W /c

c
1  M 2W /c
Vf
For Vf < 0.1c, then M = “e” = 2.7182…..
For a round trip, where 4 legs of the trip each require a factor of M:
M RT  M 4
Suppose we took a round trip to a star 5 pc away:
Via Chemical Rocket
Vf / c ~ 10-5
MRT = 55 (=e4)
t = 3 million years
Via Nuclear Rocket
Vf / c ~ 10-1
MRT = 55
t = 300 years
iClicker Question
What does the letter “e” represent in
these equations?
A Speed of light
B The natural logarithm base
C An irrational number
D A rational number
E Both B and C are correct
Energy Costs of Interstellar Travel
Example: Controlled Nuclear Fusion (can’t do this yet!)
1000 ton payload
55,000 tons fuel in the form of H, dissociated from
440,000 tons of H2O ice mined from one of Saturn’s
moons
Dissociating 440,000 tons of ice requires 1016 Joules
(Watt-sec) = 3x109 kW-hours = 3000 GW-h ~ 0.1% total
annual energy consumption in the USA
But it won’t go very fast.
iClicker Question
When do you think the USA will develop
a feasible nuclear fusion reactor?
A Within the next 10 years
B Within the next 20 years
C Within the next 30 years
D Within the next 50 years
E Never
Vf
c
1  M 2

1  M 2


c2
x(dist.) 
M  M 1  2
2a
2c
T (earth) 
M  M 1
a
c
t (crew) 
ln(M )
a
Matter/Antimatter Rockets
W=c


Illustration - flat-out acceleration (No stopping, drifting, or return).
Vf/c = 0.1
a = 0.01 g
M = 1.1
Tcrew = 9.7 y
tearth = 39 y
Vf/c = 0.98
a = 0.01 g
M = 9.95
Tcrew = 230 y
tearth = 2000 y
Vf/c = 0.1
a=1g
M = 1.1
Tcrew = 0.1 y
tearth = 0.4 y
Vf/c = 0.98
a=1g
M = 9.95
Tcrew = 2.3 y
tearth = 20 y
The fuel supply needed to reach Vf / c=0.98 for a round-trip (MRT=M4=9,800)
10-ton payload requires 100,000 tons matter-antimatter
mc2  E  10 25 Joules
About 1 million times the annual energy consumption in the USA
iClicker Question
What is the value of v2/c2 when v is
very small compared to c?
A Near zero
B Near one
C Effectively infinite
iClicker Question
What is the value of (1 - v2/c2) when v
is very small compared to c?
A Effectively zero
B Effectively one
C Effectively infinite
iClicker Question
What is the value of (1 - v2/c2) when v
is approaching the speed of light?
A Effectively zero
B Effectively one
C Effectively infinite
iClicker Question
What is the value of 1 / (1 - v2/c2) when
v is approaching the speed of light?
A Effectively zero
B Effectively one
C Effectively infinite
Project Orion - detonate nuclear
bombs to provide thrust (motion
picture “Deep Impact”)
iClicker Question
Do you support the use of nuclear
weapons for space travel?
A Yes
B No
Solar Sailing
Solar wind 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
Suppose we start at 1 AU
from the Sun (i.e. Earth's
orbit), a sail area A and a
payload (plus sail mass) M.
v 
2x
R1AU
ALSun
x
M 2 c
10-ton payload, sail 1000 km x 1000 km in size. v∞ is then only 0.04 c.
It would take roughly 3/0.04 = 75 years to get anywhere, i.e. 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!
Science fiction story - sails from star to star in a day or two (1/300th of a year),
This is impossible by a factor of 300 x 300 = 90,000 times! Such trips are,
therefore, unrealistic fantasy.
Yet other "Possibilities" for Interstellar Flight
Ships pushed by X-ray lasers
A rear reflector plays the same role to a
powerful planet-based light source as the solar
sail did to sunlight.
Interstellar Ramjets
This uses interstellar gas as fuel. You no longer
need to carry it with you. Avoid low-density
regions? How do you get the fuel into the
engine?
FTL (Faster-Than-Light)
Warp drives, etc. Contrary to all known
physics. Sorry.
Exploration by Proxy - Robotics
Von Neumann Machines/Probes - self-replicating:
1. Travel to a destination
2. Mine resources
3. Make copies of itself
4. Send copies out to new destination
5. Spread though the Galaxy as exponentially growing
fleet of machines that consume raw resources
Is this really a good idea?
Commentary on Interstellar Space
Travel
•
•
Unless there is a major revolution in our
understanding of the laws of nature, space travel is
likely to be confined to the solar system, unless
someone wants to launch "generation ships" where
only their distant descendents will see arrive
somewhere.
IF interstellar travel were to become a reality, but
still limited to relatively slow travel, all trips will be
1-way. For M="e", M1way = M2 = 7.4, while MRT = M4 =
55. Also, why return? Everyone you know back on
Earth will be dead. You will be an anachronism (how
would your great-great-great-great grandparents fit
into today's society?), or worse, a specimen in a zoo.
iClicker Question
You take a spaceship to Alpha Centaurus and
return to Earth. Which of the following is the
case when you return to Earth?
A All who knew you will be dead.
B There will be no time noticed to have
passed on Earth.
C All who knew you will be alive.
D This is not possible.
D More information is needed.
Another Interstellar Flight Hazard
A 1-mm grain (mass of 0.012 grams) hit by a spacecraft
traveling 0.1 c - energy (E=1/2 mv2) of 5.4x109 J.
Same energy as a 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.
iClicker Question
If you double the mass of a moving
object, the force needed to accelerate
it would
A be doubled.
B be tripled.
C be quadrupled.
D decrease.
E Cannot be determined, more
information is needed.
iClicker Question
If you double the mass of a moving
object, its kinetic energy will
A be doubled.
B be tripled.
C be quadrupled.
D decrease.
E Cannot be determined, more
information is needed.
Past "Attempts" at
Physical Contact
The Pioneer 10 spacecraft plaque
The Voyager 1 and 2
spacecraft - gold record (and
stylus for "playing") with
images and sounds of Planet
Earth.
iClicker Question
Do you believe it’s easy to construct a
message for another civilization?
A True
B False
More Scenes of Earth
Voyager
Trajectories
– Interstellar
Spacecraft
Neither of these are targeted at
any specific star. Their
trajectories were constrained by
their science missions to the
jovian planets.
Will the Pioneer & Voyager Spacecraft ever “get anywhere”?
To come within 1 AU of a star & accidentally be found:
“Mean Free Path” (how far to go in order to hit something)
x=1/(n)
n = number of systems per pc3
 = "target area" to be hit.
(For a circle, the target area is  times the radius (here 1 AU) squared, which we will express in pc2 to
get the units we need.)
n  2.5x10 3 stars / ly 3  0.1star / pc 3
2
1


2
   1AU    
pc    2.4  1011 pc2
 206,265 
x
1
1
11


1.3x10
pc
 n 0.1pc3 7.5x10 11 pc2



The MW is less than 105 pc across (and less than 103 pc thick)
Changes of “hitting” are less than 10-6 or 0.0001%. Using
Neptune’s orbit as target - goes up to a whopping 0.1%.
iClicker Question
Can the previous calculation be applied
to the likelihood of intercepting a radio
signal from another civilization?
A Yes
B No