space travel basicphysics.gmu.edu/~hgeller/astrobiology/228travels09.pdf · 2009-04-22 · last...
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Last Words on CETI and some Space Travel Basics
HNRS 228Spring 2009Dr. H. Geller
What I Will Cover Today
Final words about CETISpace TravelSpace EnvironmentSpaceflight ProjectsSpaceflight Operations
A Cartoon about CETI
What does a telescope do?
Collect electromagnetic wavesCollecting ability proportional to the square
of the diameter of the objective
Resolve electromagnetic sourcesRelated 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 Radio- telescope at Green Bank, WV
The 100- meter 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
Space Environment
Solar SystemReference SystemGravity and MechanicsTrajectoriesPlanetary OrbitsElectromagnetics
Solar System Considerations
DistanceFrom Sun
Energy, temperature, condensation of matter
Hostile EnvironmentRadiation (gamma
ray)Radiation (x-ray)Radiation (UV)
Coordinate Reference Systems
GeographicCelestialPrecession
Gravity and Mechanics
OrbitsKeplerNewton
Orbital Transfers
Planets and Gravity
Flight Projects
Mission InceptionExperimentsSpacecraft ClassificationTelecomOnboard SystemsScience InstrumentsNavigation
Mission Inception
Instruments
Telecommunications
Onboard Systems
Operations
LaunchCruiseEncounterExtended OperationsDeep Space Network
Launch
Cruise
Encounter
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
1 atc
2
x(t) c2
a1 at
c 2
1
c = speed of light
Crew Duration (yr) Earth Duration (yr) Range (pc)1 1 0.02
10 24 3 - nearest stars20 270 4240 36,000 5,400 - center of Galaxy
Accelerating at 1g = 9.8 m/s2:
iClicker Question
What does the letter “c” stand for in the equations shown?A Speed of soundB Speed of lightC A constant of unknown valueD A generic constantE Speed of time
Considerations for Interstellar Travel
Unless there is a MAJOR revolution in technology - rockets are all we have.
Three considerations for interstellar travel
1. Imagination - not a problem today
2. Technology - constantly improving
3. Laws of Nature - may provide ultimate limits
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:
Vfc
1 M 2W /c
1 M 2W /c
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 Via Nuclear Rocket
Vf / c ~ 10-5 Vf / c ~ 10-1
MRT = 55 (=e4) MRT = 55t = 3 million years t = 300 years
iClicker Question
What does the letter “e” represent in these equations?A Speed of lightB The natural logarithm baseC An irrational numberD A rational numberE 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 H2 O 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 yearsB Within the next 20 yearsC Within the next 30 yearsD Within the next 50 yearsE Never
Matter/Antimatter RocketsW = c
V fc
1 M 2
1 M 2
x ( d i s t . ) c 2
2 aM M 1 2
T ( e a r t h ) 2 ca
M M 1 t ( c r e w )
ca
l n ( M )
Illustration - flat-out acceleration (No stopping, drifting, or return).
Vf /c = 0.1 Vf /c = 0.98 Vf /c = 0.1 Vf /c = 0.98a = 0.01 g a = 0.01 g a = 1 g a = 1 gM = 1.1 M = 9.95 M = 1.1 M = 9.95Tcrew = 9.7 y Tcrew = 230 y Tcrew = 0.1 y Tcrew = 2.3 ytearth = 39 y tearth = 2000 y tearth = 0.4 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 1025 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 zeroB Near oneC Effectively infinite
iClicker Question
What is the value of (1 - v2/c2) when v is very small compared to c?A Effectively zeroB Effectively oneC Effectively infinite
iClicker Question
What is the value of (1 - v2/c2) when v is approaching the speed of light?A Effectively zeroB Effectively oneC Effectively infinite
iClicker Question
What is the value of 1 / (1 - v2/c2) when v is approaching the speed of light?A Effectively zeroB Effectively oneC 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 YesB No
Solar Sailing
Planetary Society - Cosmos 1
June 21, 2005, launched on Volna rocket from Russian sub. Failed to reach orbit
Solar wind only reaches 0.003c, need to use sunlight
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
x ALSunM 2c
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 Hazard of interstellar flight
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 wouldA 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 willA 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 TrueB 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 2.5x103stars / ly3 0.1star / pc3
1AU 2 1
206,265pc
2 2.4 1011 pc2
x 1n
1
0.1pc3 7.5x1011 pc2 1.3x1011 pc
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 YesB No
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