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Interstellar Travel

Interstellar Travel. ASTR 1420 Lecture 2 5 Sections 13.1 & 13.2. Four spacecrafts flying away from the Sun. Voyagers : 1977 Pioneers : 1972 & 1973 Voyager 1 reached the e nd of Solar System i n 2010. Pioneer 10 (& 11). Launched on March 1972 (moving away from the Sun at 12.2 km/sec)

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Interstellar Travel

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  1. Interstellar Travel ASTR 1420 Lecture 25 Sections 13.1 & 13.2

  2. Four spacecrafts flying away from the Sun Voyagers : 1977 Pioneers : 1972 & 1973 Voyager 1 reached the end of Solar System in 2010

  3. Pioneer 10 (& 11) • Launched on March 1972 (moving away from the Sun at 12.2 km/sec) now at ~28 light minutes away!

  4. Scenes from Earth from the Pioneer Plaque will take ~120,000 years to the nearest star (if it were aimed directly at it).

  5. Rockets = Newton’s 3rd Law • For every action, there is an equal and opposite re-action!

  6. 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

  7. Chemical Rockets limited by Mass ratio • Need to accelerate fuel also! Mass ratio • mass ratio = weight of a rocket with fuel / without Current technology • To escape from Earth: mass ratio = 39 • Best single-stage rocket: mass ratio < 15 • Multi-staged rockets • Necessary, and used, to leave Earth, or even for intercontinental ballistic missiles • Interstellar travel: impractical, hundreds to thousands of stages required • Most powerful rocket engine with 100 stages • achieve 0.001c • takes 4,000 yrs to the nearest star!

  8. Space Elevator • SpaceElevator Competition ($1M USD) “Elevator:2010”

  9. Nuclear Rockets • Method • Advantage: higher energy/mass ratio of nuclear reactions • Disadvantage: difficult to controll especially fusion • Maximum speed: ~ 0.1c, • Project Rover • Fission rocket • Achieve speeds 23 times those of chemical rockets • Application: manned mission to Mars, since abandoned Nuclear Thermal Rocket Engine

  10. 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”

  11. 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

  12. Solar Sailing Solarwindonly 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?

  13. 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!

  14. Antimatter rocket • Antimatter rocket • Exists: all matter has antimatter • Matterantimatter annihilation: all mass converted into light! (100% change versus < 0.7% mass-to-energy conversion in nuclear reactions) • Problem: controlled storage • Edward Purcell (1952 Novel Prize for nuclear magnetic resonance ): Imaginary antimatter rocketship with 100% engine efficiency… • maximum speed of 0.99c  still requires 14 time more mass in fuel than the payload. • if we want to stop at the designation  14 times more fuel to stop  14 times 14 = 196 times more fuel • 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!!

  15. Interstellar Ramjets : collect fuel during flight! • Ramjets • Collect Hydrogen from the interstellar medium and fuse it • 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

  16. 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.

  17. One advatange : Travel Time gets shorter at high-speed! Time dilation time is different for high-speed travelers than for people stay at home

  18. 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 5.4x109J.  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!)

  19. High-speed interstellar travel is impractical! • Difficulties of high-speed interstellar travel • Fuel issue (for accelerate and decelerate) • Speed limit • Travel time (round-trip possible?) • Space hazard

  20. Interstellar Arks OK, fast traveling is challenging. How about slow traveling over long time? • Hibernation of crews • How do we put people to sleep?  hibernation gene from animal? • How do we wake them up? • Long life (make human’s life longer!) • Pure speculation at the moment • Robotic missions would be simpler • Multi-generational trip: • Perseverance in the mission and/or infighting • Loss of expertise • Moral issue (1st generation=volunteers, later generations?)

  21. Energy use of an Interstellar Ark • Speed for interstellar travel • Escape velocity from Earth: 11 km/sec • Travel velocity, say 0.001c = 300 km/sec, dominates energy requirement • Kinetic energy • 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 • Add cost of provisions, energy efficiency, deceleration!

  22. 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).

  23. How about a short-cut? Hyperspace and Wormholes • Hyperspace • General theory of relativity (1916): space is warped by gravity • Wormholes • 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

  24. In summary… Important Concepts Important Terms Time dilation Mass ratio • Difficulties of high speed space travel • Difficulties of low speed space travel • Speed limit • Fuel and rocket • Chapter/sections covered in this lecture : 13.1 & 13.2 • Terra-forming and Colonization : next class!

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