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) Launched on March 1972 (moving away from the Sun at 12.2 km/sec) now at ~28 light minutes away!

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

Rockets = Newton’s 3 rd Law For every action, there is an equal and opposite re-action! 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? If we build a larger version of this rocket, can our descendants travel among the stars?No!

Chemical Rockets limited by Mass ratio Need to accelerate fuel also!  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 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” SpaceElevator Competition ($1M USD) “Elevator:2010”

Nuclear Rockets Method Method o Advantage: higher energy/mass ratio of nuclear reactions o Disadvantage: difficult to controll especially fusion o Maximum speed: ~ 0.1c, Project Rover 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 Use pellets of 2 H and 3 He, ignited by an electron beam from the spacecraft UK plan to reach Barnard’s star (5.9 Lyrs away)

Solarwind only reaches 0.003c need to use sunlight Solar Sailing 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. 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) It would take roughly 75 years to reach the nearest star (3 Ly away; ignoring deceleration & stopping) Oops! The SAIL ALSO has mass! 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 c. Now it takes over 300 years to get anywhere!

Antimatter rocket

Interstellar Ramjets : collect fuel during flight! Ramjets 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: 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. 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? 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

HAZARD of interstellar flight A spacecraft hit by an 1-mm dust grain (mass of grams) while traveling at 0.1c A spacecraft hit by an 1-mm dust grain (mass of grams) while traveling at 0.1c  collision energy = (E=1/2 mv 2 ) of 5.4x10 9 J.  Same as an 1-ton object hitting at Mach 9.5 (7,000 mi/hr)!!  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 Difficulties of high-speed interstellar travel o Fuel issue (for accelerate and decelerate) o Speed limit o Travel time (round-trip possible?) o Space hazard

Interstellar Arks OK, fast traveling is challenging. How about slow traveling over long time? Hibernation of crews 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!) Long life (make human’s life longer!) o Pure speculation at the moment o Robotic missions would be simpler Multi-generational trip: Multi-generational trip: o Perseverance in the mission and/or infighting o Loss of expertise o Moral issue (1 st generation=volunteers, later generations?)

Energy use of an Interstellar Ark Speed for interstellar travel 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 Kinetic energy o Mass: say 10 8 kg (10 5 ton) for 5,000 people o = 9 x Joules = 2.5 x kW hr o 1% of the world annual energy consumption o 250 billion dollars $0.10/kW hr) o 1/5 of US GDP o Add cost of provisions, energy efficiency, deceleration!

Ion Engine Continuous firing…but weak thrust! Continuous firing…but weak thrust! Need to be free from other ions… (i.e., in space) Need to be free from other ions… (i.e., in space) Much more efficient! Much more efficient! Already used by NASA (1998, Deep Space 1) and ESA (2004, SMART-1, lunar orbiter). Already used by NASA (1998, Deep Space 1) and ESA (2004, SMART-1, lunar orbiter).

Ion Engine

How about a short-cut? Hyperspace and Wormholes Hyperspace Hyperspace o General theory of relativity (1916): space is warped by gravity Wormholes Wormholes o Rotating black holes connect to another flat space o Other flat space may connect to ours somewhere, but may be not o We will know only after we go through the wormhole o Stellar black holes: have too strong a tidal force, which would rip us apart o Massive black holes: only known in galactic nuclei, have to get there

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