Interstellar Travel

ASTR 1420

Lecture 25

Sections 13.1 & 13.2

Four spacecrafts flying away from the Sun

Voyagers : 1977Pioneers : 1972 & 1973

Voyager 1 reached theend of Solar System in 2010

Pioneer 10 (& 11)• 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 3rd Law• 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?

No!Saturn V rocket

Chemical Rockets limited by Mass ratio• Need to accelerate fuel also! Mass ratioo mass ratio = weight of a rocket with fuel / without Current technologyo To escape from Earth: mass ratio = 39o Best single-stage rocket: mass ratio < 15

• Multi-staged rocketso Necessary, and used, to leave Earth, or even for

intercontinental ballistic missileso Interstellar travel: impractical, hundreds to

thousands of stages required

o Most powerful rocket engine with 100 stagesachieve 0.001ctakes 4,000 yrs to the

nearest star!

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

Nuclear Rockets

• Methodo Advantage: higher energy/mass

ratio of nuclear reactionso Disadvantage: difficult to controll

especially fusiono Maximum speed: ~ 0.1c,

• Project Rovero Fission rocketo Achieve speeds 23 times those of

chemical rocketso Application: manned mission to

Mars, since abandoned

Nuclear Thermal Rocket Engine

Project OrionExplode 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 2H and 3He, 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.• 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!

Antimatter rocket• Antimatter rocketo Exists: all matter has antimattero Matterantimatter annihilation: all mass converted into light! (100% change

versus < 0.7% mass-to-energy conversion in nuclear reactions)o Problem: controlled storage

• Edward Purcell (1952 Novel Prize for nuclear magnetic resonance ): Imaginary antimatter rocketship with 100% engine efficiency…o maximum speed of 0.99c 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 times 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!!

Fuel to accelerate & decelerate is the main issue!!

Interstellar Ramjets : collect fuel during flight!• Ramjetso Collect Hydrogen from the interstellar medium and fuse ito 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 theimaginary interstellar

ramjet rocketship

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.

One advatange : Travel Time gets shorter at high-speed!

Time dilationtime 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 0.012 grams) while

traveling at 0.1c collision energy = (E=1/2 mv2) of 5.4x109 J.

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 travelo Fuel issue (for accelerate and decelerate)o Speed limito Travel time (round-trip possible?)o Space hazard

Interstellar Arks

OK, fast traveling is challenging. How about slow traveling over long time?

• Hibernation of crewso 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!)o Pure speculation at the momento Robotic missions would be simpler

• Multi-generational trip:o Perseverance in the mission and/or infightingo Loss of expertiseo Moral issue (1st generation=volunteers, later generations?)

Energy use of an Interstellar Ark• Speed for interstellar travelo Escape velocity from Earth: 11 km/seco Travel velocity, say 0.001c = 300 km/sec, dominates energy requirement

• Kinetic energyo Mass: say 108 kg (105 ton) for 5,000 peopleo = 9 x 1018 Joules = 2.5 x 1012 kW hro 1% of the world annual energy consumptiono 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!• 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).

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

• Wormholeso Rotating black holes connect to another flat spaceo Other flat space may connect to ours somewhere, but may be noto We will know only after we go through the wormholeo Stellar black holes: have too strong a tidal force, which would rip us aparto 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 & 13.2Terra-forming and Colonization : next class!