Search for Life in the Universe

Part 1 - Interstellar Travel

04/22/23 AST 248, Fall 2005 2

Skyline Staff Meeting

Outline• Challenges of Interstellar Travel

– Distance– Speed– Energy

• “Conventional” Interstellar Spacecraft– Chemical Rockets– Nuclear Rockets– Ions, Sunlight, and Lasers– Interstellar Arks

• Relativistic Travel– Time Dilation– Antimatter and Ramjets– Hyperspace and Wormholes

Distance

• Pioneer 10 example:– Jupiter: 21 months– Alpha Centauri (not aimed there): 115,000 yr– Nearest star to trajectory: 3.3 ly in 2 myr

• Messages:– Where we are: positions relative to pulsars– Who we are: simple pictures– Our culture: music samples

Speed• Speed of light

– Special theory of relativity (1905): speed of light is the ultimate speed that cannot be exceeded

– Nearest stars 4.4 ly Minimum roundtrip travel time is 8.8 yr

• Time dilation– Simultaneity is not universal– Allows enormous reduction in travel time for the

traveler, but not for the folks back home

Energy

• Velocity for interstellar travel– Escape velocity from Earth: 11 km/s– Travel velocity, say 0.1c = 30,000 km/s,

dominates energy requirement– ~ 100 x world annual energy consumption– Add cost of provisions

Chemical Rockets• Newton’s Third Law

– To every action there is an equal and opposite reaction

– Shoot mass out of the back propel forward• Mass ratio

– Escape from Earth: 39– Best single-stage rocket: < 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

Nuclear Rockets• Method

– Advantage: higher energy/mass ratio of nuclear reactions– Disadvantage: controlled use, especially fusion– Maximum speed: ~ 0.1c, i.e., minimum travel time of decades

• Project Rover– Fission rocket– Achieve speeds 23 times chemical rockets– Application: manned mission to Mars, since abandoned

• 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 2H and 3He, ignited by an electron beam from the

spacecraft

Ions, Sunlight, and Lasers• Ions

– Same as a TV ion gun, but ions released into space– NASA tested a low-power version, Deep Space 1

• Sunlight– Radiation pressure on large solar sails– Need sails hundreds of kilometers wide– Bulk of acceleration near the Sun

• Lasers– Laser on Earth: continual acceleration– Mirror size: hundreds of kilometers– Power needed: >1,000 x total Earth production– Travel relies on continuation of project– Slow down and then return: propellant on board heated by

laser?

Interstellar Arks• Ideas

– Hibernation: long sleep– Long life: slow down aging– Multi-generational: accept many generations

• Hibernation– How do we put people to sleep?– How do we wake them up?

• Long life:– Pure speculation– Robotic mission would be simpler

• Multi-generational:– Perseverance in the mission and/or infighting– Loss of expertise

Time Dilationv/c Earth Time

[yrs]Spacecraft Time [yrs]

5 x 10-5 5 x 105 5 x 105

0.1 250 249

0.5 50 430.7 36 260.9 28 12

0.99 25 3.50.999 25 1.1

0.9999 25 0.35

Antimatter and Ramjets• Antimatter

– Exists: all matter has antimatter– Matterantimatter annihilation: all rest mass released

as rays, cf., < 0.8% in nuclear reactions– Problem: controlled storage

• Ramjets– Collect H from the interstellar medium and fuse it– Need scoops hundreds of kilometer wide

• Danger of high speed– Collisions with dust particles cause enormous

damage– Need heavy shielding

Hyperspace and Wormholes• Hyperspace

– General theory of relativity (1916): space is warped by gravity– Detailed experimental tests in weak gravity:

• Solar system• Binary pulsar

– Black holes: stellar (~10 MSun) and galactic (~106109 MSun) shown to exist

• Wormholes– Rotating black holes connect to another flat space– Other flat space may connect to ours somewhere, but may 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

Inventing Alien Life forms

• This activity is from the Univ. of Washington http://www.astro.washington.edu/labs/clearinghouse/activities/aliens.html

• Take one dice for each group of 2 students and try the evolution experiment to create your own alien. The rolling of the dice reproduces the random elements in evolution.

• When you have finished, draw a picture of your alien and give it a a name

Inventing Alien Life forms• Write a paragraph that has the following

information:– Describe the environment your creature

needs to survive. – Where in our solar system would you be

most likely to find such a creature? – What sort of food source might your

creature need? – Is your creature alone in its environment? If

not, how does it coexist with other species?