Interstellar Travel Galactic Colonization Terrestrial

Interstellar TravelGalactic ColonizationTerrestrial Habitability

Final exam schedule at

http://www.artsandscience.utoronto.ca/current/exams

(also linked from course web site)

AST 251 Life on Other Worlds Lecture 12

Exam Review Tutorials:

Tomorrow (Tues Apr 5) 11am-12noonThis Friday (Apr 8) 12-1pm

McLennan 15th floor conference room






Cosmic rays hit our atmosphere and produce the gamma rays high above the Earth.

Atmosphere blocks these (harmful) cosmic rays from reaching the Earth's surface

Gamma Ray Views of the Earth

Habitable Zone around a Red Giant Star

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QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Interstellar Travel

Spacecraft Propulsion Basics

Like everything else in the Universe, space vehicles are constrained by two basic rules:

1. Momentum is conserved. The product of mass and velocity is momentum, and cannot be created or

destroyed (just traded between objects). Momentum has direction (it’s a vector), so you can create oppositely-directed momenta that cancel. (Example: in an explosion, the total momentum can be zero even though each piece of shrapnel has its own momentum.)

2. Energy is conserved. (Or, after Einstein, Mass-energy is conserved.) You can’t make things happen that you don’t have the energy (e.g., fuel)

for.

Spacecraft Propulsion Basics

Mass M

Mass 0.5M, velocity - 0.5vex Mass 0.5M, velocity + 0.5vex

Old exhaustMass 0.5M, velocity –0.5vex

Mass 0.25M, velocity 0

0. Start with a rocket in space.

1. Spit out half of the mass as exhaust at relative speed vex.

2. Do it again!Mass 0.25M, velocity vex

In this example, you have to throw away 75% of the original mass to increase the cockpit speed by an amount vex!

In a real rocket, you throw the exhaust out slowly. This improves things a little bit: you only have to throw away 63% of the total mass to make the cockpit go at vex.

If you want the cockpit to go at speed 2vex, you have to throw away 63% of the remaining 37%, leaving only 14% of the original mass. For 3vex, 5% remains…

Lessons from “Spacecraft Propulsion Basics”:

You can’t make a rocket go much faster than its exhaust speed.

Also: you have to fight against gravity, your rocket has to get up to orbital velocity (about 8 km/s) in a short amount of time.

[Why short? Every 10 minutes, Earth’s gravity saps all of the speed you’ve built up.]

Chemical reactions produce vex ≈ 1-4.5 km/s. This is why rockets are built in stages.

Gravitational assist involves slinging a rocket around a planet. This can increase the speed up to about 30 km/s, enough to get out of the Solar System.

[After climbing out of the Sun’s gravity field, only about 8 km/s would remain!]

Let’s consider how this compares to the needs of interstellar travel.

Interstellar Travel: questions

1. How far do we have to go? To reach another star: Centauri, 4.2 light years away. To reach a known planetary system: Eridani, 10 light years away To reach a planet like Earth: who knows?

2. How long do we want to wait? Adult life: 50 years Multigeneration travel: 30 generations = 1000 years? (Linguistic & cultural stability becomes an issue…)

3. How long will it take? At 30 km/s: 4.2 light years in 42,000 years

At 3000 km/s: 4.2 light years in 420 years At almost the speed of light: 4.2 light years in 4.2 years the Galactic Centre in 24,000 years

Fast spaceflightCosmic Speed Limit

Einstein’s Relativity theory: everyone measures the speed of light c to have exactly the same value, no matter how they move relative to one another.

Q. How is this possible? A. Both space and time must get “squeezed”…

Strange Implications:

1. Time Dilation: if someone travels past you at speed v, their clock will appear to slow down by a factor (v)…. but time will seem normal to those on-board

2. Length Contraction: You’ll also notice that they seem flattened by the same factor (v) along the direction of motion.

3. Energy: An object’s mass increases as it approaches c, so the energy cost rises (the same rocket thrust generates less and less acceleration)

Fast spaceflightCosmic Speed Limit

Einstein’s Relativity theory: everyone measures the speed of light c to have exactly the same value, no matter how they move relative to one another.

Q. How is this possible? A. Both space and time must get “squeezed”…

Strange Implications:

1. Time Dilation: if someone travels past you at speed v, their clock will appear to slow down by a factor (v)…. but time will seem normal to those on-board

2. Length Contraction: You’ll also notice that they seem flattened by the same factor (v) along the direction of motion.

3. Energy: An object’s mass increases as it approaches c, so the energy cost rises (the same rocket thrust generates less and less acceleration)

Traveling at light speed takes an infinite amount of energy.

Getting Close to Light Speed

If one accelerated at the same rate a falling stone does on Earth, g = 980 cm/s per second,

Then you’d get close to the speed of light in just 1 year.

And, this rate of acceleration would produce a very natural “artificial gravity” for the spacecraft – like being in an upward-accelerating elevator.

Half the time would be spent accelerating, and the other half decelerating.

Time dilation and length contraction would bring hundreds of stars within reach in a human lifespan.

Is it really that easy? No.

Getting Close to Light Speed

Mass-Limited RocketsThe rocket can go as fast as its exhaust, or faster. Then, the mass of fuel for exhaust must be very large. This is unavoidable if you want to move close to c. Examples: ordinary rockets, antimatter rockets

Energy-Limited RocketsThe rocket doesn’t go as fast as vex. The energy required to throw the exhaust out the back still must be very large. Example: ion rockets, nuclear-thermal rockets

Externally Powered RocketsThe rocket does not carry its energy supply and may not need its own fuel either. Energy comes from outside. Examples: Jet airplanes, interstellar ramjets, solar and laser sails.

Ion Drives

Strategy: use energy from a nuclear generator to throw ions out the back of the spacecraft at high speed.

Ion drives are currently in use. One was sent to the Moon recently

Disadvantage: Low thrust compared to conventional rocketsAdvantage: High final speed. Maximum possible speed: about 0.1% c (We’re nowhere close to that!)

Antimatter Drives

Antimatter is a lot like normal matter. The two annihilate in a burst of gamma rays if they are brought together…

This makes antimatter the most efficient of all possible fuels. Exhaust velocity: c (the highest possible value!)

Even so, interstellar travel is not easy. To reach 0.99c and slow down again requires that 95% of the initial mass be fuel!

Solar Sails

The force of sunlight can be collected on a giant sail. This can be used as a gentle push to raise a spacecraftfrom one orbit to another.

Advantages: No fuel on board.

Disadvantages: Speeds above ten km/s (0.003% c) are very hard to attain. Very thin, extremely large sails required to get out of Solar System at all. Mostly useful for interplanetary travel only.

Plans exist to test solar sail technology.

Nuclear Pulse Propulsion:Project Orion

A rocket can be built by throwing nuclear bombs out the back, exploding them, and using the explosion to propel a plate forward. Shock absorbers would berequired to smoothly accelerate the cockpit.

Maximum speed: 0.5% c at best. Suitable for the construction of a low-tech “interstellar ark”

Project Orion

1954: in the Bikini Atoll nuclear test, steel spheres were thrown far awaywithout being damaged.1957: By accident, a steel plate was accelerated well above Earth’s escapevelocity during the “Plumbbob” nuclear test. 1959: Battleship-sized spacecraft planned. A test flight with chemical explosives was conducted. 1963: Nuclear Test Ban Treaty ends Project Orion.

Orion-type propulsion would be useful for emergency asteroid interceptions.

Interstellar Ramjet

Rather than bringing the fuel along, how about gathering it along the way?

Concept: 1. scoop up interstellar hydrogen; 2. collect it together and burn it to helium or further; 3. spit it out the back as exhaust.

Problems: A lot of interstellar space must be traversed to find enough material. Worse: drag from collecting fuel slows down the spacecraft as well. This limits the maximum speed to a pretty small value, <0.1% c.

Wormholes

Modern physics leaves open the possibility that black holes can be bridged by a “wormhole” through which the distance is shorter than going across regular space.

This requires weird “negative mass” and may not be stable enough to cross even if it existed.

If it did, it could be used for time travel as well…

This exotic possibility is considered very remote by most physicists. Nevertheless, it is quite exciting.

Why go fast?

Slow transport is also possible. To carry any significant amount of payload, it seems slow is the only option.

Multigenerational craft or “space arks”: carry an entire ecosystem and civilization.

Stasis craft: carry crew in hibernation.

Clone craft: carry just the genetic information to create a colony at the destination (and good enough androids to raise them!)

Robotic craft: No crew, but robots that can make more robots and do the dirty work of terraforming distant planets on arrival.

Galactic Colonization

About 5-10% of G and K stars have detectable Jupiter-mass planets.

About 10% of them are in orbits that seem to allow terrestrial planets to exist.

So perhaps 1 in 100 or 200 of G & K stars have planets that can be terraformed for human habitation.

Can we spread across the Galaxy in these conditions?

Distance between suitable planets: about 30 parsecsMaximum speed of most conceivable multigenerational craft: about 1/100th to 1/1000th of c, giving travel times of 1,000 to 10,000 years. (This assumes we can figure out how to slow down.)

If new craft are built and sent out as soon as a new planet is colonized, the Galaxyis ours in only about 10 Megayears. More realistically, 100 Myr – 1 Gyr.

Can we imagine a civilization coherent enough to carry out this task?

If other civilizations exist…

Nicolai Kardashev’s classification of civilizations:

Type 0: not in complete control of planet’s energy Chemical and nuclear propulsion, solar sails

Type I: harnesses energy output of an entire planet Laser sails

Type II: harnesses entire output of their host starAntimatter drives

Type III: colonizes and harnesses output of an entire galaxy?

Type II civilizations: the Dyson Sphere

Freeman Dyson has suggested that mankind may someday build a coherentsphere enclosing the Sun and harnessing all its energy.

If civilizations have already done this, their optical radiation will be stopped, making them invisible to optical telescopes.

However, the sphere cannot avoid emittinginfrared radiation due to the heat from thestar.

This makes the spherelook a lot like a forming protostar that is stillenclosed in dust…

Question:

What do you foresee for Humanity’s long term future?Give your rationale.

1. 50 years

2. 5000 years

3. 5 Myr

4. 5 Gyr

Water, Plate Tectonics, and HabitabilityHow important is plate tectonics? It makes the planetary surface “alive” in the sense of constant renewal. 1. Plate subduction and volcanoes give rise to a constant recycling of volatile substances. 2. It constantly produces carbon dioxide that increases the global greenhouse, so it warms the planet - this is part of the CO2-Silicate (thermostat) cycle that regulates global temps. - this allows the surface to recover from “Snowball Earth” glaciations 3. It allows crustal rock to accumulate, leading to an increase in the land mass and changing ocean circulation 4. It creates linear mountain ranges (Rockies, Himalaya, Andes…) that help widen the variety of environmental niches for species.

Plate tectonics may have slowed down the increase of oxygen in the atmosphere… because the gases and magma released by volcanoes are reducing, so they must be oxidized before oxygen can rise.

Issues for terrestrial planet habitability:

Water, Plate Tectonics, and Habitability

How important is water to plate tectonics? 1. Water softens rock and makes the crust soft enough that it can buckle and be

subducted. 2. Without surface subduction, mantle convection might stall (Mars) or occur violently

(Venus – entire surface replaced 1 Gyr ago!)3. The CO2-Silicate cycle relies on aqueous weathering of continental rocks.


How important is the Moon?

The Moon was apparently formed in a violent collision.

This may have removed a lot of volatiles from early Earth The Moon itself is completely dry (except for a few craters) and very depleted in volatile chemicals, including organic compounds. Perhaps Earth was spared from a runaway greenhouse (like Venus) due to the removal of volatiles in this event.

But we must have preserved some volatiles, or perhaps we got them from comets, so that we can have our atmosphere and oceans.

Nowadays the Moon raises tides and stabilizes the Earth’s obliquity.


Thought experiment 1: Plate tectonics stops


Thought experiment 1: Plate tectonics stops

No seafloor spreading

Carbon dioxide is removed from atmosphere: planet freezes

Continents erode

No runoff of nutrients from continents: difficult times for ocean life


Thought experiment 2: No Jupiter


Thought experiment 2: No Jupiter

Mars might be bigger, and the asteroid belt might have formed another planet

But, the rate of impacts would be 100 times higher: probably lethal for a long time.


Thought experiment 3: Jupiter & Saturn are bigger or closer


Thought experiment 3: Jupiter & Saturn are bigger or closer

The Solar System is no longer stable!

Jupiter and Saturn will perturb each others’ orbits significantly, and Saturn is likely to go onto an eccentric orbit.

This is bad news for Earth.


Thought experiment 4: A 15-km-diameter asteroid or comet strikes Earth


Thought experiment 4: A 15-km-diameter asteroid or comet strikes Earth

1. Vast explosion of rock into atmosphere 2. Huge earthquakes and tidal waves 3. Boulders kicked upward fall back down and ignite fires all over the planet4. Dust and smoke in upper atmosphere; planet cools for six months 5. Nitrous oxide produced in heated atmosphere: produces nitric acid and acid rain – ocean surface becomes acidic 6. After dust settles, carbon dioxide from the fires leads to long-term heating until

the carbon dioxide can be removed from the atmosphere.


Is global risk a necessity for complex life to develop?

This is a central tenet of Ward & Brownlee’s “Rare Earth” hypothesis

Mass extinctions open niches for new types of animals to gain hold.

These have to be quite serious, yet leave enough species alive for a new wave.


End of lecture 12

End of course

UofT Department of Astronomy & Astrophysicsin partnership with

Ontario Science Centre and Astronomy & Space Exploration Societypresents

Cosmic Frontiersa series of public lectures celebrating

a century of astronomy at UofT

Sep 21: “Dark side of the universe”Rocky Kolb, University of Chicago

Sep 28: “Way too cool: Tales of stellar corpses”David Helfand, Columbia University

Oct 14: “Quest for other worlds and prospects for life”Debra Fischer, San Francisco State University

Oct 21: “In search of the cosmic dawn”Bob Abraham, University of Toronto

All lectures will be Fridays 7pm in Convocation Hall

Documents

Interstellar Travel Galactic Colonization Terrestrial