Starships

1. Starships Lecture Twenty-Eight, Apr. 11, 2003

2. Pluto • In 1991, the United States Postal Service issued a series of stamps commemorating American space exploration. The stamp for Pluto consisted of a painting of a grayish disk with an apology of sorts underneath: "Not Yet Explored." • Spacecraft have visited other planets.

3. Pluto’s discovery • Clyde W. Tombaugh spent years searching for a hypothesized ninth planet to explain small perturbations in the orbits of Uranus and Neptune, and then on Feb. 18, 1930, he found Pluto. Note, the perturbation calculations were wrong, and Pluto, turned out to be smaller than Earth's Moon — too small to account for the perturbations, in any case. • In June 1978, James W. Christy noticed that in one image, Pluto's point of light seemed to have a bump on it. He went back and looked at other photographs and Pluto seemed to have a bump in many of those images, too, and the bump seemed to be moving around. This was discovery of moon Charon.

4. Is Pluto a Planet? • It is very small. Much smaller then several moons. • It has an odd orbit that is tilted out of the plane by 17 degrees and is very eccentric. So eccentric that it crosses the orbit of Neptune. • Pluto discovered 72 years ago. In recent time we have found many other minor planets even farther from the sun. Some of these are quite large (although none yet as large as Pluto). • Many astronomers think we will soon find an object as large or larger than Pluto, far from sun in Kuiper belt. • Pluto is a planet because have called it so for 70 years! • If Pluto were to be discovered today it might be classified as a minor planet, just one of many in the Kuiper belt.

5. Pluto and its moon Charon are a “double planet”

7. Pluto • Has an atmosphere and mysterious bright and dark marking with large contrast. • Charon probably formed in a large collision like earth’s moon. • Spectra suggest nitrogen, methane and carbon monoxide frost on Pluto, and water frost on Charon.

8. New Horizons Mission to Pluto • Planned for 2006 launch and 10 year journey to Pluto. • Need very small spacecraft that can be accelerated to high speed. • Uses nuclear power and miniature instruments. • Plan is to flyby Pluto and explore one or more other Kuiper belt objects.

9. Interstellar Flight • Can we search beyond our solar system for life near other stars? • Can we go to another star? • What can we learn about life near other stars by using sensitive instruments (without leaving the solar system). • Can we leave our Milky Way galaxy and search for life in other galaxies? (Probably no.)

10. A Light Year • Light travels 186,000 miles per second. • In one year light travels 6 £ 1012 miles. • Thus a light year is 6 trillion miles! • The nearest star is 4.3 light years away. • Interstellar journeys need to cover many trillions of miles!

11. Where would you like to go today?

12. Andromeda Galaxy with hundreds of billions of stars

14. Interstellar flight is hard • Need to go fast given huge distances. • It takes a very big and efficient rocket to go fast. • Suppose you have a 1 ton payload and a 9 ton rocket can boost the payload to a speed v. • How big a rocket does it take to go 2v? • Answer 100 tons total and a 1000 ton rocket is needed to go 3v!

15. 2 stage rocket 1 ton payload 9 ton 2nd stage rocket • A 10 ton system (9 ton rocket plus 1 ton payload) can go a speed v. • Then a 90 ton booster rocket can boost this 10 ton payload to a speed v. • After booster runs out of fuel fire 9 ton 2nd stage to accelerate from v to 2v. 90 ton booster rocket

16. Giant Saturn 5 Moon Rocket and Apollo spacecraft is 363 feet tall.

17. Rockets are gigantic • Because first stage must accelerate both payload and all the weight of the upper stages and their fuel. • A rocket spends most of its energy accelerating the fuel rather then accelerating the payload.

18. Options for Interstellar flight • Use a fuel with much more energy per pound compared to conventional chemical fuels such as hydrogen and oxygen. • Use some scheme where it is not necessary to carry the fuel. • Be prepared for a very long trip!

19. Slow Boats • Can a mission of 1,000 or 10,000 years succeed? • Automated probes to explore another solar system. • Generation ships (or space colonizes). How large would they have to be and with how many people? How to accelerate this large mass? • Suspended animation (send people in hibernation). • Send frozen fertilized human eggs and specialized machines.

20. More Powerful Rockets • Nuclear power such as project Orion. • Antimatter rockets. • Laser acceleration: Place a large mirror on the starship and bounce a powerful ground based laser off of it to accelerate the ship. Don’t need to accelerate the fuel on earth! Problem, how to stop?

21. Nuclear Explosive Power • Explode a whole series of small nuclear weapons behind a pusher plate. • Very high temperatures reached in nuclear explosions accelerate debris to very high speeds. • This makes an incredibly powerful rocket if your spacecraft survives.

23. Antimatter Rockets • Each particle has a corresponding antiparticle with the same mass but opposite charge. The anti-electron is called a positron. A hydrogen atom has an electron orbiting a proton. • An antihydrogen atom has a positron orbiting an antiproton. This is the simplest atom of antimatter. • When antimatter and matter meet they annihilate into gamma rays releasing giant amounts of energy.

24. Making antimatter • Accelerators can make antiparticles by colliding high energy particle beams. • We can collect these antiparticles using magnetic fields and other means and accelerate them to produce beams of antiparticles. SLAC in California collides a positron beam with an electron beam. • To date we have only been able to make a few atoms of antihydrogen by combining positrons and antiprotons.

25. Antimatter rockets • Antimatter is perhaps the ultimate fuel because it can release the most energy per mass. • However very very hard to make the antimatter, store it and annihilate it in such a way that all the gamma rays go backwards so the rocket will recoil forward.

26. Relativistic Time Dilation • If a rocket has enough energy to go a significant fraction of the speed of light strange things happen. • Time seems to slow down on the rocket. • Trocket = [1-(v/c)2]1/2 Tearth • Here v is speed of rocket and c is speed of light. • Example v=0.9c: Trocket=0.43 Tearth • Passangers age only 4 years during a 10 year trip at 0.9 c.

27. ’39 by Brian May of Queen In the year of '39 assembled here the volunteers In the days when the lands were few Here the ship sailed out into the blue and sunny morn Sweetest sign ever seen And the night followed day And the story tellers say That the score brave souls inside For many a lonely day sailed across the milky seas Ne'er looked back, never feared, never cried Don't you hear my call though you're many years away Don't you hear me calling you Write your letters in the sand For the day I take your hand In the land that our grandchildren knew

28. Aah, aah, aah, aah In the year of '39 came a ship in from the blue The volunteers came home that day And they bring good news of a world so newly born Though their hearts so heavily weigh For the earth is old and grey, little darling we'll away But my love this cannot be Oh so many years have gone though I'm older but a year Your mother's eyes from your eyes cry to me Don't you hear my call though you're many years away Don't you hear me calling you Write your letters in the sand for the day I take your hand In the land that our grandchildren knew Don't you hear my call though you're many years away Don't you hear me calling you All your letters in the sand cannot heal me like your hand For my life Still ahead Pity me

29. Starships • Are very very hard to build because the distances are so great. • It might cost 100 billion $ to send people to Mars. This is comparable to the space station or the Apollo moon landings. • A starship might cost our full gross national product??? • Only for a far future time when worlds economy is larger.

30. Justifying the Cost • Maybe starships are too expensive just to explore other stars. • Colonization of new worlds could justify great cost. Or avoiding disaster on earth. • For now work hard to explore other stars with sensitive instruments from within our solar system.