[FPSPACE] FW: Centauri Dreams - The Interstellar Conundrum Reconsidered

[LARRY KLAES]

>From: Centauri Dreams <gilster at mindspring.com>
>Reply-To: Centauri Dreams <gilster at mindspring.com>
>Subject: Centauri Dreams
>Date: Thu, 21 Aug 2008 14:02:22 -0500 (CDT)
>
>Centauri Dreams
>
>///////////////////////////////////////////

>The Interstellar Conundrum Reconsidered
>
>Posted: 20 Aug 2008 03:09 PM CDT

>http://www.centauri-dreams.org/?p=2670
>
>
>Just how hard would it be to build a true interstellar craft? Im not 
>talking about a spacecraft that might, in tens of thousands of years, drift 
>past a star by happenstance, but about a true, dedicated interstellar 
>mission. Those of you whove been following my bet with Tibor Pacher on Long 
>Bets (now active, with terms available for scrutiny on the site) know that 
>I think such a mission will happen, but not any time soon. And the 
>proceedings of the Joint Propulsion Conference, held last month in 
>Hartford, go a long way toward explaining why the problem is so difficult.
>
>Wired looked at the conference results in a just published article, the 
>most interesting part of which contained Robert Frisbees speculations about 
>antimatter rocketry. Two things have been clear about antimatter for a long 
>time. The first is that producing sufficient antimatter is a problem in and 
>of itself, one that may keep us working with tiny amounts of the stuff for 
>some time to come. Even so, interesting mission concepts, like Steve Howes 
>antimatter-energized sail, have grown out of the studies that have been 
>performed on possible hybrid systems.
>
>As to antimatter itself, while the annihilation of matter and antimatter 
>releases vast amounts of energy, controlling the result is even more 
>difficult than producing antimatter in quantity in the first place. 
>Proton/anti-proton annihilation is preferable to electron/positron because 
>the gamma rays produced by the latter cant be directed to produce thrust, a 
>problem Eugen Sänger wrestled with fifty years ago. But the former is a 
>possibility because the reaction products (pions) can be directed and 
>confined electromagnetically. The idea here is to transfer some of that 
>vast energy of annihilation to a propellant working liquid.
>
>Even so, our rocket still has problems. Check our friend Adam Crowls recent 
>piece on antimatter for several good links and some musing on the 
>relatively poorer performance with antimatter than one might have expected 
>(an exhaust velocity of 0.33 c may itself be a surprise, but take a look at 
>this Frisbee presentation). Frisbee (NASA, Jet Propulsion Laboratory) has 
>been studying the interstellar conundrum for a long time, with particular 
>attention to antimatter. The design he presented at the conference, a stack 
>of linked components designed to keep radiation away from crew or payload, 
>is summed up by Wired this way:
>
>At the rocket end, a large superconducting magnet would direct the stream 
>of particles created by annihilating hydrogen and antihydrogen. A regular 
>nozzle could not be used, even if made of exotic materials, because it 
>could not withstand exposure to the high-energy particles A heavy shield 
>would protect the rest of the ship from the radiation produced by the 
>reaction.
>
>A large radiator would be placed next in line to dissipate all the heat 
>produced by the engine, followed by the storage compartments for the 
>hydrogen and antihydrogen. Because antihydrogen would be annihilated if it 
>touched the walls of any vessel, Frisbees design stores the two components 
>as ice at one degree above absolute zero.
>
>So far, so good. We then include basic spacecraft systems in front of the 
>tanks of propellant and then our payload. But theory meets a grim reality 
>in the numbers: Frisbee is talking about an 80 million metric ton starship 
>(the Space Shuttle weighs in at 2,000 metric tons), with another 40 million 
>metric tons each of hydrogen and antihydrogen. The payoff is a forty year 
>mission to Alpha Centauri.
>
>At least its designed as a rendezvous mission. A forty year flyby to the 
>Centauri stars would be moving at something better than a tenth of 
>lightspeed once it gets up to cruise. Even if exquisitely targeted, such a 
>probe would operate within 1 AU of the target system (lets say Centauri B) 
>for something less than three hours. Ponder the challenge presented by 
>collecting imagery and data from Centauri planets in such a scenario.
>
>What to do? These results reinforce much that we already knew about the 
>difficulty of coming up with an interstellar mission design that is 
>remotely affordable, and everything comes down to energy. As noted by 
>Wired, interstellar theorist Brice Cassenti (Rensselaer Polytechnic 
>Institute) comes up with a minimum value of the current energy output of 
>the entire world to send a probe to the Centauri system, a figure Cassenti 
>is quick to note could easily swell to 100 times that value.
>
>Its useful to ponder the size of the challenge as we continue to scout for 
>concepts that can overcome these problems. The dual track that interstellar 
>studies takes continues to work this way: 1) Push concepts constructed 
>under the parameters of known physics to their utmost, to see where they 
>might lead. Antimatter rockets, laser sails, pulsed fusion and their ilk 
>all fall under this category. 2) Investigate potential concepts that might 
>extend our knowledge of known physics. Here we turn to studies like those 
>sponsored by, among others, NASAs now defunct Breakthrough Propulsion 
>Physics project. The Tau Zero Foundation hopes to bring philanthropic 
>support to both approaches.
>
>No one can say whether interstellar missions will ever be feasible. What we 
>can insist is that studying physics from the standpoint of propulsion 
>science may tell us a great deal about how the universe works, whether or 
>not we ever find ways of extracting propulsive effects from such futuristic 
>means as dark matter or dark energy. And if it turns out that our 
>breakthroughs fail to materialize, the potential of multi-generational 
>missions supported by human crews still exists. They will be almost 
>inconceivably demanding, but nothing in known physics says that a 
>thousand-year mission to Centauri is beyond the reach of human technology 
>within a future we can still recognize.
>
>How big would an interstellar mission be? Let me close by quoting Robert 
>Frisbee himself, from a presentation he gave at the 2003 iteration of the 
>Joint Propulsion Conference:
>
>In the long term, it will represent a Solar System civilizations defining 
>accomplishment in much the same way we look to the past accomplishments of 
>humanity, like the Pyramids, Stonehenge, the great medieval Cathedrals of 
>Europe, the Great Wall of China and, not so long ago, a space program 
>called Apollo.
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