Physics Today

Commentary: Let’s send the DOE to Alpha Centauri Edwin Kite and Andrew Howard

Citation: Physics Today 66(9), 8 (2013); doi: 10.1063/PT.3.2095 View online: http://dx.doi.org/10.1063/PT.3.2095 View Table of Contents: http://scitation.aip.org/content/aip/magazine/physicstoday/66/9?ver=pdfcov Published by the AIP Publishing

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.215.71.79 On: Thu, 12 Dec 2013 15:33:38 readers’ forum Commentary Let’s send the DOE to Alpha Centauri

he announcement of an Earth-mass planet orbiting the closest Sunlike T 1 star has renewed discussion of the costs and benefits of an interstellar probe. The planet, Alpha Centauri Bb, is in an extremely hot orbit, but its existence increases the probability that a planet with a liquid-water ocean orbits Alpha Centauri. Data from NASA’s Kepler tele- scope now show that there are about as many habitable-zone, Earth-sized planets as stars,2,3 and in April NASA decided to fund a follow-up spacecraft, the Transiting Exoplanet Survey Satellite. Before this decade is out, we will prob- ably discover an ocean-bearing planet orbiting a nearby star. When we do, should we send a mission? Scientific rewards from such a mis- sion would extend to all fields. Earth’s climatic complexity, biosphere, and geo- dynamics are unique in the solar system. Artist's impression of the Earth-mass planet (crescent) and two stars (center and If we could double our sample of planets lower left) in the Alpha Centauri triple-star system. A spacecraft propelled by a fusion that have surface oceans, geophysicists or antimatter rocket could travel the 4.4 light-years from Earth to capture detailed would have a chance to understand images and spectra of the planets in this system. From this vantage point, our own whether Earth’s environmental stability sun (upper right) is the bright star piercing the Milky Way in the constellation is an inorganic phenomenon, a result Cassiopeia. (Image courtesy of ESO/L. Calçada/Nick Risinger, skysurvey.org). of global biological feedback (the Gaia

hypothesis), or a statistical fluke that probe carried a Hubble-class telescope, and Ve is engine exhaust velocity. Ion en- −4 allowed us to survive to witness it. As- such as the two recently made available gines reach Ve of approximately 10 c; tronomers and space physicists would to NASA by the National Reconnais- chemical rockets are even less efficient.

gain from study during a probe’s travels, sance Office (see PHYSICS TODAY, July Much higher Ve is needed to reach the both interstellar cruise and the explo- 2012, page 26), it would not be able to stars in a reasonable time, and that re- ration of a second circumstellar environ- send back high-resolution images of the quires antimatter or fusion propulsion.5 ment—especially the environment of target star early enough for humans to Turning a high-energy physics ex- Alpha Centauri, a multiple-star system. update the encounter plans, so some au- periment into a propulsion system has Topics would include the planet’s chem- tonomy is essential. been done before: Within six years of its istry, orbital environment, and climate. Engineering and materials science inception in 1949, the Naval Reactors Finally, a mission would address ques- challenges are involved in building a organization, then led by Hyman Rick- tions that can never be convincingly an- spacecraft that would be reliable for the over, developed the first fission reactors swered by near-Earth telescopes: Could long term and rugged enough to with- for the US Navy’s nuclear fleet. How- the planet be made habitable? Is it al- stand collisions with interstellar dust at ever, the problem is not one that NASA ready inhabited? speeds much greater than 103 km/s, and is designed to solve. Any robotic flyby of Alpha Centauri the limited number of received photons Developing technology for an inter- in the foreseeable future would be led by will force a tradeoff between spectral stellar rocket makes scientific sense the US. Compared to the technologies of resolution and spatial detail for the when there is a compelling scientific NASA’s Voyager 1, currently 0.7 light- flyby. But all those challenges are reason, when the physical principles of days from Earth, improved guidance, within NASA’s reach, and they could the technology are well understood, navigation, control, and autonomous indeed provide a new stimulus and when going still faster would require operations would be required. For ex- focus for the agency. unanticipated new physics, and when ample, longer-baseline parallax meas- However, NASA alone cannot reach the technology will not be developed urements are needed to reduce errors in Alpha Centauri. The rocket equation for another purpose. All four conditions

calculating Alpha Centauri’s position. states Mf /Msc = exp(V/Ve), where Mf is are true for fusion and antimatter Optical links are better suited to inter- fuel mass, Msc is spacecraft dry mass in- propulsion. With fusion or antimatter stellar communications than is Voy- cluding payload, V is cruise speed (10−2 c propulsion, V is approximately 0.03 c ager’s radio transmitter.4 Even if the to 10−1 c, where c is the speed of light), or, very optimistically, 0.1 c or higher.

This8 articleSeptember is copyrighted 2013 as Physicsindicated Todayin the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. www.physicstoday.org Downloaded to IP: 131.215.71.79 On: Thu, 12 Dec 2013 15:33:38 Laboratory Could an interstellar mission hold the unacceptably ambitious. But powerful Cryogenic public’s interest for the duration—two images reset assumptions: Pictures of or three times as long as the Voyager Earth—Blue Marble and Earthrise, for Systems missions? Then again, since the costs of example—taken by Apollo astronauts construction would exceed the costs of helped to launch the global environ- operation, sustained public interest mental movement. Similarly, an image Ultra Low Vibration might not matter. of an ocean planet around another star6 The US Department of Energy and would unsettle our assumption that its international partners have commit- Earth is the only planet in our own ted approximately $100 billion to fusion long-term future. Such a widening of power research over the years. In doing horizons has happened before: The de- so, they have gambled that fusion tection of the New World broke Euro- power will not face the same public op- pean scholasticism and contributed to position that has stymied fission power. the Copernican revolution. The tenor of the recent debates leading Four hundred years later, we scien- to nuclear switch-off in Germany and tists still wonder if Earth and life on it Japan suggests that fission versus fu- resulted from chance or necessity.7 Until sion is a fine distinction that will not be SETI succeeds or we can build optical made. A fusion propulsion system telescopes the size of Los Angeles, an in- would be assembled from radioactively terstellar probe is the only experiment inert components in high orbit—that is, that can end that speculation. Galileo MicroRaman Galilei wrote that living beings beyond in no one’s backyard. Fusion propulsion MicroPL requires only that ejecta momentum is Earth “would be extremely diverse, and aimed in one direction, and it does not far beyond all our imaginings.” Let’s put 3-5 nm require net power output. his hypothesis to the test. Inertial confinement fusion—and pulsed fusion more generally—is a step References toward interstellar flight. Although the 1. X. Dumusque et al., Nature 491, 207 (2012). main purpose of the National Ignition 2. T. D. Morton, J. J. Swift, http://arxiv.org Facility (NIF) is to sustain an arsenal of /abs/1303.3013; C. D. Dressing, D. Char- nuclear weapons, Lawrence Livermore bonneau, http://arxiv.org/abs/1302.1647v2. National Laboratory has promoted 3. W. J. Borucki et al., Science 340, 587 (2013). 4. A. W. Howard et al., Astrophys. J. 613, 1270 peaceful uses for NIF’s successor. Even (2004). from a narrow national-security per- 5. T. Kammash, ed., Fusion Energy in Space spective, that approach is reasonable: A Propulsion, American Institute of Aeronau- revolutionary mission with peaceful tics and Astronautics, Reston, VA (1995). purposes is valuable for attracting tal- 6. O. Guyon et al., Astrophys. J., Suppl. 167, Atomic Resolution Non Optical 81 (2006). Helitran® CCR ented people, demonstrating continued 1.7K - 400K 1.7K - 350K US leadership, and maintaining a tech- 7. J. Monod, Chance and Necessity: An Essay on the Natural Philosophy of Modern Biology, 1.5W @ 4.2K 40mW @ 2K nological edge. The national laboratories A. Wainhouse, trans., Vintage, New York most associated with weapons design— (1972). Los Alamos, Lawrence Livermore, and Edwin Kite Sandia—take pride in solving complex ([email protected]) physics and engineering problems. But California Institute of Technology the National Ignition Campaign has not Pasadena succeeded as scheduled (see PHYSICS Andrew Howard TODAY, June 2013, page 20), and the post- ([email protected]) Fukushima opposition to nuclear power University of Hawaii Honolulu is hardening. Interstellar propulsion of- fers a substitute complex problem, so exoplanet discoveries provide an alter- Letters and commentary are encouraged native focus for fusion research. and should be sent by email to If the extremely high efficiencies [email protected] (using your surname needed for interstellar travel cannot be as the Subject line), or by standard mail achieved with a fusion rocket, NASA to Letters, PHYSICS TODAY, American Center and DOE would still have produced an for Physics, One Physics Ellipse, College extremely capable vehicle for travel to Park, MD 20740-3842. Please include Mars, asteroids, and the moons of Sat- urn and Jupiter; it could be used for sci- your name, work affiliation, mailing ence, terraforming, or settlement. address, email address, and daytime Clearly, an interstellar probe would phone number on your letter and represent something beyond the contin- attachments. You can also contact us uation of long-running programs at online at http://contact.physicstoday.org. Advanced Research Systems NASA and DOE. In the current fiscal We reserve the right to edit submissions. Tel: 610.967.2120 climate it is tempting to dismiss it as www.arscryo.com This articlewww.physicstoday.org is copyrighted as indicated in the article. Reuse September of AIP content 2013 is subject Physics to Todaythe terms9 at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.215.71.79 On: Thu, 12 Dec 2013 15:33:38