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Starshot: Laser Sailing to Alpha Centauri

by John G. Cramer

Alternate View Column AV-184
Keywords:  interstellar, travel, laser, propulsion, light, sail
Published in the October-2016 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 04/18/2016 and is copyrighted ©2016 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

The major news headlines in the second week of April 2016 included the announcement that billionaire science philanthropist Yuri Milner has allocated $100 million for the Starshot Project, a plan for probing nearby stars.  Milner's funds are seed money for research and development leading to the launch of a fleet of postage-stamp-size star probes aimed at the Alpha Centauri system, 4.3 light-years from Earth.  In addition to Milner, theoretical physicist Steven Hawking and Facebook founder Mark Zuckerberg form the Starshot Project board of directors.  The Starshot Project will be directed by Pete Worden, former director of NASA/Ames.  It also has an impressive list of advisors including Freeman Dyson, Martin Rees, James Benford, and Geoff Landis.

Tiny Starshot nano-probes, packed with microelectronic smarts, would be boosted by laser-illuminated light sails to about 1/5 of the speed of light.  The light pushing the sail of a Starshot probe would come from a large array of individual lasers, all phased to produce a combined single coherent laser beam.  The probe would arrive in about 21 years.


Using lasers to boost starships and probes to the stars is a familiar idea in hard science fiction.  Not long after the invention of the laser, Robert Forward suggested its use for launching and returning a starship, and he used it in his Rocheworld novels.  More recently in his novel Accelerando, Charles Stross described the launch and return of a coke-can-size interstellar ship containing downloaded virtual replicas of the active crew members.  The Starshot scheme is a simplified one-way version of these ideas taken from hard science fiction.


The laser boost of a given Starshot probe would last only about two minutes, by which time the Starshot probe would be 3.6 million kilometers from Earth and the laser beam would begin to lose focus on the light sail.  Moving at 1/5 of the speed of light, the probes could travel one astronomical unit (one Earth-Sun distance) in about 42 minutes, so the viewing of the Alpha Centauri system would last only a few hours.  With no way to stop on arrival, they could only make quick measurements during the fly-by and beam pictures and data back to Earth before passing beyond the system and continuing outward.  Any radio or laser signals from the Starshot probes would, of course, require another 4.3 years to reach Earth, so the information payoff from an Alpha Centauri launch of a Starshot probe cluster would require about 25 years. 


Light sailing, as envisioned by the Starshot project, works because a beam of light carries a small amount of momentum as well as energy.  Now let's consider some numbers based on Newtonian mechanics.  The parameters of the launch are not well specified at the Starshot web site, but references are made to 100 gigawatt beaming lasers, gram-mass probes, and launch durations between 2 and 10 minutes.  We will here assume a 100 gigawatt laser that operates for a 2 minute launch duration to give the probe velocity of 0.2 c, where c is the speed of light.  If the light beam is emitted with power P, when it bounces from the perfectly reflective surface of a light sail and reverses its direction, it will exert a force F equal to 2P/c on the sail's surface.  Division by the speed of light, which is a very large number, gives a small force.  It requires reflection of 150 megawatts of light to exert 1 newton of force on a light sail.  To accelerate each gram of mass to 0.2 c in 2 minutes requires a continuous force on the light sail of 500 newtons, acting on the probe as it moves from near-earth orbit to a distance 3.6 million kilometers away. At that distance the laser beam becomes too diffuse for further useful acceleration.  The available force must be produced by reflecting a laser power of 100 gigawatts from the light sail for two minutes.  For comparison, the largest nuclear power plant, the Palos Verde plant in Arizona , operates three nuclear reactors and produces a combined electric power output of 3.9 gigawatts.  The Starshot web site discusses some hypothetical storage system that could accumulate the needed launch power over a longer period of time and deliver it in two minute bursts.  With such storage, an electric power source comparable to Palos Verde might be adequate.  However, the hypothetical energy storage system would probably be as large and expensive as the nuclear power plant itself.  A Starshot probe, including its light sail, would also have to withstand an acceleration of about 51,000 gees, a tall order for a fragile sail structure, and would have to retain a stable orientation in the laser beam during acceleration, which is problematical.  The probe, to meet the above performance numbers, could have a mass of only 1.33 grams.

The eventual cost of the Starshot project is estimated to be around $5-10 billion.  The main expense of the system is the enormous laser array that the project requires.  A graphic at the Starshot project web site shows 135 phase-locked lasers in a 9 by 15 array beaming their power to the small probe.  If such an array was to boost a 1.3 gram Starshot probe, each of the lasers in that array, for the ten minute launch duration, would have to deliver 740 megawatts of power.  The power requirement scales directly with mass, so nano-engineering a lighter probe would require less launch power, placing a large premium on nano-scale integration.

The Starshot probes themselves, although requiring much thought and design, would be a small part of the overall cost of the project.  The fortunate consequence of this is that once such a laser system is in place with the proper power supplies and environmental impact statements, it could be used to launch a very large number of mass-produced Starshot probes, sending them in clusters to a given target. Such targets might include distant parts of our own Solar System and other nearby stars in our galactic neighborhood, provided they are in the sky at the site of the launch laser.

The other interesting star systems in our neighborhood are farther away than the 4.3 light years to Alpha Centauri, but if we were willing to wait 50 to 60 years for the Starshot results, the systems of Sirius, Epsilon Eridani, Procyon, Cygni 61, Epsilon Indi, and Tau Ceti could also be explored.  Tau Ceti is particularly interesting because it is a solitary (i.e., non-binary) star much like our own Sun.  Astronomers have found evidence from radial velocity variations that it is orbited by at least five planets.  On the other hand, Tau Ceti is about 1.2 billion years older than our Sun and is metal-poor (28% that of the Sun).  The system also differs from ours by having a debris cloud in orbit that contains about ten times as much material as similar debris in our Solar System, a potential issue for a Starshot probe.


What problems must the Starshot project overcome before Alpha Centauri be probed?  The Starshot web site lists 24 challenges that must be faced by a light-propelled nanocraft mission, along with comments from interested parties about these challenges.  The listed challenges include issues in fabricating the probe itself and its on-board power system, issues involving the structure and deployment of the light sail, issues involving the light beamer system and its power supply, issues during the launch, cruise, and flyby, and issues in communicating the accumulated data back to Earth.

One very significant problem involves collision with debris and gas.  At a speed of 0.2 c, even a collision with a milligram dust particle would liberate 1.8 billion joules of kinetic energy and could be catastrophic.  Further, radiation damage arising from cosmic rays and from collisions with interstellar gas molecules will be significant.  To a Starshot probe moving at 0.2 c, a hydrogen atom at rest looks like an 18 million electron-volt proton from a particle accelerator.  An 18 MeV proton is easily energetic enough to make nuclear reactions with most nuclei in the probe, producing secondary neutrons and gamma rays.  About 2 millimeters of aluminum is needed to range out and stop an 18 MeV proton.  Further, there is enough interstellar gas between Earth and Alpha Centauri to significantly slow the Starshot probe due to collision momentum transfer.

Another problem is that of communicating significant data from the fly-by over a separation distance of 4.3 light years.  The Starshot web site suggests that the large array of launch lasers will also serve double duty as a phased array of optical and radio wave receivers for the returning signals from the probes.  Another suggestion is to wait until the outgoing Starshot probe reaches the gravitation focal point at which Alpha Centauri's gravity focuses light rays on the Earth and to send the fly-by data from there.  This would greatly increase the received strength of the signal.

An issue that is curiously not identified among these listed challenges is the problem of accurately aiming at and passing through the inner Alpha Centauri system from a distance of 4.3 light years.  To come within one AU of one of the stars would require a pointing accuracy of about 4.8 arc-seconds.  That seems challenging, considering that the probes may be sent slightly off course by differential variations in the illumination of the light sail during launch and may be slightly deflected gravitationally by Jupiter and other massive bodies while exiting our Solar System.  The challenge discussion refers to four "light thrusters" that are part of each Starshot probe.  These are four on board multi-watt laser diodes that can do some steering by using the recoil momentum from emitting light.  One watt of laser light emitted by such a thruster produces a force of 3.3 micro-newtons.  It is not clear if this very small thrust can keep a star probe that is moving at 0.2 c on course to its target.


Is our technology up to the challenges presented by the Starshot Project?  Not at the moment, but we are working on it.  Starshot is billed as requiring work that will be ongoing for a generation, implying a time scale of 20 to 30 years before the first launch.  Further, there is a looming economic challenge, because the $100 million of seed money that Milner has provided is dwarfed by the eventual cost of the launch laser system.  As I said earlier, the current cost estimate for Starshot is $5-10 billion.  Probably some combination of national governments and other billionaires will have to fill this financial gap.  Moreover, I doubt if the ultimate cost of the project will turn out to be this low, unless there is some major breakthrough in the cost of energy generation and/or storage.

But in any case, we stand at the dawning of a new era in which some significant part of the technological effort of mankind is now aimed at sending objects to the stars.  I really like the feeling I get when considering that.


Reference:

Breakthrough Starshot web site: http://breakthroughinitiatives.org/Concept/3 .


John Cramer's new book:  a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406

SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 180 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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 This page was created by John G. Cramer on 01/31/2016.