Because I get irritable when I don’t get my walking in every day, I made sure when I arrived in Palo Alto to cram as much as I could into the day before Breakthrough Discuss began. That meant heading out from the hotel just after noon and putting in about five miles. Palo Alto is a very walkable place and I found myself ambling up and down shady streets past gardens bright with spring flowers. We had superb weather for the entire conference, but naturally when things got going, both days were crammed with talks and long walks were out of the question.
But the day before, as I walked, I pondered the schedule of the conference, wondering how a mission to Alpha Centauri fit into the overall plan. In addition to the $100 million going toward Breakthrough Starshot, Breakthrough Initiatives has also put up $100 million for its SETI project, which has already begun operations at Green Bank (West Virginia) and is slated to operate at the Parkes dish in Australia as well, giving SETI the services of two of the largest steerable dishes in the world. But as we’ve seen, SETI fit into the Starshot discussion because beamed laser technologies are an observable, and one that can be seen from a long way off.
The focus on Alpha Centauri also turned out to be a natural for Starshot. If you’re hoping to build a mission, one of your first priorities is to understand the target. Here we’re very close to learning what kind of planets are available around the three stars in the Alpha Centauri system, and what other challenges by way of dust and debris we may encounter with a fast probe through the plane of any planets found there. And of course a technology that can get us to Alpha Centauri is an enabler for a gravitational lens mission, which I’ll discuss more on Monday.
Miracles Required?
In more than one overheard conversation a sentence more or less like this one occurred: “Just how many miracles are required to get this plan to work?” The theory being that one miracle is OK, because we can’t discount how fast technology is developing, and two miracles might also work, because we can’t know in which direction we’re going to be surprised. But as miracles pile up, the odds become more daunting. That’s why Breakthrough Starshot is spending $100 million. That’s far less than the actual mission would require, but it’s a more than workable amount to create the kind of concept study which is what is in Starshot’s immediate future.
As Pete Worden, former director of NASA Ames and now executive director of Breakthrough Initiatives made clear, there is a process here through multiple grants, research and experiments that will attack the problem of sail beaming at every level, addressing along the way the issues of data return, dust mitigation during the flight, beamer construction, sail materials technology and all the other matters that bedevil so futuristic a concept. Breakthrough Starshot will hope to complete the study with a scaled-down prototype of the mission hardware.
Kepler’s famous letter to Galileo came up not just in Worden’s keynote on day two but in several other talks, it being the first time using sails in space in any realistic way was introduced. Here’s the relevant quote:
There will certainly be no lack of human pioneers when we have mastered the art of flight… Let us create vessels and sails adjusted to the heavenly ether, and there will be plenty of people unafraid of the empty wastes. In the meantime, we shall prepare for the brave sky-travelers maps of the celestial bodies. I shall do it for the moon, you Galileo, for Jupiter.
That was written in 1610, and it reminds us that despite their misconceptions about what they would find in space, the early pioneers of astronomy could draw sound conclusions from their observations. Kepler had studied comets and viewed the fact that comet tails always pointed away from the Sun as evidence for a ‘solar wind’ that could be harnessed by space travelers. We might also remember that Kepler wrote an early science fiction tale called the Somnium, a journey to the Moon accomplished through supernatural means, but with an attempt to be as scientifically accurate as possible given knowledge of the day.
Image: In a new and much smaller room, day two of Breakthrough Discuss began with Pete Worden’s keynote.
Today’s beamed sail draws on trends that were not obvious even decades ago when science fiction writers and physicists alike described the sail missions we might one day fly. Today nanotechnology is helping us build materials one molecule at a time, Worden said, while we have a continuing decrease in size and increase in complexity in electronics, along with advances in lasers and fiber optics that point toward creating phased laser arrays. A key question, of course, is this: Does something like Moore’s Law apply to laser technologies? And how far can we extrapolate it taking us?
The Starchip we would like to see deployed on the Starshot is about the size of a postage stamp, for as I’ve said before, this is a spacecraft more like a smartphone than a macro-scaled capsule with beams and struts. In Worden’s words:
“The Starchip. enabled by phone technologies, is under 1 gram in mass. That makes it 1 million times lighter than an ordinary spacecraft, and on this chip we can carry cameras, thrusters, navigation and communications equipment at the cost of an iPhone. NASA doesn’t like to build anything small, but I’m now seeing JPL doing all sorts of things with smaller satellites. As to the sail, nanotechnology is key to building a sail 300 atoms thick that has to hold together and not absorb much energy, but transmit and reflect energy. Experiments will start soon.”
These are huge requirements, for we need a sail as light as smoke that can nonetheless maintain its structural integrity when hit by the beam from a phased laser array that will accelerate it at 60,000 g’s to twenty percent of the speed of light. The slightest absorption of heat will fry the sail, and it had better be engineered to the tolerances needed to stay on the beam, which is going to involve a great deal of investigation. The only laboratory work on microwave sail beaming I’m aware of, conducted by the Benford brothers, implies the need for a cone-shaped sail of a specific topology that will also spin, riding the beam rather than being deflected from it. What we need now is lab work involving laser-beaming to clarify these matters.
Can the sail and chip withstand the beating they’ll take from the laser array? 60,000 g’s seems astonishing, but Worden noted that the chip has to survive only a few minutes of this, and the technology to protect it is developing rapidly, there being artillery rounds that do guidance and propulsion and withstand 100,000 g’s even now. The starships would be deployed by a ‘mothership’ in a highly elliptical orbit, after which the mothership would depart the area to allow the beam to work its magic on the sails. Two minutes to 20 percent of the speed of light.
A Full Array of Challenges
A four-meter sail produced in great numbers and capable of riding a powerful laser beam would be the kind of enabler Robert Forward used to think about, though in the days when he took an interstellar program to the US Congress (see his “A National Space Program for Interstellar Exploration,” as discussed in Roadmap to the Stars, a 2013 Centauri Dreams entry), he was talking about much larger ships. In his document, which was published by the House Committee on Science and Technology, Forward talked about sending automated probes to nearby star systems by the turn of the 21st Century, beginning with a 15-year period of mission definition studies and work on key technology areas. This was in 1975, and I can only imagine what Forward would have thought of having $100 million to pursue the idea.
Now we think small on the one end — the sail, the payload — and large on another — the beamer. Phase locking combines the output of an array of lasers into a single focused beam, with Worden describing “network effects that amplify the beam into a laser wind of 50 gigawatts.” If we can build something like this, perhaps in Chile’s Atacama desert, perhaps in Antarctica, we could theoretically create a modular and scalable system that could make Forward’s dream a reality, sending fast flybys to a host of nearby stars at tens of thousands of kilometers per second.
It was great to talk to Mason Peck at the Breakthrough Discuss meetings, especially since his work at Cornell on the tiny spacecraft he calls ‘Sprites’ has led directly to ideas like Starchip. Both Peck and Zac Manchester, also at the conference, have worked in the Cornell lab on the Sprite technology, each of these 32 x 32 x 4mm in size and weighing less than 7.5 grams. You can see a rundown of these chip-sized satellites in Sprites: A Chip-Sized Spacecraft Solution, which I wrote in 2014. At lunch on the second day of the conference, Peck showed several of us a Sprite, whose implications in swarm technology missions continue to fascinate me.
But back to the beamer. Worden said that the Atacama desert might be the best location of those considered — the Kalahari, the Australian outback, Antarctica — because it has more infrastructure, and power might be bought from utilities or produced through systems built by the project itself, perhaps solar, perhaps nuclear. Space hasn’t entirely been ruled out, but it’s hard to see us ready to build a huge, privately funded laser installation in space even within several decades. For that matter, you can imagine the political issues involved in placing a laser that could be weaponized anywhere near Earth orbit.
The Starshot sail would fly edge-on to minimize the cross-section exposed to matter in the interstellar medium. Here we’re dealing with a lot of unknowns because we’ve only gotten one mission out beyond the heliopause, and it — Voyager — wasn’t designed to do the kind of measurements we’d like to have about the Local Interstellar Medium (LISM). But based on what we do know about local ‘bubbles’ in the medium and our Sun’s position in them, a fast mission to Alpha Centauri seems survivable at least by some of the craft thrown at it. Redundancy thus becomes crucial, which is why the plan is to send a large number of sails.
And here we arrive at yet another challenge, or ‘miracle’ if you will. We’ll look at getting a signal back to Earth on Monday, but the plan is to use the sail itself as an optical element, turning it into a phased receiver as well as a transmitter. The tolerances needed in doing this, and the technologies required to shape the sail at its destination, remain unexplored territory. We have to ensure that this element is not the showstopper. As you might expect, data reception back on Earth is to be handled through the enormous laser array that sent the craft.
That array also serves as a kilometer-class telescope, meaning it would have a useful future of continuing astronomical observation. And as a beamer, says Worden, the laser array is multi-purpose. A successful beamer could make possible any number of missions within the Solar System and beyond, including the gravitational lens FOCAL mission. We have to remember we’re not just targeting Alpha Centauri. “We’re convinced we can contemplate in this century, and perhaps in a single generation, expanding the human reach to the stars.” Note the plural.
I’ve only mentioned some of the larger challenges Breakthrough Starshot will face. At the Yuri’s Night celebration, Worden showed a slide listing nineteen, as seen in the image above. These suggest and certainly do not exhaust the list of issues that Starshot raises. In my last Breakthrough Discuss report on Monday, I’ll look at how the FOCAL mission to the Sun’s gravitational lens fits into the mission, with thoughts on where we go from here.
I invariably get goosebumps everytime I read that Kepler quote. His mind certainly was able to see over the empty wastes of the centuries
“The Starshot sail would fly edge-on to minimize the cross-section exposed to matter in the interstellar medium. ”
Doesn’t that trade fewer impacts, for much greater damage?
At that kind of speed, each impact produces a narrow “jet” of debris, itself capable of producing substantial damage. In a face on scenario, each impact blows a tiny conical hole in the sail, but the volume of damaged material is going to be quite small, because it IS a narrow cone, and the sail is quite thin. The debris jet exits the sail long before it’s energy is spent.
But, edge on, you’re going to get a long triangle of damage from each impact. A much greater volume of material will be damaged, much of it volatilized. I suppose the sail is large enough it won’t actually be cut in half, but the leading edge will fray very badly.
And unless your edge on orientation is maintained to a very high precision, it’s all leading edge.
I’ve got serious doubts about an unshielded object making it to Alpha at that kind of speed, unless it’s capable of active self-repair. Which I suppose is under consideration.
There’s a lot of this work, though, that would be applicable to mass beam propulsion, though, where the beam consists of a stream of sails. So the research won’t be wasted even if the original concept proves to be infeasible.
@Brett Bellmore:
Interesting point! It looks like there’s an optimum angle, based on the various system parameters. Because we have to be able to line up the trajectories with decent precision (else we’ll be wildly off-target after years of travel), the idea of a leading “shield group” of sacrificial chips might be of benefit, to attempt to clear the path.
Great idea. Assuming very accurate navigation, the leading sails would blow a tunnel through the interstellar medium that the trailing sails could safely travel through. A sacrificial shield.
I would think that such a tunnel would quickly be filled again in a small fraction of a second due to the movement of the ISM particles. Both thermal motion and transverse “wind”. The latter should be of the order of tens of km/s, as this is the average relative velocity of stars. This would mean the “tunnel” would last only for 100 microsecond or so.
This depends very much upon the prevailing direction of the ISM wind. Best case for the sacrificial shield scenario is for a minimum transverse component. Of course, we don’t know much about this since nobody has yet made any maps! This highlights the breakthrough nature of this new mode of interstellar exploration. There are all sorts of new things to discover along the way.
‘“The Starshot sail would fly edge-on to minimize the cross-section exposed to matter in the interstellar medium. ”
Doesn’t that trade fewer impacts, for much greater damage?’
Gas and dust will have a lot of material to go through edge on and most of the debris will most likely go to the sides missing the rest of the sail. You are correct about the face on sail hitting more objects and punching very small holes through it, all good and well, but the electronics will be more badly hit by gas ions in a face on configuration than with an edge on one.
In Robert Charles Wilson’s science fiction novel “Burning Paradise” (highly recommended like everything from RCW), nasty aliens use the Atacama desert to launch spore packages to other stars, with a technique essentially similar to Starshot. They don’t use microsails though (I guess RCW didn’t think of that) but shine high power lasers directly on the packages. The book has impressive descriptions of the Atacama desert.
I think, however, that the Starshot team will be forced to reconsider the location of the beamer. Which country will accept the responsibility to protect the beamer from terrorists that could use it to shoot planes in the sky, satellites etc.?
I hope they will consider a launch site on the far side of the Moon. Other parties including national space agencies could be persuaded to contribute to the establishment of a moonbase on the far side, which could conceivably raise its own funding in full. Perhaps Starshot could be the catalyst that makes a moonbase happen.
Space is filled with microwaves. A SpaceChip might benefit from this effect
http://www.eetimes.com/document.asp?doc_id=1329498&piddl_msgid=358190
Sounds like a potential synergy here:
Space is not “filled with microwaves”. Microwaves is one of the quieter areas of the spectrum, I believe. In any case, extractable energy would be minuscule. Less than photovoltaic cells would produce from starlight, probably.
“Within a few years of the discovery of the CMBR, it was established … the specific intensity of the radiation is … 4 x 108 photons m-3, and an energy density … ~ 4 x 10-14 J m-3, which can also be expressed as a mass density … ~ 5 x 10-31 kg m-3, much less than the critical density required to close the Universe.”
http://ned.ipac.caltech.edu/level5/Birkinshaw/Birk1_1.html
Not if this would be enough to benefit the Star Chip or not.
While the mere success in reaching exostellar systems would be fantastic, I’m missing any talk of instrumentation and measurements on target. Given the uncertainties in trajectory and velocity, a precise intersect to, say, within 10 million km of a selected target may be very unlikely. At distances of 1 AU a goal I think Dr. Loeb postulated, even a very precise microcamera will be resolving nothing. What options for instrumentation are there for planetary science? Please post your ideas. (Aside from the obvious statistical solution of mass armadas achieving at least one close encounter)
Could a hundred balloons with jet turbine electric generators be used to create the laser beams? The thrust downwards to generate the electricity could be used to generate lift to reach high into the stratosphere, as well as waste heat for added lift. Great coordination will be required but only for a brief time.
Any good?
At first blush, no, because we need rock solid pointing accuracy. You don’t get that when waving about at the end of a balloon tether.
Even though the lasers may be rock solid on the ground I doubt the sail will be. A stratosphere laser system is more powerful due to less atmospheric disturbances and the balloons would be very mobile, anywhere in the world.
Funny, I pinged office@breakthroughprize.org an e-mail but still no reply, must be really busy.
I asked them if there were any videos of the conference. Also no reply. Their Youtube channel Breakthrough has a few 2 minute interviews but no extended coverage.
On the optical side, (should have calculated this first) at 1AU a Jupiter equivalent would appear a little over 3′ in size. So, more than a single pixel. Surprised me. But, a 1.5R Super Earth would be only 25″. Expanding that to more than a pixel will be a stretch for a micro-optical instrument. Thoughts?
May 7th, 2015
Washingtontimes.com
China and Russia have announced plans for a joint space exploration project…They aim for the moon and a joint lunar base…perhaps on the far side of the moon…starting work as soon as 2020…
Bigalow has made progress with their crewed inflatable space room…the room can be adapted for the startup core of a major lunar base…
Hope there is progress forthcoming on the Webster Cash star-shade…We need a specific target in the Centauri system…whereupon political backing and funding will come together to end the technological speculation and you can begin testing ideas…Carl Sagan mentioned taking little baby steps to minimize the mishaps along the way…like the Navy Vanguard rocket exploding on the launch pad in 1957…Remember that? Stuff happens…
Enjoyed the Kepler Galileo background as these type of accounts win lasting points in the political arena…Citizens must have a way to relate to the enterprise you will propose…after we have the target…
Time is being well spend in Palo Alto…
The key man is also the advance man…
Tossing these little ships at the stars is going to be so much fun. We’ve never had the luxury of lots of disposable spacecraft before. We can afford to make tons of mistakes and try any number of crazy experiments that might just work. It’s a whole new ballgame for space research.
John, you sparked an interesting question. What is the current physical size of a pixel in modern micro-optical hardware? Will truly nanoscale picture elements be required to deliver the most information from a flyby scan?
The pixels in cellphone cameras are around 1.2 microns, I think. There are CMOS cameras used by amateur astronomers for planetary photography that have pixels as small as 2.4 microns. Visible light is 0.4-0.7 microns, so I doubt it’s practical to go smaller.
If there’s one thing I have learned living on this beautiful planet is that it seems … there are no shortage of miracles. The fact that we have even lived lives in ponderance is clearly one. Onto, unto and forward.
Re: that Kepler quote.
How our imaginations fly unimpeded even by the laws of the universe. I guess this is both a blessing and a curse.
P
Another beamer location to consider: the Central Otago region of New Zealand. It’s further south than the other sites considered (so Alpha Centauri is nearer the zenith); has an arid climate with many cloudless, windless days; and has good infrastructure including abundant hydropower from local rivers.
One thing is clear. This is a break throw similar to invention of radio, internet or a first orbital flight. Getting to even 1% of c would change everything.
Ability to deflect asteroids using the same laser beam might one day save the Earth. This is the most important side-effect.
Ability to deploy 1000s of fast probes (1 week to target) in Solar system is priceless.
Ability to deploy 1000-2000 kg payloads to Mars in one or two weeks on a regular basis will probably be a deciding factor to allow manned missions. It will probably be possible to break there as there is no fuel required to accelerate.
Having a telescope with multi kilometre size aperture will probably allow seeing planets around closest stars in some detail.
Great potential for communicating with other civilisations with a laser this big can come useful soon.
Did I forget anything?
Projecting the Bat-Signal onto the Moon.
: ^ )
Slow down or destroy any swarm of alien laser-microsails a civilisation from Alpha Centauri would have launched towards us through a similar Breakthrough Starshot project :)
Read original(?) Roadmap to Stars here:
http://www.deepspace.ucsb.edu/wp-content/uploads/2015/04/A-Roadmap-to-Interstellar-Flight-15-d.pdf
A lot of questions about Starshot are raised and answered there.
I am sincerely moved by that Kepler quote, as well as by some of the comments here.
It is great to be alive now and a part of this.
True visionairies, such as Kepler and Galileo, will always be remembered as such.
Another wrinkle in these discussions that sorta gets overlooked is the “who’s nipping at our heals” question. Assuming we are mostly “westerners” here (which I hope is incorrect) when we speak of these technological miracles as though they were optional lofty and even romantic challenges we abet our worst enemy…our sublime conceit. Apollo is now lost in the mists of deep time. And that exemplary team returned to and is now largely one with the good Earth and yet it is still fashionable to argue “We can put a man on the moon there fore we can do [pet objective].” Sorry but we can not put a man on the space station let alone the moon.
And our enemies like communist red China, the USSR and our friends like the ESA, JAXA et al are all working apace to outpace that ever so drowsy, overfed, risk-averse giant, the USA.
Do people respond to fear? Yes they do. Should people fear their betrayal at the hands of leadership that fails to put measure to our sloth? Yes they should. Are they aware of our situation analysis? No, I don’t think so because I don’t think there is an such an analysis or if there is they are not privy to it.
I suggest that people who go to these “think tank” type boiler sessions fold more fear into the motivational batter because it is warranted as a valid ingredient for consideration
Just curious, why 2 minutes of laser time to reach 20% of c? Who picked this beam time and cruising speed and why? Why not some faster speed? Say 30 or 40% of c?
The short time is dictated by the need to keep the sail close to the laser during acceleration. The beam spreads with distance, and quickly becomes too wide to use efficiently. The speed is just an arbitrary goal, a wild guess at the optimal compromise between feasibility and desirability. First tests will be done at much lower speed, and we’ll work our way up as technology permits.
This sort of observations are why it is so important that the Breakthrough Board heading up the StarShot initiative (Hawking, Milner, Zuckerberg) put their signatures to clear policy statements that answer the questions people have about the intent, strategy, and scope of this project. These people have high public profile which they are already putting behind this initiative. While policy alone is not enough to safeguard its lofty goals, statements signed by these individuals carry significant weight and, if violated, send a serious indication of the project’s health (or rather the lack of health) to the public and all involved.
We need to consider that the StarShot technology is potentially dual use as not only a planetary defence, but also as a weapon that could be used by one nation against another.
DE-Star as a a potential pretext for weaponisation of space has been previously discussed on this forum.
https://centauri-dreams.org/?p=33450
Besides the phased-array beaming technology, the probes launched can carry a multi-megaton kinetic energy, either of which could be used to deflect asteroids, destroy satellites, or — if reflected back to the ground from an orbiting mirror, destroy cities.
Some questions that could be answered by policy statements from the Breakthrough Board are:
Is this an international project, a US project, a DOD funded project?
Will the beaming infrastructure that is built, or the intellectual property developed, be spun off for military purposes?
How can the beaming infrastructure be protected from being nationalised by a host country or even seized by terrorists?
What controls can be put in place prevent misuse?
@Mark:
The reason for the cutoff has more to do with pointing accuracy. Already with the spec’d parameterss, we’re talking a powered distance of about 10^9 metres, which calls for a pointing accuracy of about 1 nanoradian. That’s at the limit of current tech.
This project is just not about A.C but also P.C, a small deviation from the trajectory to A.C could easily land in P.C they are that close.
Paul –
Forgive me if I have nothing substantive to add to the conversation – yet. I just wanted to impart that I am so amazed that we are even able to consider this, and glad that you are there to write about it and relay the discussions and news to us. Please continue, and I will find a way to constructively contribute when I get over my stunned reaction.
Steven, you’re welcome here any time. Thanks for being a part of things!
“Worden said that the Atacama desert might be the best location of those considered — the Kalahari, the Australian outback, Antarctica — because it has more infrastructure, and power might be bought from utilities or produced through systems built by the project itself, perhaps solar, perhaps nuclear.”
If Starshot is going to be used for multiple targets the location of the beamer will need to carefully consider celestial coordinates of targets to maximise the potential number of targets.
Top 10 target stars for a Southern Hemisphere beamer.
Proxima Centauri ?62° 41?
Alpha Centauri ?60° 50?
Barnard’s Star +04° 42?
Luhman 16 ?53° 19?
WISE 0855-0714 ?07° 15?
Wolf 359 +07° 01?
Sirius ?16° 43?
Luyten 726-8 ?17° 57?
Ross 154 ?23° 50?
Epsilon Eridani ?09° 27?
Average ?24° 2?
Mid point ?27° 50?
Northern Hemisphere targets unlikely to be accessible from a Southern Hemisphere beamer.
Lalande 21185 +35° 58?
Ross 248 +44°10?
The mid-point at almost 28° South would result in a beamer having an angle to Proxima Centauri of almost 35° from the Zenith. To minimise the amount of atmospheric impact on the laser beam it is likely an angle greater than 30° would be sub-optimal.
Seems like the optimal location may be somewhere around 33° South.
Western Australia has numerous locations suitable for a beamer around 33° South, infrastructure is readily available and it is one of the locations for the Square Kilometre Array. Solar energy for the beamer would not be a problem. Security (from terrorists or a hostile government) would be amongst the best in the world. The only negative is that most locations suitable for a beamer are below 1000 m above sea level.
Elevation will be a very important factor, right after southern location. Skipping the lowest parts of the atmosphere by starting high up is worth more than a vertical angle. That, I suppose, is why the Atacama desert was fingered in the first place.
Starshot isn’t a direct enabler for FOCAL mission. The latter almost certainly will be a heavy mothership, with some kind of nuclear power aboard. FOCAL mission must send all the data directly to Earth, without the aid of focal retranslator (FOCAL mission itself being a necessary retranslator for Starshot), and it takes quite a big mirror to resolve Einstein ring, which is necessary for imaging, too [http://arxiv.org/abs/1604.06351].
But Focal mission requires delta-V on order of hundreds km/s – photon thrust is very energy-inefficient for it. FOCAL is somewhere on the border between advanced nuclear-powered electric propulsion and beamed propulsion with a stream of slow (~0,005c) particles deflected by m2p2 or electric sail. But there are challenges too, for the driver accelerator must be built in space (or at least on the surface of airless body), and to accelerate a 1-ton probe to 300 km/s both GW-level power and microradian-level beam control are needed. Maybe there is a feasible solution in some corner of “power – beam quality” space, but if we can install a square kilometer of solar arrays or a hundred-MW reactor on the Moon, we can launch a nuclear-powered VASIMR as well, and for FOCAL that would work.
(if we can build a square kilometer of solar arrays on the Moon, we can also launch FOCAL mission with laser sail acting like a photovoltaic arrays, and power it up with a laser beam from Earth, but that requires some kind of both high I_sp and high thrust electric propulsion onboard)
I believe the FOCAL mission specifically is a radio mission and as such does not require resolving the Einstein Ring. When intended purely for communication (as opposed to imaging), a gram-scale craft with a small radio antenna might make sense. You’d need to launch two for testing, though, in opposite directions, otherwise you’d have nothing to communicate with before the actual Starshot.
Still, as you say, you’d need an antenna (or laser?) large enough to communicate with Earth, which could drive the minimum mission mass way above the gram scale. Maybe a small radioisotope power source coupled with a pulsed laser operated intermittently could work and be made very small. Probably needs some leaps and bounds of improvements in small radioisotope power sources. Lasers today are probably up to it.
Another factor to consider is that the gravitational focus communicator must be launched in the opposite direction, so in the case of AC the main laser would not work. There’d have to be a second one in the northern hemisphere.
If we had laser hardware such as this, what would stop us pointing it at an asteroid, accelerating it onto an intercept course for Europa, Enceladus etc. in order to punch a hole through it’s ice crust just ahead of a lander mission? Maybe we could get a sub into those oceans this way.
Depending on the beamer parameters, maybe we could directly burn a hole through the crust.
A 100 GW DE-4 projecting onto a 1 Km x 1 Km surface patch would melt the ice with a flux of 100 KW/m^2 at a distance of 10 AU. Jupiter is 4 AU distant at closest approach to Earth, so it’s more flux than that.
Ice is quite reflective as well as thermal conductivity and rotational effects would conspire against the concept.
Suggest installations on Mount Cooke, Mount Dale, and Mount Gunjin; all situated near Perth near 32 degrees south latitude, with altitudes from 1,300 to 1900 feet.
Stirling Mountains, near Toolbrunup Peak. Latitude 34 deg, altitude 3,451 feet, near the Southern Ocean port of Albany, Western Australia.
https://www.google.com.au/maps/place/Toolbrunup+Peak/@-34.40556,117.951928,3a,75y,90t/data=!3m8!1e2!3m6!1s38593849!2e1!3e10!6s%2F%2Flh5.googleusercontent.com%2Fproxy%2FdIh4-VxvFvvcB35bjIic9UdNFGg5fA5IlmmEozMFnnWjeYqBBIaIBzaEkdxZNpK4nAxWKkTNDnIODxXQ9xJFRLFYIorLPg%3Dw295-h100!7i2269!8i768!4m2!3m1!1s0x2a38501aaf03c7c3:0x33001dfc4269f540!5m1!1e4?hl=en
I wonder, if the Starchips could be aimed directly at Alpha Centauri A and the sails turned away from the star a few days or weeks before arrival, could they use the light pressure from the star to slow down? It would need to be a very close approach, to within 1 star radius, like a sungrazing comet. If they arrive at the right time they could be deflected by Cen A and head toward Cen B to repeat the deceleration, perhaps enough to enter a figure-8 orbit around the two stars. That would allow lots of time to explore the systems, using the sails to modify course toward any planets they find.
Sadly no. There’s not nearly enough stellar intensity to make a dent in that 0.2c relative speed. And even magsail braking (which isn’t going to fit in the mass budget anyway) is pretty ineffective below 0.1c.
A quick calculation based on Alpha Cen A shows that the max possible braking to be no more than roughly 0.5% off the 0.2c incoming speed using the suggested 20 m^2 sail. Plus, the electronics would be fried by the star at that close range.
Spaceships will need precise navigation, especially when in interstellar space where the targets are really far apart. Pulsars may fill that bill:
http://www.spaceflightinsider.com/missions/solar-system/pulsar-based-spacecraft-navigation-system-one-step-closer-reality/