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Toward a High-Velocity Astronomy

Couple the beam from a 100 gigawatt laser with a single-layer lightsail and remarkable things can happen. As envisioned by scientists working with Breakthrough Starshot, a highly reflective sail made incredibly thin — perhaps formed out of graphene and no thicker than a single molecule — could attain speeds of 20 percent of c. That’s good enough to carry a gram-scale payload to the nearest stars, the Alpha Centauri triple system, with a cruise time of 20 years, for a flyby followed by an agonizingly slow but eventually complete data return.

A key element in the concept, as we saw yesterday, is the payload, which could take advantage of microminiaturization trends that, assuming they continue, could make a functional spacecraft smaller than a cell phone. The first iterations of such a ‘starchip’ are being tested. The Starshot work has likewise caught the attention of Bing Zhang, a professor of astrophysics at the University of Nevada, Las Vegas. Working with Kunyang Li (Georgia Institute of Technology) Zhang explores in a new paper the kind of astronomy that could be done by such a craft.

For getting to Proxima Centauri for the exploration of its interesting planet involves a journey that could itself provide a useful scientific return. The paper’s title, “Relativistic Astronomy,” flags its intent to study how movement at relativistic speeds would affect images taken by its camera. As Zhang explains in a recent essay on his work in The Conversation, when moving at 20 percent of lightspeed, an observer in the rest frame of the camera would experience the universe moving at an equivalent speed in the opposite direction to the camera’s motion.

Relativistic astronomy, then, explores these different spacetimes to observe objects we are familiar with from our Earth-based perspective as they are seen in the camera’s rest frame. Zhang and Li consider this “a new mode to study astronomy.” Zhang goes on to say:

…a relativistic camera would naturally serve as a spectrograph, allowing researchers to look at an intrinsically redder band of light. It would act as a lens, magnifying the amount of light it collects. And it would be a wide-field camera, letting astronomers observe more objects within the same field of view of the camera.

Image: Observed image of nearby galaxy M51 on the left. On the right, how the image would look through a camera moving at half the speed of light: brighter, bluer and with the stars in the galaxy closer together. Zhang & Li, 2018, The Astrophysical Journal, 854, 123, CC BY-ND.

Such observations become intriguing when we consider how light from the early universe is red-shifted as a result of the expansion of the cosmos. Zhang and Li point out that a camera moving at the relativistic speeds of the Proxima Centauri probe sees this redshifted light becoming bluer, counteracting the effect of the universe’s expansion. Light from the early universe that would have had to be studied at infrared wavelengths would now be susceptible to study in visible light. The camera, then, becomes a spectrograph allowing the observation of everything from remote galaxies to the cosmic microwave background.

Moreover, other relativistic effects come into play that add value to the fast camera. From the paper on this work:

…unique observations can be carried out thanks to several relativistic effects. In particular, due to Doppler blueshift and intensity boosting, one can use a camera sensitive to the optical band to study the near-IR bands. The light aberration effect also effectively increases the field of view of the camera since astronomical objects are packed in the direction of the camera motion, allowing a more efficient way of studying astronomical objects.

Let me depart for a moment from the Zhang and Li paper to pull information from a University of California at Riverside site, a page written by Alexis Brandeker, and presumably illustrated by him. In the figure below, we see only the effect of aberration at a range of velocities. Notice how the field becomes squeezed at we move from 0.5 c to 0.99 c. At 0.99 c, almost all visible radiation from the universe is confined to a region 10 degrees in radius around the direction of travel.

Image: This figure shows aberration effects for the ship travelling towards the constellation of Orion, assuming a 30 degrees field of view. The field of view is kept constant, only the speed is changed from 0 to 0.99c, showing dramatic effects on the perceived field. No radiative effects are considered, only geometrical aberration. Credit: Alexis Brandeker/UC-R.

But to get the overview, we have to fold in Doppler effects as infrared radiation is shifted into the visible. If we combine these effects in a single image, we get the startling view below.

Image: Both relativistic effects switched on. Credit: Alexis Brandeker/UC-R.

But back to Zhang and Li, whose camera aboard the probe is a spectrograph, a lens, and a wide-field camera all in one. The authors make the case that fast-moving cameras can likewise be used to probe the so-called ‘redshift desert’ (at 1.4 ≲ z ≲ 2.5) that coincides with the epoch of significant star formation (the name comes from the lack of strong spectral lines in the optical band here). Lacking data, we have no large sample of galaxies in a particular range of redshifts, which hinders our understanding of star formation.

Zhang and Li consider relativistic observations of gamma-ray bursts (GRBs) at extreme redshifts, as well as tracing the electromagnetic counterparts to gravitational wave events. Thus a Breakthrough Starshot payload enroute to Alpha Centauri offers a new kind of astronomy if we can master the construction of a camera that can withstand a journey through the interstellar medium without damage from dust as well as one that can transmit its data back to Earth.

What struck me as I began reading this paper is that when it comes to relativistic effects, 20 percent of lightspeed is actually on the slow side, making me wonder how much better the kind of observations the authors describe would be at higher velocities. But Zhang and Li move straight to this question, describing the relativistic effect of a Starshot probe as ‘mild,’ and noting that a Breakthrough laser infrastructure might be used for faster, dedicated astronomy missions.

If one drops the goal of reaching Alpha Centauri, cameras with even higher Doppler factors may be designed and launched. A Doppler factor of 2 and 3 (which gives a factor of 2 and 3 shift of the spectrum) is available at 60% and 80% speed of light, respectively. More interesting astronomical observations can be carried out at these speeds.

While probes in this range would demand ever more powerful acceleration from their laser energy source, they might actually be easier to build, for the need for cosmic ray shielding on a long cruise or data transmission at interstellar distances would be alleviated by sending them on missions closer to home. Of course, pushing probes to speeds much higher than 20 percent of c is even more problematic than the Centauri mission itself. Beyond Starshot, the authors argue that relativistic astronomy will repay the effort if we continue to push in the direction of beamed laser probes with an eye toward ever faster, more capable missions.

The paper is Zhang & Li, “Relativistic Astronomy,” Astrophysical Journal Vol. 854, No. 2 (20 February 2018). Abstract.


Comments on this entry are closed.

  • john walker May 15, 2019, 14:53

    In the Breakthrough Starshot concept 100 GW is needed to accelerate a gram scale probe with subgram scale optics to “just” 0.2c . Deep field observations like those Hubble undertook will need several orders of magnitude larger optics and thus also mass. The power generation requirement for a directed energy solution (or any other) that would suffice for large “relativistic” effects is simply utopian.

  • Alex Tolley May 15, 2019, 14:55

    At this point, it is still more of a thought experiment than a practical system. But the idea is conceptually elegant. The article did not cover a camera looking backward so that high frequency em radiation could be made visible. This would allow UV, x-ray, and gamma-ray astronomy to be achieved with simpler tools, assuming the vehicle itself and the needed velocities were achievable.

    • George King May 15, 2019, 17:49

      Would add mass, but particularly for a pure relativistic astronomy mission rather than necessarily a Proxima Centauri trek, one could have instruments facing both forward and backward to do both.

      Janus would be the perfect name for the craft.

      • ljk May 16, 2019, 9:07

        Except it would be abused by those who know squat about history and still think saying the name Uranus is funny. Which means most people.

        • Michael May 16, 2019, 11:59

          Yip still the butt of many a joke.

  • ljk May 15, 2019, 17:30
  • Anton M. May 15, 2019, 17:40

    Interesting point: you do not need to accelerate a whole camera. Throwing a mirror really fast will suffice. The light reflected from the moving mirror will be Doppler-shifted. Turning the mirror into plasma in the process is OK. Actually, you can make the mirror from electrons, ions or whatnot. Just saying.

  • Brett Bellmore May 15, 2019, 19:32

    I don’t see how you couple a molecule thick sail to a separate payload at these sorts of accelerations.

    But such sails would make a great mass beam for a more massive probe.

  • Andrew Palfreyman May 16, 2019, 0:01

    The sticking point is the price of the power (not the energy). I estimate the future cost of delivering 100 GW at 50% efficiency using batteries to be somewhere around $50-100 Billion. Looks like we’ll have to wait for fusion power to make this dream affordable.

  • djlactin May 16, 2019, 3:26

    I’m surprised that everybody seems to assume that the proxCen flyby will be the first test. I do not doubt that numerous dry-runs will examine nearby bodies, like say Pluto and Haumaea, with returns on investment orders of magnitude faster.

    • Ron S. May 16, 2019, 9:38

      Even that is overly optimistic. They’re a long way from having a system whereby a star chip could capture a low distortion image the Moon at moderately fast launch velocity and successfully return that image to us.

  • Michael Fidler May 16, 2019, 9:07

    PROCSIMA: Diffractionless Beamed Propulsion for
    Breakthrough Interstellar Missions
    NIAC Phase I Report
    Christopher Limbach and Ken Hara
    March 25, 2019

    Executive Summary
    The following report describes a new propulsion concept based on self-guiding of a combined light and particle beam and explores the physics, technology and design principles needed to implement such a system for an interstellar fly-by mission to Proxima b. While the relevant self-focusing mechanism has been considered in an optical context, this is the first application to space propulsion known to the authors.
    The purpose of the present study is to provide a broad overview of the pertinent physics and design principles, credibly assess propulsion capabilities, and lay a comprehensive foundation for further, more targeted investigations of critical system elements and processes.
    Starting from basic principles, this report describes the equations of motion and physical phenomena needed to establish the feasibility of self-guiding and furthermore analyze the production and sustainment of the self-guided beam. Compared with laser or particle beam propulsion alone, the self-guided beam concept introduces a plethora
    of light-matter interactions and additional complexities, imposing certain constraints on the geometric and physical characteristics of the beam sources. In particular, we have the identified the particle beam as a crucial element of the proposed concept. System constraints are quantitatively analyzed and then explored by developing and applying a
    mission design process to a Proxima b flyby mission as well as a nearer-term mission to the solar gravitational lens point.
    Through this study, we conclude that propulsion by self-guided beams is credible based on an analysis of the governing equations and literature review. A quantitative analysis of phenomena including light scattering/absorption, photoionization, heating, collisions, gravity, reference-frame effects, and beam riding revealed no “show-stoppers”. Missions design indicates that propulsion capability is strongly affected by 1) the particle beam source characteristics, specifically a parameter
    known as brightness, and 2) power constraints and mission constraints on the final velocity. An analysis of the Proxima b mission demonstrated that while self-guided beamed propulsion enables a 0.1� fly-by with a 5 gram probe, a larger 0.6 kilogram payload is obtained for a 0.075� mission. Both optimizations were performed for 50 GW of available beam power and a transmitter area of only 1 m (, the latter a reduction of > 10* relative to comparable laser propulsion concepts. Observed payload mass scaling with velocity to the power of ∼ 8.8 results in dramatic improvements for slightly extended mission duration, and suggests game-changing capability for near term low velocity missions throughout the solar system.


  • ljk May 16, 2019, 9:59

    I have posed the following questions before here in this blog regarding the method of propulsion for Breakthrough Starshot and need to do so again. For the moment we will ignore its numerous technical and physics issues and cut right to the heart of the matter:

    * Who is going to build this gigalaser?

    * Who is going to pay for this gigalaser? And not just the initial funding but all the money it is going to need after to keep it operating for decades if not centuries?

    * Who is going to maintain this gigalaser?

    * Where will this gigalaser be based – on Earth or in space? And where exactly then in either of those places?

    * Who is going to ensure that the gigalaser is not turned into the very deadly weapon it could become? I am not even thinking of some ambiguous band of terrorists: I am thinking of a government that gets taken over by a dictatorial regime.

    This is why I cannot get currently behind Breakthrough Starshot as much as I want to. While I am very happy to see an actual plan for an interstellar space mission that isn’t just the equivalent of a white paper put together by a group of space fans who have neither the money nor the clout to put their design into action (I am looking at you, BIS Daedalus), the fact that the gigalaser concept is going to require so much to happen makes me think it will either never happen or not become a reality until the far, far future.

    Same with the Daedalus’ fusion engine. Billions of dollars and decades have already been spent on huge facilities on the ground, with only a few seconds worth of fusion to show for it. Now we want to make this happen on a starship that will need to operate for decades or more?

    I am not even going to bother to ask how that NASA plan to get a probe to Alpha Centauri launched by 2069 is going because I already know the answer.

    There is one STL interstellar propulsion method that could be done now using technology that exists now. We started back in the 1950s with a lot of real promise, only to have it derailed for reasons that had nothing to do with physics or technology. If we are serious about reaching the stars we can and should start it up again.

    As I have also said elsewhere in this blog, if the US and Europe are unwilling to do this, I know one country that won’t have any qualms about building it. A country that is currently going all out to be the most advanced nation on Earth.

    So, if you REALLY want us to go to the stars within this century…


    Of course if we keep falling for this kind of thinking and project it into space, we won’t be going anywhere and when the human population reaches a critical mass, it will be too late for civilized measures:


    • Alex Tolley May 16, 2019, 14:58

      Orion will not get you to the stars anytime soon. It is still subject to teh rocket equation. From the projectrho site the Orion has an exhaust velocity of around 33 km/s. To reach 0.2c = 60,000 km/s the mass ration is impossibly large. Orion is good for throwing huge masses to orbit and traveling to targets in the solar system. But not the stars unless you can do it really slowly. But the cost of worldships is well beyond any economy we have have today.

      The questions you ask about the GW laser array is relevant, but what about all those nukes needed for even an Earth to orbit Orion transport? It is almost Strangelovian.

      The advantage of beamed craft is scalability of the beamer. We can start “small” to test concepts for near Earth space and the nearer planets with much more modest power output and terminal velocities. Costs can be kept low by having these beamers on the ground, on a high plateau. If they prove useful, then later we can talk about space based, higher power ones, and with them propelling larger vehicles even to the stars.

      The size of the economy needs to be much, much larger to afford an Orion. Didn’t Dyson think it might be affordable around 2500 with 3% pa sustained growth? More likely the global economy will prove logistic and not far from where we are today, cutting off that avenue. That leaves requiring a path to achieve a solar system wide economy with billions of people living in space (living on Earth and consuming space resources will not work). That path may be closed off.

      Reducing mass constraints by extreme miniaturization where possible seems to me to be the best path to reach the stars. Craft that can avoid the constraints imposed by the rocket equation seem like the best option, IMO.

      • ljk May 16, 2019, 16:53

        While I am not ignoring the obvious potential destruction of a nuclear bomb, one could easily say that anything can be turned into a weapon. Lord knows humans are really good at that ingenuity.

        I am just concerned and frustrated that Breakthrough Starshot will focus on a propulsion method that will lead us down a rabbit hole where we could be complaining in another forty years why someone has not developed even a relatively efficient STL motor.

        • Alex Tolley May 16, 2019, 22:43

          It needn’t be a single choice. There will be a number of propulsion methods developed for different applications. At this time sails of different types seem more versatile, technologically simpler, and with likely higher terminal velocities than other methods. But who knows what might emerge at some point? Don’t forget that the Lubin laser array was pitched for planetary defense which would make it a multiple use device, even excluding a weapon (which I am sure that is of interest to the DoD to fund it).

        • Michael C. Fidler May 18, 2019, 10:22

          Can not be weaponized, it’s free, it’s very close, it has great potential. :-) Add a few probes to study the dynamics maybe even some that can control the electron jet, this is not to different from a cathode ray tube. We have not even explored it at sunspot MAX, so no telling how much potential and power. Did I tell you it was FREE, Earth’s MAGNETIC RE-CONNECTION in our Magnetosphere. Any body get the connection???

          Maybe even an experiment with using the earth’s magnetic re-connection in our Magnetosphere.

          Dr. Torbert and colleagues found that the symmetrical reconnection events last only a few seconds, producing extremely high velocity electron jets — over 9,320 miles per second (15,000 km per second) — and intense electric fields and electron velocity distributions.



          • Michael C. Fidler May 18, 2019, 10:26

            How about a magnetic bubble to sail on the earth’s magnetic re-connection winds…

            • Michael C. Fidler May 18, 2019, 10:37

              In 2011, in an attempt to survey a wider area of the Earth’s magnetosphere, the THEMIS team repositioned two of its five spacecraft into lunar orbits, creating a new mission dubbed ARTEMIS after the Greek goddess of the hunt and the moon. From afar, these two spacecraft provided a unique global perspective of energy storage and release near Earth.
              Similar to a pebble creating expanding ripples in a pond, magnetic reconnection generates expanding fronts of electricity, converting the stored magnetic energy into particle energy. Previous spacecraft observations could detect these energy-converting reconnection fronts for a split second as the fronts went by, but they could not assess the fronts’ global effects because data were collected at only a single point. By the summer of 2012, however, an alignment among THEMIS, ARTEMIS, the Japanese Space Agency’s Geotail satellite and the U.S. National Oceanic and Atmospheric Administration’s GOES satellite was finally able to capture data accounting for the total amount of energy that drives space weather near Earth. During this event, reported in the current Science paper, a tremendous amount of energy was released.

              The amount of power converted was comparable to the electric power generation from all power plants on Earth — and it went on for over 30 minutes. The amount of energy released was equivalent to a 7.1 Richter-scale earthquake. Trying to understand how gigantic explosions on the sun can have effects near Earth involves tracking energy from the original solar event all the way to Earth.



      • Alex T. May 17, 2019, 2:38

        It is not usual, but on this issue I fully support ljk’s point of view.
        While Breakthrough Starshot concept can be attractive as some technical and technologic testing platform , it is almost useless for scientific researches.
        Due to weight limitation, the best optical resolution of this probe should be worst than human’s eye, it will never produce fine galaxy pictures like posted in this article.
        Long distance communication with such probe also under huge question.

    • Michael May 17, 2019, 9:41

      “I have posed the following questions before here in this blog regarding the method of propulsion for Breakthrough Starshot and need to do so again. For the moment we will ignore its numerous technical and physics issues and cut right to the heart of the matter:”

      * Who is going to build this

      The laser system is designed to be modular so many companies could build the components without stretching their financial systems.

      * Who is going to pay for this gigalaser? And not just the initial funding but all the money it is going to need after to keep it operating for decades if not centuries?

      As above many companies and institutions with vested interests could sponsor its construction, this system is not just about an interstellar mission it has enormous potential in the solar system.

      * Who is going to maintain this

      Maintenance will be part of the costs of launches.

      * Where will this gigalaser be based – on Earth or in space? And where exactly then in either of those places?

      Earth is the best place with a suitable high altitude, Chile looks very good.

      * Who is going to ensure that the gigalaser is not turned into the very deadly weapon it could become? I am not even thinking of some ambiguous band of terrorists: I am thinking of a government that gets taken over by a dictatorial regime.

      Starshot would a giant with a glass jaw.

      This is why I cannot get currently behind Breakthrough Starshot as much as I want to.

      It is difficult and that is why you should get behind it by solving not just the physical issues but the opportunities it could provide.

      • Alex T. May 17, 2019, 16:21

        Michael , you did not answer any of questions you quote.
        To invent or invest into development of high power laser, there is no need to any company to be somehow connected to Breakthrough project, the problem is money, not purpose, I am sure the pure military purpose will bring more funds than 1g of load sent to Proxima…
        Breakthrough project is not about good capital investment it is about charity, even theoretically it cannot bring any input and gain for any investor…
        May be theoretically Chile is good place for high power laser, but do you asked from Chile government and citizens agreement for such problematic things?

        • Michael May 18, 2019, 1:21

          You are forgetting the lasers are not high powered they are low powered but there is many of them. There are many companies that would invest as the project as it is modular and there are many types of paying missions, debris’s deorbiting, moon paving, atmosphere vehicles, and heavy spacecraft movement to name a few. Starshot is not the beginning and end all of the project, once we think like this without taking our eyes off the ultimate goal we will succeed.

      • ljk May 17, 2019, 17:32

        While I appreciate the time you took to reply to each part of my commentary, the problem is this is still just a general outline bordering on wishful thinking.

        Perhaps in one sense it is still too soon for such details, yet if we do not start making concrete such things as who will build and pay for it, this project runs the risk of being pushed farther and farther into the future, with each new generation assuming the next one will take care of it. Which means no one will.

        I do support Breakthrough Starshot and any other viable interstellar mission projects, even the NASA one that say Alpha Centauri by 2069. However, I want to see some real foundations being built, not a bunch of white papers and endless conferences.

        Where is the interstellar equivalent of Elon Musk and Jeff Bezos? Yes, I know about Yuri Milner of course, but as you said above it is going to have to be a cooperative effort.

    • Charley May 22, 2019, 13:46

      @ljk ,
      for the greatest amount of time I was considerably on the fence concerning the use of nuclear pulse units as a means to achieve practical interstellar flight-but now I’m happy to report that I have in a tentative manner come down on your side in this issue. The negatives that have been raised against such a propulsion scheme seeing more politically motivated than grounded in any kind of physical inability to achieve the desired end. The questions of fallout and contamination in an earthly environment can easily be dismissed and countered by building the Orion ship in a lunar orbit by using localized lunar materials and processing them.

      If there’s any seriousness in this idea, then I think the creation of the space elevator could essentially nullify any difficulties that would come from transporting nuclear materials from the surface of the earth to outer space for use in nuclear pulse units-thus taking away the argument of launch failures of nuclear materials from the surface of the earth. In addition, one could set up the actual beginning of the voyage in in orbit out beyond Jupiter, which would prevent large plumes of radioactive materials from accumulating in the near earth environment.
      Finally, and most importantly, studies indicate that with the proper design they are looking at achieving velocities better than 10 percent the speed of light as your terminal velocity. This is exceedingly better on a mass/speed ratio, then we can even hope to obtain in any time in the near future. Your space probe could reach alpha Centauri in 40 years and it might be able to even achieve orbit in a properly designed system and you would be carrying tons and tons of payload to be delivered to the star system in contrast to this Breakthrough Starshot that has been touted endlessly here in this blog. Also too in terms of the delivery of mass/cost ratio you really can’t do better than this in the near term than such a system as has been talked about. If I was a betting man, this is where I would put my money.

      • ljk May 23, 2019, 9:09

        Yes, what has held us back in this arena was a combination of politics and general ignorance more than technological know-how.

        We could have had Orion or some equivalent decades ago, but the forces that killed it are the same kind that kept us from doing more with Apollo after 1972.

        I see potential with Breakthrough Starshot, but if the questions I posed for the gigalaser are not seriously answered and resolved, and within a reasonable time period, then this will just be another interstellar rat hole we go down.

        I can see we will go to the stars someday, too, but talk is cheap. Show me the money and the tech instead. And oh yes, the will, drive, and real support.

  • Gary Wilson May 16, 2019, 13:49

    Some very interesting material in the way of comments here. I’m all for preserving 7/8th’s of the solar system. I think that would be wonderful. However trying to predict 400 years into the future (the approximate time estimated to exhaust the solar system’s resources in the article above) is futile at best. First of all unless we get a handle on human induced climate change (remember the required turnaround point is 2030), and we show no intentions of doing that, there is no possible way to make any predictions about how fast resources will be exploited in the future. We might not have a global civilization by 2100 let alone exhaust the entire solar system’s resources in 400 years. Some things to keep in mind: many of the world’s leaders are relentlessly greedy and selfish and have no interest in turning the corner on climate change. I could name a few of these leaders but that might get embarrassing and upsetting. I could mention Justin Trudeau but he’s only a bit player in this mess. We as a species have a median IQ of 100. That means many, many voters in democratic countries are very easily convinced to vote against their own self interest including supporting leaders who for reasons of individual greed want to maintain the status quo including insisting climate change is a Chinese hoax. So lets start with the near future shall we? How about reducing greenhouse gas emissions rapidly and extensively in the next 11 years before runaway greenhouse climate effects begin and become unstoppable. Then we can think about the solar system and its resources.

  • Bruce D. Mayfield May 16, 2019, 13:58

    In trying to rap my brain around the relativistic effects on photos taken from a fast moving camera the blueshift of light in the direction of flight is easy to understand, but the compression of off-center objects (fisheye lens effect) is harder to fathom. Can someone provide a little more explanation on why this would be the case please?

  • Michael May 17, 2019, 2:56

    Starshot can be used to more much more massive objects to high velocity. If a disc stream is shoot out towards a spacecraft each alternate discs can be slowed down by an onboard or even from the home system laser, they will impact at ionisation velocity and can be repelled by a magnetic field.

  • Michael May 17, 2019, 3:15

    Could we not create a light compressor camera?

    If we had a stationary mirror with a hole in it to let the light in and then the high velocity mirror is shot towards it. The light will be come trapped between the mirrors and blue shifted on each reflection to higher wavelengths. The mirrors could be destroyed but an image would be obtained at the edge.

  • ljk May 30, 2019, 12:22

    Breakthrough Needed? Starshot Interstellar-Flight Project Faces Challenges

    By Mike Wall a day ago Tech

    Humanity’s quest to explore other star systems up close is still in its infancy.


    To quote:

    Over the past few years, the Breakthrough Starshot team has identified about 25 potentially significant technological hurdles, Loeb said. And the assessment work is ongoing.

    “We are currently in the process of research and development, trying to figure out whether these challenges can be met,” Loeb said during a talk at the The Humans to Mars Summit in Washington, D.C., earlier this month.

    He laid out a few of the more notable challenges. It’ll be tough, for example, to make a sail that’s light enough and strong enough to do the job and that has the right shape to “ride” the laser beam in a stable fashion for the few minutes required. Producing a sufficiently coherent beam from a 100-gigawatt laser network consisting of many individual components may also prove difficult — and so might convincing the world that such a powerful instrument is safe and will never be turned to nefarious ends.

    “The main challenge may be policy, because obviously, the same laser system could have other capabilities,” Loeb said.