One of the great problems of lightsail concepts for interstellar flight is the need to decelerate. Here I’m using lightsail as opposed to ‘solar sail’ in the emerging consensus that a solar sail is one that reflects light from our star, and is thus usable within the Solar System out to about 5 AU, where we deal with the diminishment of photon pressure with distance. Or we could use the Sun with a close solar pass to sling a solar sail outbound on an interstellar trajectory, acknowledging that once our trajectory has been altered and cruise velocity obtained, we might as well stow the now useless sail. Perhaps we could use it for shielding in the interstellar medium or some such.
A lightsail in today’s parlance defines a sail that is assumed to work with a beamed power source, as with the laser array envisioned by Breakthrough Starshot. With such an array, whether on Earth or in space, we can forgo the perihelion pass and simply bring our beam to bear on the sail, reaching much higher velocities. Of the various materials suggested for sails in recent times, graphene and aerographite have emerged as prime candidates, both under discussion at the recent Montreal symposium of the Interstellar Research Group. And that problem of deceleration remains.
Is a flyby sufficient when the target is not a nearby planet but a distant star? We accepted flybys of the gas giants as part of the Voyager package because we had never seen these worlds close up, and were rewarded with images and data that were huge steps forward in our understanding of the local planetary environment. But an interstellar flyby is challenging because at the speeds we need to reach to make the crossing in a reasonable amount of time, we would blow through our destination system in a matter of hours, and past any planet of interest in perhaps a matter of minutes.
Robert Forward’s ingenious ‘staged’ lightsail got around the problem by using an Earth-based laser to illuminate one part of the now separated sail ring, beaming that energy back to the trailing part of the sail affixed to the payload and allowing it to decelerate. Similar contortions could divide the sail again to make it possible to establish a return trajectory to Earth once exploration of the distant stellar system was complete. We can also consider using magsail concepts to decelerate, or perhaps the incident light from a bright target star could allow sufficient energy to brake against.
Image: Forward’s lightsail separating at the beginning of its deceleration phase. Laser sailing may turn out to be the best way to the stars, provided we can work out the enormous technical challenges of managing the outbound beam. Or will we master fusion first? Credit: R.L. Forward.
But time is ever a factor, because you want to reach your target quickly, while at the same time, if you approach it too fast, you’re incapable of creating the needed deceleration. Moreover, what is your target? A bright star gives you options for deceleration if you approach at high velocity that are lacking from, say, a red dwarf star like Proxima Centauri, where the closest terrestrial-class world we know is in what appears to be a habitable zone orbit. In Montreal, René Heller (Max Planck Institute for Solar System Research), a familiar name in these pages, laid out the equations for a concept he has been developing for several years, a mission that could use not only the light of Proxima itself but from Centauri A and B to create a deceleration opportunity. You can follow Heller’s presentation at Montreal here.
Remember what we’re dealing with here. We have two stars in the central binary, Centauri A (G-class) and Centauri B (K-class), with the M-class dwarf Proxima Centauri about 13000 AU distant. Centauri A and B are close – their distance as they orbit around a common barycenter varies from 35.6 AU to 11.2 AU. These are distances in Solar System range, meaning that 35.6 AU is roughly the orbit of Neptune, while 11.2 AU is close to Saturn distance. Interesting visual effects in the skies of any planet there.
Image: Orbital plot of Proxima Centauri showing its position with respect to Alpha Centauri over the coming millennia (graduations are in thousands of years). The large number of background stars is due to the fact that Proxima Cen is located very close to the plane of the Milky Way. Proxima’s orbital relation to the central stars becomes profoundly important in the calculations Heller and team make here. Credit: P. Kervella (CNRS/U. of Chile/Observatoire de Paris/LESIA), ESO/Digitized Sky Survey 2, D. De Martin/M. Zamani.
Using a target star for deceleration by braking against incident photons has been studied extensively, especially in recent years by the Breakthrough Starship team, where the question of how its tiny sailcraft could slow from 20 percent of the speed of light to allow longer time at target is obviously significant. Deceleration into a bound orbit at Proxima would be, of course, ideal but it turns out to be impossible given the faint photon pressure Proxima can produce. Investing decades of research and 20 years of travel time is hardly efficient if time in the system is measured in minutes.
In fact, to use photon pressure from Proxima Centauri, whose luminosity is 0.0017 that of the Sun, would require approaching the star so slowly to decelerate into a bound orbit that the journey would take thousands of years. Hence Heller’s notion of using the combined photon pressure and gravitational influences of Centauri A and B to work deceleration through a carefully chosen trajectory. In other words, approach A, begin deceleration, move to B and repeat, then emerge on course outbound to Proxima, where you’re now slow enough to use its own photons to enter the system and stay.
Working with Michael Hippke (Max Planck Institute for Solar System Research, Göttingen) and Pierre Kervella (CNRS/Universidad de Chile), Heller has refined the maximum speed that can be achieved on the approach into Alpha Centauri A to make all this happen: 16900 kilometers per second. If we launch in 2035, we arrive at Centauri A in 2092, with arrival at Centauri B roughly six days later and, finally, arrival at Proxima Centauri for operations there in a further 46 years. That launch time is not arbitrary. Heller chose 2035 because he needs Centauri A and B to be in precise alignment to allow the gravitational and photon braking effects to work their magic.
So we have backed away from Starshot’s goal of 20 percent of lightspeed to a more sedate 5.6 percent, but with the advantage (if we are patient enough) of putting our payload into the Proxima Centauri system for operations there rather than simply flying through it at high velocity. We also get a glimpse of the systems at both Centauri A and B. I wrote about the original Heller and Hippke paper on this back in 2017 and followed that up with Proxima Mission: Fine-Tuning the Photogravitational Assist. I return to the concept now because Heller’s presentation contrasts nicely with the Helicity fusion work we looked at in the previous post. There, the need for fusion to fly large payloads and decelerate into a target was a key driver for work on an in-space fusion engine.
Interstellar studies works, though, through multiple channels, as it must. Pursuing fusion in a flight-capable package is obviously a worthy goal, but so is exploring the beamed energy option in all its manifestations. I note that Helicity cites a travel time to Proxima Centauri in the range of 117 years, which compares with Heller and company’s now fine-tuned transit into a bound orbit at Proxima of 121 years. The difference, of course, is that Helicity can envision launching a substantially larger payload.
Clearly the pressure is on fusion to deliver, if we can make that happen. But the fact that we have gone from interstellar flight times thought to involve thousands of years to a figure of just over a century in the past few decades of research is heartening. No one said this would be easy, but I think Robert Forward would revel in the thought that we’re driving the numbers down for a variety of intriguing propulsion options.
The paper René Heller drew from in the Montreal presentation is Heller, Hippke & Kervella, “Optimized Trajectories to the Nearest Stars Using Lightweight High-velocity Photon Sails,” Astronomical Journal Vol. 154 No. 3 (29 August 2017), 115. Full text.
It will be like planting a shade tree with the knowledge that one will not sit in its shade. Unless progress is made in work on human longevity.
Is that correct? 0.056c for 4.3 ly = 77 years, so a launch in 2035 has a ship arrive in the Centauri system in 2113 with another 46 years to reach Proxima.`
Both Forward’s approach and this maneuver are very limited. Forward’s, because the beam divergence over the distance will mean a much-reduced power at the destination, even if the small craft can use all the beam energy reflected back by the main sail. This Centauri system maneuver simply because it will apply to very few stars and is therefore not a universal solution, just one with favorable target system features. Nice to have if available, but not generally applicable.
Light sails may be far more useful when target stars have already been visited and beams established at the other end. Then acceleration and deceleration can be achieved using beams. So not for exploration, but when a transportation infrastructure can be created for settlement or exploitation.
Such long travel times even to the Centauri system seem to rule out human crews unless the vastly larger energies for generation ships are available (c.f. KSR’s Aurora). Just how sophisticated would a robotic probe be by the design and launch dates arrive?
I see from the presentation that the slides make the same error despite noting that the travel time to the system is 75 years from 2035 – i.e. arrival should be 2110, not 2092.
OTOH, they do suggest this type of maneuver can be used for a number of nearby stars, so I stand corrected as to application.
Dear Alex, I think you’re right and you identified a mistake in our animation, which I also used in my presentation at the 8th Interstellar Symposium in Montreal. The minimum travel time to α Cen A that we estimate for a graphene sail is ~75 yr. The optimal time of arrival is 2092. At this point there will be a favorable alignment of α Cen A and B for an inbound light sail from Earth that’s supposed to be deflected to α Cen C (Proxima Cen). So calculating back to the launch date, 2092 – 75 = 2017. Not 2035. I just noticed 2092 – 2035 = 57. Maybe we produced transposed digits (75 vs. 57) when we created the animation? Thanks for spotting that! It’s a very welcome feedback. Also great to see that the comments section here at Centauri Dreams is so fruitful.
Hi Paul
That last link to the paper seems to be broken. Here’s the correct link:
https://iopscience.iop.org/article/10.3847/1538-3881/aa813f
Plus it was uploaded on the arXiv.
Thanks, Adam. I’ll get that fixed.
Perhaps we could have the sail at a slight angle of attack and have the leading edge highly positively charged. The high positive charge would deflect the ISM ions towards a energy generation device such as a coil or impact heating device. The enegy generated could then be used to drive onboard particle accelerators to slow the sail down.
https://www.sciencealert.com/scientists-have-built-a-nanoscale-particle-accelerator-that-fits-on-a-silicon-chip
Had not heard about this work. Thanks!
This paper is a cornucopia of fascinating ideas, too many to mention. Imagine getting a probe to a white dwarf in 69 years! (But alas, not dog years, despite the choice of star!) Because I’m skeptical of laser launches (will such military hardware ever really be made public?) my thoughts go elsewhere – but Heller delivers! This paper cited a melting point for a graphene solar sail of 4510 K, and describes using graphene microwires that can be strong, conductive and reflective. The Heller paper stops its simulations at 100 C according to silicon chip requirements, but couldn’t a graphene Dewar flask protect components for a little while? If you could protect the control systems that way, or design the logic into a graphene structure, it’s a probe that could touch down on a sunspot and coast around on the Sun’s magnetic fields. And the top speed for an unaided solar sail launch is inversely proportional to the distance at closest approach.
But why stop there? I daydream such a probe might surf along a filament out of the sunspot on Alfven waves, extracting kinetic energy from the Sun much like the particles of solar wind. I’m not sure, but I think https://iopscience.iop.org/article/10.3847/2041-8213/ac25fa might extrapolate that these would be very fast where magnetic reconnection occurs near the Sun. (Also, similar reconnection occurs in the Earth’s magnetosphere to cause the aurora borealis. Could a superconducting probe get a sustained Meissner effect boost from this? I’d call that one the Bifrost Drive!)
Good point about graphene chips. They may well be ready, even mature, by the time the design phase is in process.
Tiny graphene microchips could make your phones and laptops thousands of times faster, say scientists . (Not the most reliable source, but indicative.)
If the microchips can be made of graphene, that offers the possibility of making the whole sail a computer and probably quite intelligent. Reminds me of the AI sail in Baxter’s “Proxima”.
Getting around a white dwarf would be amazing if it can get near it’s gravitational solar lens distance of under a AU. The probe could start scanning the distance universe at a very high rate indeed and transmit the information back better ! I am surprised tungsten has not been looked at as although it’s denser by a factor of around 10 it’s vapour pressure is much better by a factor of around 1000 to 10000. And its reflectivity is quite good from around 65 to 95 % better in the infrared part of the spectrum and much better than graphenes paltry 3%.
I saw quite a bit is out there about tungsten chalcogenide monolayers; I’m curious what you had in mind. Heller cited a non-Arxiv paper ( https://ui.adsabs.harvard.edu/abs/2013PhRvB..88p5116S ) about a silicon metamaterial coating that is 99.999% reflective with a 450-nm coating, against 1500-nm infrared light. I still wonder whether conductive carbon “nanowires” might be used in a metamaterial of their own.
To me the ulterior appeal of carbon is that the Sun is more than 0.4% carbon by mass (perhaps much more; there seems to be ongoing confusion: https://www.sciencenews.org/article/neutrino-sun-carbon-nitrogen-metals-elements ) If I can daydream of a probe that can survive sunspot temperatures, is light and strong enough to resist 28 g, and can still contain electronics, magnetic field generators, and reflective mechanisms all made of graphene, well, NOW it’s time to start wondering about indigenous solar ecosystems. :) Could the dominant extraterrestrial community truly be “from the stars”?
Mike, using the tungsten in a grating fashion would greatly increase the reflectively and we can make layers quite thin and strong less than a 100nm.
It’s however the absorbtion that is a problem with all materials, but tungsten may be able too handle it as the melting point is quite high.
A tungsten sail could approach promixa quite close as the spectrum of the star is shifted into the infrared where tungstens reflective properties are better.
Just a matter of clarification here.
Agreed that a close up study of a white dwarf star would be interesting in many respects, planets or not, and might even have environmental effects which even assist the mission. But nominally, this article is about Proxima Centauri, a red dwarf still on the main sequence and likely remain there long after the sun departs that domain. Is it suggested somewhere that a similar mission be aimed at Sirius A and B or Procyon A and B, where the B elements are red dwarfs? Or perhaps something else?
@wdk: The Heller paper is the one linked at the end of the article. Although Sirius is twice the distance of Proxima Centauri, its intense light allows such rapid deceleration that, IF we suppose a .12c launch via laser array, it is faster to get to a bound orbit there. Indeed, they talk about using the “Sirius afterburner” for faster trips to many other destinations! By contrast, the trip to Procyon takes 154 years, and no, that’s not in raccoon years. :) Either probe might make fundamental physics advances studying the cooling of a white dwarf by axions… unless the questions are resolved before they arrive.
Michael Serfas,
Thank you. Followed the link to the Heller paper and now understand the context. And consequently have a better understanding of why the white dwarf targets would have flight advantages ( at Sirius and Procyon) vs. the red dwarf at Proxima – interfacing with propulsion. However, as to the merits of those stars as mission targets I do have some “relative” reservations compared to the Centauri system (Alpha A&B plus Proxima). From a celestial mechanics and environmental standpoint, these two binaries are significantly less stable abodes for HZ planets than the former with Sirius as the worst. Decades back when I did simulations of such, in the Sirius System, HZs would last as planets in a stable orbit just an order of magnitude or two longer than the prospective trip. At Alpha Centauri A & B, there were pronounced cyclic variations of eccentricity and periastron node point in the orbital plane about the primary, but these cycles were repeated cycles and closed – barring other bodies intervening. In the Sirius case, a temperate zone planet about Sirius was disrupted and then ejected in the space of a few thousand terrestrial years.
At Sirius, perhaps a “Mercury” would be safe. As for Procyon, what seemed like minor instabilities could increment over geologic time scales. So in comparison to Alpha Centauri A and B where we don’t have reliable data,
there is a better exoplanet argument to explore. I guess this will be a continuing community debate: propulsion and vehicle design with operational radii vs. targets recommended.
The only way I can imagine Proxima b isn’t the first stop, is if the astronomers can prove it doesn’t have an atmosphere. The way some of the papers about red dwarfs have been going, that may not be unlikely. But Sirius seems, at least, the clear second choice target of a laser array that is so massive and powerful that (if it is used for basic science at all…) it seems necessary to use it hundreds of times to justify the investment. The way this paper tells it, the Dog Star is the closest thing we’ve got to a Star Gate, and the sooner someone dials in a destination and sees if our probe can make its way through “alive”, the sooner people can start using it routinely.
Perhaps we could have paddles coated in radioactive decay products that can be moved around for attitude control and slowing the craft down, alpha emission decay products have quite high expulsion velocities and could be quite useful.
The emissions would have to be asymmetric, i.e. there would need to be a particle deflector to prevent symmetric emissions. (An absorber, e.g. to block alpha emissions is insufficient, it must reflect or deflect the particles.)
If this is possible, then the emissions must be sufficient to accelerate the sail at the beginning, or 1st half, of the mission, no beam would be required. Then, to be efficient, the life of the emitter would need to be short to ensure that there was no parasitic mass on the sail that hadn’t yet contributed to the velocity change. Yet this implies that it would be used up during the early mission.
I would have thought that some small radioactive emitter would best be used as a power source during the cruise phase of the flight.
Alex, if a graphene sail is used a powerful magnetic field could be applied to the sail via the circulation of a current in the sail, this would align a significant amount of the alpha particles in the direction required. Choosing the correct alpha emitter would be easy enough, perhaps a high emitter for the start of the journey and a lower one for the end of the journey and yes it could also provide power for the journey. We could once the sail is deployed in space spray the radioactive decay product on to the sail for even coverage safely from earth. I thought about a probe idea where we would approach the sun quite closely deploying a sail to be accelerated by the sun’s light and then at a certain distance spray the radioactive material on to the sail for the extra boost going out.
If you can generate that magnetic field, you will create a magsail. Rather than align the ions being emitted, wouldn’t the induction of a larger field composed of the star’s ion wind be more effective? This is the plasma magnet/wind rider concept where the size of the induced magnetic sail is inversely proportional to the external ion density – very large in the ISM and smaller as it gets closer to the star, maintaining a constant thrust (theoretically).
The radioactive decay thrust and energy production to form a large magnetic field could compliment each other.