Can we use a laser array to get a fast probe to another star? Breakthrough Starshot relies upon the notion, which was first advanced by Robert Forward all the way back in 1962, and subsequently considered by George Marx in 1966, along with hosts of researchers since. With beamed energy we leave the propellant behind, but as we’ve seen in our discussions of deceleration, there remains the problem of slowing down at the target. Breakthrough Starshot assumes a flyby, but the paper we looked at yesterday works out strategies for braking into orbit at the target star. Or more accurately, at the target stars, for multiple systems are assumed.
Let’s dig back into that paper today, but first, let me make a brief administrative comment. The upcoming Breakthrough Discuss meeting in Palo Alto (I covered last year’s sessions) occurs at exactly the wrong time for me — I’m locked into long-standing travel plans elsewhere. While I travel, there will be no Centauri Dreams posts for the rest of this week, though I will try to keep up with comment moderation. Things return to normal next Monday.
Now, as to multiple stars and our target list. René Heller, Michael Hippke and Pierre Kervella use multiple stars as ‘photon bumpers,’ depending upon steep deceleration at one or more in order to slow the craft and deflect it on to a final target. Alpha Centauri is the obvious case in point, for using these methods we can consider a probe that brakes into orbit around Proxima b after decelerating encounters with Centauri A and B. The new paper revises a January paper from Heller and Hippke by offering trajectories that provide for faster cruise times.
Image: This image of the sky around the binary Alpha Centauri AB also shows the much fainter red dwarf star, Proxima Centauri, the closest star to the Solar System. The picture was created from images forming part of the Digitized Sky Survey 2. Credit: Digitized Sky Survey 2. Acknowledgement: Davide De Martin/Mahdi Zamani.
Interstellar Destinations
As we saw yesterday, the authors believe they can cut the travel time to Centauri A and B from 95 to 75 years (with an additional 46 years necessary for the crossing to Proxima Centauri). All this depends upon maximizing the arrival speed at the first star while producing enough deceleration through photon braking to make the needed speed and course adjustments. And if this can be done at Alpha Centauri, it can be done with other nearby systems, but we would experience much longer travel times with single stars like Tau Ceti or Epsilon Eridani.
Fortunately, multiple star systems abound, offering interesting possibilities. Having optimized the trajectories for Proxima Centauri, the authors consider Sirius A, which is actually about twice the distance from the Sun as Alpha Centauri. The star is bright enough (24.2 solar luminosities) that maximum insertion speed into the Sirius system can be high, some 12.5% of the speed of light. We get a much faster cruise speed coupled with much more dramatic deceleration, and the travel time of 69 years is actually shorter than our Proxima mission.
Sirius is itself a binary, with a white dwarf, Sirius B, that would be useful to study. The paper’s Table 2 is worth reproducing here, as it broadens the range of targets still further.
Image: The paper’s Table 2. Caption: An interstellar travel catalog to use photogravitational assists for a full stop. Credit: Heller, Hippke and Kervella.
Notice that Epsilon Eridani is 10.5 light years away, while Sirius A is 8.58 light years distant, but the journey to Epsilon Eridani is a whopping 363 years compared to the 68.9 to Sirius. Here again we’re relying on the innate brightness of Sirius to allow the fast insertion speed that will accommodate maximum cruise velocity plus deceleration and trajectory change at destination — we don’t have this at Epsilon Eridani. But that also presupposes that we have the ability to launch a craft at a blistering 12.5 percent of c. Breakthrough Starshot talks about 20% of c through the use of an Earth-based laser array. For their part, Heller, Hippke and Kervella think a hybrid accelerative boost might be the best approach:
A graphene-class sail could have a maximum ejection speed of about 11,500 km s ?1 from the solar system if it was possible to bring it as close as five solar radii to the Sun and then initiate a photogravitational launch (Heller & Hippke 2017). This is much less than the maximum injection speed of 17,050 km s ?1 that can be absorbed by successive photogravitational assists at ? Cen A to C. If sunlight were to be used to push a lightsail away from the solar system, then its propulsion would need to be supported by a second energy source, e.g. a ground-based laser array, to fully exploit the potential of photogravitational deceleration upon arrival. A combination with sunlight might in fact reduce the huge energy demands of a ground-based laser system.
Lessening the requirements for the array is all to the good considering the challenge of building it and powering it up. Of course, the hybrid approach assumes huge advances in sail materials, though the authors believe that current progress with graphene will make a workable sail for these purposes feasible within the next few decades. However we reach the necessary speeds, we can look at the prospect of even faster speeds with the assistance of other stars.
The Sirius Afterburner
Which gets us back to Sirius, where the authors consider what they call the ‘Sirius afterburner,’ in which the star becomes the instrument for achieving higher speeds than we can achieve with Earth-based technologies. The authors’ numerical simulations show that minimizing deceleration during approach while maximizing the acceleration after the flyby can increase the velocity of the lightsail up to 27,000 km s-1. That’s a boost of 9 percent of the speed of light. Sirius, then, with its proximity to the Sun and its brightness, becomes what the paper calls “the most natural choice for an interstellar photogravitational hub for humanity.”
Taking that broad view of a possible future is what the paper’s next figure is all about.
Image: Travel times (as hour angles) versus stellar distance to the solar system (along the radial coordinate) for stars in the solar neighborhood. Symbols refer to numerical trajectory simulations to individual targets, lines refer to the logarithmic spiral derived in Equation (4). The spirals are parameterized using M2V (red), K5V (orange), G2V (yellow), F3V (green), and A0V (blue) template stars from Pecaut & Mamajek (2013). Square symbols represent stars with known exoplanets. The black star symbol represents ? Cen. Left: All 117 stars within 21 ly around the Sun. Right: 22,178 stars out to 316 ly around the Sun. Credit: Heller, Hippke & Kervella.
A photogravitational hub like this can yield an interesting target list indeed. We have a total of 328 known exoplanet host stars within 316 light years, and many stars of astrophysical interest in themselves. Among these, the authors suggest such destinations as TV Crateris, which is a quadruple system of young T Tauri stars about 150 light years away. 36 Ophiuchi is a triple system of K stars some 19.5 light years out. Also intriguing: Fomalhaut, 25 light years away and another triple, with exoplanet Fomalhaut b and protoplanetary disks around Fomalhaut A (Alpha Piscis Austrini) and C (LP 876-10).
We would like to reach any number of single star targets as well, especially nearby red dwarfs of the sort that we will soon be analyzing for biosignatures, but achieving a bound orbit around such a star would involve extremely long travel times. A flyby more like the basic Breakthrough Starshot concept seems to make more sense for these because unlike Proxima Centauri, they have no nearby bright stars to slow the incoming craft and alter its trajectory.
The paper is sanguine about the prospects for developing the needed technologies for the missions examined herein. Components currently under development include:
…procedures for the large-scale production of graphene sheets, nanowires with the necessary electronic properties consisting of single carbon atom layers, gram-scale cameras and lasers (for communication between the sail and Earth), or sub-gram-scale computer chips required to perform on-board processing etc. We thus expect that a concerted effort of electronic, nano-scale, and space industries and research consortia could permit the construction and launch of ultra-light photon sails capable of interstellar travels and photogravitational assists, e.g. to Proxima b, within the next few decades.
Making any of these missions happen, though, demands the highest degree of aiming accuracy, which we lack even for the Alpha Centauri stars (and bear in mind that Gaia will not observe Centauri A or B). To pull off a successful ‘bank shot’ in the Centauri system, we will have to nail the proper motion and binary orbital motion of the AB binary. Right now our data are simply not sufficient: The authors note that we lack the current capability to achieve orbital injection and swing-by to Proxima, something that will have to be remedied through highly precise astrometry.
The paper is Heller, Hippke and Kervella, “Optimized trajectories to the nearest stars using lightweight high-velocity photon sails” (preprint).
Why isn’t GAIA observing the Centauri stars?
From what I understand, they are too bright for it.
This is from an earlier paper from Pierre Kervella:
“In addition, the ? Cen pair is one of the principal benchmark stars of the Gaia mission (Heiter et al. 2015; Jofré et al. 2015). An extremely accurate calibration of its fundamental parameters is essential for the validation of the data analysis methods that are currently applied to the fainter targets of the Gaia catalog (see e.g. Bailer-Jones et al. 2013).”
Though the one under discussion today, of which Kervella is a co-author, states that Gaia will not observe Centauri A and B. So I’ll have to track down an answer here.
They may be too close together at this point.
Here you can find Alpha Centauri in GAIA’s first data release:
http://gea.esac.esa.int/archive/
Search for “alf Cen A” or “alpha Centauri A”, not simply “alpha Centauri”. There is data for brightness and position (Gaia Source) but not for distance and velocity (TGAS).
I think it’s amazing that we can even think about launching a mission in the next few decades that would end with orbital insertion in another star system. The question we must be working on in parallel is even more awe-inspiring:
What is the smallest craft that could land on an asteroid there and begin to build tools from the local materials? Once it could boot up to something like a tiny factory, it could make tools that build just about anything. One priority would be high bandwidth antenna for receiving various software/recipes from Earth, which will hopefully have made substantial progress since mission launch, in AI, synthetic biology, materials science, etc.
Let’s say the mission would be en route for a century, equipped with a minimal toolkit adequate only for rough start. That would give Earth scientists a century to figure out how to program the primitive gear on board for building the really fancy stuff, like living cells, a gene sequencer, etc. And by “etc” I mean: The stuff you would need to build a healthy human colony, along with all the other terrestrial lifeforms that humans like to have around (gut bacteria, rice, kitties, earthworms, and the rest). We will be able to create such a human colony long before it will make sense to transport actual humans through interstellar space. Yes, it will take lots of science, but most of that science (nanotech, ai, biotech) is advancing rapidly already. Space propulsion is the laggard, but these bankshots give me hope that we won’t need an extra revolution in propulsion when the other tech is ready.
It’s a pity that Sirius is unlikely to host any planets: the progenitor binary would have been very hostile to accretion and there is no indication of metal pollution in the white dwarf that would indicate the presence of asteroid-like objects in the system.
This strikes me as being a stepping stone in between where we are now and breakthrough starshot. An alternative ‘mission profile that might be realisable sooner and cheaper, but with research and kit investment that could be built upon. The results from the missions would be complementary to each other.
Like breakthrough starshot this is really a story about building out the required Sol-side infrastructure. Once we have the ability to launch the first one, the marginal cost for additional probes is loose change so we might as well shotgun them out there by the crateload.
Sol photo assist gets us launching probes sooner with a lower upfront phased array laser/power supply cost, leaving us with the option to extend the array
and power supply at a later date to do high speed flyby missions.
Project planning with timescales in decades and later-launched probes arriving earlier than probes launched sooner is going to be interesting. Makes me think of ‘The Forever War”!
They assume a magical two atom thick substance two orders of magnitude more opaque than single layer graphene, which I find impossible to believe. The original version of the paper assumed graphene, while failing to take into account the fact that single layer graphene is very transparent (only about 2.3% of light is absorbed/reflected). They attempted to handwave some sort of magical augmentations to improve reflectivity by two orders of magnitude without much added mass, but I see no reason to believe any of that.
After taking the true optical properties of atom-thick graphene into consideration, I calculate that you can get 50 layer thick graphene up to around 1800km/s with sunlight. That’s really good for a lot of purposes, but it’s still 700 years to Alpha Centauri.
If you want to get to Alpha Centauri in less than a century with “hybrid” solar/laser acceleration, then the solar portion only reduces your laser array energy requirements by about 10%. I don’t think it really helps at all once you factor in the various design problems with designing your sail to operate with sunlight as well as laser acceleration.
However, development of thin film probes might actually help something like starshot in a different way – forming extremely lightweight solar powered microwave beam drones or solar powered lasers. 50 layer graphene cells could be only 40mg/m^2. A single 20 ton payload could launch enough cells to cover 500 km^2, or roughly the area of California. Even in Earth orbit, this could translate to capturing 650GW of sunlight. In orbit around Mercury, we’re talking 4000GW. That’s for just a single 20 ton payload! Three of these payloads would capture as much sunlight as our global energy consumption.
Thank you for looking at the details. It’s a bit of a buzzkill, but at least we’re smarter for it.
The reflectivity issue presupposes (I think) that the light–whether it’s from the Sun, a laser array, or other stars–can be made to reflect off extremely thin sail materials almost completely, when the incident illumination is 90 degrees or less. With one-atom or two-atom thick sails, that is indeed a “tall order,” *but*:
Even transparent materials reflect much if not most of the light that falls upon them, when the angle of illumination is very shallow (imaging X-ray telescopes in X-ray astronomy satellites use such “grazing incidence reflection” off metal mirrors in order to bring the rays to a focus), so:
Might an extremely thin sail, that looked like “/\/\/\/\/\/\” (in an edge-on view, that is), be able to reflect most of the light that fell upon it? The angles of these “accordion folds” could be chosen to ensure that re-reflection onto adjacent “folds” (which could cause over-heating of the sail material) would not normally occur, and would be minimized even at extreme “tack angles” of the sail. (It might even be possible to incorporate tiny, either active or passive [perhaps bi-metallic strips?] actuators into the sail material, which would automatically change the “fold angles” as required, in order to avoid over-heating and to minimize photon thrust loss at various sail tack angles.)
Geometrically, a thin sail at a shallow angle is equivalent to a much thicker and smaller sail normal to the incident light. Only the latter is easier to make.
But a smaller, thicker sail–even though it’s more reflective than a very thin sail–produces less thrust, plus the thicker sail has a greater areal density. The “corrugated” thin sail would effectively be similar (because having the fold-like “corrugations,” it would contain more sail material per unit area), but by making the corrugations a small as possible–and choosing their fold angles carefully–it might be possible to get a compromise between reflectivity and “effective areal density” with extremely thin sail material, which could yield acceptably high performance.
I’m extremely dubious about laser launched atomically thin sails that can also function as probes. Even Forward’s Starwisp wasn’t THAT thin.
OTOH, if you can build such a sail with even rudimentary course correction capabilities, you’d have a fantastic mass beam to push much more massive probes up to decent speeds.
As always, accelerating is just a matter of energy resources, and we’ll have those coming out of our ears by the end of the century, once we get working Von Neumann machines churning out solar power satellites. Slowing down at the other end of the trip is indeed the real problem, as it’s resource limited at that end. I think we really are going to have to crack the problem of working antimatter rockets.
I continue to think the real priority here is the Von Neumann machines. Without them we’ll never have the ratio of infrastructure to population to permit even extensive colonization of the Solar system, let alone exponentially more energy intensive ventures like interstellar colonization. With them we can afford to do these things.
Thank you for the reply. One minor point: I haven’t gone into the details on this, but I would question whether stacking up additional graphene layers would necessarily be the most effective way to increase reflectivity. Optical dielectric coatings might be a better way: stack up layers of materials (probably rare earth oxides) with different, alternating refractive indices.
I completely agree that Magic will be necessary , but not just for lightsails ..Without Magic , Starshot (as described so far) is going to have some hard problems as well : how exactly will a 10 or hundred gram ‘spacecraft’ communicate over 5 Lightyear distance ?(perhabs by having a two atoms thick antenna !) …but in the case of Starshot , at least it is possible to imagine what KIND of Magic we are talking about …it must somehow be connected to the kind of Magic we call Swarm-behaviour …only by finding each other from the start and cooperating in an ALLMOST magic way , will gram-scale spacecraft be capable of doing anything usefull
A pinch of powdered alicorn (unicorn horn) in the graphene would provide the requisite magic… But there may be another way to make atom-thick sails sufficiently reflective (which the article about superconducting lithium-doped graphene, which Michael posted a link to below, made me think of):
It may be possible to develop such ultra-thin materials (made using multiple chemical elements) whose optical properties–such as reflectivity–could be controlled by passing electric currents through them (phase-changing materials, which change from liquid to solid when electricity is passed through them, already exist). A very thin sail made of such a variable-reflectivity material could get its required electrical power from IKAROS-type, thin-film integrated solar cells, or from passing through the solar–and later (near its destination), stellar–magnetic fields.
I mentioned a similar idea on the ‘Starshot’ website the potential of using electrical impulses to change the transmission of light through the sail in certain locations which could be used to control the attitude of the sail to high accuracy.
Good! JAXA’s IKAROS solar sail, which incorporates both thin-film solar cells *and* variable-albedo (reflectivity) Liquid Crystal panels (that worked well for steering this spin-rigidized square sail, see: http://en.wikipedia.org/wiki/IKAROS ), demonstrated that the electrically-controlled panels had good control authority and steering accuracy. Also:
In a non-spinning, 3-axis controlled sailcraft, such variable-albedo panels would likely be able to change the sail’s orientation more quickly (although spinning sails can be lighter). In the spin-stabilized IKAROS, the sailcraft’s spin axis had to be precessed to the desired orientation.
Gaia will provide astrometric data for Alpha Centauri.
Gaia does not have an input catalog, and thus at some level observes every source that passes through its telescopes. It was originally thought it would not be able to handle sources (like Alpha Centauri) brighter than some magnitude (originally 5, then 3) but the DR-1 paper (Section 6.5) says:
“The 230 brightest stars in the sky (G < 3 mag, loosely referred
to as very bright stars) receive a special treatment to ensure
complete sky coverage at the bright end." and "end-of-life astrometry with standard errors of a few dozen ?as."
http://www.aanda.org/articles/aa/pdf/2016/11/aa29272-16.pdf
It may be better to accelerate out a graphene sail using a very reflective heavier sail which is then ejected so that the graphene sail goes on its way to the target star. The graphene layer sail which is doped say with lithium can then behave as a super conducting magsail which allows a large magnetic field to be generated to slow the sail down against the target stars stellar wind. These two processes of mag and photonic braking could potentially generate better results.
https://www.extremetech.com/extreme/213816-lithium-doping-turns-graphene-into-a-superconductor
Michael–thank you for posting the article link! This is, I think, a *very* important thing to know in connection with solar sail, lightsail (laser sail), and combined-function (solar/lightsail) design. Now:
Being able to also operate a solar sail or a lightsail as a magnetic sail (magsail) would also be useful for many applications *within* our solar system–for propulsion, braking, orbital maneuvers (in solar orbit and in orbit around planets with magnetic fields, particularly in the outer solar system), and generating onboard electrical power for the spacecraft systems. This “combination sail” technology, after it has matured and had its risks retired through such close-to-home uses of it, would be very useful and versatile indeed for interstellar probes.
There is also other alkaline metals that allow superconductivity and high reflectance, Calcium mixed with a few layers of graphene has a surprising amount of reflectance but unfortunately in the infrared (Proxima maybe good to slow it down). I am sure there are other combinations out there that may be suitable to the wavelength of the target star or our laser propulsion system or both.
https://www.nature.com/articles/srep23254/figures/3
https://www.nature.com/articles/srep23254
This is most interesting! Thank you for posting these article links. M giant stars (which also radiate a lot of infrared) would also give such sails quite a kick, either to decelerate for arrival or to accelerate away. Other elements mixed in with graphene may provide superconductivity along with high reflectivity in visible light (and possibly also in ultraviolet light, which might provide additional thrust depending on which stars were the destinations of the solar sail probes).
Ca-GIC is pretty good even in the optical part of the spectrum as well but I don’t know what the absorption is like. The UV part of the spectrum is pretty harsh ~30000 K for the greatest reflectivity, that’s a hot star! M giants also pump out a huge stellar wind which would aid a slow down via the magnetic field component. And as you say the material has many combinations that could be looked at which could be very useful.
This–basic research on the reflection and absorption characteristics of graphene when doped with different elements or compounds, as well as manufacturing methods–sounds like something that NASA should fund, perhaps through SBIR/STTR grants (see: http://sbir.nasa.gov/ ), and:
Not only superconducting dopes, but highly-reflective non-superconducting dopes would be useful (particularly for interplanetary solar sails intended for use as far out as the asteroid belt), although the “superconducting, electrically-isolated rim” combination photon/magnetic sail that you suggested for interstellar missions would also be very useful for operations around the giant outer planets, where the magnetic thrust would compensate–and perhaps *more* than compensate–for the dim sunlight out there. (Plus, that is good to know about M giant stars’ powerful stellar winds; not only would that aid a combination photon/magnetic sail’s braking on arrival, but it could also enable the sail–once in orbit–to generate electricity due to its motion through the star’s magnetic field.) Also:
I had only mentioned the ultraviolet reflectivity of doped graphene sails as an aside, for a hypothetical sail probe intended for operations around Sirius (exploring Sirius and its white dwarf companion star would be an interesting “exotic astrophysics” interstellar mission [as Eugene Mallove and Gregory Matloff categorized it in “The Starflight Handbook”]). Having seen what even the relatively mild, heavily-attenuated UV light of *our* Sun does to paints, plastics, and rubber & fabric items that remain outdoors, any non-metallic sail material (something other than a refractory metal foil, perhaps) that can “shrug off” the harsh ultraviolet light of Sirius will be tough indeed!
Once these sails are in orbit around Sirius b they could use its powerful ‘gravilens’ to see great detail much further out into space as the focal line is much, much closer in. If these graphene films are this reflective on the 1- 10 nm scale as they appear to be then the sail discussed becomes viable by a significant margin. These sails been so thin could approach the central stars quite close and still maintain superconductivity as there will be little light absorption over the nm thickness. Yes they could be tested around our solar system and in particular the outer gas/ice giants, for now they could use the suns light to propel them and later a laser system.
And thanks for the link, I wonder if a University could be pointed to look into this potential idea using the funding available.
You’re welcome for the link, Michael, and the gravitational lens utility of Sirius hadn’t occurred to me–that observation opportunity would be another excellent reason to go there! Hmmm…universities that have conducted other space hardware development work, and/or those that haven’t but which are looking for new materials science work, would be good candidates to seek the NASA grants. Also, there is another, local application of these sails that could serve as a catalyst for their development (and it could also be employed around other stars, especially–and possibly with greater effectiveness–at Sirius):
These graphene sails should be able to be made light enough to function as statites (light-levitated solar/stellar sails, see: http://www.google.com/#q=statite+solar+sail ). Not only can a statite hover above the Sun or another star, but it can also hover over the north or south pole of a planet (here’s a Centauri Dreams article about this: https://centauri-dreams.org/?p=13631 ). This “pole-sitting” station-keeping should be particularly easy to achieve over the Moon, where a statite follow-on to JPL’s Lunar Flashlight orbiter mission could do multiple things, including:
A pole-hovering statite sail could–in addition to reflecting sunlight into perpetually-dark lunar craters to map their interiors–also serve as the equivalent of a geostationary satellite for landers, rovers, and human expeditions on the lunar farside. The sail itself could be used as a passive, Echo satellite-like reflector for radio and laser signals from/to such spacecraft or lunar bases, and the statite could also carry signal relay equipment. Statites emplaced high enough above the lunar poles would provide coverage for virtually the entire Moon (and they could avoid losing “lift” in the Earth’s penumbra and umbra most if not all the time).
My sense is that many low orbit comsats would be a better solution to coverage. High inclination and polar orbits would satisfy the need for coverage.
Earth GPS coverage works very well with LEO satellites. I’m not clear what a statite[s] would do to improve on this performance.
Sails should be used where they have operational advantages. At this point, fuel-free propulsion. and with beaming, potentially very high velocities and low flight times within the solar system.
LLO (Low Lunar Orbit) comsats would offer shorter (almost certainly unnoticeable) signal relay delay times, and such comsats–if equipped with solar sails–could remain in orbit indefinitely, using their photon thrust to keep their orbits from being “warped down to impact” by the lunar mascons’ gravitational “tugs” (plus the sails could provide attitude control, and graphene ones could last for decades or even longer).
Lunar “pole-sitter” solar sail statites would enable antennas on the lunar surface (at bases and mobile exploration units on the nearside and the farside) to be simpler, as they need not be movable since they wouldn’t have to track the motionless statites (this would be convenient for TV and radio broadcast services provided to lunar settlements). Lunar GPS systems could also work with statites, although I imagine being able to get simultaneous fixes on multiple LLO GPS navsats would (as is the case with Earth-orbiting GPS satellites) yield more accurate position determinations.
There are many techniques that this laser and reflective material can be used for, it is this multi use which is its greatest strength and will drive it’s development. One could be that it is coated onto satellites outer surfaces at a specific reflective frequency so that their orbits can be pumped up or down allowing a controlled deorbit when they are no longer useful.
Now *that’s* an application that should be suggested to folks in the aerospace industry, academia, and space agencies whose work concerns the space debris problem. Highly-reflective, doped graphene blankets wrapped around parts of spacecraft (as metallized Mylar and Kapton ones are) would not only last longer than them (and would likely resist monatomic oxygen corrosion better than them, too), but such graphene blankets would reflect laser light rather than (probably, as with metallized Mylar and Kapton) undergoing melting and ablation after getting heated up (which would generate more bits of space debris; this undesirable effect would likely occur with metallized Mylar and Kapton insulation blankets).