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Proxima Centauri: The Problem of Arrival

Given his key role in the development of sail ideas for interstellar flight, Robert Forward inevitably comes up in any discussion of deep space missions. The late physicist put forward a number of sail concepts and mission ideas, including a laser-driven lightsail to Epsilon Eridani with return capability that would travel at 50 percent of the speed of light. Those were numbers that made a manned mission theoretically possible, though demanding a huge sail (1000 kilometers in diameter) and a mind-bending space-based 75,000 TW laser system.

Yesterday we looked at the critical problem of deceleration in a sail-based interstellar mission, with reference to the new paper by René Heller and Michael Hippke. I only wish Forward were here to give us his thoughts on the newly proposed ‘photogravitational assist’ method of deceleration, because for years his own method for the Epsilon Eridani mission — a ‘staged’ sail that separates, so that one sail ring reflects laser light back onto another — has been the only method I’ve seen for slowing a sailcraft down for orbital insertion at another star.


Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. In the paragraph above, I didn’t even mention the ‘paralens,’ a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus. The structure would be used to collimate the laser beam.

To my knowledge, Forward never considered the possibility of using stellar photon pressure combined with gravity assists as a means of deceleration. The method wouldn’t have occurred to him in relation to Epsilon Eridani in any case. For one thing, moving at 50 percent of c, his sailcraft would be unable to achieve the needed braking from the method, and for another, Epsilon Eridani, a single star, is the wrong kind of target for this type of maneuver. As Heller and Hippke explain, a multiple star system is the destination of choice.

This quote from the paper gets the point across. In the passage, L refers to stellar luminosity:

In multi-stellar systems, successive fly-bys at the system members can leverage the additive nature of photogravitational assists. For multiple assists to work, however, the stars need to be aligned within a few tens of degrees along the incoming sail trajectory of the sail. Such a successive braking is particularly interesting for multi-stellar system, where bright stars can be used as photon bumpers to decelerate the sail into an orbit around a low-luminosity star, such as Proxima (0.0017 L) in the α Cen system or the white dwarf Sirius B (0.056 L) around Sirius A.

Sirius A? Indeed. For the paper notes that other nearby stars offer more favorable conditions even than the Alpha Centauri triple system for decelerating an incoming lightsail. Sirius A is about twice the distance from the Sun as Alpha Centauri but offers an extremely bright target (25 L) for deceleration, making the maximum injection speed into the system almost 15 percent of lightspeed. It would take something other than a solar photon sail to get the initial payload up to cruise speed for such a journey, but deceleration upon arrival is possible.

We need to learn everything we can about deceleration given the advantages of a sail that operates for years in a bound orbit within a stellar system (and even around a target planet like Proxima b) vs. a flyby mission. Early probes to nearby stars might well be flyby missions, particularly if we build the Breakthrough Starshot infrastructure, which would also be useful here in our own Solar System. But detailed follow-ups could come through decelerating lightsails in those destinations most suited for such methods. Fortunately, the nearest stars to our own form one such system.

I refer you back to yesterday’s post if you’re just coming into the discussion, but the brief summary is that the combination of the gravitational pulls of Centauri A and B along with their photon pressures is what makes deceleration of Heller and Hippke’s 316-meter sail possible. Centauri A is thus the first target, with the flyby there being manipulated through autonomous onboard technologies to maximize the braking effect before sending the sail on to Centauri B.

With the help of Centauri B, we slow from 4.6% of c to about 1280 kilometers per second, the figure that Heller and Hippke have determined would allow entry into a bound orbit around Proxima Centauri. A flight time of 46 years to Proxima ensues. At the destination, the resulting highly elliptical orbit is then circularized over time using photon pressure; we wind up with a functioning, data-returning probe in the star’s habitable zone. This obviously demands extreme and precise maneuvering but needs no onboard fuel.


Image: Artist’s concept of Proxima b orbiting Proxima Centauri. (Image: ESO./L. Calçada/Nick Resigner).

Navigation during the critical period of the photogravitational assists demands careful attention. The paper argues that multiple spacecraft may be one way to handle this. In the passage below, rmin refers to the sail’s minimum distance from the star:

Regarding the nautical issues of an A-B-C trajectory, communication among sails within a fleet could support their navigation during stellar approach, as it will be challenging for an individual sail to perform parallel observations of both the approaching star and its subsequent target star or of other background stars. Course corrections will need to be calculated live on board. In particular, the location of rmin will need to be determined on-the-fly as it will depend on the actual velocity and approach trajectory and, hence, on the local stellar radiation pressure and magnetic fields (Reiners & Basri 2008) along this trajectory.

I mentioned yesterday the question of what any beings on Proxima b might see if a sail like this one were headed for them. In a Frequently Asked Questions document timed for release with the paper, the authors point out that the sail would indeed be observable, appearing as a new star in Proxima b’s skies that would have the same electromagnetic spectrum as Proxima Centauri itself, although blue-shifted. There’s also this:

…any time variability of their host star’s spectrum would be delayed in that star — initially by years, later only by months, weeks, and finally just days or seconds. This new star would also become brighter as the sail approaches Proxima b, and the blue-shift would decrease until, upon the sail’s arrival at Proxima b, the blueshift would disappear and the time delay would be very short, e.g. seconds only. At some point, when the sail would reorient itself into an oblique angle to transfer into an orbit at Proxima b, this fake star would suddenly disappear for an observer on Proxima b. As the sail would orbit the planet over the next months or years, it could occasionally reappear for just a few seconds as a very bright star-like dot in the sky. In principle, if these potential inhabitants of Proxima b were able to identify the sail as being artificial, they might conceive of a way to deliberately betray their presence to the cameras aboard the sail.

Interesting fodder for science fiction! I can recall the incoming lightsail seen by characters in Niven and Pournelle’s The Mote in God’s Eye (Simon & Schuster 1993), but I’m hard pressed to think of other science fictional treatments of this scenario. Perhaps the readers can help me out. Meanwhile, have a look at the Heller and Hippke paper, whose methods offer serious hope for solving the critical question of slowing down at another star.

The paper is Heller, R., & Hippke, M. (2017), “Deceleration of high-velocity interstellar photon sails into bound orbits at α Centauri,” The Astrophysical Journal Letters, Volume 835, L32, DOI:10.3847/2041-8213/835/2/L32 (preprint).


Comments on this entry are closed.

  • DJ Kaplan February 2, 2017, 13:32

    No matter what we find out about the AC system, it’s got to be a better target for observation than Proxima. Can’t Proxima’s luminosity and/or gravity be used as a brake for arriving at the AC system?

  • Robert February 2, 2017, 13:48

    I question how much more we can learn by physically sending a probe there vs. advancing techniques for remote sensing especially given the long travel time vs. the rate of technological development of sensing at a distance.

  • Charlie February 2, 2017, 14:32

    ” … and for another, Epsilon Eridani, a single star, is the wrong kind of target for this type of maneuver.”

    why ??

    • Paul Gilster February 2, 2017, 16:41

      The answer is in the following paragraph in the article, quoting from the paper about the need for multiple star systems to make maximum use of photogravitational effects. Here it is again:

      “As Heller and Hippke explain, a multiple star system is the destination of choice.

      “This quote from the paper gets the point across. In the passage, L☉ refers to stellar luminosity:

      “In multi-stellar systems, successive fly-bys at the system members can leverage the additive nature of photogravitational assists. For multiple assists to work, however, the stars need to be aligned within a few tens of degrees along the incoming sail trajectory of the sail. Such a successive braking is particularly interesting for multi-stellar system, where bright stars can be used as photon bumpers to decelerate the sail into an orbit around a low-luminosity star, such as Proxima (0.0017 L☉) in the α Cen system or the white dwarf Sirius B (0.056 L☉) around Sirius A.”

      Epsilon Eridani is not a multiple star system.

  • Micky Badgero February 2, 2017, 14:52

    To me, lasers make the most sense for flyby probes or for later interstellar infrastructure, where there is a laser at both stars. Send robots there to build the laser, with fusion engines or a laser for acceleration and fusion for deceleration. Then all you need is the very small center sail.

    • Antonio February 2, 2017, 18:04

      Huh??? You send a fusion starship, able to stop at destination and carrying robots, and thus at least 50,000 tonnes at launch time, in order to be able to stop a very small sail 50 years later???

    • Ralph February 2, 2017, 19:04

      The problem with accelerating a fusion-powered spacecraft with lasers and a light sail to have it decelerate with fusion at the destination is that you’d have to accelerate the fusion drive itself, which is dead weight until it is used, if it still functions after that long in space. Another approach is to use both lasers and fusion during acceleration, so the mass of the fusion drive does more overall. The sail and fusion drive would be involved in acceleration and deceleration, even without a laser system at the destination.

      Re using Proxima to get to the other systems, Proxima’s luminosity is probably not enough to provide sufficient braking. Proxima is the location where a world is known to exist. We’d still have flybys of both brighter stars, a close observation point for all three stars, and the light sail could probably still enable travel to the brighter stars, albeit at a much slower speed. By the time any of this could happen, we’d also have time to shift the target if something more interesting were spotted from here.

  • RobFlores February 2, 2017, 16:48

    Maybe this “laser” can be expanded in stages. and as the laser power grows so can the goals.

    First stage to get a 5Kg , proof of concept, modest
    analysis/survey tools obviously.

    Second Stage get 500 kg craft, that can report detailed info
    to determine suitable human exploration sites within the extra solar system.


    Using Long Sleep/life extension tech send a crew 12-18 on a
    1,000 metric ton ship. Only 3 crew awake during voyage. explore
    nearby system fully

    Duration of mission. Well, until nano assembly machines brought along
    build a Mega Laser on the nearby system, for return trip.

  • Bill February 2, 2017, 16:52

    Did the authors consider the use of a solar electric sail? The technology has yet to be proven but assuming it works would it result in a faster transit and improved braking at the destination?

  • James Benford February 2, 2017, 19:01

    , I might point out that there are three minor related papers about Aplha C deceleration that you may not know of. Two of them are associated with Project Icarus, a design study for a fusion rocket to Alpha Centauri. Pat Galea and myself produced Notes related to the use of solar sails for Alpha Centauri. (P. Galea, “Solar Sail Technology for the Icarus Interstellar Mission”, Project Icarus Study Group, 2011.)

    In my case, I was looking at deceleration of sail probes from a fusion-powered starship as it approaches. The starship uses its power to slow down diaphanous sails to interplanetary velocities and does not decelerate itself. Assuming optimistic parameters for the technologies assesses practicality. But it doesn’t turn out to be practical: results are that <10% of light speed can be subtracted only for very small payloads using sails well beyond the state-of-the-art, driven by very high power beams.(J. Benford, “Beamed Energy Propelled Sails for Deceleration of Sub-Probes”, Project Icarus Research Note, 2012.)

    Greg Madoff looked at deceleration of solar sails in the following the following reference: G. L. Matloff, “Solar Photon Sail Deceleration at Alpha Centauri A”, IAC-09-C4.6.5, 2009.

    • Michael February 3, 2017, 13:21

      Would it be possible to drop magsails into the exhaust, the fusion exhaust could be salted with extra ion seeders like potassium to help ionisation?

      • Michael Fidler February 4, 2017, 23:46

        I’ve been thinking of magsails also and wondering if a rotating wire frame for the light sail, if energized might generate a magnetic field. The closer to the star the greater the braking, especially if it could catch a solar storm! Could the graphene be doped to turn it into a giant solar cell? Plenty of power if it can all be integrated into a light weight sail.
        Our View of a Decelerating Magsail
        by PAUL GILSTER on MARCH 25, 2015
        Thinking about Magnetic Sails
        by PAUL GILSTER on AUGUST 28, 2014

  • Christian G February 2, 2017, 19:03

    The bit with a light-sail looking like a blue-shifted reflection of the target star also appeared in Niven’s “The Fourth Profession”

  • Michael T February 2, 2017, 23:29

    Interesting article.
    Admittedly pedantic quibble: doesn’t
    ‘L☉’ refer to our sun’s luminosity, as ‘1.0’, properly speaking?

  • Charlie February 3, 2017, 12:54

    This ongoing discussion concerning a method of reaching it distant star system lends me to more and more coming down on the side of massive starships that are required to actually accomplish the missions. The reason why I tend to support this, rather than the small sail concept is the fact that the small sails concept seems to require a large expenditure for what appears to be a very limited return on investment.
    Also too, as I previously pointed out, but I believe there’s been insufficiently considered as a true source and problems is the problem concerning Galactic Cosmic Rays, as I previously pointed out in a two or three blocks sessions previously. This is no small matter whatsoever and is a significant source of threatening radiation that can only be reasonably overcome by the use of massive vehicles, which can successfully shield animate (read living here) and non-animate objects. Also, the massive vehicles will carry far more scientific components than a small solar sail that can be considered at this time.

    In addition to what all I have said above, there is also the idea that I have not seen elsewhere (at least as far as I know) of the idea of using a variant of the Bussard type of space drive, which suggested that you could collect fuel on Route and have it undergo nuclear fusion, such that the hydrogen collected could be used as a propellant.
    This got me to thinking;. why not use this mechanism as a means to collect hydrogen atoms, but use this hydrogen not as a source of energy, but rather as a source of rejection mass that can be heated by a nuclear fusion reactor that is fueled by on board deuterium ? In other words, the hydrogen collected could serve as expulsion mass that could be vented out the back far reaction. This would allow you to harvest interstellar medium for fuel, as well as the onboard fuel itself, thus presumably allowing more extensive acceleration of the ship. Just a thought. As a way to overcome the limitations that solar sailing seems to engender.

  • Charlie February 3, 2017, 13:06

    Very interesting article, Indeed!

    Use of multiple sails makes one acutely aware of the fact that we are making a large number of assumptions which might not be true concerning the solar environments that these probes are going to sail into. With only a weight of 90 grams you’d have to be pretty doggone certain of yourself to allow capture by the star system in question, while at the same time avoiding incineration by the intense radiation that’s expected to be present in the solar coronas !

    But what is a problem, but if not for it being challenging …

  • Gerry February 3, 2017, 13:30

    Are any of the Project Icarus Study Group research papers are available to read online?

    Also, many interesting papers on interstellar topics are to be found in the Journal of the British Interplanetary Society. This seems like the place to ask if anyone knows if that publication is available in an online journal format or only in print?


    • Paul Gilster February 3, 2017, 17:39

      Gerry, JBIS papers are not available in full-text form online, a true gap in our coverage, but one that will perhaps one day be fixed. The individual issues are available for purchase through JBIS, but not in an online database. As to the Icarus papers, I assume these will become available as soon as the final report on Icarus is released, and will keep readers posted.

  • spaceman February 3, 2017, 13:51

    Wow, a 75000 TW laser system! Would such a huge laser be powered by the Sun? Couldn’t a laser of this size or even one somewhat smaller serve as a potent weapon too?

    • Paul Gilster February 3, 2017, 17:40

      Huge system, no? And yes, it would have to be powered by an installation in a close orbit of the Sun, inside the orbit of Mercury. Forward always thought big…

  • Steven Torry Rappolee February 4, 2017, 6:43

    check out Ken MacLeod’s “Learning the World” where alien bats discover incoming starships using astroarchaeology

  • Robert Clark February 10, 2017, 4:30

    Thanks for the link. A recent accidental discovery may make for even faster graphene solar sail flights:

    May 28, 2015
    Graphene sponge can absorb light and emit energetic electrons for breakthrough solar sail propulsion.

    The graphene unexpectedly ejected electrons when exposed to intense light. Also, unexpectedly a recoil force was observed on the graphene.
    The researchers considered the force might be akin to solar light pressure, but the observed force was orders of magnitude higher. They concluded it was due to reaction of the electrons emitted at high speed.

    In any case the force on the graphene used as a sail in this fashion would be orders of magnitude higher than just a solar sail operating by photon pressure alone.

    About the issue of slowing down in general, could we use the density of the solar wind reacting against the large sail at relativistic speeds to slow down?
    Heller and Hapke mentioned in their FAQ that the interstellar wind outside the star system wouldn’t be enough to slow it down, but the stellar wind inside a star system is much denser.
    For the Sun, at 1 AU the density is about 10 particles, mostly protons, per cubic centimeter. However, the density increases as the inverse square on distance. That is, it’s a hundred times denser at ten times closer to the star.
    Doing a rough calculation using the simple approximation that the drag force is proportional to the product of the density, area, and velocity squared, I think it might work to slow it don for a kilometer scale sail, moving at relativistic speeds, at a tens of grams scale mass.

    Bob Clark

  • ljk May 4, 2017, 9:01

    Taking a cue from the US Ranger Block 3 lunar probes of the 1960s…

    Kamikaze Starshot: Will Some Interstellar Probes Slam into Their Target Planets?

    By Mike Wall, Space.com Senior Writer | May 3, 2017 06:54 am ET

    The first fleet of robotic spacecraft that humanity launches to explore exoplanets may include a few kamikazes.

    The $100 million Breakthrough Starshot initiative is working to develop an interstellar spaceflight system that would use powerful lasers to accelerate tiny, sail-equipped probes to 20 percent the speed of light.

    The baseline plan calls for the robotic nanocraft to hunt for signs of life and gather other data during flybys of nearby alien worlds such as Proxima b, a potentially habitable planet located just 4.2 light-years from Earth. But the scientific return would be even greater if some of the 1-gram probes actually slammed into their target worlds, some researchers said.

    “I want 10 — at least 10 ships, not just one,” Harvard University astronomy professor Dimitar Sasselov said April 20 during a panel discussion at the annual Breakthrough Discuss conference at Stanford University.

    “So then, two of them I’ll point at the planet, straight into the planet, hit the planet and create a thermal event in the atmosphere,” added Sasselov, who’s also the founding director of the Harvard Origins of Life Initiative. The other eight probes would “beam the data back, because you’ll get a lot better characterization of what’s in the atmosphere,” he said.

    Full article here:


    To quote:

    This suicide strategy was also raised as a possibility by Philip Lubin, a physics professor at the University of California, Santa Barbara, who’s the driving force behind Breakthrough Starshot’s planned laser-propulsion system.

    “Maybe you have a small mothership that sends out little daughters [and] drops them in the atmosphere,” Lubin said during a separate presentation at Breakthrough Discuss on April 20. “And then perhaps you could telemeter data back from the daughters to the mother saying, ‘I’m sacrificing myself for the good of science and humanity back home.’ You could possibly do spectroscopy from the impact on the atmosphere and the surface, and there are ways to measure gravity.”


    One of the biggest challenges facing the Breakthrough Starshot program is ensuring that the tiny probes can relay their images and other data back to their handlers here on Earth, team members have said. If the project cannot solve that problem, suicide dives could at least provide confirmation that an interstellar mission reached its target, Kuhn said.

    “If we could string out a bunch of these … maybe we could set it up so that, with all of our ground-based telescopes, we could get the self-destructive signal that said, ‘Yeah, we made it,'” he said. “It might start interstellar warfare, but it would satisfy that.”

    As that last remark suggests, the kamikaze strategy would likely be controversial and provoke spirited debate, especially considering that Starshot aims to prioritize the study of worlds that may be capable of supporting life. (Indeed, Sasselov’s comments were greeted with nervous chuckles by some of his fellow panelists at the Breakthrough Discuss conference, one of whom said, “I’m picturing ‘Independence Day.'”)

    But the Starshot team and the world at large have some time to chew over such ethical issues. The first interstellar nanoprobes probably won’t launch for another 20 years or so, even if the team makes good progress and everything goes well, Starshot team members have said.

  • J. Jason Wentworth May 20, 2017, 5:25

    Another (relatively) nearby, possibly life-bearing triple-star system that photogravitational braking could be used at is Omicron² Eridani, also called 40 Eridani (see: http://en.wikipedia.org/wiki/40_Eridani ). Lying approximately 16.45 light-years away, this system consists of a K-class orange dwarf (star A), which is distantly orbited by a binary whose components are a white dwarf (B) and a red dwarf (C). 40 Eridani A, in “Star Trek,” is orbited by the planet Vulcan. (Although no planets have been found in the system, this component could have a terrestrial planet in its habitable zone, 0.6 AU out, and the B – C binary orbits A approximately 400 AU out, so B’s red giant phase probably did no harm to any close-in planets–if there are any–orbiting A.)