By ‘Photogravitational Assists’ to Proxima b

by Paul Gilster on February 1, 2017

Given the distances involved, faster would always seem to be better when it comes to interstellar flight. Voyager, which took 12 years to get to Neptune and roughly 35 years to encounter the heliopause, would take 75,000 years to cross the 4.22 light years to Proxima Centauri. Voyager’s 17 kilometers per second clearly doesn’t cut it, but how fast can we realistically hope to go?

Let’s say we manage to build the phased laser array contemplated in the early Breakthrough Starshot discussions. Starshot’s researchers contemplate driving small sails to 20 percent of the speed of light, a figure that should allow safe passage through the interstellar medium for a large percentage of the sails sent. But get to Proxima Centauri in 20 years and another problem arises: Each sail blows through the system in mere hours. In fact, at 0.2c, these sails cross a distance equivalent to the Moon’s orbit around the Earth in six seconds. Hence the huge problem: How to explore the system we’ve reached?

A new paper from René Heller (Max Planck Institute for Solar System Research, Göttingen), working with German colleague Michael Hippke, gives us another way to frame the matter. I would say that it’s not so much an alternative to Starshot as an idea that could be pursued along with it, and perhaps implemented as a follow-on to any early sail flybys of Proxima. For Hippke and Heller believe a somewhat slower craft could make the Proxima crossing, but also achieve a bound orbit around the star and perhaps even its planet, Proxima Centauri b.


Image: Artist concept of an Autonomous Active Sail (AAS) approaching the potentially habitable exoplanet Proxima b. The reflection of Proxima Centauri and background stars are seen on the mirror-like surface of the sail. Four communication lasers beams are shown firing from its corners to transmit information back to Earth. The lower right panels of the sail are in the process of becoming darker to change its direction and orientation from differences in radiation pressure. Credit: Planetary Habitability Laboratory, University of Puerto Rico at Arecibo.

At the heart of the concept are what the duo call ‘photogravitational swings,’ which are used to decelerate an incoming light sail and deflect it. Here things get more interesting still, because Heller and Hippke believe the proper use of these maneuvers will allow flybys of both Centauri A and B enroute to Proxima itself. With much higher levels of brightness than the red dwarf Proxima Centauri, Centauri A and B are used as ‘photon bumpers’ to slow the spacecraft, dropping it from the 13,800 kilometers per second of cruise to 1280 km/sec.

Launching from our Solar System involves the Sun’s photons alone. The numbers the authors put forth show a graphene sail closing to within 5 solar radii receiving enough of a ‘sundiver’ style boost to reach 4.6% of lightspeed. Made of graphene, the sail, some 316 meters to the side, takes 95 years to make the crossing to Centauri A, where it uses both photon pressure and the gravitational pull of the star to reduce speed. A second encounter, with Centauri B, allows the sail to drop to 1280 km/sec for transfer into a bound orbit at Proxima, one that could gradually be adjusted into a planetary orbit around Proxima b.

The paper calculates 46 years to make the crossing between the AB binary and Proxima, making for a total travel time of 141 years. That’s a good bit more than the lifetime of a researcher, the figure often cited as acceptable for a deep space mission, but if we abandon that preconception, the advantages are considerable. From the paper:

In a more general context, photogravitational assists of a large, roughly 105 m2 = (316 m)2 -sized graphene sail could (1.) decelerate a small probe into orbit around a nearby exoplanet and therefore reduce the technical demands on the onboard imaging systems substantially; (2.) in principle allow sample return missions from distant stellar systems; (3.) avoid the necessity of a large-scale Earth-based laser launch system by instead using the sun’s radiation at departure from the solar system; (4.) limit accelerations to about 1,000 g compared to some 10, 000 g invoked for a 1 m2 laser-riding sail; and (5.) leave of the order of 10 gram for the sail’s reflective coating and equipment.

These are powerful advantages, especially if they forego the need for a phased laser array on the Earth as the launch system (although it should be pointed out that such an array, once built, would have myriad uses for exploration in the Solar System as well as interstellar applications). And the prospect of a platform in another star system, able to return data for years in a period of close observation, is a huge incentive. It could be argued that we are far from being able to craft the graphene sail depicted in this paper, but several decades of technological development could well make graphene our tool of choice for sail missions.


Image: An interstellar mission of an Autonomous Active Sail (AAS) to the nearest three stars. The sail uses an active reflective surface to change its direction and orientation from photogravitational assists from the stars, including the Sun. A light 90 grams sail could take nearly 100 years to reach Alpha Centauri A and another 46 years to Proxima Centauri. Many engineering challenges will need to be solved to pack enough communication and science instruments in such light but wide interstellar probes. Credit: PHL @ UPR Arecibo.

The paper points out that the maximum injection speed at Centauri A for a photogravitational assist to Centauri B and then Proxima depends on the mass-to-surface ratio of the sail, the idea being to maximize the photon force on the sail and yield the highest decelerations. But can even a graphene sail handle the conditions this one would be exposed to? Returning to the paper:

Close stellar encounters necessarily invoke the risk of impacts of high-energy particles and of thermal overheating. On the one hand, impacts of high-energy particles could damage the physical structure of the sail, its science instruments, its communication systems, or its navigational capacities. On the other hand, if those impacts could be effectively absorbed by the sail, they could even help to decelerate it. As shown in Section (3), heating from the stellar thermal radiation will not have a major effect on a highly reflective sail. However, the electron temperature of the solar corona is > 100,000 K at a distance of five solar radii. The Solar Probe Plus (planned launch in mid-2018) is expected to withstand these conditions for tens of hours (Fox et al. 2015), although the shielding technology for an interstellar sail would need to be entirely different (Hoang et al. 2016), possibly integrated into the highly reflective surface covering.

Also present is the issue of stellar alignments. Heller and Hippke’s analysis found that the optimal conditions for a photogravitational assist to work at Centauri A are when all three Centauri stars are in the same plane as the incoming sail, which minimizes the deflection angle required by the sail to reach the next star, while maximizing the injection speed for the first encounter (this, in turn, makes for the fastest possible travel time from Earth):

Proxima is not located in the orbital plane of the AB binary, but for a distant observer all three stars align about every 79.91 yr (the orbital period of the AB binary). From the perspective of an incoming probe from Earth, the alignment occurs near the time of the AB periastron, the next of which will take place on June 24, 2035 (Beech 2015).

Thus we can define a launch window involving the position of the Centauri stars. The next alignment comes in 2035, clearly out of reach to a probe with such long travel times, but there is another in 2115, likewise unreachable because we would have to launch in 2020 to take advantage of it. The 86 gram ‘fiducial’ sail analyzed by Heller and Hippke would thus have a launch window at the end of this century to make it to destination for the following alignment, though dropping to a 57 gram sail of equivalent size would allow faster travel times, and in some cases allow a launch within 25 years. Thus we have a bit of flexibility depending on advances in material sciences and lightsail technologies in the intervening years.

There is a good deal more to discuss, and rather than trying to cram everything into a single post, I want to go deeper into the photogravitational assist idea tomorrow, with renewed attention to the sail itself and in particular the question of navigation. We’ll also entertain an interesting thought — what would such a sail look like to any observers on Proxima b as it approached their star system?

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).


{ 15 comments… read them below or add one }

Albert A Jackson February 1, 2017 at 10:41

Boy! Can’t get ahead of the game! I was researching this same topic for a paper!
Did radiation forces calculations for interplanetary dust* for years and a little back of the envelope suggests this might be possible.
Have to try something else, now!

*The capture of interstellar dust: the pure Poynting-Robertson case, Planetary and Space Science, Volume 49, Issue 5, p. 417-424, 2001.


Michael February 1, 2017 at 11:35

We could go much faster than that if we have MEMS actuators with films of radioactive decay products on them to slow and manoeuvre the graphene sail. The reflective sail is powered by the powerful earth based laser and on the back of it we have a much smaller thinner graphene secondary sail with these decay product actuators attached. We only need to jump off at some point and then the sail will decelerate quite fast due to the low masses involved.


Alex Tolley February 1, 2017 at 13:36

So now we are getting more ways to decelerate. From the original Robert Forward design with Earth-based lasers, to magsails, and now clever use of a nearby star.

This last is an approach with limited, rather than general, application. Nevertheless, while we are still in the mode of looking for the most attainable approaches with the least amount of technology development and risk, these sorts of innovative approaches are well worth exploring.


Jens Knudsen February 1, 2017 at 16:26

Can we make graphene sails?


Micky Badgero February 1, 2017 at 17:20

The short answer is yes:
But how big sheets have actually been made is a good question. A Google search for the largest graphene sheet yields only old results.


Alex Tolley February 1, 2017 at 19:55

No. The size we are talking about cannot be manufactured as sheets. A new process has just been invented that makes cheap graphene, but sheets would have to be made by bonding the fragments. Graphene sails would need to have a reflective surface added to reflect light, although I think they should be able to reflect microwaves.


Micky Badgero February 2, 2017 at 12:08

Alex is right, we can’t make graphene solar sails this big. Or anything graphene this big. To date, solar sails have only been made of thin, metalized plastic.

True, being able to make the material is not the same as being able to make the sail. I recently read an article that claimed square-meter graphene, but I have not been able to find it again, so I do not know if it was confirmed. This would at least be on the order of Breakthrough Starshot, if not this large enough for Photogravitational Assists. Perhaps this is the ‘cheap graphene’ you are referring to, Alex? I did find this:, but it does not appear to produce large sheets.

For Breakthrough Starshot, the original idea was to use dielectric sails, which are much less absorbing of the incoming light than metalized sails. Adding a reflective coating to graphene would also increase the weight many times. Perhaps dielectric sails could be made with a process similar to this:, for two layers of graphene separated by a vacuum dielectric. This might work for a single frequency laser, but it would not work for sunlight.

I have also read about graphene sails with no reflective metal, but I don’t remember where. Seems that with lasers, they would vaporize, but they might work better for solar sails just because of their lower mass.


Alex Tolley February 2, 2017 at 12:53

James Benford has demonstrated carbon fiber sails with microwaves in the lab. In this case, the sails sublimate to eject mass for high acceleration, rather than reflect energy.

Making large sheets of single atom thick, defect free graphene is not yet possible. The problems are no doubt similar to making single strand carbon nanotubes. This has resulted in focussing the applications on short tubes that can be made readily. (So space elevators have stalled.) But bonding fragments of graphene into a larger sheet should be possible, as well as vapor deposition of metal to make a reflective surface for laser light.


Gary Wilson February 1, 2017 at 17:53

Another brilliant article. This looks extremely promising. As always it will depend on sorting out the technical details, but the idea of being able to remain in the Centauri system makes it extremely important. Years of data if it can be done.


Charlie February 2, 2017 at 1:40

This looks promising, but I would add one conditional to the program:

I suggest that once the probe had slowed down into the Centauri system to begin capture, that once it had performed a single orbit of the star and was proceeding to come around the star back into the direction in which it came from Earth, at that point, a substantial laser blast directed from earth years before could be shined upon the sail to permit a substantial de-acceleration of the probe by the laser, so that it would be able to achieve capture by the primary star.
Such a procedure with shining laser on the sale might, just might preclude the necessity to have the probe be in such an initial close knit orbit to the Centauri star. Thus, the probe would be able to be captured at a greater radius from the star itself and would lessen the chance of it being damaged during the capture phase. That’s just a thought here. And since a solar system laser array would be useful within the solar system proper, then this would be an additional bonus.

I didn’t understand the portion of the paragraph where it said that the photon graphene sail would need to be at facing the plane of the three star in the system. Does that mean that the spacecraft would have to be in a plane parallel to the plane of the three stars ?


Joe February 4, 2017 at 0:35

This is a great idea, but it would require superb artificial intelligence on the starchip, as well as perfect aim and timing of the laser shot from earth.


Charlie February 2, 2017 at 1:46

To be honest on further reading a 90 gram sail sounds awful lite for a space probe able to do any substantial type of work, even if it achieves orbit in a distant star system. Is there any reason to feel that a somewhat larger type of sale would be advisable doesn’t wowing for a somewhat greater payload to do the science here ??
Under this original plan of a 90 gram sail just how much of a payload are they expecting to be able to connect to the sail to allow to do science?


Gabriel Thelen February 2, 2017 at 20:51

Perhaps this technology might have some minor auxiliary uses at some point, most likely within a solar system, but not as a flagship exploration technology. For that this seems pathetically and unacceptably slow and awkward and comprehensively uninspiring.
If we were ever to launch one of these, I’m confident we would have nuclear fusion powered probes overtake it long before ever reaching Alpha Centauri, especially with the recent progress regarding fusion, especially in South Korea.

If we image a habitable planet in the Alpha Centauri system, there will be huge pressure to get there, and get there fast.


David H February 3, 2017 at 23:38

Your optimism is refreshing, but I really stopped expecting quick progress in reaction propulsion technology. On the other hand, miniaturization keeps happening and will continue. That’s why I think the best bet is to expect tiny probes (that will still be able do a lot).


ljk March 1, 2017 at 0:33

Exploring the climate of Proxima B with the Met Office Unified Model

Ian A. Boutle, Nathan J. Mayne, Benjamin Drummond, James Manners, Jayesh Goyal, F. Hugo Lambert, David M. Acreman, Paul D. Earnshaw
(Submitted on 27 Feb 2017)

We present results of simulations of the climate of the newly discovered planet Proxima Centauri B, performed using the Met Office Unified Model (UM). We examine the responses of both an `Earth-like’ atmosphere and simplified nitrogen and trace carbon dioxide atmosphere to the radiation likely received by Proxima Centauri B. Additionally, we explore the effects of orbital eccentricity on the planetary conditions using a range of eccentricities guided by the observational constraints.

Overall, our results are in agreement with previous studies in suggesting Proxima Centauri B may well have surface temperatures conducive to the presence of liquid water. Moreover, we have expanded the parameter regime over which the planet may support liquid water to higher values of eccentricity (>= 0.1) and lower incident fluxes (881.7 Wm-2) than previous work. This increased parameter space arises because of the low sensitivity of the planet to changes in stellar flux, a consequence of the stellar spectrum and orbital configuration. However, we also find interesting differences from previous simulations, such as cooler mean surface temperatures for the tidally-locked case.

Finally, we have produced high resolution planetary emission and reflectance spectra, and highlight signatures of gases vital to the evolution of complex life on Earth (oxygen, ozone and carbon dioxide).

Comments: Astronomy and Astrophysics, in press

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:1702.08463 [astro-ph.EP]
(or arXiv:1702.08463v1 [astro-ph.EP] for this version)

Submission history

From: Ian Boutle [view email]

[v1] Mon, 27 Feb 2017 19:00:05 GMT (1251kb,D)


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