Remembering Voyager: Triton’s New Map

I’m glad to see Ralph McNutt quoted in a recent news release from the Johns Hopkins Applied Physics Laboratory. McNutt has been working on interstellar concepts for a long time, including the Innovative Interstellar Explorer mission that could become a follow-up to New Horizons. But he’s in the news in late August because of Voyager, and in particular Voyager 2, which made its flyby of Neptune on August 25, 1989, some 25 years ago. McNutt recalls those days, when he was a member of the Voyager plasma-analysis team:

“The feeling 25 years ago was that this was really cool, because we’re going to see Neptune and Triton up-close for the first time. The same is happening for New Horizons. Even this summer, when we’re still a year out and our cameras can only spot Pluto and its largest moon as dots, we know we’re in for something incredible ahead.”

I can only envy someone who was up close with the Voyager outer planet flybys and is now a key player on New Horizons, for which McNutt leads the energetic-particle investigation team. The image below is a long way from the much closer views Voyager gave us of Neptune, but it’s what New Horizons could make out with its Long-Range Reconnaissance Imager in mid-July. It’s what NASA’s Jim Green calls a ‘cosmic coincidence’ that New Horizons crossed the orbit of Neptune on the 25th anniversary of the Voyager flyby.


Image: The New Horizons spacecraft captured this view of the giant planet Neptune and its large moon Triton on July 10, 2014, from a distance of about 3.96 billion kilometers — more than 26 times the distance between the Earth and sun. The 967-millisecond exposure was taken with the New Horizons telescopic Long-Range Reconnaissance Imager (LORRI). New Horizons traversed the orbit of Neptune on Aug. 25, 2014 — its last planetary orbit crossing before beginning an encounter with Pluto in January 2015. In fact, at the time of the orbit crossing, New Horizons was much closer to its target planet — just about 440 million kilometers — than to Neptune.

I can remember staying up late the night of the Neptune encounter, being most curious not about Neptune itself but its moon Triton. We had already learned to expect surprises from Voyager — Io alone made that point — and Triton did not disappoint us with its unanticipated plumes, signs that the frozen world was active, and its odd ‘cantaloupe’ terrain. A bit larger than Pluto, Triton serves as a rough guide for what to expect at Pluto/Charon, but it’s also a point of departure, given its evident capture by Neptune and the resulting tidal heating.

Remember, this is a world that follows a retrograde orbit, moving opposite to Neptune’s rotation. The odds are strong that we’re looking at an object captured from the Kuiper Belt. Gravitational stresses would account for melting within this ice world, and explain the fractures and plume activity, evidently geysers of nitrogen, that Voyager saw. A newly restored Triton map, produced by Paul Schenk (Lunar and Planetary Institute) has a resolution of 600 meters per pixel and has been enhanced for contrast.


Image: The best-ever global color map of Neptune’s large moon Triton, produced by Paul Schenk. This map has a resolution of 600 meters per pixel. The colors have been enhanced to bring out the contrast but are a close approximation to Triton’s natural colors. Voyager’s “eyes” saw in colors slightly different from human eyes, and this map was produced using orange, green and blue filter images. Credit: Paul Shenk/LPI.

The video using the same data is a bit breathtaking. Have a look.

Keep in mind the limitations of the imagery. In 1989, the year of the Voyager flyby, Triton’s northern hemisphere was swathed in darkness, allowing the spacecraft to have a clear view of only one hemisphere during its closest approach. Now we wait to see what views New Horizons will generate of Pluto/Charon next summer. Given that Triton and Pluto are similar in density and composition, with carbon monoxide, carbon dioxide, nitrogen and methane ices on the surface, we may see some similar features. Will there be plumes on Pluto?


Thinking about Magnetic Sails

Magnetic sails — ‘magsails’ — are a relative newcomer on the interstellar propulsion scene, having been first analyzed by Dana Andrews and Robert Zubrin in 1988. We saw that the particle beam concept advanced by Alan Mole and discussed this week by Jim Benford would use a magsail in which the payload and spacecraft were encircled by a superconducting loop 270 meters in diameter. The idea is to use the magnetic field to interact with the particle beam fired from an installation in the Solar System toward the departing interstellar craft.

Within our own system, we can also take advantage of the solar wind, the plasma stream flowing outward from the Sun at velocities as high as 600 kilometers per second. A spacecraft attempting to catch this wind runs into the problem that sunlight contains far more momentum, which means a magnetic sail has to deflect a lot more of the solar wind than a solar sail needs to deflect sunlight. A physical sail, though, is more massive than a spacecraft whose ‘sail’ is actually a magnetic field, so the magsail spacecraft can be the less massive of the two.

Science fiction began exploring basic solar sails in the 1960s through stories like Clarke’s “Sunjammer” and Cordwainer Smith’s “The Lady Who Sailed the Soul.” In fact, SF writers have done an excellent job in acquainting the public with how solar sails would operate and what their capabilities might be. But magsails are hard to find in science fiction, and the only novel that springs readily to mind is Michael Flynn’s The Wreck of the River of Stars, whose haunting title refers to a magsail passenger liner at the end of its lifetime.

Here’s Flynn in ‘Golden Age’ Heinlein style introducing the tale:

They called her The River of Stars and she spread her superconducting sails to the solar wind in 2051. She must have made a glorious sight then: her fuselage new and gleaming, her sails shimmering in a rainbow aurora, her white-gloved crew sharply creased in black-and-silver uniforms, her passengers rich and deliciously decadent. There were morphy stars and jeweled matriarchs, sports heroes and prostitutes, gangsters and geeks and soi-disant royalty. Those were the glamour years, when magsails ruled the skies, and The River of Stars was the grandest and most glorious of that beautiful fleet.


Image: There are few science fiction stories involving magsails, and even fewer visual depictions. The cover art for Michael Flynn’s book, by the artist Stephan Martiniere, is a striking exception.

The novel takes place, though, many years later, when the grand passenger liner has become no more than an obsolete freighter whose superconducting sail structure has been decommissioned in favor of newly developed fusion drives. What happens when she needs to power up the sail again because of a fusion emergency makes up the bulk of the tale. The Wreck of the River of Stars is not about an interstellar journey but a highly developed infrastructure within the Solar System that, for a time, used the solar wind. It will be interesting to see what science fiction tales grow out of the current interstellar thinking.

For magsails emerged in an interstellar context, and if it was Robert Zubrin and Dana Andrews who worked through the equations of what we conceive today as a magsail, it was Robert Bussard who first brought life to the idea through his notion of an interstellar ramjet that would use magnetic fields to scoop up fuel between the stars. Both Zubrin and Andrews saw the potential uses of a magsail for deceleration against a stellar wind. If beam dispersal cannot be prevented to allow an interstellar magsail to be accelerated by particle beam, we might still consider equipping a beamed laser sailcraft with magsail capabilities for use upon arrival.

And when it comes to magsails closer to home, one cautionary note is provided by a 1994 paper from the Italian physicist Giovanni Vulpetti, who describes the problems we may have operating superconductors within the orbit of Mars. The paper notes that superconductivity can be lost this close to the Sun unless massive thermal shielding is applied and that, of course, ramps up the spacecraft mass. This evidently does not preclude outer system work, but it could serve as a brake on using magsails near the Earth, at least until we make considerable advances in superconductor technology.

The Vulpetti paper is “A Critical Review on the Viability of Space Propulsion Based on the Solar Wind Momentum Flux,” Acta Astronautica 37 (1994), 641-642.


A Response to Comments on ‘Sails Driven by Diverging Neutral Particle Beams’

Jim Benford’s article on particle beam propulsion, published here last Friday and discussed in the days since, draws from the paper he will soon be submitting to one of the journals. I like the process: By running through the ideas here, we can see how they play before this scientifically literate audience, with responses that Jim can use in tweaking the final draft of the paper. Particle beam propulsion raises many issues, not surprising given the disagreements among the few papers that have tackled the subject. Are there ways of keeping the beam spread low that we haven’t thought of yet? Does a particle beam require shielding for the payload? Does interplanetary particle beam work require a fully built infrastructure in the Solar System? We have much to consider as the analysis of this interesting propulsion concept continues. Dr. Benford is President of Microwave Sciences in Lafayette, California, which deals with high power microwave systems from conceptual designs to hardware.

by James Benford


Let me first say that I appreciate the many comments on my piece on neutral particle beam propulsion. With so many comments I can react in only a limited sense. I appreciate in particular the many comments and suggestions by Alex Tolley, swage, Peter Popov, Dana Andrews, Michael, Greg (of course), Project Studio and David Lewis.

Galacsi: The launch system as envisioned by Dana Andrews and Alan Mole would be affixed to an asteroid that would provide sufficient mass to prevent the reaction from the launch of the beam from altering the orbit of the Beamer and changing the direction of the beam itself. No quantitative valuation of this has been provided to date.

James Messick says we can have thrusters to maintain the Beamer in place, but the thrusters must have the same thrust as the Beamer in order to prevent some serious motion.

Rangel is entirely right; one has to start at lower power nearer objectives, as we have to do for all interstellar concepts.

Alex Tolley is quite correct that what is envisioned here is a series of beam generators at each end of the journey for interplanetary missions, which means a big and mature Solar System economy. That’s why I placed this in future centuries. And I agree with him that in the short term beamed electromagnetic or electric sails are going to be much more economic because they don’t require deceleration at the destination.

Adam: the Beamer requirement if the magsail expands as the pressure falls off probably doesn’t scale well, as B falls off very quickly- I don’t think the scaling justifies any optimism.

There are certainly a lot of questions about the solar wind’s embedded magnetic field. All these requirements would benefit from a higher magnetic field from the magsail, which unfortunately also increases the mass of the probe.

Alex Tolley correctly points out that deflecting high-energy particles produces synchrotron radiation, which will require some shielding of the payload. Shielded payloads are available now, due to DOD requirements. [Jim adds in an email: “Shielding is needed for the payload while the beam is on. Keep it, don’t discard, as there are cosmic rays to shield against on all flights].

Swage is correct in saying that we need to start small, meaning interplanetary, before we think large. Indeed lasers are far less efficient than the neutral beam concept. That’s because deflecting material particles is a much higher efficiency process than deflecting mere photons. Swage is completely correct about the economics of using beam propulsion.

And using multiple smaller beams doesn’t reduce divergence. ‘Would self focusing beams be an option?’ No. Charged beams don’t self-focus in a vacuum, they need a medium for that and it isn’t easy to make happen. Charged particle beams can be focused using their self-generated magnetic field only when some neutralization of charges is provided. There is also a large set of instabilities that can occur in such regimes. That’s a basic reason why charged particle beams are not being seriously considered as weapons and neutral beams are the only option.


Image: The divergence problem. A charged-particle beam will tend naturally to spread apart, due to the mutually repulsive forces between the like-charged particles constituting the beam. The electric current created by the moving charges will generate a surrounding magnetic field, which will tend to bind the beam together. However, unless there is some neutralization of the charge, the mutually repulsive force will always be the stronger force and the beam will blow itself apart. Even when the beam is neutralized, the methods used to neutralize it can still lead to unavoidable beam divergence over the distances needed for interstellar work. Image credit: Richard Roberds/Air University Review.

Peter Popov asked whether you could focus sunlight directly. You can’t focus sunlight to a smaller angular size than it fills in your sky. (That is because the sun is an incoherent source. The focusability of sunlight is limited by its incoherence, meaning that the radiation from the sun comes from a vast number of radiating elements which are not related to one another in a coherent way.) Therefore the ability to focus sunlight is limited, and is in no way related to the focusing of coherent light. However, you can increase the focusing aperture, collecting more light, making the power density higher, but the spot size doesn’t grow.

Dana Andrews’ comment that the neutral “atoms with any transverse velocity are eliminated before they are accelerated” means that you throw away all but one part in a million of the initial beam: Suppose this device, which separates particles out, reduces the divergence by 3 orders of magnitude. That implies, for a beam uniform in angular distribution, a reduction in Intensity of 1 million (because the solid angle scales with the square of the opening angle). Such a vast inefficiency is unaffordable.

For Dana & Alex Tolley, re-ionizing the beam as it reaches the magsail will not be difficult. The reason is that they are in relativistically separated frames so that the magnetic field of the magsail will appear as an electric field in the frame of the atoms, a field sufficient to ionize the atom. No on-board ionizer is required.

Michael suggests going to ultrarelativistic beams, but that means much more synchrotron radiation when the beam deflects from the magsail. Consequently, very much higher fields are necessary for deflection. That would mean either much more current or much larger diameter in the magsail. My instinct is that that does not scale well. And the divergence I described is not changed by going ultrarelativistic, as it just depends on ratios of mass and energies of electron to ion. Also, using heavier atoms helps but, with a square root dependence, not enough.

ProjectStudio also advocates that an ultrarelativistic neutral beam would have a reduced divergence, for which see above. I note again the enormous amount of radiation they produce whenever they are either deflected by the magnetic field or collide with matter. In fact, going in the Andrews/Mole concept from 0.2 c to 0.9c means the synchrotron radiation increases by a factor of 2300! That bathes the payload, as the ions swing round.

Alex Jolie is also correct in saying that we need to look into the development of beam power infrastructure. Once it’s in place economics drives down the price of transportation; the same was true for the railroads.

David Lewis seems to get the concept entirely.


Beaming to a Magnetic Sail

Jim Benford’s work on particle beam propulsion concepts, and in particular on the recent proposal by Alan Mole for a 1 kg beam-driven interstellar probe, has demonstrated the problem with using neutral particle beams for interstellar work. What we would like to do is to use a large super-conductor loop (Mole envisions a loop 270 meters in diameter) to create a magnetic field that will interact with the particle beam being fired at it. Benford’s numbers show that significant divergence of the beam is unavoidable, no matter what technology we bring to bear.

That means that the particle stream being fired at the receding starship is grossly inefficient. In the case of Mole’s proposal, the beam size will reach 411 kilometers by the end of the acceleration period. We have only a fraction of the beam actually striking the spacecraft.

This is an important finding and one that has not been anticipated in the earlier literature. In fact, Geoffrey Landis’ 2004 paper “Interstellar Flight by Particle Beam” makes the opposite statement, arguing that “For a particle beam, beam spread due to diffraction is not a problem…” Jim Benford and I had been talking about the Landis paper — in fact, it was Jim who forwarded me the revised version of it — and he strongly disagrees with Landis’ conclusion. Let me quote what Landis has to say first; he uses mercury as an example in making his point:

[Thermal beam divergence] could be reduced if the particles in the beam condense to larger particles after acceleration. To reduce the beam spread by a factor of a thousand, the number of mercury atoms per condensed droplet needs to be at least a million. This is an extremely small droplet (10-16 g) by macroscopic terms, and it is not unreasonable to believe that such condensation could take place in the beam. As the droplet size increases, this propulsion concept approaches that of momentum transfer by use of pellet streams, considered for interstellar propulsion by Singer and Nordley.

We’ve talked about Cliff Singer’s ideas on pellet propulsion and Gerald Nordley’s notion of using nanotechnology to create ‘smart’ pellets that can navigate on their own (see ‘Smart Pellets’ and Interstellar Propulsion for more, and on Singer’s ideas specifically, Clifford Singer: Propulsion by Pellet Stream). The problem with the Landis condensed droplets, though, is that we are dealing with beam temperatures that are extremely high — these particles have a lot of energy. Tomorrow, Jim Benford will be replying to many of the reader comments that have come in, but this morning he passed along this quick response to the condensation idea:

Geoff Landis’ proposal to reduce beam divergence, by having neutral atoms in the particle beam condense, is unlikely to succeed. Just because the transverse energy in the relativistic beam is only one millionth of the axial energy does not mean that it is cool. Doing the numbers, one finds that the characteristic temperature is very high, so that condensation won’t occur. The concepts described are far from cool beams.

Where there is little disagreement, however, is in the idea that particle beam propulsion has major advantages for deep space work. If it can be made to work, and remember that Benford believes it is impractical for interstellar uses but highly promising for interplanetary transit, then we are looking at a system that is extremely light in weight. The magsail itself is not a physical object, so we can produce a large field to interact with the incoming particle stream without the hazards of deploying a physical sail, as would be needed with Forward’s laser concepts.


Image: The magsail as diagrammed by Robert Zubrin in a NIAC report in 2000. Note that Zubrin was looking at the idea in relation to the solar wind (hence the reference to ‘wind direction’), but deep space concepts involve using a particle stream to drive the sail. Credit: Robert Zubrin.

Another bit of good news: We can achieve high accelerations because unlike the physical sail, we do not have to worry about the temperature limits of the sail material. The magnetic field is not going to melt. Although Landis is talking about a different kind of magsail technology than envisioned by Alan Mole, the point is that higher accelerations come from increasing the beam power density on the sail, and that means cruise velocity is reached in a shorter distance. That at least helps with the beam divergence problem and also with the aiming of the beam.

Two other points bear repeating. A particle beam, Landis notes, offers much more momentum per unit energy than a laser beam, so we have a more efficient transfer of force to the sail. Landis also points to the low efficiency of lasers at converting electrical energy, “typically less than 25% for lasers of the beam quality required.” Even assuming future laser efficiency in the fifty percent range, this contrasts with a particle beam that can achieve over 90 percent efficiency, which reduces the input power requirements and lowers the waste heat.

But all of this depends upon getting the beam on the target efficiently, and Benford’s calculations show that this is going to be a problem because of beam divergence. However, the possibility of fast travel times within the Solar System and out as far as the inner Oort Cloud make neutral particle beams a topic for further study. And certainly magsail concepts retain their viability for interstellar missions as a way of slowing the probe by interacting with the stellar wind of the target star.

I’ll aim at wrapping up the current discussion of particle beam propulsion tomorrow. The image in today’s article was taken from Robert Zubrin and Andrew Martin’s “The Magnetic Sail,” a Final Report for the NASA Institute of Advanced Concepts in 2000 (full text). The Landis paper is “Interstellar flight by particle beam,” Acta Astronautica 55 (2004), 931-934.


Beamed Sails: The Problem with Lasers

We saw on Friday through Jim Benford’s work that pushing a large sail with a neutral particle beam is a promising way to get around the Solar System, although it presents difficulties for interstellar work. Benford was analyzing an earlier paper by Alan Mole, which had in turn drawn on issues Dana Andrews raised about beamed sails. Benford saw that the trick is to keep a neutral particle beam from diverging so that the spot size of the beam quickly becomes much larger than the diameter of the sail. By his calculations, only a fraction of the particle beam Mole envisaged would actually strike the sail, and even laser cooling methods were ineffective at preventing this.


It seems a good time to look back at Geoffrey Landis’ paper on particle beam propulsion. I’m hoping to discuss some of these ideas with him at the upcoming Tennessee Valley Interstellar Workshop sessions in Oak Ridge, given that Jim Benford will also be there. The paper is “Interstellar Flight by Particle Beam” (citation below), published in 2004 in Acta Astronautica, a key reference in an area that has not been widely studied. In fact, the work of Mole, Andrews and Benford, along with Landis and Gerald Nordley, is actively refining particle beam propulsion concepts, and what I’m hoping to do here is to get this work into a broader context.

Image: Physicist and science fiction writer Geoffrey Landis (Glenn Research Center), whose ideas on particle beam propulsion have helped bring the idea into greater scrutiny.

Particle beams are appealing because they solve many of the evident limitations of laser beaming methods. To understand these problems, let’s look at their background. The man most associated with the development of the laser sail concept is Robert Forward. Working at the Hughes Aircraft Company and using a Hughes fellowship to assist his quest for degrees in engineering (at UCLA) and then physics (University of Maryland), Forward became aware of Theodore Maiman’s work on lasers at Hughes Research Laboratories. The prospect filled him with enthusiasm, as he wrote in an unfinished autobiographical essay near the end of his life:

“I knew a lot about solar sails, and how, if you shine sunlight on them, the sunlight will push on the sail and make it go faster. Normal sunlight spreads out with distance, so after the solar sail has reached Jupiter, the sunlight is too weak to push well anymore. But if you can turn the sunlight into laser light, the laser beam will not spread. You can send on the laser light, and ride the laser beam all the way to the stars!”

The idea of a laser sail was a natural. Forward wrote it up as an internal memo within Hughes in 1961 and published it in a 1962 article in Missiles and Rockets that was later reprinted in Galaxy Science Fiction. George Marx picked up on Forward’s concepts and studied laser-driven sails in a 1966 paper in Nature. Remember that Forward’s love of physical possibility was accompanied by an almost whimsical attitude toward the kind of engineering that would be needed to make his projects possible. But the constraints are there, and they’re formidable.

Landis, in fact, finds three liabilities for beamed laser propulsion:

  • The energy efficiency of a laser-beamed lightsail infrastructure is extremely low. Landis notes that the force produced by reflecting a light beam is no more than 6:7 N/GW, and that means that you need epically large sources of power, ranging in some of Forward’s designs all the way up to 7.2 TW. We would have to imagine power stations built and operated in an inner system orbit that would produce the energy needed to drive these mammoth lasers.
  • Because light diffracts over interstellar distances, even a laser has to be focused through a large lens to keep the beam on the sail without wasteful loss. In Forward’s smaller missions, this involved lenses hundreds of kilometers in diameter, and as much as a thousand kilometers in diameter for the proposed manned mission to Epsilon Eridani with return capability. This seems highly impractical in the near term, though as I’ve noted before, it may be that a sufficiently developed nanotechnology mining local materials could construct large apertures like this. The time frame for this kind of capability is obviously unclear.
  • Finally, Landis saw that a laser-pushed sail would demand ultra-thin films that would need to be manufactured in space. The sail has to be as light as possible given its large size because we have to keep the mass low to achieve the highest possible mission velocities. Moreover, that low mass requires that we do away with any polymer substrate so that the sail is made only of an extremely thin metal or dielectric reflecting layer, something that cannot be folded for deployment, but must be manufactured in space. We’re a long way from these technologies.

This is why the particle beam interests Landis, who also looked at the concept in a 1989 paper, and why Dana Andrews was drawn to do a cost analysis of the idea that fed into Alan Mole’s paper. Gerald Nordley also discussed the use of relativistic particle beams in a 1993 paper in the Journal of the British Interplanetary Society. Here is Landis’ description of the idea as of 2004:

In this propulsion system, a charged particle beam is accelerated, focused, and directed at the target; the charge is then neutralized to avoid beam expansion due to electrostatic repulsion. The particles are then re-ionized at the target and reflected by a magnetic sail, resulting in a net momentum transfer to the sail equal to twice the momentum of the beam. This magnetic sail was originally proposed to be in the form of a large superconducting loop with a diameter of many tens of kilometers, or “magsail” [7].

The reference at the end of the quotation is to a paper by Dana Andrews and Robert Zubrin discussing magnetic sails and their application to interstellar flight, a paper in which we learn that some of the limitations of Robert Bussard’s interstellar ramjet concept — especially drag, which may invalidate the concept because of the effects of the huge ramscoop field — could be turned around and used to our advantage, either for propulsion or for braking while entering a destination solar system. Tomorrow I’ll continue with this look at the Landis paper with Jim Benford’s findings on beam divergence in mind as the critical limiting factor for the technology.

The Landis paper is “Interstellar flight by particle beam,” Acta Astronautica 55 (2004), 931-934. The Dana Andrews paper is “Cost considerations for interstellar missions,” Paper IAA-93-706, 1993. Gerald Nordley’s 1993 paper is “Relativistic particle beams for interstellar propulsion,” Journal of the British Interplanetary Society 46 (1993) 145–150.