Interstellar Journey: Shrinking the Probe

by Paul Gilster on July 14, 2014

We’ve all imagined huge starships jammed with human crews, inspired by many a science fiction novel or movie. But a number of trends point in a different direction. As we look at what it would take to get even a robotic payload to another star, we confront the fact that tens of thousands of tons of spacecraft can deliver only the smallest of payloads. Lowering the mass requirement by miniaturizing and leaving propellant behind looks like a powerful option.

Centauri Dreams regular Alex Tolley pointed to this trend in relation to The Planetary Society’s LightSail-1 project. In a scant ten years, we have gone from the earlier Cosmos 1 sail with an area of 600 square meters to LightSail-1, with 32 square meters, but at no significant cost in scientific return because of continuing miniaturization of sensors and components. We can translate that readily into interstellar terms by thinking about future miniature craft that can be sent out swarm-style to reach their targets. Significant attrition along the way? Sure, but when you’re building tiny, cheap craft, you can lose some and count on the remainder to arrive.

The Emergence of SailBeam

I inevitably think about Jordin Kare’s SailBeam concepts when I hear thinking like this. Kare, a space systems consultant, had been thinking in terms of pellet propulsion of the kind that Clifford Singer and, later, Gerald Nordley have examined. The idea here was to replace a beam of photons from a laser with a stream of pellets fired by an accelerator — the pellets (a few grams in size) would be vaporized into plasma when they reached the spacecraft and directed back as plasma exhaust. Nordley then considered lighter ‘smart’ pellets with onboard course correction.

I’m long overdue for a re-visit to both Singer and Nordley, but this morning I’m thinking about Kare’s idea of substituting tiny sails for the pellets, creating a more efficient optical system because a stream of small sails can be accelerated much faster close to the power source. Think of a solar sail, as Kare did, divided into a million pieces, each made of diamond film and being accelerated along a 30,000 kilometer acceleration path, all of them shot off to drive a larger interstellar probe by being turned into a hot plasma and pushing the probe’s magnetic sail.


Image: Jordin Kare’s ‘SailBeam’ concept. Credit: Jordin Kare/Dana G. Andrews.

Kare, of course, was using his micro-sails for propulsion, but between Nordley and Kare, the elements are all here for tiny smart-probes that can be pushed to a substantial fraction of the speed of light while carrying onboard sensors shrunk through the tools of future nanotechnology. Kare’s sails, in some designs, get up to a high percentage of c within seconds, pushed by a multi-billion watt orbiting laser. Will we reach the point where we can make Kare’s sails and Nordley’s smart pellets not the propulsion method but the probes themselves?

In that case, the idea of a single probe gives way to fleets of tiny, cheap spacecraft sent out at much lower cost. It’s a long way from LightSail-1, of course, but the principle is intact. LightSail-1 is a way of taking off-the-shelf Cubesat technology and giving it a propulsion system. Cubesats are cheap and modular. Equipped with sails, they can become interplanetary exploration tools, sent out in large numbers, communicating among themselves and returning data to Earth. LightSail’s cubesats compel anyone thinking long-term to ask where this trend might lead.

A Gravitational Lensing Swarm

In Existence, which I think is his best novel, David Brin looks at numerous scenarios involving miniaturization. When I wrote about the book in Small Town Among the Stars, I was fascinated with what Brin does with intelligence and nanotechnology, and dwelled upon the creation of a community of beings simulating environments aboard a starship. But Brin also talks about a concept that is much closer to home, the possibility of sending swarms of spacecraft to the Sun’s gravitational focus for observation prior to any star mission.

We normally speak about the distance at which the Sun’s gravity bends light from objects on the other side of it as being roughly 550 AU, but effects begin closer than this if we’re talking about gravitons and neutrinos, and in Brin’s book, early probes go out here, between Uranus and Neptune, to test the concept. But get to 550 AU and beyond and photon lensing effects begin and continue, for the focal line goes to infinity. We have coronal distortion to cope with at 550 AU, but the spacecraft doesn’t stop, and as it continues ever further from the Sun, we can be sampling different wavelengths of light to make observations assisted by this hypothesized lensing.

Before committing resources to any interstellar mission, we want to know what targets are the most likely to reward our efforts. Why not, then, send a swarm of probes. Claudio Maccone, who has studied gravitational lensing more than any other physicist, calls his design the FOCAL probe, but I’m talking about its nanotech counterpart. Imagine millions of these sent out to use the Sun’s natural lens, each with an individual nearby target of interest. Use the tools of future nanotech and couple them with advances in AI and emulation and you open the way for deep study of planets and perhaps civilizations long before you visit them.

The possibilities are fascinating, and one of the energizing things about them is that while they stretch our own technology and engineering well beyond the breaking point, they exceed no physical laws and offer solutions to the vast problems posed by the rocket equation. Perhaps we’ll build probes massing tens of thousands of tons to deliver a 100 kilogram package to Alpha Centauri one day, but a simultaneous track researching what we can do at the level of the very small could pay off as our cheapest, most effective way to reach a neighboring star.

More on this tomorrow, as I take a longer look at Clifford Singer and Gerald Nordley’s ideas on pellet propulsion. I want to use that discussion as a segue into a near term concept, Mason Peck’s ideas on spacecraft the size of computer chips operating in our Solar System.

And today’s references: Cliff Singer’s first pellet paper is “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107–115. Gerald Nordley’s ideas can be found in “Beamriders,” Analog Vol. 119, No. 6 (July/August, 1999). Jordin Kare’s NIAC report “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion,” (Final Report, NIAC Research Grant #07600-070, revised February 15, 2002) can be found on the NIAC site.



Exploring the Galaxy’s Outer Halo

by Paul Gilster on July 11, 2014

Not long ago we talked about what the Milky Way would look like when seen from afar. I had mentioned Poul Anderson’s World Without Stars, which appeared in Analog in 1966 under the title The Ancient Gods. In the Anderson tale, a starship crew is sent to make contact with a recently discovered technological civilization that lives on a world hundreds of thousands of light years from the galactic core. Now a recent paper deepens our understanding of this environment deep in the galaxy’s outer halo.

Recall that the Milky Way is about 100,000 light years in diameter, and that the distance to the nearest large galaxy is roughly 2,500,000 light years. Anderson’s crew is over 200,000 light years from the core, which puts them in the outer halo, a sparse spherical volume of space that stretches out 500,000 light years, well beyond the familiar, highly visible disk. While the stars in the galactic disk are on nearly circular orbits in the plane of the galaxy, the halo stars are on more elliptical orbits that are randomly oriented, so that while inner halo stars can pass through the disk, most of their lives are spent well above or below the plane of the galaxy. The inner, visible part of the halo is where we find the ancient, metal-poor globular clusters.


Image: Structure of the Milky Way, showing the inner and outer halo. Credit: NASA, ESA, and A. Feild (STScI).

Astronomer John Bochanski (Haverford College, PA) and team, however, are looking out well beyond the globular clusters. The researchers note in their paper in The Astrophysical Journal Letters that there are few known outer halo stars at distances over 120 kiloparsecs, which works out to about 390,000 light years — in fact, the list of known halo stars at this distance yields a grand total of seven, with the paper adding an additional two. The galaxy’s outer halo, we learn, is largely unexplored, but as we’ll see, it holds implications for galaxy formation theories.

The team’s recent paper outlines the discovery of two cool red giants — ULAS J0744+25 and ULAS J0015+01 — that appear to be the most distant Milky Way stars yet detected, at distances of 775,000 and 900,000 light years respectively. The work draws on observations from the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey, with spectroscopic studies using the 6.5m telescope at the MMT Observatory in Arizona. The newly discovered stars are five times more distant than the Large Magellanic Cloud and almost a third of the way to the Andromeda galaxy. At these distances, both Andromeda and the Milky Way should appear quite faint in the visible spectrum. If Anderson’s crew were here, the night sky would be dark indeed.

The image below brightens the Milky Way to give some sense of its distance from these stars.


Image: This simulated image demonstrates how small the Milky Way would look from the location of ULAS J0744+25, nearly 775,000 light years away. This star, along with ULAS J0015+01, are the most distant stars ever associated with our Galaxy, and are about five times further away than the Large Magellanic Cloud, one of the Milky Way’s closest galactic neighbors. Credits: Visualization Software: Uniview by SCISS Data: SOHO (ESA & NASA), John Bochanski (Haverford College) and Jackie Faherty (American Museum of Natural History and Carnegie Institute’s Department of Terrestrial Magnetism).

Bochanski’s team has been looking at formation models for the Milky Way, an interesting issue given that, as he explains, “Most models don’t predict many stars at these distances. If more distant red giants are discovered, the models may need to be revised.” The halo itself may be the result of mergers over the galaxy’s lifetime with numerous smaller galaxies, with outer stars the remnant population of what had once been intact dwarf galaxies. If this is correct, we can study these outer halo stars as a way of probing the formation history of the entire spiral. The team hopes to identify up to 70 red giants in the halo, refining its selection criteria and aiding next generation surveys like Gaia that will help us deepen our catalog in this distant region.

The paper is Bochanski et al., “The Most Distant Stars in the Milky Way,” The Astrophysical Journal Letters, Volume 790, Issue 1, article id. L5 (abstract / preprint).



Reflections on the LightSail Project

by Paul Gilster on July 10, 2014

I was delighted to see Doug Stetson, the program manager for The Planetary Society’s LightSail effort, quoting Johannes Kepler in last night’s webcast. If you missed the Pasadena event, which took place at the KPCC Crawford Family Forum in Pasadena, CA, you can watch the recorded session here, and I highly recommend it. Kepler’s 1610 letter to Galileo added context to the excitement over LightSail, for solar sailing has a rich history. Kepler wrote of providing “ships or sails adapted to the heavenly breezes,” and added that “there will be some who will brave even that void.”

It’s an inspiring thought as we now look into the launch of a privately funded sail that can join IKAROS and NanoSail-D in a series of operational experiments that will hone our sail knowledge. The chief news of the Pasadena meeting was the announcement of an approximate launch date. LightSail-1 is scheduled to go into space aboard a SpaceX Falcon Heavy rocket in April of 2016. The quick video Mat Kaplan showed illustrating the Falcon Heavy in flight was likewise energizing: This is a booster made up of three Falcon 9 cores, meaning 27 rocket engines. The Planetary Society’s Jim Bell calls it a rocket that ‘opens up the outer Solar System.’


Image: Artist’s concept of LightSail in space. Credit: Josh Spradling / The Planetary Society.

But let’s clear up that problem of sail nomenclature I referenced yesterday. What is being called LightSail-1 is the designation for two solar sail CubeSats, LightSail-A and LightSail-B. Whether LightSail-A or LightSail-B is the one to fly next depends upon systems tests that will be completed in coming months, as Jason Davis explains in LightSail Update: Launch Dates:

If LightSail-A goes to space, it won’t reach a high enough altitude for the momentum it gains from solar sailing to overcome atmospheric drag. The spacecraft will deploy its sails, capture images, and communicate with the ground, giving engineers a chance to work through any problems en route to a full-fledged solar sailing flight.

LightSail-B, on the other hand, will embark upon true solar sailing, using the momentum of solar photons to increase its velocity. Usefully, both missions now seem likely to fly. Stetson told the Pasadena audience that an opportunity exists for launching LightSail-A to a lower orbit, a test flight that would occur in May of 2015. Joining the crowd by Skype, he went on to say that a test like this would validate the sail by putting it through a full deployment in lower Earth orbit.

The image below, shown by Stetson during the presentation, shows LightSail-B capturing forward momentum from solar photons. You can see the changes in attitude as the sail, orbiting the Earth, adjusts its position to maximize the effect of sunlight. Ultimately, turning the sail at key points in the orbit allows the orbit to be raised, allowing the prospect of escaping Earth’s orbit entirely. We can envision a day when CubeSats, those tiny marvels of inexpensive engineering, could be equipped with sails and sent out in swarms to explore the Solar System.


Image: In order to capture forward momentum from the sun’s rays, LightSail-1 will make two 90 degree pitches during an orbit. Credit: LightSail Team / The Planetary Society.

Planetary Society CEO Bill Nye refers to solar sailing in terms of power, utilizing ‘a fusion reactor at a safe distance, the Sun.’ Assuming we get LightSail-B into space in the spring of 2016, the surrounding package will be quite interesting. LightSail will launch embedded within a satellite called Prox-1. The latter, developed at Georgia Tech, was designed to conduct autonomous proximity operations in which two spacecraft operate at close quarters. Prox-1 will release the sail and then rendezvous with it, closing to within fifty meters and capturing images of LightSail. The actual sail deployment will occur after several weeks of operations.

We’re going to get some remarkable video of sail deployment, both from the cameras aboard the LightSail craft as well as those aboard Prox-1. System engineer Barbara Plante (Boreal Space/Ecliptic Enterprises) went on to discuss spacecraft attitude control after showing the crowd a large piece of the 4.5 micron sail material, roughly 1/50th the width of a human hair. Sail packaging and deployment is complicated business, but Plante said she was struck by the elegance of the design, referring to solar sailing as a ‘green method’ for exploration.

Looking to the future, David Brin joined the group by Skype as the panel discussed the implications of today’s sail experiments. Solar sails run into the obvious problem of diminishing sunlight the farther one goes from our star. The way around that problem is to imagine a future space infrastructure for power generation, one in which power is readily available for large laser arrays that can illuminate a sail and push it entirely out of the Solar System. Brin suggested the Sun’s gravitational lens at 550 AU and beyond as a natural target for later generation sails, a way of exploiting the extreme magnifications this natural lensing provides for our instruments.

For me, the word ‘lightsail’ has always referred to a sail under this kind of beamed power, and now I’ll have to be careful not to confuse that kind of sail with the experiments LightSail-1 will offer in nearby space. But we always need to think about how today’s efforts play into a larger scenario, including so-called ‘Sundiver’ missions that unfurl their sails after a close pass by the Sun to achieve high accelerations. It seems fitting to end this piece on LightSail-1 with Louis Friedman’s thoughts on such missions, he being a key player in making this happen.

Friedman, as we’ve seen, worked at the Jet Propulsion Laboratory on the design of a large sail targeting Comet Halley, and although that mission never flew, he has been an advocate of sail designs for decades, anticipating that we would one day understand the virtues of leaving propellant behind and shedding the rocket equation altogether. In his 1988 book Starsailing: Solar Sails and Interstellar Travel, he brings his experience to the long view:

[The sail] would provide a nearly perfect means of interstellar travel — except that it requires sunlight, and we will run out of that as we leave the solar system, a circumstance that could leave us becalmed, like a terrestrial sailboat on a windless ocean. We could, however, use a series of gravity assist maneuvers to keep ourselves moving along. We could fly around Jupiter, for instance, then head in toward the sun, picking up a lot of speed with our sail. Then we could swing around and head out past Jupiter and Saturn, picking up even more speed. With a large sail and with a close fly-by of the sun, assuming good thermal protection, we could conceivably exit the solar system at a rate of 40 astronomical units a year — in other words, we could travel through the whole solar system in about 1 year. Even at that great a speed, however, we would not reach the second star for 6,600 years! Not so good. Nevertheless, this use of gravity assist and solar sailing combined offers real promise for the use of solar sails even for traveling to the outer planets, which may extend the range of the interplanetary shuttle.

But Friedman has interstellar thinking in his bones. Intimately familiar with the work of Robert Forward, he goes on to describe the latter’s concept of a 3.6 kilometer sail that would, using laser beaming technologies, get a one-ton payload to Alpha Centauri in about 40 years, reaching a bit more than one-tenth of lightspeed. It was one of many Forward designs that speculate on what a human presence in space and the immense power of directed light could accomplish.

After the years at JPL, working with Bruce Murray and Carl Sagan, the time on Voyager, the 32 years as executive director of The Planetary Society, Louis Friedman, one of its founders, is seeing yet another result of his efforts pay off. I can assure you that when LightSail-1 flies, we’ll pop open a bottle of Champagne at my house, and the first toast of the evening will go to him.



Focus on LightSail-A

by Paul Gilster on July 9, 2014

As Cosmos 1 demonstrated, launching solar sails isn’t always easy. The Planetary Society’s sail perished thanks to a malfunctioning Volna booster not long after launch in 2005. When NASA attempted to launch its NanoSail-D in 2008, a problem aboard the Falcon 1 booster destroyed the craft. And when the agency launched the backup, NanoSail-D2, in December of 2010, the CubeSat-based sail failed to eject from the FASTSAT satellite it was aboard. Just when all seemed lost, NanoSail-D2 ejected on its own on January 17, 2011 and deployed its sail soon after.

Now we’re looking at a new Planetary Society venture called LightSail-A, which grows out of the NanoSail-D project and, according to news that should be firmed up tonight, should be ready for launch in the near future. As with Cosmos 1, the funding for LightSail-A was raised from private sources and Planetary Society membership dues, with the spacecraft itself being built by Stellar Exploration Inc. out of San Luis Obispo, CA. With mylar sails 4.5 microns thick, the sail will extend upon deployment to cover 32 square meters. Three CubeSat spacecraft form a ‘bus’ about the size of a shoebox that weighs in at no more than 4.5 kilograms.


Image: The Planetary Society’s LightSail-1 will test out solar sail technologies in Earth orbit as a prelude for later missions including solar storm monitoring at L1. Credit: The Planetary Society/Rick Sternbach.

Back in June, the Planetary Society’s Jason Davis described recent LightSail-A activity:

During the past two years, LightSail has come of age. The solar sail itself demonstrated a full deployment back in 2011, but the guts of the spacecraft were far from mature. Project manager Doug Stetson and his team have been shaking out bugs and overhauling LightSail’s electronics, attitude control, software and communications systems. Next up is a full “day in the life” flight system test on Wednesday, June 4 that includes another sail deployment and full operation of the spacecraft as if it were in orbit. I’ll be on hand at Cal Poly San Luis Obispo to observe and report.

The test in question had to be delayed because of problems involving the TRAC (Triangular Rollable and Collapsible) boom system so crucial in proper deployment of the sail. A breakdown of the three deployment problems the team uncovered is here. Later testing on June 24 showed that fixes for the power anomalies, spacecraft processor overload and other boom deployment issues seem to have worked. Davis has made a video of the June 24 deployment test available; he’s also offering regular updates explaining technical features of the diminutive spacecraft.

The LightSail project involves three craft, the goal of the first being to test deployment and basic operations at an altitude of 800 kilometers. A second mission, LightSail-B, will collect scientific data and demonstrate controlled solar sailing, while a third has the ambitious goal of reaching the L1 Lagrangian point, a useful position from which to detect solar activity producing geomagnetic storms.

At this point it’s always a good idea to distinguish between the phenomenon LightSail-A will be exploiting — the momentum imparted by solar photons — and that other means of ‘sailing’ through space, the solar wind. The sailing metaphor is what can make this confusing. The solar wind consists of particles streaming out from the Sun, moving much slower than the speed of light and offering a push to spacecraft designed to exploit them. So-called ‘magsails’ are under study that could use the solar wind for fast interplanetary transport, but the force from the solar wind is a thousand times less than the photon force a solar sail can draw on in its operations. By ‘solar sailing,’ then, I refer to drawing on the momentum imparted by massless photons.

We’ll know more about LightSail-A’s shakedown cruise this evening, when The Planetary Society hosts a live webcast from 2200-2330 EDT (0200-0330 UTC), billed as the venue for “a major announcement about our solar sail spacecraft.” Expect a tour inside the cubesat and presentations by the engineers behind it, as well as an announcement about its launch. Confusingly, The Planetary Society is referring to its sail as both LightSail-A and LightSail-1, but I suspect that by the time the evening is over, we’ll have the basic nomenclature set.



Cosmos 1 in Context

by Paul Gilster on July 8, 2014

We’re coming up on the tenth anniversary of Centauri Dreams, and it doesn’t surprise me even remotely that two of the earliest stories I ever wrote for the site involve solar sails. August 17, 2004’s Solar Sail Test by Japan talks about the Japanese Institute of Space Astronautical Science testing sail deployment strategies, and the August 14 entry, Cosmos 1 Solar Sail Closer to Launch, briefly describes the privately funded sail that was developed by The Planetary Society’s efforts, with financial contributions from members and major backing from Ann Druyan’s Cosmos Studios.


I’ve been fascinated with space sailing since first encountering the notion in 1960s-era science fiction, particularly Clarke’s “The Wind from the Sun” and Cordwainer Smith’s “The Lady Who Sailed the Soul.” What I lacked back then, though, was an appreciation for the challenge posed by the rocket equation, which demands so much more propellant the faster you want to go. Space missions demand huge mass ratios (the weight of the fueled rocket compared to the same rocket without fuel), and a chemical rocket on a deeply ill-advised Alpha Centauri mission would, to reach the target within a human lifetime, demand an amount of propellant larger than all the mass in the observable universe. Rocket scientists call that sort of thing a ‘showstopper.’

Image: An artist’s conception of Cosmos 1. Credit: Rick Sternbach.

So using solar photons to impart momentum while leaving heavy propellant out of the vehicle entirely makes eminent sense, but it’s been a long, slow road from theory to implementation. We saw yesterday that sail ideas got a serious bump in interest when Jerome Wright, who had been studying sail techniques, began to look into a possible rendezvous mission to Comet Halley, and soon NASA had Louis Friedman’s team hard at work investigating the possibilities. Budgeting issues, a tight schedule and an untested technology accounted for the decision not to pursue the sail mission.

Image: Artist’s conception of the Cosmos 1 sail deployed in orbit. Credit: Cosmos Studios.

We did see what could be described as a solar sail test in 1993, when Russian scientists deployed Znamya 2 from the Mir space station, although Znamya was not a free-flying device and was designed as a thin-film mirror. The idea was to experiment with beaming solar power to the ground, but dealing with large, lightweight reflective material and unfurling it in space makes for a reasonable demonstration of some aspects of sail technology. The Japanese experiments in 2004 involved deployment of sail materials from a sub-orbital sounding rocket.

And I should mention the two communications satellites — INSAT 2A and 3A — launched by India in 1992 and 2003, each of which used sail material to offset the torque from solar pressure on their multi-panel solar arrays. You see, we did have plenty of data about the pressure photons could exert on objects in space — controllers had used vanes mounted on the Mariner 10 spacecraft for attitude control, and Louis Friedman recounts in his book Starsailing: Solar Sails and Interstellar Travel that NASA, experimenting early in the space era with small metal needles launched into the ionosphere to study communications, found that sunlight pressure was a significant factor in their trajectories.

So when The Planetary Society, of which Friedman was then executive director, took up private sail work that had been developed through the World Space Foundation after the latter group’s demise in 1998, it was building on known physics and an idea in need of deployment and testing in space. A suborbital test of the deployment system, using a sail with only two blades, was attempted in 2001, only to fail when the spacecraft did not separate from the Volna booster. The full 100-kilogram spacecraft, built at the Babakin Space Center and the Space Research Institute in Russia, was paired with another Volna, a former intercontinental ballistic missile.

Cosmos 1 was launched on June 21, 2005 from a Russian submarine in the Barents Sea, but failure aboard the booster prevented the spacecraft from reaching orbit. Had it succeeded, this would have been the first orbital test of a true solar sail. The $4 million project was tiny by government space agency standards, but it produced an eight-blade sail, with each blade fifteen meters in length and a total surface area of some 600 square meters. The plan would have been to attain a circular orbit at 800 kilometers, at which point the sail blades would have deployed.

It surely would have been a test as exciting as the Japanese deployment of the IKAROS sail had everything gone according to plan. The deployed sail was to have used the momentum of solar photons to raise its orbit by 50 to 100 kilometers over the course of the 30-day mission. There were also plans to bounce microwaves from NASA’s Goldstone facility of the Deep Space Network off the sail to measure the effect of microwave beaming, the physics of which had already been demonstrated in laboratory work by James and Gregory Benford.

The Cosmos 1 launch failure was hard to take, but the idea of privately funding a significant space mission certainly did not die here, and as we’ll see tomorrow, The Planetary Society has continually renewed the effort to get a craft called LightSail into space in the context of a multi-vehicle project. As the space agencies continue their work, it’s heartening to see the commitment of volunteers and contributors to private projects that leverage public interest in these technologies and show what can be done on shoestring budgets and sheer dedication.



Sailing to Halley’s Comet

by Paul Gilster on July 7, 2014

We have interesting solar sail news coming up later this week, so it seems a good time to lead into it with some thoughts on NASA’s early solar sail work. For the theoretical work for a sail rendezvous with Halley’s Comet was well along in the 1970s, when Louis Friedman, later a founder and executive director of The Planetary Society, led what would become the first space agency attempt to develop an actual sail mission. Friedman’s interest in and commitment to sail ideas will become apparent as this week progresses and we look at other sail designs.


Much of the impetus for a NASA sail study in the 1970s — a $4 million effort at the Jet Propulsion Laboratory in 1977 and 1978 — can be traced back to Jerome Wright, an engineer at the Battelle Memorial Institute in Ohio. In his book Starsailing: Solar Sails and Interstellar Travel (John Wiley & Sons,1988), Friedman recalls a meeting of the Advanced Projects Group at the Jet Propulsion Laboratory in which he learned about Wright’s newest idea from Chauncey Uphoff, a senior member of JPL’s mission design section. Friedman describes the event:

Then, at the group meeting, Chauncey Uphoff and Phillip Roberts dropped the bombshell. “Jerry Wright has found a possible way to rendezvous with Halley’s Comet,” Chauncey announced.

“You mean fly-by, not rendezvous,” I said.

“No, I mean rendezvous.”

“With a trip time of ten years?”

“Would you believe four years?”

Image: Louis Friedman, former executive director of The Planetary Society and one of its founders, who led NASA’s study of a solar sail mission to Halley’s Comet.

Wright’s book Space Sailing (Routledge, 1992) lays out the basic concepts behind solar sails and the principles behind navigating with a sail in space — it was Wright who first realized that Halley’s Comet’s 1986 appearance in the inner Solar System offered a chance to try the technology out on an actual mission. Wright would be invited to conduct a seminar at JPL on sail techniques in May of 1975 that led to his joining the small team there in December of that year, with NASA’s approval of a study to design a vehicle and mission concept to the comet.

We’ve looked at solar sailing frequently on Centauri Dreams and traced the development of the idea from the first technical paper, Carl Wiley’s “Clipper Ships of Space,” which appeared in Astounding Science Fiction in May of 1951. Wiley, of course, built on ideas dating back to the time of Kepler, with major work accomplished by Konstantin Tsiolkovsky and Fridrickh Arturovich Tsander in the early part of the 20th Century. It’s interesting to learn from Friedman’s book that Wiley, then working as an engineer at Rockwell, came to some of the technical presentations at JPL as the Halley’s Comet idea was discussed.

Friedman credits Ted Cotter, working at Los Alamos, with the first presentation of a spinning sail. See A Sail Mission Emerges for more on Cotter’s ideas, and be aware that his 1958 memorandum “An Encomium on Solar Sailing,” distributed internally at Los Alamos, is now online. Spinning the sail helps because it allows the sail to be stabilized without the help of an external structure. I should also mention Richard Garwin, whose paper on solar sails that same year led to a lifetime of interest in the concept. Friedman credits Garwin with helping to inspire NASA’s later interest in a sail mission, leading NASA administrator James Fletcher to commission studies on sails that were assigned to Jerome Wright during his days at Batttelle.

So we can see a flurry of sail interest in the agency in the mid-1960s — an early meeting on solar sail design was actually held at NASA’s Langley Research Center as early as 1960 — but with the end of the Apollo missions, a dwindling space program had no plans for specific missions. Thus at the time Jerome Wright began talking about a Halley’s Comet mission in the mid-1970s, little other work was being done on sail technologies. Wright was more or less the only game in town.

Friedman recalls the situation:

Then Wright found the Halley rendezvous opportunity. By the time the JPL study team began its task, two major developments had occurred. First, NASA was developing the space shuttle, which promised to carry large-volume payloads into orbit. Second, there had been great advancements in the technology of deploying huge structures in space. The shuttle also made it possible for scientists to test space concepts, and the JPL study team hoped to test the solar sail from a shuttle in orbit.

The positive results of the 1976 and early 1977 JPL studies captured the imagination of the new director of the Jet Propulsion Laboratory, Dr. Bruce Murray. With his approval, the study team made a major effort to put together a project plan for a rendezvous with the comet. This work, however, had to be done rapidly. In order to launch in late 1981, the project would have had to start moving by the end of 1978. 1 was put in charge of the study and we quickly wrote a proposal for a one-year study and gave it to NASA.

The original NASA design called for a sail of 800 meters to the side supported by four diagonal cross beams, to be launched by the Space Shuttle in 1981. But in 1977 a ‘heliogyro’ approach invented in the mid-1960s by Richard MacNeal and John Hedgepath won out, featuring 12-kilometer long sails made up of narrow strips that would spin up like helicopter blades to keep the sail rigid. Unlike the earlier design, the heliogyro could use centrifigual force from its spin to unreel the blades from storage drums, thus requiring no human assembly in orbit.


Image: An artist’s conception of the Halley’s Comet heliogyro design. Credit: JPL.

NASA’s acceptance of the proposal led to the design study that brought in industrial contractors, included support from NASA Ames and Langley, work that resulted in the conclusion that solar sailing was indeed feasible. The question, though, was whether it was practical to speak in terms of a Halley’s Comet rendezvous given the time constraints involved. NASA would ultimately turn down the proposal as being based on a technology that was not sufficiently mature. When I asked Dr. Friedman about this at the last 100 Year Starship symposium, he said NASA was probably right. The Halley’s sail was pushing too far, too fast for its time.

Cost was a factor as well, with a price tag estimated at $500 million — remember the budgetary morass NASA found itself in with Space Shuttle cost overruns in this era. Other comet missions were considered, but in the end the United States did not launch a dedicated mission to Halley’s Comet. The story of comet rendezvous, the Giotto mission from ESA, the Soviet Vega spacecraft and the Japanese Suisei and Sakigake probes, makes for fascinating reading, but in terms of solar sails, it would be a long wait before JAXA’s IKAROS sail took flight in 2010. Before then, though, Louis Friedman would be involved in other attempts, one of which we’ll discuss tomorrow.



A Manifesto for Expansion

by Paul Gilster on July 4, 2014

Michael Michaud gave the speech that follows in 1988 at the 39th International Astronautical Congress, which met in Bangalore, India in October of that year. Reading through it recently, I was struck by how timely its theme of spaceflight advocacy and human expansion into the cosmos remains today. When he wrote this, Michaud was director of the Office of Advanced Technology for the US Department of State, though he reminded his audience that the views herein were his own and not necessarily those of the US government. Michaud’s support of spaceflight and his determinedly long-term approach to our possibilities as a species has distinguished his space writing, which has been prolific and includes the essential Contact with Alien Civilizations (Copernicus, 2006). Although I had thought of updating some of the references below, it seems unnecessary. What counts are the themes. Working well before the recent surge in interstellar interest, Michael here explains why humans need to develop and strive for goals among the stars.

by Michael A.G. Michaud


The history of astronautics is not only a history of scientific and technological progress, but is also a history of persuasion. Advances in astronautics have sprung not only from steady technical advance, but also from advocacy, led by individuals and groups with deep-seated motivations. Those advocates, while often frustrated in the near term, laid the philosophical and cultural foundations that helped speed the coming of the Space Age.(1)

While many justifications have been put forward for space activities, two motivations have consistently underlain the leading edge of the space advocacy: the exploration of the universe around us, and human expansion into it. Throughout the history of astronautics, other motivations have appeared and disappeared, but these two always have been identifiable.

The Spaceflight Advocacy

The Spaceflight advocacy began with visions and ideas, initially in science fiction. Serious theoretical work began with Konstantin Tsiolkovski in the late l9th and early 20th centuries. Hermann Oberth and Robert Goddard further developed the theoretical structure necessary for spaceflight. Initial rocket experiments were conducted by Goddard and others in the United States, by members of the Verein Fur Raumschiffahrt in Germany, and by members of rocket societies in the Soviet Union. The VFR in Germany, the American Interplanetary Society in the United States, and the British Interplanetary Society were advocating interplanetary travel in the early l930′s. Yet rockets of significant scale were not launched until World War II. Despite far-seeing work such as the 1946 RAND study(2), the use of the rocket to enter space had little political support in the 1940s. Yet, a decade later our machines had entered space to stay, and two decades later we landed humans on the Moon.

The original space advocacy, directed toward exploration and expansion beyond the Earth’s atmosphere into cislunar space and later into the solar system, has been spectacularly successful. By 1989, our unmanned spacecraft will have visited every planet in our solar system except Pluto. Both the Soviet Union and the United States — with its allies — are establishing a permanent human presence in low Earth orbit. The industrialization of near-Earth space has been conceptualized since the early 1970s, and the idea of human colonies in free space has been shown to be technically feasible. Advocacies have crystallized around the long-visualized Moon Base and manned mission to Mars; though neither goal has been achieved yet, it is widely expected that both will be early in the 21st century. The U.S. National Commission on Space has proposed an elaborate space transportation infrastructure linking the Earth to the Moon and Mars.(3) We are well advanced in exploring the solar system, and are close to expanding into it.

A major symbolic turning point occurred in February, 1988, with the release of a new U.S. national space policy document. That document committed the United States to a new long-term goal: the expansion of human presence and activity beyond Earth orbit into the solar system.(4) This had been a goal of the spaceflight advocacy for many years, identifiable implicitly in the writings of Tsiolkovski and explicitly at least as far back as the 1920s. In two to three generations (depending on the starting point one chooses), the spaceflight advocacy had won a policy endorsement that would have seemed inconceivable to any but its most optimistic original members: the expansion of the human species outward from Earth.

The Interstellar Advocacy

In recent years, we have seen a small but active advocacy for interstellar flight. In many ways, the interstellar advocacy of today is similar to the spaceflight advocacy of the 1920s and the 1930s. Dedicated and believing that what it advocates is not only right but inevitable, the members of that advocacy are doing the theoretical work and are laying out plans for interstellar exploration and travel. However, they lack the credibility needed to win funding and political support for their proposals.

Like the spaceflight advocacy, the interstellar advocacy first appeared in science fiction, in the 1930s and 1940s. The first significant non-fiction paper was published in 1950.(5) Scattered works appeared during the next two decades, but it was not until the mid-1970s that the interstellar advocacy achieved some public recognition. Landmarks were Forward‘s paper “A National Program for Interstellar Exploration,” published in 1975, and the British Interplanetary Society’s Project Daedalus study, published in 1978.(6) Papers on the theory and technology of interstellar flight began to appear more frequently, particularly in Astronautica Acta and the interstellar studies issues of the Journal of the British Interplanetary Society. The literature grew to the point that bibliographies were published.(7) Yet the number of advocates remains small, the literature still is specialized and narrowly circulated, and political and financial support is essentially non-existent.

The interstellar advocacy has reflected both the motivation to explore our larger environment and the motivation to expand human presence and activity beyond our solar system. Many of the proposed missions, particularly the early ones, are unmanned probes of nearby star systems. Others are missions of manned exploration, and still others are explicitly intended to carry human colonists to other systems, beginning the human colonization of the galaxy. None of these missions are known to be on the agenda of any space agency. However, more modest precursor missions have been proposed, such as an extrasolar probe, an Oort cloud mission, or the Thousand Astronomical Unit probe, which are extensions of existing solar system exploration technology.

In its 1988 report titled Space Science in the 21st Century, the Space Science Board of the U.S. National Academy of Sciences endorsed an interstellar probe. The Board envisioned a spacecraft that would escape the solar system at a velocity of about 80 kilometers a second, and enter the interstellar medium within 10 years. Such a spacecraft, if launched in the year 2000, would pass the Pioneer and Voyager spacecraft new proceeding slowly toward the stars.(8) This endorsement would have been inconceivable only a decade earlier.

The interstellar advocacy is not yet taken seriously by the opinion leaders of any nation, and has yet to win support from any government. That advocacy might do well to reflect on the history of the successful spaceflight advocacy, which took decades to sell its ideas, and only won success in stages. Its progress was not smooth, but was marked by raised hopes and disappointments, starts and stops. Political events and cultural change had significant impacts on the process. The advocacy could not force events beyond what was known of the physical environment and foreseeable technology at any given time. Yet it succeeded in transferring its aspirations to many people, and in eroding conceptions of what was not possible in others. Many of its views have been adopted by the governments of major nations, and are part of popular culture.

The Larger Context

Both the spaceflight advocacy and the interstellar advocacy reflect larger paradigms of human exploration and expansion.(9) We humans, who have explored and expanded into new environments on Earth, have nearly completed our initial reconnaissance of the solar system, and are on the verge of expanding into it. The day may come when we find the solar system as limiting to our aspirations as the Earth was thirty years ago. We will look outward into an even larger environment, sprinkled with stars and planetary systems. We will explore the nearer parts of the interstellar environment through space-based astronomy and unmanned probes. Then, with our improved knowledge of that environment, our improved technological capabilities, our expanded economic base, and our changed point of view, we may choose to continue the expansion. With that decision, we will assure that humans and their cultures will free themselves of dependence on one star, as they are now freeing themselves of dependence on one planet,

This vision will not be accepted easily by the public — even the informed public. While some individuals accept the outward-looking paradigms of exploration and expansion, most do not, and must be persuaded in stages to at least tolerate such ventures. If the advocacy of interstellar exploration and colonization is to succeed, it must have a long perspective, and must maintain a certain degree of continuity. Yet it also must be ready to seize on events that will speed the coming of interstellar flight. In doing so, it will demonstrate a continuity with the spaceflight advocacy that rests on the shared aspirations of exploration and expansion.

A Manifesto

Humanity should adopt expansion beyond Earth as a major organizing theme for its future. Evidence is strong that life tends to expand into new ecological niches when that is possible, and that such expansion is advantageous for the species. Expansion opens new opportunities for evolution and diversification, and for access to larger resources of materials and energy. Space is the macro-environment for life, the ultimate extension of our ecological range.

Exploration precedes expansion. Even more than the other forms of life we know, humans are motivated to expand by their improving perceptions of their larger environment. They deliberately explore the larger environment of space in the belief that they will benefit from improved knowledge. Astronomy and the unmanned exploration of space are allies of human expansion.

We should be conceptualizing the expansion in stages. The rate of human expansion is constrained by our perceptions, by our technologies, by our economic and human resources, and by our cultures, particularly by the predominant conceptions of what is possible. While there is a growing perception that a permanent human presence on the Moon and the human exploration of Mars are feasible, the creation of a solar system civilization still seems beyond our reach. In the early stages of developing a solar system civilization, we may reject the idea of human interstellar flight: later, with an expanded economic and technical base and greater confidence in our abilities, interstellar voyages will seem more feasible. Each stage will grow from the perceptions and capabilities created in the preceding stage.

Humanity needs to develop the technologies of expansion. Humans dreamed of voyages beyond the Earth for centuries, but could not accomplish those dreams until the technologies of the 20th century made them possible. Without telescopes, we would not have been tantalized by Mars; without rocketry, we could not have seen it in detail; without improved life support systems, we will not be able to journey there ourselves. Technology enables expansion.

Expanding into the Galaxy is an appropriate long-term goal for humanity. To rise above their intra-species disputes, humans need purposes that transcend their divisions. The expansion of humanity outward from the Earth and later outward from our solar system would be a grand shared enterprise for humanity, extending over many generations and giving us a long-term continuity of purpose.

Human expansion will require a continuity of intelligent advocacy. While the drive to expand is strong, cultural values vary with time and place, and the degree of support for expansion will vary with them. At each stage of the expansion there will be arguments against the next stage, which will be called too expensive or impractical; there also may be arguments against expansion because of our own moral imperfection. Advocates will be needed at every stage.


Astronautics already has brought great benefits to humanity, in our ability to communicate with each other, to navigate safely, to preserve the peace, to observe the Earth and the atmosphere, to study the solar system, the Galaxy and the Universe, and to perceive of ourselves as a species. Implicit in astronautics is the idea of expansion, which will bring benefits we can only dimly perceive today. Some have sensed all along that astronautics is linked to our destiny as a species.

We who are advocates of spaceflight, of interplanetary flight, and of interstellar flight are part of a great continuity. We are expressions both of powerful human drives and of an intellectual tradition. We have an endless mission before us, one marked by a succession of stages in which resistance, doubt, and delay must be overcome. Now that the first Spaceflight Revolution is behind us, we have reached a stage of maturity in which we can consciously decide and declare our intent to be human expansionists. We should not be embarrassed to be advocates of human expansion; we should be proud.



1. For studies of the spaceflight advocacy, see William S. Bainbridge, The Spaceflight Revolution, New York, John Wiley and Sons, 1976; Frank H. Winter, Prelude to the Space Age: The Rocket Societies, 1924-1940, Washington, D.C., Smithsonian Institution Press, 1983; Michael A.G. Michaud, Reaching for the High Frontier: The American Pro-Space Movement, 1972-1984, New York, Praeger, 1986.

2. Report Number SM-11827, Preliminary Design of an Experimental World-Circling Spaceship, May, 1946.

3. National Commission on Space, Pioneering The Space Frontier, New York, Bantam, 1986.

4. White House Fact Sheet on Presidential Directive on National Space Policy, February 11, 1988.

5. Leslie R. Shepherd, “Interstellar Flight,” Journal of the British Interplanetary Society, Volume 11 (1950), Pages 149-55.

6. Robert L. Forward, “A National Program for Interstellar Exploration,” in Future Space Programs 1975, a compilation of papers prepared for the Subcommittee on Space Science and Applications of the Committee on Science and Technology of the U.S. House of Representatives, Volume II, Washington, D.C., U.S. Government Printing Office, 1975, pages 279-326; Project Daedalus: The Final Report on the BIS Starship Study, supplement to the Journal of the British Interplanetary Society, 1978.

7. For example, see Eugene F. Mallove, Robert L. Forward, Zbigniew Paprotny, and Jurgen Lehmann, “Interstellar Travel and Communication: A Bibliography,” Journal of the British Interplanetary Society, Volume 33 (1980), entire issue.

8. Space Science Board, Space Science in the Twenty-First Century: Imperatives for the Decades 1995 to 2015 — Overview, Washington, D.C., National Academy Press, 1988, page 34.

9. For further elaborations by this author on the theme of human expansion, see Michael A.G. Michaud, “Spaceflight, Colonization, and Independence,” Journal of the British Interplanetary Society, Volume 30, Number 3 (March, 1977), 83-95 (Part One); volume 30, Number 6 (June, 1977), 203-212 (Part Two); Volume 30, Number 9 (September, 1977), 323-331 (Part Three); Michael A.G. Michaud, “The Extraterrestrial Paradigm,” Interdisciplinary Science Reviews, Volume 4, Number 3 (September, 1979), 177-192; Michael A.G. Michaud, “Four-Dimensional Strategy,” in Jerry Grey and Christine Krop, Editors, Space Manufacturing Facilities 3: Proceedings of the Fourth Princeton /AIAA Conference, New York, American Institute of Aeronautics and Astronautics, October 31, 1979, 49-61; Michael A.G. Michaud, “Improving the Prospects for Life in the Universe,” in William A. Gale, Editor, Life in the Universe: The Ultimate Limits to Growth, Boulder, Colorado, Westview, Press, 1979, 107-117.



The Milky Way from a Distance

by Paul Gilster on July 3, 2014

Growing up in the American Midwest, I used to haunt the library in Kirkwood, Missouri looking for books on astronomy. I had it in mind to read all of them and I pretty much did, looking with fascination at fuzzy images of distant objects I yearned to see close up. What did Saturn look like from Titan? What would it be like to be close enough to see the Crab Nebula fill the sky? Breathtakingly, what would it look like to be inside one of the great globular clusters?

Early on in Vernor Vinge’s A Fire Upon the Deep the character Ravna finds herself looking out a window at the entire Milky Way from a distance sufficient to view it whole:

She’d guessed right: tonight the Galaxy owned the sky… Without enhancement, the light was faint. Twenty thousand light-years is a long, long way. At first there was just a suggestion of mist, and an occasional star. As her eyes adapted, the mist took shape, curving arcs, some places brighter, some dimmer. A minute more and … there were knots in the mist … there were streaks of utter black that separated the curving arms … complexity on complexity, twisting toward the pale hub that was the Core. Maelstrom. Whirlpool. Frozen, still, across half the sky.

When I read that, all I could think was that I’d like to know what Chesley Bonestell would have done with the scene. The memory stuck with me, enough so that last weekend, I went digging through my collection of old magazines in search of a particular cover. We had been talking on Centauri Dreams about a Poul Anderson story called World Without Stars, and having been reminded of it by several readers, I kept thinking it was in a 1960s era Analog, and that Bonestell had indeed done a cover showing the Milky Way.


Soon I had magazines spread all over my office floor but couldn’t put my hands on the right one. The problem was that while I remembered (vaguely) the plot, about a crew that found itself tens of thousands of light years outside the galaxy on a mission to an isolated technical civilization, I had forgotten the original title. Finally I did the obvious and ran World Without Stars through the Internet Speculative Fiction Database, where I found the issue numbers. It turns out the story was serialized in Analog’s June and July, 1966 issues as The Ancient Gods, and my memory of a Chesley Bonestell cover was correct.

But serendipitous things happen when you collect old magazines. In leafing through the June issue, I came across a letter Poul Anderson had written to John Campbell, then in the latter part of his long run at the magazine — Campbell published it in Brass Tacks, his letter column, even though it was really just directed at the editor. Anderson talks about having been at Bonestell’s place and seeing the cover art for The Ancient Gods.

Did you notice in the Vinge paragraph above how dim the galaxy is depicted as being? Ravna thinks to herself that 20,000 light years is a long way out, and Anderson’s crew was to be a lot further out than that. Bonestell had been thinking all this over and had to find a way to work it into his painting. Thus Anderson to Campbell in the letter:

One point came up which may interest you. Though the galaxy would be a huge object in the sky, covering some 20⁰ of arc, it would not be bright. In fact, I make its luminosity, as far as this planet is concerned, somewhere between 1% and 0.1% of the total sky-glow (stars, zodiacal light, and permanent aurora) on a clear moonless Earth night. Sure, there are a lot of stars there — but they’re an awfully long ways off!

If you look at the Bonestell cover, it does appear to be glowing more brightly than this, but Anderson says this is not a contradiction. His imagined planet’s natives, he points out, are adapted to the dim light of the red dwarf they orbit (no tidal lock here, evidently), and the galaxy in their night would appear luminous enough. Anderson adds: “To us, galaxies look brilliant in an astronomical photograph — but that picture involved a huge light-gathering mechanism plus hours of exposure. We could make the Milky Way look just as bright if we wanted to.”

Anyway, Bonestell had to come up with something that would be bright enough to be interesting while still suggesting the distances involved, and he certainly manages this. For the humans who have just landed on this remote world, the galaxy at 200,000 light years out is indeed a dim spectacle, as described in the story:

This evening the galaxy rose directly after sunset. In spite of its angular diameter, twenty-two degrees along the major axis, our unaided eyes saw it ghostly pale across seventy thousand parsecs. By day it would be invisible. Except for what supergiants we could see as tiny sparks within it, we had no stars at night, and little of that permanent aurora which gives the planets of more active suns a sky-glow.

And later:

Vast and beautiful, it had barely cleared the horizon, which made it seem yet more huge. I could just trace out the arms, curling from a lambent nucleus… yes, there was the coil whence man had come, though if I could see man by these photons he would still be a naked half-ape running the forests of the Earth…

Keep going further out and galaxies tend to all but disappear. Greg Laughlin noted this back in 2005 on his systemic site, where he discussed the beauty of objects like M104, the Sombrero Galaxy, which seen from a distance a bit further than Anderson’s 200,000 light years, would appear as “only a faintly ominous, faintly glowing flying saucer.” M31, the great Andromeda galaxy, is larger than the full Moon in the sky, but go out tonight and try to find it. Laughlin adds: “Our Galaxy, the Andromeda Galaxy, and the Sombrero Galaxy are all essentially just empty space. To zeroth, to first, to second approximation, a galaxy is nothing at all.”

More power, then, to long CCD time exposures and big mirrors, allowing us to see the things our eyes would perceive only dimly. Part of the thrill of those early library days was in looking at spectacular objects and trying to imagine them close up. But it was a thrill of equal measure to begin to learn about the wavelengths of light, about the gathering of photons, and the way we can tease out information about the universe with instruments designed to surmount our limitations.



Fritz Zwicky On Moving Stars

by Paul Gilster on July 2, 2014

The great Ukrainian mathematician Israil Moiseyevich Gelfand was famous for his weekly seminars in Moscow, where sudden switches in topics and impromptu presentations were the norm. Although his listeners had heard it many times, Gelfand liked to tell this story: In the early 20th Century, a man approaches a physicist at a party and says he can’t understand how the new wireless telegraphy works. How is it possible to send a signal without using wires?

The physicist tells him it is simple. “To understand wireless telegraphy, you must first understand how the wired telegraph system works. Imagine a dog with its head in London and its tail in Paris. You yank the tail in Paris and the head in London barks. That is wired telegraphy. Wireless telegraphy is the same thing except without the dog.”

It always got Gelfand a laugh, but he liked to use the story for a deeper purpose. According to Edward Frenkel, who in his youth attended and presented at some of Gelfand’s seminars, Gelfand would use the tale whenever he thought a problem needed a more radical solution than anyone had proposed. “What I am trying to say,” he invariably added, “is that we need to do it without the dog.” Read Frenkel’s charming Love & Math: The Heart of Hidden Reality (Basic Books, 2013) for a look at Soviet-era mathematicians and the world they worked in.

I’m thinking that if there was ever a man who worked without the dog, it was Fritz Zwicky (1898-1974). The name of the Bulgarian-born physicist who spent his career in the United States inevitably came up in the light of our discussions on moving entire stars. This was a man with a deeply independent mind whose six-volume catalog of 30,000 galaxies, based on the Palomar Observatory Sky Survey, remains a touchstone in the study of galaxy clusters. But he was also a thorn in the side of many astronomers, routinely disparaging their work and their characters.

Zwicky 2

If a combative colleague — he coined the term ‘spherical bastards’ to describe his fellow astronomers, who he said were bastards from any angle you chose to observe them — Zwicky was kind to students, university administrators and people outside his profession. He was a man who liked to think big. Zwicky broached the subject of what we might call ‘stellar propulsion’ in a May, 1948 lecture at Oxford University, where he said there was a possibility of:

“…accelerating…[the Sun] to higher speeds, for instance 1000 km/s directed toward Alpha Centauri A in whose neighborhood our descendants then might arrive a thousand years hence. [This one-way trip] could be realized through the action of nuclear fusion jets, using the matter constituting the Sun and the planets as nuclear propellants.”

Zwicky’s lecture was published later that year in The Observatory (68:121-143). In a June, 1961 article in Engineering and Science called “The March Into Inner and Outer Space,” he followed up on the idea, although as before only in broad terms shorn of detail. In Zwicky’s view, a journey to the stars should not necessarily demand leaving the Earth behind. Instead, accelerate the Sun, letting it pull the planets along with it, and you maintain your own environment on the most comfortable of all generation ships. As to how to do it:

In order to exert the necessary thrust on the sun, nuclear fusion reactions could be ignited locally in the sun’s material, causing the ejection of enormously high-speed jets. The necessary nuclear fusion can probably best be ignited through the use of ultrafast particles being shot at the sun. To date there are at least two promising prospects for producing particles of colloidal size with velocities of a thousand kilometers per second or more. Such particles, when impinging on solids, liquids, or dense gases, will generate temperatures of one hundred million degrees Kelvin or higher-quite sufficient to ignite nuclear fusion. The two possibilities for nuclear fusion ignition which I have in mind do not make use of any ideas related to plasmas, and to their constriction and acceleration in electric and magnetic fields.

Zwicky would amplify his stellar propulsion ideas in his book Discovery, Invention, Research through the Morphological Approach (Macmillan, 1969), where he described how these directed exhaust jets would accelerate the Sun to a velocity sufficient to reach Alpha Centauri in about fifty human generations. Left for the reader’s imagination is the question of how a moving Solar System would decelerate once it arrived in the vicinity of the Alpha Centauri stars, presumably to join them to form a new triple (or quadruple, counting Proxima Centauri) star system.

We’ve seen how Leonid Shkadov conceived of wrapping a thruster around a star in such a way as to create propulsive forces, an idea now explored in the Greg Benford and Larry Niven novels Bowl of Heaven and Shipstar. But it’s clear that a star journey via a propulsive Sun is yet another idea that Zwicky had early, although he never went ahead to work out all the ramifications. In addition to manipulating the Sun’s own fusion, Zwicky’s Engineering and Science article described other broad concepts: The benefits of taming fusion for power and rocket propulsion, the need for a human presence in nearby space, particularly the Moon, and the possibility of using what he called ‘terrajet engines’ to burrow into the interior of the Earth.

In tribute to Zwicky, it’s necessary to mention several of his insights, including the notion that cosmic rays are produced in the explosion of massive stars which he began, in 1931 lectures, to describe as ‘supernovae,’ as opposed to the more common and less powerful novae. Working with Mount Wilson Observatory astronomer Walter Baade, he went on to extrapolate the creation of neutron stars that were dense as atomic nuclei but only a few kilometers in diameter, an idea that was met with skepticism.

The Palomar Observatory Sky Survey grew out of Zwicky’s work, and Zwicky himself discovered 122 supernovae. But his work hardly ended with exploding stars. Investigating the Coma cluster of galaxies, he worked out that the mass of the cluster was far too little to produce the gravitational forces needed to keep the cluster together. Unseen matter — and he coined the term ‘dark matter’ — must be making up the difference in mass. How to investigate the idea? Zwicky suggested gravitational lensing, now a common technique but a novel solution in his day.

In his dark matter work in particular, Zwicky can be said to have done what Israil Moiseyevich Gelfand urged. He learned how to do it without the dog, to take an enormous conceptual leap into an answer that later observation would prove suggestive and worthy of intense follow-up. He also had, at times, to do it without his colleagues thanks to his aggressive demeanor. Remember the ‘spherical bastards’ line? One night Zwicky and his wife Dorothea were having a group of graduate students over for dinner. Opening the door, Dorothea saw them and called back to her husband, “Fritz, the bastards are here!” Zwicky’s assertive language had evidently become the norm in their household. He was a stormy genius, an irritated, irritating treasure of a man.



Building the Bowl of Heaven

by Paul Gilster on June 30, 2014

Because his new novel Shipstar had just reached the top of my reading stack, and because I had been writing about Shkadov Thrusters last week, I asked Gregory Benford if he could provide a deeper explanation of how these enormous structures might work. Greg had already noted in an email to me that a Shkadov Thruster is inherently unstable, and earlier discussions of the idea on Centauri Dreams had raised doubts about the acceleration possible from such a device. However, I’ve referred to what Benford and Larry Niven have created as a ‘modified’ Shkadov Thruster, and I was anxious to hear their thinking on what might be possible. Greg, an award-winning science fiction author and physicist, here offers his insights into — and reservations about — a propulsion scheme capable of moving stars.

by Gregory Benford


Physicist Leonid Shkadov first described in 1987 a stellar propulsion system made by putting an enormous mirror in a static, fixed position near a star. To stay there it had to balance gravitational attraction towards and light pressure away from the star, exactly—or else it would either fall into or away from the star. Since the radiation pressure of the star would be asymmetrical, i.e. more radiation is being emitted in one direction as compared to another, the excess radiation pressure acts as net thrust, so tiny that the Sun would, after a million years, have speed of 20 m/s, and have moved 0.03 light years—far less than its orbital speed around the galaxy, ~100 km/sec.

Surely we can do better, I thought back in the early 2000s. So I mentioned some ideas to Larry Niven, and eventually we wrote two novels about a different sort of stellar thruster — Bowl of Heaven and Shipstar. Here’s an explanation from the Afterword to Shipstar:

We think of such engines as Smart Objects–statically unstable but dynamically stable, as we are when we walk. We fall forward on one leg, then catch ourselves with the other. That takes a lot of fast signal processing and coordination. (We’re the only large animal without a tail that’s mastered this. Two legs are dangerous without a big brain or a stabilizing tail.) There’ve been several Big Dumb Objects in sf, but as far as I know, no smart ones. Our Big Smart Object is larger than Ringworld and is going somewhere, using an entire star as its engine.

Our Bowl is a shell more than a hundred million miles across, held to a star by gravity and some electrodynamic forces. The star produces a long jet of hot gas, which is magnetically confined so well it spears through a hole at the crown of the cup-shaped shell. This jet propels the entire system forward – literally, a star turned into the engine of a “ship” that is the shell, the Bowl. On the shell’s inner face, a sprawling civilization dwells. The novel’s structure doesn’t resemble Larry’s Ringworld much because the big problem is dealing with the natives.

The virtue of any Big Object, whether Dumb or Smart, is energy and space. The collected solar energy is immense, and the living space lies beyond comprehension except in numerical terms. While we were planning this, my friend Freeman Dyson remarked, “I like to use a figure of demerit for habitats, namely the ratio R of total mass to the supply of available energy. The bigger R is, the poorer the habitat. If we calculate R for the Earth, using total incident sunlight as the available energy, the result is about 12 000 tons per Watt. If we calculate R for a cometary object with optical concentrators, travelling anywhere in the galaxy where a 0 magnitude star is visible, the result is 100 tons per Watt. A cometary object, almost anywhere in the galaxy, is 120 times better than planet Earth as a home for life. The basic problem with planets is that they have too little area and too much mass. Life needs area, not only to collect incident energy but also to dispose of waste heat. In the long run, life will spread to the places where mass can be used most efficiently, far away from planets, to comet clouds or to dust clouds not too far from a friendly star. If the friendly star happens to be our Sun, we have a chance to detect any wandering life-form that may have settled here.”

This insight helped me think through the Bowl, which has an R of about 10-10!


Image: Artwork by Don Davis, as are all the images in this essay.


Shdakov thrusters aren’t stable. They are not statites, Bob Forward’s invention, because they’re not in orbit. Push them, as the actual photon thrust will do, and they’ll fall outward, doomed. So how to build something that harvests a star’s energy to move it and can be stabilized?

I worried this subject, and thought back to the work my brother Jim and I had done on speeding up sails by desorption of a “paint” we could put onto a sail surface, to be blown off by a beam of microwave power striking it. This worked in experiments we did at JPL under a NASA grant, with high efficiency. Basically, throwing mass overboard is better than reflecting sunlight, because photons have very little momentum. The ratio of a photon’s momentum to that of a particle moving at speed V is

(V/c)(2Ep )/EM

where Ep is the photon energy and EM the kinetic energy of the mass M. So if those two energies are the same, the photon has a small fraction of the mass’s momentum, V/c.

So don’t use photons. Use a jet of the mass brought out from the star by forcing it to eject a jet—straight through the center of the Bowl. Jets must be confined by magnetic fields, or else they spray outward like a firehose. Get the magnetic fields from where the reflecting band of mirrors on the Bowl focuses it—on the nearest part of the star. Create a jet from that reflected energy. Make the jet push the star away. Use the jet’s magnetic fields to entwine with fields built into the Bowl itself. Let the jet hug the Bowl toward the star. Only by shaping the magnetic fields of star and jet can we move the Bowl, with constant attention to momentum and stability.

Stable, if you manage it. Who does that? How?


Larry Niven and I started building the Bowl in our minds:

The local centrifugal gravity avoids entirely the piling up of mass to get a grip on objects, and just uses rotary mechanics. So of course, that shifts the engineering problem to the Bowl structural demands.

Big human built objects, whether pyramids, cathedrals, or skyscrapers, can always be criticized as criminal wastes of a civilization’s resources, particularly when they seem tacky or tasteless. But not if they extend living spaces and semi-natural habitat. This idea goes back to Olaf Stapledon’s Star Maker: “Not only was every solar system now surrounded by a gauze of light traps, which focused the escaping solar energy for intelligent use, so that the whole galaxy was dimmed, but many stars that were not suited to be suns were disintegrated, and rifled of their prodigious stores of sub-atomic energy.”

Our smart Bowl craft is also going somewhere, not just sitting around, waiting for visitors like Ringworld–and its tenders live aboard.

We started with the obvious: Where are they going, and why?
Answering that question generated the entire frame of the two novels. That’s the fun of smart objects – they don’t just awe, they intrigue.

My grandfather used to say, as we headed out into the Gulf of Mexico on a shrimping run, A boat is just looking for a place to sink.

So heading out to design a new, shiny Big Smart Object, I said, An artificial world is just looking for a seam to pop.

You’re living just meters away from a high vacuum that’s moving fast, because of the Bowl’s spin (to supply centrifugal gravity). That makes it easy to launch ships, since they have the rotational velocity with respect to the Bowl or Ringworld… but that also means high seam-popping stresses have to be compensated. Living creatures on the sunny side will want to tinker, try new things…

“Y’know Fred, I think I can fix this plumbing problem with just a drill-through right here. Uh—oops!”

The vacuum can suck you right through. Suddenly you’re moving off on a tangent at a thousand kilometers a second—far larger than the 50 km/sec needed to escape the star. This makes exploring passing nearby stars on flyby missions easy.

But that easy exit is a hazard, indeed. To live on a Big Smart Object, you’d better be pretty smart yourself.


Mechanical Engineering

Very smart, it turns out.

As we explained in Shipstar:

We supposed the founders made its understory frame with something like scrith–a Ringworld term, greyish translucent material with strength on the order of the nuclear binding energy, stuff from the same level of physics as held Ringworld from flying apart. This stuff is the only outright physical miracle needed to make Ringworld or the Bowl work mechanically. Rendering Ringworld stable is a simple problem—just counteract small sidewise nudges. Making the Bowl work in dynamic terms is far harder; the big problem is the jet and its magnetic fields. This was Benford’s department, since he published many research papers in Astrophysical Journal in the like on jets from the accretion disks around black holes, some of which are far bigger than galaxies. But who manages the jet? And how, since it’s larger than worlds? This is how you get plot moves from the underlying physics.

One way to think of the strength needed to hold the Bowl together is by envisioning what would hold up a tower a hundred thousand kilometers high on Earth. The tallest building we now have is the 829.8 m (2,722 ft) tall Burj Khalifa in Dubai, United Arab Emirates. So for Ringworld or for the Bowl we’re imagining a scrith-like substance 100,000 times stronger than the best steel and carbon composites can do now. Even under static conditions, though, buildings have a tendency to buckle under varying stresses. Really bad weather can blow over very strong buildings. So this is mega-engineering by master engineers indeed. Neutron stars can cope with such stresses, we know, and smart aliens or even ordinary humans might do well too. So: let engineers at Caltech (where Larry was an undergraduate) or Georgia Tech (where Benford nearly went) or MIT (where Benford did a sabbatical) take a crack at it, then wait a century or two—who knows what they might invent? This is a premise and still better, a promise—the essence of modern science fiction.

Our own inner solar system contains enough usable material for a classic Dyson sphere. The planets and vast cold swarms of ice and rock, like our Kuiper Belt and Oort Clouds—all that, orbiting around another star, can plausibly give enough mass to build the Bowl. For alien minds, this could be a beckoning temptation. Put it together from freely orbiting sub-structures, stuck it into bigger masses, use molecular glues. Then stabilizes such sheet masses into plates that can get nudged inward. This lets the builders lock them together into a shell–for example, from spherical triangles. The work of generations, even for beings with very long lifespans. We humans have done such, as seen in Chartres cathedral, the Great Wall, and much else.



Still: Who did this? Maybe the Bowl was first made for just living beneath constant sunshine. Think of it as an interstellar Florida, warm and mild, with a fantastic night sky. Which keeps moving, over time.

At first the builders may have basked in the glow of their smaller sun, developing and colonizing the Bowl with ambitions to have a huge surface area with room for immense natural expanses. But then the Bowl natives began dreaming of colonizing the galaxy. They hit on the jet idea, and already had the Knothole as an exit for it. Building the mirror zone took a while, but then the jet allowed them to voyage. It didn’t work as well as they thought, and demanded control, which they did by using large magnetic fields.

The system had virtues for space flight, too. Once in space, you’re in free fall; the Bowl mass is fairly large but you exit on the outer hull at high velocity, so the faint attraction of the Bowl is no issue. Anyone can scoot around the solar system, and it’s cleared of all large masses. (The Bowl atmosphere serves to burn any meteorites that punch through the monolayer.)

The key idea is that a big fraction of the Bowl is mirrored, directing reflected sunlight onto a small spot on the star, the foot of the jet line. From this spot the enhanced sunlight excites a standing “flare” that makes a jet. This jet drives the star forward, pulling the Bowl with it through gravitation.

The jet passes through a Knothole at the “bottom” of the Bowl, out into space, as exhaust. Magnetic fields, entrained on the star surface, wrap around the outgoing jet plasma and confine it, so it does not flare out and paint the interior face of the Bowl — where a whole living ecology thrives, immensely larger than Earth’s area. So it’s a huge moving object, the largest we could envision, since we wanted to write a novel about something beyond Niven’s Ringworld.

For plausible stellar parameters, the jet can drive the system roughly a light year in a few centuries. Slow but inexorable, with steering a delicate problem, the Bowl glides through the interstellar reaches. The star acts as a shield, stopping random iceteroids that may lie in the Bowl’s path. There is friction from the interstellar plasma and dust density acting against the huge solar magnetosphere of the star, essentially a sphere 100 Astronomical Units in radius.

So the jet can be managed to adjust acceleration, if needed. If the jet becomes unstable, the most plausible destructive mode is the kink – a snarling knot in the flow that moves outward. This could lash sideways and hammer the zones near the Knothole with virulent plasma, a dense solar wind. The first mode of defense, if the jet seems to be developing a kink, would be to turn the mirrors aside, not illuminating the jet foot. But that might not be enough to prevent a destructive kink. This has happened in the past, we decided, and lives in Bowl legend.

The reflecting zone of mirrors is defined by an inner angle, Θ, and the outer angle, Ω. Reflecting sunlight back onto the star, focused to a point, then generates a jet which blows off. This carries most of what would be the star’s solar wind, trapped in magnetic fields and heading straight along the system axis. The incoming reflected sunlight also heats the star, which struggles to find an equilibrium. The net opening angle, Ω minus Θ, then defines how much the star heats up. We set Ω = 30 degrees, and Θ = 5 degrees, so the mirrors subtend that 25 degree band in the Bowl. The Bowl rim can be 45 degrees, or larger.

The K2 star we gave the Bowl is now running in a warmer regime, heated by the mirrors, thus making its spectrum nearer that of Sol. This explains how the star can have a spectral class somewhat different from that predicted by its mass. It looks oddly colored, more yellow than its mass would indicate.

For that matter, that little sun used to be a little bigger. It’s been blowing off a jet for many millions of years. Still, it should last a long time. The Bowl could circle the galaxy itself several times.

The atmosphere is quite deep, more than 200 km. This soaks up solar wind and cosmic rays and makes the Bowl toasty through greenhouse effect. Also, the pressure is higher than Earth normal by about 50%, depending on location in the Bowl. It is also a reservoir to absorb the occasional big, unintended hit to the ecology. Compress Earth’s entire atmosphere down to the density of water and it would only be 30 feet deep. Everything we’re dumping into our air goes into just 30 feet of compressed nitrogen and oxygen, then. The Bowl has much more, over a hundred yards deep in equivalent water. Too much carbon dioxide? It gets more diluted.

This deeper atmosphere explains why in low-grav areas surprisingly large things can fly–big aliens and even humans. We humans Earthside enjoy a partial pressure of 0.21 bars of oxygen, and we can do quite nicely in a two-bar atmosphere of almost pure oxygen (but be careful about fire). The Bowl has a bit less than we like: 0.18 bar, but the higher pressure compensates. This depresses fire risk, someone figures out later.

Starting out, we wrote a background history of where the Builders came from, which we didn’t insert into the novel. It lays out a version of what made the Builders do all this.


Is this plausible?

Not really. It demands the scrith, for example, which nobody knows how to make.

And the Bowl is a vast accident waiting to happen. You can’t just say Don’t blame me, it’s nonlinear. Somebody has to manage that jet forever. The natives get to take part in slow-motion starflight, but they’re always in danger. Their society must keep this from being obvious, or they’d all go crazy.

Our goal in writing the two novels, and perhaps stories to follow, was to show how strange an alien mindset could be, by giving it a real, physical presence, in the Bowl. Also, we wanted to see what it felt like to think of where humanity itself might go, given time, purpose, and the true essential, imagination.

© 2014 by Gregory Benford