Frank Malina: Texas Rocket Grandmaster

by Paul Gilster on June 12, 2017

It’s wonderful to have my friend Al Jackson back at the top of the site with a look at the career and times of JPL’s Frank Malina. Al’s service in the Apollo program came as astronaut trainer on the Lunar Module Simulator; he then spent 40 more years at Johnson Space Center, mostly for Lockheed working the Shuttle and ISS programs. His doctorate was in 1975 from the University of Texas at Austin. The author of numerous scientific papers on interstellar concepts, Al is a fixture at deep space conferences and a continuing source of inspiration on matters scientific as well as science fictional. Today Al gives us an overview of a man who played a key role in the sounding rocket era following World War II, as the infant Jet Propulsion Laboratory began its rich history of exploration and technical development.

by Al Jackson

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I travel from Houston to Austin by Highway 290 fairly often, and sometimes I stop at Brenham, Texas for lunch. I skip the fast food joints on 290 and go downtown. It is a beautiful small town with a charming old downtown (founded in 1844). Only recently have I become aware that a native Texan from Brenham fulfilled a dream started by Robert Goddard, in fact doing in 10 months what Goddard had for twenty years tried to accomplish. Even more than that, he was co-founder of the Jet Propulsion Laboratory, JPL, and co-founder of Aerojet General. By 1945 he had eclipsed Goddard as the most important American rocket scientist. He was a consummate researcher in the theoretical engineering of rocketry and a master manager of several rocket and rocket vehicle projects for the U.S. Army. So who was the Texas pioneer ‘Wernher von Braun’? Dr. Frank J Malina from Brenham, Texas.

Malina was the originator and leader of a project whose anniversary is today, June 12. It is the 70th anniversary of the last launch of a Wac-Corporal sounding rocket at White Sands. We tend to forget that Robert Goddard had a solid scientific use for his development of rockets — to explore the Earth’s upper atmosphere. We all love Goddard for his inventiveness in rocket hardware and his stubborn individualism, and given time he may have realized his sounding rocket dream. However, while he struggled in the New Mexico desert in 1936, Frank Malina, still a graduate student at Caltech, had put on the wall of his office a chart of how a successful sounding rocket project might be accomplished. Unlike Goddard, he recognized the need for a team and a choice of team captains.

Malina’s dream was interrupted by World War II. Along with his mentor Theodore von Kármán (the great 20th century aerodynamicist), he directed the development of the Jet Assisted Take Off (JATO) rockets for use by the Army Air Force. This work for the U.S. Army led to the formation of JPL, and Malina became its first director. There is a straight line heritage from the solid rocket JATO motors to the intercontinental missiles in the American defense arsenal and even up to the Space Shuttle booster motors. His involvement in this project alone is enough to have made him a famous rocketeer.

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Image: Dr. Theodore von Kármán (black coat) sketches out a plan on the wing of an airplane as his JATO engineering team looks on. From left to right: Dr. Clark B. Millikan, Dr. Martin Summerfield, Dr. Theodore von Kármán, Dr. Frank J. Malina and pilot, Capt. Homer Boushey. Captain Boushey would become the first American to pilot an airplane that used JATO (Jet Assisted Take-Off) solid propellant rockets. Credit: NASA/JPL.

In 1944 Dr. Malina was sent to England and France to inspect salvaged V2s and V1 launch sites. Returning by plane over the Atlantic, he decided to ask the Army ordnance department to fund his cherished goal of building and launching a vehicle to sound the upper atmosphere in regions that could not be reached by balloons. This was December of 1944. From designs by H.S. Tsien and Malina, he and Homer Stewart submitted and got approval on a proposal to launch a sounding rocket with a 25 lb payload to 100,000 ft.

There had already been a program started at the newly founded JPL to build military rockets. Malina organized a team to use components developed from this program. It is amazing that the von Kármán-Malina program at JPL during WWII accomplished, on a smaller scale, almost the same technical objectives as von Braun’s huge V2 project. A viable liquid rocket motor using nitric acid and aniline with 1500 lbs of thrust was developed, as was the Private-series of missiles. The main difference being the V2’s much larger rocket motor and especially the guidance system, which was still being researched at JPL by the end of the war.

Once the project was approved, Malina and his JPL crew turned over several ideas for the sounding rocket. It turned out that the solid rocket motors would be too heavy for the flight. They needed a long burn light weight rocket. So a liquid motor powered vehicle boosted quickly to a high speed was designed. They needed the initial boost in order to gain a sufficient amount of stability from the vehicle fins since they had yet to developed an active onboard guidance system. The booster system used some of the solid rocket technology in the JATO units that JPL had already fashioned. The booster and 2nd stage liquid rocket were to be launched using a 60 ft tower.

In July of 1945 the flight characteristics of the booster were tested with a 1/5 scale model at Goldstone Lake, California. The tests showed the viability of the solid booster system and a three fin stabilization system rather than four fins favored by ordnance experts. One wonders: Did any copies of this ‘baby Wac Corporal’ survive to the present?

Nine months after Malina had proposed it the vehicles were taken to the new facility at White Sands Proving Grounds, New Mexico.

Four rounds of the booster called Tiny Tim were launched off the tower. Two dummy rounds of the WAC were boosted and then two with only partially filled fuel tanks were flown to get experience with the radar tracking.

These must have been counted as rounds 1 through 4 because on October 11, 1945 a fully loaded round 5 was made ready. The 16 foot long 1 foot in diameter rocket stood flight ready. It weighed 665 lbs and would be boosted by 50,000 lbs of thrust before the 1500 lb thrust liquid motor took over. In Malina’s words the flight went like this:

“11 October 1945 became our great day for the first flight of the WAC (round 5) fully charged with propellant. It was a clear day. We craned our necks to watch the WAC’s smoke trail until the engine stopped at around 80,000 ft. On the basis of radar tracking data for the 6th round of the WAC, it was estimated that the maximum altitude reached was between 230,000 and 240,000 ft. The total time of flight was about 450 sec. or 7.5 min. the velocity of the WAC at the end of the burning was about 3,100 ft per sec. The impact point of the first round was around 3,500 ft. from the launcher, which meant that the WAC had maintained a very satisfactory vertical path. Success!”

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Image: Project director Frank J. Malina (a former JPL Director) poses with the fifth WAC Corporal at the White Sands Missile Range. The solid-propellant booster is not shown. Credit: NASA/JPL.

That 43 mile flight was a world record, for even the more advanced V2 had not been launched to such an altitude yet. It was an amazing achievement. In 10 months, Malina and his crew had designed and built the sounding rocket Goddard had dreamed of and made such a contribution to. Soon there followed the captured V2 flights from New Mexico and other sounding rocket programs.

Malina headed a large team of people working together just as von Braun had run a much larger team in Germany (Malina and von Braun were almost the same age). Malina remarked:

“The large number of people involved in this (WAC Corporal) program indicates why the dreams of individuals and small groups of rocket enthusiasts in the 1920’s and 1930’s to design, construct and test a high altitude sounding rocket had little chance of success. Fortunately, most pioneers do not foresee all of the practical implications of their dreams. No doubt if they were able to do so, few new wild ideas will ever be tried.”

It is good to remember a fellow Texan, Dr. Frank Malina, a man not as well-known as Dr. Goddard, or Dr. von Braun, but a rocketeer who had profound and lasting impact on the American development of rocket vehicles, astronautics and spaceflight.

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Planet Formation around TRAPPIST-1

by Paul Gilster on June 9, 2017

Just how did the seven planets around TRAPPIST-1 form? This is a system with seven worlds each more or less the size of the Earth orbiting a small red dwarf. If these planets formed in situ, an unusually dense disk would have been required, making planet migration the more likely model. But if the planets migrated from beyond the snowline, how do we explain their predominantly rocky composition? And what mechanisms are at work in this system to produce seven planets all of approximately the same size?

New work out of the University of Amsterdam attempts to resolve the question through a different take on planet formation, one that involves the migration not of planets but planetary building blocks in the form of millimeter to centimeter-sized particles. Chris Ormel (University of Amsterdam) and team note that thermal emission from pebbles like these has been observed around other low-mass stars and even brown dwarfs. The researchers believe these migrating particles become planetary embryos as they reach the snowline, which at TRAPPIST-1 occurs at about 0.1 AU.

Once within the snowline, the embryos would grow by the accretion of rocky pebbles from the inner circumstellar disk, with inward migration eventually stopping at the inner edge of the disk. A key assumption here is that the planets of TRAPPIST-1 formed sequentially rather than simultaneously, a novel concept indeed. So let me go to the paper at this point:

In our model we assume that the H2O iceline is the location where the midplane solids-to-gas ratio exceeds unity, triggering streaming instabilities and spawning the formation of planetesimals. These planetesimals merge into a planetary embryo, whose growth is aided by icy pebble accretion. Once its mass becomes sufficiently large, it migrates interior to the H2O iceline by type I migration, where it continues to accrete (now dry) pebbles until it reaches the pebble isolation mass.

The process then begins again for a second planet:

After some time, a second embryo forms at the snowline, which follows a similar evolutionary path as its predecessor. Even though the inner planet’s growth could be reduced by its younger siblings’ appetite for pebbles, it always remains ahead in terms of mass. Planet migration stalls at the inner disk edge, where the planets are trapped in resonance.

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Image: Astronomers from the University of Amsterdam (the Netherlands) present a new model for how seven earth-sized planets could have been formed in the planetary system Trappist-1. The crux is at the line where ice changes to water. Credit and copyright: NASA/R. Hurt/T. Pyle. And please note this JPL news release on the artists who produced this image. All too often, artists like Tim Pyle and Robert Hurt receive scant attention in the stories that run their work. It’s excellent to see their background and methods explained.

The TRAPPIST-1 planets, indeed, form what the authors call ‘a resonant convoy,’ with the outer planets ‘pushing’ on the inner ones. The paper’s numerical simulations produce the observed planetary system with the exception that a 3:2 mean motion resonance emerges among planets b and c, as well as among c and d. Although neither pair is presently at the 3:2 MMR, the authors argue that during the disk dispersion phase of the system’s formation, the 3:2 MMRs of these pairs were broken, leaving us with the overall architecture we see today.

The paper’s most radical contention is that planets have assembled at a specific location, the snowline, as opposed to forming in situ or migrating from their formation regions beyond the snowline. Clearly, many questions remain, including how the streaming instabilities induced at the snowline operate in the presence of planetary embryos. The paper does, however, make a prediction: If a giant planet forms rapidly at the snowline, it should end the flux of pebbles to the inner disk, depriving it of planet-building material. From the paper:

Hence, we expect a dichotomy: when giant planet formation fails, pebbles can drift across the iceline to aid the growth of super-Earths and mini-Neptunes. Conversely, when a giant planet forms at the iceline we expect a dearth of planetary building blocks in the inner disk. Therefore, the close-in super-Earth population found by Kepler and the cold Jupiter populations found chiefly by radial velocity surveys should be anti-correlated – a prediction that could be tested with future exoplanet surveys.

The paper is Ormel et al., “Formation of Trappist-1 and other compact systems,” accepted at Astronomy & Astrophysics (abstract). A preprint is available, but be aware that a number of internal references are not yet filled in, another reason not to assume that preprints necessarily mirror the final paper.

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Ultraviolet Insights into Red Dwarf Flares

by Paul Gilster on June 8, 2017

I seem to be reminded every day of how many discoveries are lurking in our archives. On the question of red dwarf stars and the flare activity that could compromise the habitability of planets around them, the ten year dataset from GALEX is proving invaluable. The Galaxy Explorer Evolution spacecraft was launched in 2003 and operated until 2012. Bear in mind that it was designed to study the evolution of galaxies at ultraviolet wavelengths. But now this valuable mission’s archives are helping us track the study of nearby habitable planets.

Led by first author Chase Million (Million Concepts, State College PA), a project dubbed gPhoton has set about reprocessing more than 100 terabytes of GALEX data now at the Mikulski Archive for Space Telescopes (MAST), which is maintained at the Space Telescope Science Institute in Baltimore. Million worked with STScI’s Clara Brasseur to develop custom software that could tease out the signature of flares for several hundred red dwarf stars. Dozens have been detected so far, with the prospect of far more in the GALEX archive.

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Image: Artist’s illustration of a red dwarf star orbited by a hypothetical exoplanet. Red dwarfs tend to be magnetically active, displaying gigantic arcing prominences and a wealth of dark sunspots. Red dwarfs also erupt with intense flares that could strip a nearby planet’s atmosphere over time, or make the surface inhospitable to life as we know it. Credit: NASA, ESA, and G. Bacon (STScI).

The software behind this project is significant because it can measure flare events that are much less energetic than previously detected from red dwarfs, reminding us that while major flares get our attention, persistent lower-strength flare activity could have the more dangerous cumulative effect on a closely orbiting planet. A significant amount of a flare’s total energy is released in the ultraviolet wavelengths GALEX observed, while the stars themselves are relatively dim at these wavelengths, offering contrast that made these results possible. The team was able to trace stellar variations lasting as little as a few seconds in the data.

“We have found dwarf star flares in the whole range that we expected GALEX to be sensitive to, from itty bitty baby flares that last a few seconds, to monster flares that make a star hundreds of times brighter for a few minutes,” said Million.

Many of the flares detected in the GALEX observations are similar to those that occur on our own Sun. But bear in mind that any planet in the habitable zone of a cool, dim red dwarf is going to be much closer to its host, making it subject to more of the flare’s energy than the Earth. Flares of large enough energy could play a role in stripping a planet of its atmosphere, while strong ultraviolet light from persistent flaring could damage living organisms.

The work was presented at the 230th meeting of the American Astronomical Society in Austin in early June. But the study is far from over. Clara Brasseur and team member Rachel Osten are now looking at stars that were observed both by GALEX and the Kepler mission, trying to track down similar flare activity. Hundreds of thousands of flares may be hidden in the data.

“These results show the value of a survey mission like GALEX, which was instigated to study the evolution of galaxies across cosmic time and is now having an impact on the study of nearby habitable planets,” said Don Neill, research scientist at Caltech in Pasadena, who was part of the GALEX collaboration. “We did not anticipate that GALEX would be used for exoplanets when the mission was designed.”

So we keep factors like these in mind as we assess the possible habitability of planets around stars like Proxima Centauri, LHS 1140 and TRAPPIST-1. While we’re learning that Earth-sized planets may be plentiful around red dwarfs, and that many may occur in the zone where liquid water could occur on the surface, the question of habitability is far from resolved.

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The Golden Apples of the Sun

by Paul Gilster on June 7, 2017

Alex Tolley mentioned Ray Bradbury’s story “The Golden Apples of the Sun” in connection with my first article on the Parker Solar Probe, and it’s a short tale worth remembering in connection with a mission flying so remarkably close to our star. First published in 1953 in a Doubleday collection of the same name, the tale is a short, mythic take on dangerous questing, with its main character, the unnamed captain, a figure something like Melville’s Captain Ahab. His goal is to fly to the Sun’s surface and retrieve some of its fire:

The captain stared from the huge dark-lensed port, and there indeed was the sun, and to go to that sun and touch it and steal part of it forever away was his quiet and single idea. In this ship were combined the coolly delicate and the coldly practical. Through corridors of ice and milk-frost, ammoniated winter and storming snowflakes blew. Any spark from that vast hearth burning out there beyond the callous hull of this ship, any small fire-breath that might seep through would find winter, slumbering here like all the coldest hours of February.

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The ship and crew, fighting malfunctioning equipment, fall in their cryogenic ship “like a snowflake into the lap of June, warm July, and the sweltering dog-mad days of August.” It’s Bradbury in full poetic mode, not one of his best tales but one that does capture a bit of the natural human awe at stellar immensity. As for the Parker Solar Probe and the European Space Agency’s Solar Orbiter mission, neither will ‘touch the Sun,’ though the former will close to about 6.2 million kilometers, with the Solar Orbiter at 42 million kilometers.

Image: The dustjacket of the first edition of Bradbury’s collection The Golden Apples of the Sun. The title comes from the lush Yeats lyric The Song of Wandering Aengus. In an interview late in his life, Bradbury recalled: “Maggie [his wife] introduced me to romantic poetry when we were dating, and I loved it. I loved that line in the poem, and it was a metaphor for my story, about taking a cup full of fire from the sun.”

These missions are framed so as to study the near-Sun environment from different perspectives. Both are scheduled for launch in 2018, the Solar Orbiter in October and the Parker Solar Probe in July. What I’m seeing here are two complementary ventures, both examining the interaction between the Sun’s atmospheric gases and its magnetic field. Comparing the datasets should give us our best understanding yet of the inner heliosphere.

The key issues for the Solar Orbiter are listed on this European Space Agency page:

  • What drives the solar wind and where does the coronal magnetic field originate from?
  • How do solar transients drive heliospheric variability?
  • How do solar eruptions produce energetic particle radiation that fills the heliosphere?
  • How does the solar dynamo work and drive connections between the Sun and the heliosphere?

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Image: Payload accommodation onboard Solar Orbiter. In this rendering, one side wall has been removed to expose the remote-sensing instruments mounted on the payload panel. The SPICE [Spectral Imaging of the Coronal Environment} instrument (not visible) is mounted to the top panel from below. Credit: ESA.

What Solar Orbiter brings to the mix is an orbit that will allow us to get close-up views of the Sun’s polar regions, with images from latitudes higher than 25 degrees. We should be able to see solar storms building up over an extended period from the same vantage point, because when traveling at the highest velocity along its orbit, Solar Orbiter will come close to matching the speed of the Sun’s rotation on its axis. We will be able to track solar storms for days.

Remember that many of the Sun’s jets originate in the areas around the poles, meaning we need more data about the magnetic field and plasma flows there to delve more fully into the process. These are not regions that are visible to us from Earth.

Understanding the solar wind also helps us consider the feasibility of future propulsion concepts using magnetic sail technologies to ride the solar wind. Meanwhile, the thermal issues a photon-driven sundiver mission will encounter will take center stage as we weigh the performance of the instrument packages on both these spacecraft. A sundiver would pass as close as possible to the Sun before unfurling a solar sail for maximum acceleration. We gain information from the upcoming solar missions that benefits both types of sail technology.

Solar Orbiter will carry a suite of 10 instruments to observe solar activity, 8 of them provided by ESA member states, while the other two are being developed by a European-led consortium and NASA respectively. A three and a half year approach to the operational orbit will include close flybys of Earth and Venus to insert the craft into a highly elliptical orbit. The instruments will measure solar wind plasma, fields, wave and energetic particles close to the Sun, while we can also expect images of solar features at the highest resolution ever seen.

So we’re going to have in situ measurements of the Sun’s extended corona from the Parker Solar Probe along with high-resolution studies of the inner heliosphere and the Sun itself from Solar Orbiter. Which takes me back to Bradbury’s captain, who has now ‘touched’ the Sun and is contemplating what is next:

“Well,” said the captain, sitting, eyes shut, sighing. “Well, where do we go now, eh, where are we going?” He felt his men sitting or standing all about him, the terror dead in them, their breathing quiet. “When you’ve gone a long, long way down to the sun and touched it and lingered and jumped around and streaked away from it, where are you going then? When you go away from the heat and the noonday light and the laziness, where do you go?”

We know where a sundiver sail mission would go: Outward, fast and far…

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Into the Solar Wind

by Paul Gilster on June 6, 2017

No spacecraft has ever flown as close to the Sun as the Parker Solar Probe will. The spacecraft will penetrate the outer solar atmosphere — the corona — where its measurements should help us understand the origin and characteristics of the solar wind. To put this in perspective, consider that Mercury is at 0.39 AU. The Helios-B spacecraft, launched in 1976, closed to within 43 million kilometers of the Sun (0.29 AU), the current record for a close pass. The Parker Solar Probe will fly seven times closer, moving within ten solar radii.

We need to learn a lot more about this region as we consider various mission concepts for the future. The Parker Solar Probe uses an 11.43 cm carbon-composite shield designed to keep its instrument package at room temperature, or close to it, despite outside temperatures well over 1350° Celsius. The sundiver concept we talked about yesterday — sometimes called a ‘fry-by’ — has the advantage of maximizing the effect of solar photons by unfurling a sail in close proximity to the Sun. To fly such a mission, we need to understand this environment.

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Image: The Parker Solar Probe spacecraft leaving Earth, after separating from its launch vehicle and booster rocket, bound for the inner solar system and an unprecedented study of the Sun. Credit: JHU/APL.

And let’s compare the Parker Solar Probe with the two Helios spacecraft in one other regard. Helios-B set the current speed record for spacecraft at 70.22 kilometers per second. The Parker Solar Probe should reach 194 km/sec. The goal of a future sundiver mission would be to turn solar photon pressure into maximum departure velocity out to deep space. Just how fast an outward-bound sail could fly will depend upon how close it can approach the Sun and what kind of materials it is made of, issues about which we have much to learn.

Conditions this close to the Sun are obviously going to be treacherous, and highly variable. The solar wind of electrons, protons and ionized helium nuclei flowing from the Sun can reach velocities as high as 800 kilometers per second. Unfortunately, the plasma flow is also highly variable, fluctuating over periods short and long. While we speak of ‘solar sailing’ by way of photon pressure on a sail, a truer analog to sailing is the use of a magnetic sail riding the solar wind. In the latter case, we have a highly variable ‘wind’ like mariners face on Earth, with all the attendant issues raised by its fluctuations. The study of both types of sailing should be enhanced by the data we retrieve from the Parker Solar Probe.

In Solar Sails: A Novel Approach to Interplanetary Travel (Springer, 2014), authors Giovanni Vulpetti, Les Johnson and Greg Matloff consider the positive electric charge that can emerge on a sail as ultraviolet solar photons knock electrons free from sail atoms. The problem here is that interactions with high-energy electrons can degrade sail reflectivity. A possible strategy is to deploy an electrically charged grid in front of the sail or use layers of protective plastic that evaporate rather than becoming ionized by solar ultraviolet light.

But every time we add mass to the sail, we reduce the spacecraft’s velocity outbound, meaning we’re probably going to be better served by designing solar sails that are more resistant to UV. It’s clear that we have a lot of work ahead on the interactions between spacecraft and the near-Sun environment, the subject of the Parker Solar Probe’s investigations. Of course, the upcoming probe is also set to investigate the interactions between the solar wind and the Earth’s magnetic field that produce so-called ‘space weather.’ Learning how to predict this kind of weather can protect space assets much closer to home.

A Mechanism for Solar Eruptions

We also need to consider solar activity, for flying any kind of mission within 10 solar radii is going to be dangerous if we have to contend with solar flares or coronal mass ejections. We can do our best to predict the occurrence of such dangers to the spacecraft, but CMEs appear to be random enough that the unexpected appearance of one could quickly end a mission.

On that score, a new paper in Nature looks at a single mechanism that can explain solar eruptions from small jets to coronal mass ejections, working with 3D computer simulations to reveal the underlying process at work. Peter Wyper (Durham University, UK), lead author of the study, notes that filaments — long, dark structures above the surface of the Sun consisting of dense and colder solar material — are associated with the onset of CMEs. We’re now learning that solar jets have filament-like structures as well before their eruption.

“In CMEs, filaments are large, and when they become unstable, they erupt,” said Wyper. “Recent observations have shown the same thing may be happening in smaller events such as coronal jets. Our theoretical model shows the jet can essentially be described as a mini-CME.”

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Image: A long filament erupted on the sun on Aug. 31, 2012, shown here in imagery captured by NASA’s Solar Dynamics Observatory. Credit: NASA’s Goddard Space Flight Center/SDO.

The researchers at Durham University and at NASA have produced a ‘breakout model’ showing how stressed filaments break through magnetic restraints in both solar jets and CMEs. In this model, the process at work is magnetic reconnection, in which magnetic field lines come together and realign into a new configuration. The event is powerful enough that the energy stored in the filament breaks out from the surface and is ejected into space.

The Parker Solar Probe gives us another way to obtain high-resolution observations of the magnetic field and plasma flows in the solar atmosphere. We can expect similar help from the European Space Agency’s Solar Orbiter mission, scheduled for a launch in the fall of 2018. Unlike the Parker Solar Probe, the Solar Orbiter will go into an inclined orbit that allows better imaging of the areas around the Sun’s poles. Both missions obviously deepen our resources for sundiver mission planning as we contemplate fast departures into the outer system.

Tomorrow I’ll take a look at ESA’s Solar Orbiter in the context of our continuing efforts to understand the nearest star. The paper on solar eruptions is Wyper et al., “A universal model for solar eruptions,” Nature 544 (27 April 2017), 452-455 (abstract).

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Parker Solar Probe: Implications for Sundiver

by Paul Gilster on June 5, 2017

We’re going to be keeping a close eye on what is now called the Parker Solar Probe as work continues toward a July 2018 launch. This is a mission with serious interstellar implications because it takes us into the realm of so-called ‘sundiver’ maneuvers in which a solar sail could be brought as close as possible to the Sun (perhaps behind a protective occulter) and then unfurled to get maximum effect. Velocities well beyond Voyager’s can grow from this.

Throw in the prospect of beamed propulsion and such sails could receive an additional boost. To be sure, the Parker Solar Probe is not a solar sail but an unmanned, instrumented probe designed to explore a region as close as 10 solar radii from the Sun’s surface, where temperatures can be expected to reach about 1375° Celsius. But the sundiver implications are there, and we’ll gain priceless data about operations in this extreme environment.

Why a sundiver? Voyager 1 is exiting the neighborhood of the Solar System at 17.1 kilometers per second. And while New Horizons left Earth breaking all speed records for spacecraft launched into interplanetary space, it’s worth remembering that its velocity during the Pluto/Charon encounter had fallen to about 14 kilometers per second, a consequence of the long climb out of the gravity well. So finding ways to get spacecraft to two or three times this velocity is an obvious objective as we explore ways of moving faster still.

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Image: Artist’s impression of NASA’s Solar Probe Plus spacecraft on approach to the sun. Set to launch in 2018, Solar Probe Plus will use Venus’ gravity during seven flybys over nearly seven years to gradually bring its orbit closer to the sun. The spacecraft will fly through the Sun’s atmosphere as close as 6.2 million kilometers to our star’s surface, well within the orbit of Mercury and more than seven times closer than any spacecraft has come before. Credit: NASA/Johns Hopkins University Applied Physics Laboratory.

The first use of the word ‘sundiver’ in a scientific paper that I am aware of is Greg and Jim Benford’s “Near-Term Beamed Sail Propulsion Missions: Cosmos-1 and Sun-Diver” (Beamed Energy Propulsion, AIP Conf. Proc. 664, pg. 358, A. Pakhomov, ed., 2003), though the word’s history goes back a bit further to David Brin’s 1980 novel Sundiver, which turned out to be the first book in his Uplift Trilogy. Greg Benford worked with Brin on some of the novel’s concepts, discussions that he recalled in a column in Fantasy & Science Fiction:

I called this craft the Sundiver. The term is old—I gave it to David Brin when he first came to see me, back when he was struggling with his first novel. (As he now recounts, I asked him how his craft that literally plunges into the Sun could survive. He answered that he would throw in some jargon, techtalk, whatever. I disdainfully replied, “Oh—magic.” So David went home and found a physically possible way to do it, confounding me.)

I read Sundiver not long after it came out and don’t recall anything like a close solar pass mission — instead (as Benford says above), Brin was looking for a way to actually get a craft into the Sun by way of exploring the novel’s unusual lifeforms, creatures that lived off magnetic fields in the chromosphere. But the Benford paper mentioned above (available here) discusses sail concepts using a close solar pass as well as desorption of heated embedded molecules from the hot side of the sail that can deliver a second propulsive ‘burn.’

Let’s pause on desorption, which turned out to be interesting because in their laboratory work at JPL pushing an ultra-light carbon sail with a microwave beam, the Benfords found that the beam alone could not account for the acceleration they observed. Subsequent investigation showed that embedded molecules — CO2, hydrocarbons and hydrogen that had been incorporated in the sail material lattice when it was made — could be ejected under high enough temperatures. The original sail material was left unharmed by this propulsive effect.

Incorporating that phenomenon into a sundiver mission, we get this: The sail approaches the Sun turned edge-on to minimize solar flux that would push against it. The spacecraft then turns at perihelion to get the full effect of both photons and related sail desorption, gaining velocity even as (because of the loss of some of the molecules in its fibers) it loses a bit of mass. When desorption is complete, the sail operates like any other solar sail, though one now moving fast enough to explore the outer Solar System in far less time than it took Voyager.

On the way to a sundiver, what the Parker Solar Probe gives us, among other things, is the ability to investigate the high energy particle environment that any future sundiver mission would have to cope with. An 11.43 cm carbon-composite shield will be used to protect the craft. We’ll learn much about the solar wind, findings that will also prove useful as we contemplate future magsail possibilities, in which a spacecraft might use the highly variable plasma flow to reach high velocities. More about that possibility tomorrow, when we’ll take a closer look at the conditions the Parker Solar Probe will face and ponder the insights it is certain to provide.

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SSEARS: Background of a NIAC Study

by Paul Gilster on June 2, 2017

I always keep an eye on what’s going on at the NASA Innovative Advanced Concepts office, which is where I ran into Jeff Nosanov’s Phase I study for a solar sail called Solar System Escape Architecture for Revolutionary Science. At the Jet Propulsion Laboratory, Jeff managed flight mission proposals and supported the radio isotope power program. He now lives in Washington DC, a technology entrepreneur whose fascination with spacecraft design has never diminished. In the essay below, Jeff explains the background of his first NIAC award (a second, PERIapsis Subsurface Cave Optical Explorer, would lead to Phase I and Phase II grants), and gives us an idea of the ins and outs of making ideas into reality at NIAC and JPL. For more, the website nosanov.com is about to go online, and Jeff’s new podcast debuts today.

By Jeff Nosanov

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It’s an honor and a privilege to be asked to write about my near-interstellar mission work for the Tau Zero Foundation. Marc’s book and Paul’s writing were very inspiring to me as I began working at JPL and dove into the interstellar mission community. This article will describe my journey to JPL, the process of proposing the mission concept, and the experience of managing the project. The mission concept itself can be read about here.

In summary, SSEARS (Solar System Escape Architecture for Revolutionary Science) is a solar sail-based mission to return to the heliopause (the edge of the solar system) to continue the Voyager science. The driving goal was to determine the fastest possible propulsion method to return to the outer solar system. We concluded that it was possible to get there in about 18 years (half the time it took Voyager) using a very large solar sail system. How did we get to this point? I’ll start at the beginning.

A Child’s View

One of my favorite childhood photos was taken at the Griffith Observatory in Los Angeles. I don’t quite remember this event, but I do strongly remember a later one, my visit at age 5 in 1987 around the time of the Voyager 2 Neptune encounter. My dad took me there to look through the big telescope at Neptune, and told me of the spacecraft that was nearing the ice giant planet.

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I distinctly remember the “mind blown” moment of realization that such things were possible. I also learned that day about JPL, the NASA lab just a few dozen miles from the Griffith Observatory where the Voyagers, and so many other spacecraft, were built. I didn’t know it until much later but that event turned me into a space guy. I also recall a conversation with my dad from around that time in which he told me that we can visit other planets, but not other stars. That stuck with me.

Image: Photo of Jeff as a baby at Griffith Observatory.

As a child it is easy to have a fairly distorted view of what NASA actually is, and what it does. It’s tempting to imagine something out of Star Trek, with wild and amazing technologies in development all around. In reality, only a small part of NASA, the NASA Innovative Advanced Concepts Program (NIAC), embodies that exciting vision, as I discovered as an adult.

Working at JPL

About 22 years later I started working at JPL on January 4th, 2010. Suddenly the greatest deep space exploration playground was opened to me. I was extremely eager, giddy even, to learn about this work and discover how I could contribute. I began to learn everything I could about everything I could access. I spent many hours reading books in the library (especially Frontiers of Propulsion Science) and talking to as many people, especially senior people, as possible. I especially stalked the Voyager program people.

My general knowledge about how NASA missions are developed, selected, funded, built, and operated grew. I began to pay attention to RFPs (Requests for Proposals) and BAAs (Broad Agency Announcements) that came through the daily emails. I eventually pieced together the realization that anyone could propose concepts (at nearly any stage of development) to various programs within NASA. But how to possibly find the right combination of ambition, team, capability, vision, most importantly a receptive NASA program?

I spent many hours with program managers, scientists and engineers learning about the current challenges and capabilities in deep space exploration. I was extremely grateful at the time, but in retrospect I have even greater appreciation for the time spent with me. It was only through the kind support of managers, engineers, scientists and many others that I developed the general background knowledge to attempt the work that became my project.

Eventually I came to learn about the NIAC program. That program exists to fund revolutionary concepts that significantly increase exploration capability. This is distinct from the majority of NASA projects, which largely seek to minimize technological risk. The NIAC program exists as a way to ensure that investments in promising, ambitious technology get made, and to reduce technological stagnation as the risk-minimizing forcing functions of most other NASA programs run their course.

Stalking Voyager

I knew I wanted to make a dent in interstellar flight, so I spent a lot of time with people from the Voyager program. I tried to learn everything I could about the mission to develop a mission concept that would be worth doing scientifically. Around this time NASA had been planning to fund a technology demonstration mission for a concept called SunJammer. This was to be a solar sail mission to a Lagrange point, using the sail for propulsion and station-keeping. I decided that a NIAC-worthy proposal would be to start with just how large we could reasonably build a solar sail, and then see how fast such a system could reach the heliopause with a scientifically valuable payload.

I approached Ed Stone at Caltech with child-like reverence. Stone was project scientist for Voyager and a former director of JPL itself. I wanted to ask him what he would do to follow Voyager if he had a limitless budget. To my surprise he accepted my meeting request, and we had several meetings in which he outlined a scientific payload designed entirely for the heliopause region. This became the “mission” that the hypothetical giant solar sail propulsion system would uniquely enable.

Writing the Proposal

JPL has several systems in place to help people write successful proposals. Each proposal opportunity announcement from NASA results in a mentoring and guidance resource from the responsible JPL program office. I took great advantage of this resource for the first few proposals of my career and it has made a world of difference.

The theme across many proposals for many agencies in my experience is the importance of genuine, exciting storytelling. Thinking back, that was all I had for my first effort! The process of writing the proposal exposed me to a great many inner workings of JPL including program management, costing, and even obtaining submission authority. I doggedly, perhaps obsessively, followed the process to ensure that all the rules were followed and I felt a deep sense of relief and excitement once the proposal was fully submitted. However, I did not have any way of estimating my chances for success due to my lack of experience. I moved on with my regular work and hoped.

Months later I got a most unexpected phone call telling me that my proposal had been selected! I was over the moon, and I felt something very powerful for the first time: My ideas are good and are worth pursuing. I felt a change in responsibility. The proposal development process was partly for my own personal satisfaction and growth. It was a wonderful experience. Now, having been selected, I had the privilege of having the responsibility of discovering something new about the universe and our place in it.

Outcome

One of the important lessons I learned is to keep the long term goal in mind. In our case, this specifically meant figuring out the fastest possible way back to the heliopause with a science payload worth taking there. It turned out that we could get there in 18 years with a 250m x 250m solar sail and a “Voyager on steroids” payload.

This rendering shows our vision for the spacecraft, and a small part of the solar sail behind it.

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Towards the end of the study I made another appointment with Ed Stone. I told him what we had learned. He looked me in the eye and said “Wow.” I’d like to imagine that in that one moment he was the 5 year old looking at the sky, his mind bursting with possibilities, instead of me.

Impact

We ultimately submitted a proposal for Phase 2. Unfortunately intervening events ended the possibility in the foreseeable future for significant progress on our work. We had relied upon the NASA excitement in the SunJammer program to chart the course for solar sail work. That program was cancelled and so we did not have a path forward to advance the technology, and without that potential synergy it seemed that a NIAC phase 2 award was unlikely.

Still, I was satisfied and inspired by our results. I must conclude that the relative impact, in the grand scheme of things, of our study was small, perhaps incremental. However the experience drastically changed my goals and ambition. Ultimately my study is one of many that attempted to explore the limits of what human ingenuity and curiosity can achieve.

Conclusion

I became a father on May 4, 2012 in the middle of the study period. This event fundamentally changed the way I think about space exploration and my own role in it. I enjoyed bringing my son to the Griffith Observatory for many reasons. During the Griffith Observatory visit in which this photo of my son was taken I recalled my dad’s statement that we can travel to other planets but not other stars. As of June 2nd, 2017 now that is still true…

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… but we’re working on it.

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Enceladus: Evidence for Asteroid Impact?

by Paul Gilster on June 1, 2017

How to make sense of Enceladus? The moon’s famous jets of water vapor, mixing with organic compounds, salts and silica, first revealed the possibility of an ocean beneath the icy surface, and the Cassini orbiter has treated Enceladus as a high priority target ever since. But why the asymmetry here? After all, while the south polar region includes the active ‘tiger stripe’ fractures associated with the plumes in a geologically young area, the northern pole shows much more cratering, evidence for a considerably older surface and, obviously, no plumes at all.

Perhaps, say astronomers from NASA, the University of Texas and Cornell, we’re dealing with an ancient impact, one that completely re-oriented Enceladus by tipping it about 55 degrees away from its original axis. Thus we get fractures well over 100 kilometers long in the south, evidence for an asteroid strike in what would have once been an area close to the moon’s equator. Radwan Tajeddine (Cornell University) is a Cassini imaging team associate and lead author of the paper on this work, which recently appeared online in Icarus:

“The geological activity in this terrain is unlikely to have been initiated by internal processes,” says Tajeddine. “We think that, in order to drive such a large reorientation of the moon, it’s possible that an impact was behind the formation of this anomalous terrain.”

Giving weight to the hypothesis is the presence of a series of basins that can be detected across Enceladus’ surface. The researchers believe these are traces of a previous equator and poles. The features visible to Cassini would also reflect underlying variations in the ice shell. Such an impact would have caused Enceladus to wobble, finally re-establishing stability over the course of at least a million years. The alteration to the north-south axis resulting from this is known as ‘true polar wander,’ changing the locations of north and south poles.

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Image: These maps look toward the icy moon’s southern hemisphere, with colors representing highs and lows. Purple represents the lowest elevations, while red represents the highest. The map at left shows the surface of Enceladus in its possible ancient orientation, millions of years ago. The chain of basins representing topographic lows can be seen in blue and purple, running along the equator, with an additional low region around the original south pole. The region that encloses the moon’s currently active south polar terrain, with its long, linear “tiger stripe” fractures, would have been at middle latitudes just south of the equator. The map at right shows the current orientation of Enceladus. Credit: NASA/JPL-Caltech/Space Science Institute/Cornell University.

Thus if we simply re-orient Enceladus by about 55 degrees of latitude, the pair of depressions found by the researchers lines up with a much older equator and poles. And we have an explanation for the existence of the ‘tiger stripe’ fractures and their jets. An Enceladus without jets would have been visually less dramatic, but the impact theory also reminds us that other moons without telltale geysering activity may conceal internal oceans of their own.

The paper is Tajeddine et al., “True polar wander of Enceladus from topographic data,” published online by Icarus 30 April 2017 (abstract).

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Remembering Ben Finney (1933-2017)

by Paul Gilster on May 31, 2017

Ben Finney, the editor (along with Eric Jones) of Interstellar Migration and the Human Experience, has died at age 83. A professor emeritus at the University of Hawaii, Finney died quietly at a nursing home in Kaimuki, according to this obituary in the Honolulu Star-Advertiser. There is much to be said about this visionary man, but I begin for our purposes with his contribution to deep space studies and interstellar thinking.

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For Interstellar Migration and the Human Experience, which I bought not long after it came out in 1986, turned out to be one of those key texts, and I am hardly the only person who was transformed by the ideas in its pages. I bought it purely on the basis of its title. Not yet aware of the serious studies into interstellar flight that were then being published in the journals, I marveled that here was a text that put human movement to the stars into a serious scientific and historical context and saw it as an apotheosis of the species.

Image: Anthropologist and author Ben Finney. Credit: Polynesian Voyaging Society.

Paging through the book, I quickly realized that it was a series of papers from a conference of the same name that had taken place at Los Alamos in May of 1983. The conference was a multidisciplinary event ranging from astrophysics to anthropology, looking into societal migration, emerging technologies, resource acquisition and the critical human factors underlying all exploration. Finney’s own contributions emphasized not just the technically possible but the emotionally resonant, stressing exploration as an imperative of the species.

Thus we come to the peopling of the Pacific islands. Finney had an ear for language and a sense of deep time. Here he’s talking about exploration over long time-frames:

The whole history of Hominidae has been one of expansion from an East African homeland over the globe and of developing technological means to spread into habitats for which we are not biologically adapted. Various peoples in successive epochs have taken the lead in this expansion, among them the Polynesians and their ancestors. During successive bursts lasting a few hundred years, punctuated by long pauses of a thousand or more years, these seafarers seem to have become intoxicated with the discovery of new lands, with using a voyaging technology they alone possessed to sail where no one had ever been before.

Before reading Interstellar Migration, I had given little thought to historical corollaries that could prepare us for a possible future among the stars. These days, thanks to Finney’s work, we’re used to seeing the analogy with the Pacific islands made, to the point that its strengths and weaknesses have been debated frequently in the literature (and certainly in the comments sections on this site over the years). But it was Finney’s unique background that made it possible for him to develop the Polynesian experience in deep space terms:

Once their attempts to cope with the rising sea levels of the Holocene committed them to sea, the first pioneers of this lineage of seafarers had good reasons to keep going. The continental mindset of their distant ancestors would have faded as successive generations pushed farther and farther east, to be eventually replaced by the more accurate view that the world was covered with water through which bits of land were scattered. They therefore knew that in pushing into the open ocean they were entering not a vast empty region but one teeming with islands. What is more, after leaving the Bismarck Archipelago and outdistancing their less sea-adapted rivals, they would have realized that before them lay an ocean of islands accessible to themselves alone. What more invitation did they need?

An invitation indeed. Now, as to Finney’s background: He was an anthropologist, a Harvard Ph.D who taught for thirty years at the University of Hawaii at Manoa, where his courses included Human Adaptation to the Sea and Human Adaptation to Living in Space. It was back in the 1950s that his exposure to a book by Andrew Sharp raised his hackles, arguing as it did that Polynesians had populated the Pacific islands more or less by chance, driven by the vagaries of wind and tide. Finney decided it was a theory that could be contested.

To do so, he built a Polynesian sailing canoe christened Nalehia, followed by a larger craft, the Hōkūleʻa, which Finney sailed with a small crew from Hawaii to Tahiti. A founder and first president of the Polynesian Voyaging Society, Finney was convinced that the Polynesians had been capable of extraordinary feats of navigation and deep ocean voyages in open craft. That he proved such voyages were possible is attested to by the fact that some 25 similar canoes are now making voyages in the Pacific islands. Hokule’a itself is finishing up a three-year circumnavigation of the Earth, with return to Hawaii slated for some time in June.

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Image: Hokule’a arrival in Honolulu from Tahiti in 1976. Credit: Phil Uhl/Wikimedia Commons.

Space was never far from Ben Finney’s thinking. He was a research associate at NASA Ames on SETI issues in the 1990s and held a faculty appointment at the International Space University, in addition to his other appointments at UC-Santa Barbara and the University of French Polynesia. His career was wide-ranging enough to take in nuclear waste disposal (working with Sandia National Laboratories) and robotics. But for most people, it will be the Polynesian experience in one way or another that will always define Ben Finney.

Moreover, the future he envisioned was one of endless speciation, given the distances to the stars. Let me wrap up with the conclusion to his “The Exploring Animal,” written with Eric Jones and the lead-off paper in Interstellar Migration and the Human Experience:

Human evolution in space will hardly be limited to the birth of a new species. Space is not a single environment but an Earthcentric residual category for everything outside our atmosphere. There are innumerable environments out there providing countless niches to exploit, first by humans and then by the multitudinous descendant species. By expanding through space we will be embarking on an adventure that will spread an explosive speciation of intelligent life as far as technology or limits placed by any competing life forms originating elsewhere will allow. Could the radiation of evolving intelligent life through space be the galactic destiny of this Earth creature we have called the exploring animal?

Addendum: Savage Minds, an excellent venue for discussion of anthropology, has a detailed article on Ben Finney that I highly recommend. Thanks to Randy McDonald for the tip.

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Comments on Near-Term Interstellar Probes

by Paul Gilster on May 30, 2017

If you have questions about beamed energy concepts, James Benford is your man. A plasma physicist who is CEO of Microwave Sciences, Benford has designed high-power microwave systems for the likes of NASA, JPL and Lockheed. Now Chairman of the Sail Subcommittee for Breakthrough Starshot, he is deep into the investigation of sail materials and design, as he explains below. After reading Greg Matloff’s Near-Term Interstellar Probes: Some Gentle Suggestions, Jim passed along his comments, which highlight the need for a dedicated laboratory facility to explore the Starshot possibilities. He offers as well his thoughts on where sails stand in the overall propulsion landscape, a position of growing significance.

By James Benford

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My colleague and old friend Greg Matloff has given us a well-informed broad survey of propulsion options for interstellar flight. I’m going to contribute a few comments.

Even a century-long flight to Alpha Centauri requires a velocity of ~10,000 km/sec, which is about 500 times the fastest velocity that any human object has reached by rocket propulsion — Voyager 1 is at about 17 km/sec. On the way to getting to a velocity like 10,000 km/sec, we’re going to enable fast missions for exploration and development of the solar system and the near-interstellar region around us.

Beamed Energy Propulsion

The cubesat experiments described by Greg would probably have to be fairly massive in order to produce any substantial velocity. It’s far more expeditious and far less expensive to do such experiments in evacuated chambers in the laboratory. After all, the only flight experiments that been conducted to date, which I participated in 16 years ago, were conducted in just such apparatus. That’s the most straightforward way to demonstrate stability and high acceleration. No such laboratory exists at present.

I’m hoping that Breakthrough StarShot will soon develop such a Beam-Driven Sail Test Facility, because the pressing matter of stability and high acceleration will take some time to be researched. Of course such a laboratory has to be designed and built and that would take a year at least. The most suitable beam source will be either microwave or millimeter-wave, because they can be obtained off-the-shelf at high power and have quite reasonable costs of a few dollars per watt. (Lasers are now in the hundreds of dollars per watt range.)

Such a Beam-Driven Sail Test Facility would conduct experiments on the key Starshot issues, in order of priority:

    1) Beam-riding stability of various sail shapes.

    2) Sail material and its properties: to maintain a high reflectance with very little beam absorption or transmission.

    3) Ability to sustain high acceleration, which is necessary in order to achieve high velocities.

The Cosmos-1 experiments we (the Planetary Society, Microwave Sciences and JPL) had planned to do, and which unfortunately didn’t happen because of the launcher failure, should certainly be looked at again. We were ready to carry out an experiment to irradiate the sail with the Deep Space Network beam from Goldstone. This could have demonstrated beamed propulsion of a sail in space. The 450 kW microwave beam from the large 70-m dish can show direct microwave beam acceleration of the 30-m sail by photon pressure, and we can measure that acceleration by on-board accelerometer telemetry.

The key thing to do is to measure the acceleration on the spacecraft with an on-board accelerometer. The alternative, deducing acceleration from orbit changes, would depend upon multiple transits of the sail through the beam. It would be hard to winkle out of orbital data, especially if the acceleration is small. [see “MAX-Microwave Acceleration eXperiment with Cosmos-1,” James Benford, Gregory Benford and Tom Kuiper, Proc. Fourth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, also JBIS 59, pg. 68 (2006).]

Greg Matloff is certainly correct to point out that the sail material for Starshot must be able to reconfigure its shape. It must have very precise and accurate pointing and tracking, at both Earth and the star that it is approaching. The sail must be a very smart material with embedded and distributed artificial intelligence. How to provide the energy for reconfiguring the shape of the sail is one of the many challenges of Starshot.

Thermonuclear Fusion Rockets

Greg gives a good list of the many technical problems that face fusion rockets. Of course the greatest problem is to produce a fusion reaction at all! For interstellar flight they will be very large, very inefficient and very costly.

For comparison, here are some key parameters of a US supercarrier, two Icarus design concepts and the Starshot sailship:

  • Aircraft Carrier 0.3 km, 105,000 tons, 0.01 T$
  • Firefly 0.75 km, 23,550 tons, mass fraction 0.0064, rocket cost 40 T$, 4.7% c
  • Ghost 1.2 km, 154,800 tons, mass fraction 0.0008, cost 0.02-34 T$, 6% c
  • Large Sailship 10 km, 10 tons, mass fraction ~0.1, Beamer cost 40 T$, sailship capital cost ~ 1B$, 10% c [operating cost, electricity to drive the Beamer at today’s rate (0.1 $/kW-hr) is 0.5 T$.]
  • Starshot ~3 m, ~1 gram, capital cost ~10B$ [operating cost ~6 M$.]

And for a size comparison, see Figure 1. The ships that explored the oceans, such as the Santa Maria (19 m) and Kon-Tiki scale Polynesian rafts (45 m), as well as the Breakthrough Starshot sail (~3 m) could not be visible on this scale.

With the fusion rocket approach, the infrastructure necessary to build such huge vessels and supply nuclear fuel is a fixed cost. It is not easy to estimate, but will be quite huge.

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Figure 1. Scales of Starships compared with largest Earth vessels. From top, largest Earth ocean ships, Firefly, Ghost, 10 km diameter Sailship, all to scale. The ~3m Breakthrough Starshot sail is actually not visible on this scale. (Figure from Michael Lamontagne).

The concept that Freeman Dyson originally proposed, using nuclear materials out of the nuclear arsenals to make explosives to drive starships, is a bit out of date now. As you can see in figure 2, the Cold War adversaries have been radically reducing the warheads they have, the rapid progress beginning during the Reagan–Gorbachev era. The Nunn-Lugar Cooperative Threat Reduction program deactivated more than 7,600 nuclear warheads. Highly enriched uranium contained in them was made into commercial reactor fuel which was purchased by the U.S. Few Americans realize that during the last several decades a fair amount of the electrical power the United States was generated by burning up Russian nuclear materials in fission power reactors!

There are now a few thousand nuclear explosives compared to the hundred thousand at the peak of the Cold War.

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Figure 2: History of Nuclear Stockpiles.

Conclusion

Beam-driven propulsion is more firmly grounded and credible than nuclear fusion propulsion, as in Project Icarus. Fusion rockets remain far-term, distant on a timescale of decades, if not centuries. In today’s funding environment, that’s not likely to change: Due to Starshot, sails are becoming near-term.

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