Although I’ve often seen Arthur Conan Doyle’s Sherlock Holmes cited in various ways, I hadn’t chased down the source of this famous quote: “When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.” Gerald Jackson’s new paper identifies the story as Doyle’s “The Adventure of the Blanched Soldier,” which somehow escaped my attention when I read through the Sherlock Holmes corpus a couple of years back. I’m a great admirer of Doyle and love both Holmes and much of his other work, so it’s good to get this citation straight.
As I recall, Spock quotes Holmes to this effect in one of the Star Trek movies; this site’s resident movie buffs will know which one, but I’ve forgotten. In any case, a Star Trek reference comes into useful play here because what Jackson (Hbar Technologies, LLC) is writing about is antimatter, a futuristic thing indeed, but also in Jackson’s thinking a real candidate for a propulsion system that involves using small amounts of antimatter to initiate fission in depleted uranium. The latter is a by-product of the enrichment of natural uranium to make nuclear fuel.
Both thrust and electrical power emerge from this, and in Jackson’s hands, we are looking at a mission architecture that can not only travel to another star – the paper focuses on Proxima Centauri as well as Epsilon Eridani – but also decelerate. Jackson has been studying the matter for decades now, and has presented antimatter-based propulsion concepts for interstellar flight at, among other venues, symposia of the Tennessee Valley Interstellar Workshop (now the Interstellar Research Group). In the new paper, he looks at a 10-kilogram scale spacecraft with the capability of deceleration as well as a continuing source of internal power for the science mission.
Image: Depiction of the deceleration of interstellar spacecraft utilizing antimatter concept. Credit: Gerald Jackson.
On the matter of the impossible, the quote proves useful. Jackson applies it to the propulsion concepts we normally think of in terms of making an interstellar crossing. This is worth quoting:
Applying this Holmes Method to space propulsion concepts for exoplanet exploration, in this paper the term “impossible” is re-interpreted arbitrarily to mean any technology that requires: 1) new physics that has not been experimentally validated; 2) mission durations in excess of one thousand years; and 3) material properties that are not currently demonstrated or likely to be achievable during this century. For example, “warp drives” can currently be classified as impossible by criterion #1, and chemical rockets are impossible due to criterion #2. Breakthrough Starshot may very well be impossible according to criterion #3 simply because of the needed material properties of the accelerating sail that must survive a gigawatt laser beam for 30 minutes. Though traditional nuclear thermal rockets fail due to criterion #2, specific fusion-based propulsion systems might be feasible if breakeven nuclear fusion is ever achieved.
Can antimatter supply the lack? The kind of mission Jackson has been analyzing uses antimatter to initiate fission, so we could consider this a hybrid design, one with its roots in the ‘antimatter sail’ Jackson and Steve Howe have described in earlier technical papers. For the background on this earlier work, you can start by looking at Antimatter and the Sail, one of a number of articles here on Centauri Dreams that has explored the idea.
In this paper, we move the antimatter sail concept to a deceleration method, with the launch propulsion being handed off to other technologies. The sail’s antimatter-induced fission is not used only to decelerate, though. It also provides a crucial source of power for the decades-long science mission at target.
If we leave the launch and long cruise of the mission up to other technologies, we might see the kind of laser-beaming methods we’ve looked at in other contexts as part of this mission. But if Breakthrough Starshot can develop a model for a fast flyby of a nearby star (moving at a remarkable 20 percent of lightspeed) via a laser array, various problems emerge, especially in data acquisition and return. On the former, the issue is that a flyby mission at these velocities allows precious little time at target. Successful deceleration would allow in situ observations from a stable exoplanet orbit.
That’s a breathtaking idea, given how much energy we’re thinking about using to propel a beamed-sail flyby, but Jackson believes it’s a feasible mission objective. He gives a nod to other proposed deceleration methods, which have included using a ‘magnetic sail’ (magsail) to brake against a star’s stellar wind. The problem is that the interstellar medium is too tenuous to slow a craft moving at a substantial percentage of lightspeed for orbital insertion upon arrival – Jackson considers the notion in the ‘impossible’ camp, whereas antimatter may come in under the wire as merely ‘improbable.’ That difference in degree, he believes, is well worth exploring.
The antimatter concept described generates a high specific impulse thrust, with the author noting that approximately 98 percent of antiprotons that stop within uranium induce fission. It turns out that antiproton annihilation on the nucleus of any uranium isotope – and that includes non-fissile U238 – induces fission. In Jackson’s design, about ten percent of the annihilation energy released is channeled into thrust.
Jackson analyzes an architecture in which the uranium “propagates as a singly-charged atomic ion beam confined to an electrostatic trap.” The trap can be likened in its effects to what magnetic storage rings do when they confine particle beams, providing a stable confinement for charged particles. Antiprotons are sent in the same direction as the uranium ions, reaching the same velocity in the central region, where the matter/antimatter annihilation occurs. Because the uranium is in the form of a sparse cloud, the energetic fission ‘daughters’ escape with little energy loss.
Here is Jackson’s depiction of an electrostatic annihilation trap. In this design, both the positively charged uranium ions and the negatively charged antiprotons are confined.
Image: This is Figure 1 from the paper. Caption: Axial and radial confinement electrodes (top) and two-species electrostatic potential well (bottom) of a lightweight charged-particle trap that mixes U238 with antiprotons.
A workable design? The author argues that it is, saying:
Longitudinal confinement is created by forming an axial electrostatic potential well with a set of end electrodes indicated in figure 1. To accomplish the goal of having oppositely charged antiprotons and uranium ions traveling together for the majority of their motion back and forth (left/right in the figure) across the trap, this electrostatic potential has a double-well architecture. This type of two-species axial confinement has been experimentally demonstrated .
The movement of antiprotons and uranium ions within the trap is complex:
The antiprotons oscillate along the trap axis across a smaller distance, reflected by a negative potential “hill”. In this reflection region the positively charged uranium ions are accelerated to a higher kinetic energy. Beyond the antiproton reflection region a larger positive potential hill is established that subsequently reflects the uranium ions. Because the two particle species must have equal velocity in the central region of the trap, and the fact that the antiprotons have a charge density of -1/nucleon and the uranium ions have a charge density of +1/(238 nucleons), the voltage gradient required to reflect the uranium ions is roughly 238 times greater than that required to reflect the antiprotons.
The design must reckon with the fact that the fission daughters escape the trap in all directions, which is compensated for through a focusing system in the form of an electrostatic nozzle that produces a collimated exhaust beam. The author is working with a prototype electrostatic trap coupled to an electrostatic nozzle to explore the effects of lower-energy electrons produced by the uranium-antiproton annihilation events as well as the electrostatic charge distribution within the fission daughters.
Decelerating at Proxima Centauri in this scheme involves a propulsive burn lasting ten years as the craft sheds kinetic energy on the long arc into the planetary system. Under these calculations, a 200 year mission to Proxima requires 35 grams of total antiproton mass. Upping this to a 56-year mission moving at 0.1 c demands 590 grams.
Addendum: I wrote ’35 kilograms’ in the above paragraph before I caught the error. Thanks, Alex Tolley, for pointing this out!
Current antimatter production remains in the nanogram range. What to do? In work for NASA’s Innovative Advanced Concepts office, Jackson has argued that despite minuscule current production, antimatter can be vastly ramped up. He believes that production of 20 grams of antimatter per year is a feasible goal. More on this issue, to which Jackson has been devoting his life for many years now, in the next post.
The paper is Jackson, “Deceleration of Exoplanet Missions Utilizing Scarce Antimatter,” in press at Acta Astronautica (2022). Abstract.
My family has had a closer call with ALS than I would ever have wished for, so the news of Stephen Hawking’s death stays with me as I write this morning. I want to finish up my thoughts on antimatter from the last few days, but I have to preface that by noting how stunning Hawking’s non-scientific accomplishment was. In my family’s case, the ALS diagnosis turned out to be mistaken, but there was no doubt about Hawking’s affliction. How on Earth did he live so long with an illness that should have taken him mere years after it was identified?
Hawking’s name will, of course, continue to resonate in these pages — he was simply too major a figure not to be a continuing part of our discussions. With that in mind, and in a ruminative mood anyway, let me turn back to the 1950s, as I did yesterday in our look at Eugen Sänger’s attempt to create the design for an antimatter rocket. Because even as Sänger labored over the idea, one he had been pursuing since the 1930s, Les Shepherd was looking at the antimatter prospect, and coming up with aspects of the problem not previously identified.
Getting a Starship Up to Speed
Shepherd isn’t as well known as he should be to the public, but within the aerospace community he is something of a legend. A specialist in nuclear fusion, his activities within the International Academy of Astronautics (he was a founder) and the International Astronautical Federation (he was its president) were legion, but this morning I turn to “Interstellar Flight,” a Shepherd paper from 1952. This was published just a year before Sänger explained his antimatter rocket ideas to the 4th International Astronautical Congress in Zurich, later published in Space-Flight Problems (1953).
Remember that neither of these scientists knew about the antiproton as anything other than a theoretical construct, which meant that a ‘photon rocket’ in the Sänger mode just wasn’t going to work. But Shepherd saw that even if it could be made to function, antimatter propulsion ran into other difficulties. Producing and storing antimatter were known problems even then, but it was Shepherd who saw that “The most serious factor restricting journeys to the stars, indeed, is not likely to be the limitation on velocity but rather limitation on acceleration.”
This stems from the fact that the matter/antimatter annihilation is so mind-bogglingly powerful. Let me quote Shepherd on this, as the problem is serious:
…a photon rocket accelerating at 1 g would require to dissipate power in the exhaust beam at the fantastic rate of 3 million Megawatts/tonne. If we suppose that the photons take the form of black-body radiation and that there is 1 sq metre of radiating surface available per tonne of vehicle mass then we can obtain the necessary surface temperature from the Stefan-Boltzmann law…
Shepherd worked this out as:
5.7 x 10-8 T4 = 3 x 1012 watts/metre2
with T expressed in degrees Kelvin. So the crux of the problem is that we are producing an emitting surface with a temperature in the range of 100,000 K. The problem with huge temperatures is that we have to find some way of dissipating them. We’d like to get our rocket operating at 1 g acceleration so we could tour the galaxy, using relativistic time dilation to send a crew to the galactic center, for example, within a human lifetime. But we have to dispose of waste heat from the extraordinarily hot emitting surfaces of our spacecraft, because with numbers like these, even the most efficient engine is still going to produce waste heat.
Image: What I liked about the ‘Venture Star’ from James Cameron’s film Avatar was that the design included radiators, clearly visible in this image. How often have we seen the heat problem addressed in any Hollywood offering? Nice work.
Now we can look at Robert Frisbee’s design — an antimatter ’beamed-core’ starship forced by its nature to be thousands of kilometers long and, compared to its length, incredibly thin. Frisbee’s craft assumes, as I mentioned, a beamed-core design, with pions from the annihilation of protons and antiprotons being shaped into a stream of thrust by a magnetic nozzle; i.e., a superconducting magnet. The spacecraft has to be protected against the gamma rays produced in the annihilation process and it needs radiators to bleed off all the heat generated by the engine.
We also need system radiators for the refrigeration systems. Never forget that we’re storing antimatter within a fraction of a degree of absolute zero (-273 C), then levitating it using a magnetic field that takes advantage of the paramagnetism of frozen hydrogen. Thus:
…the width of the main radiator is fixed by the diameter of the superconductor magnet loop. This results in a very long main radiator (e.g., hundreds of km in length), but it does serve to minimize the radiation and dust shields by keeping the overall vehicle long and thin.
Frisbee wryly notes the need to consider the propellant feed in systems like this. After all, we’re trying to send antimatter pellets magnetically down a tube at least hundreds of kilometers long. The pellets are frozen at 1 K, but we’re doing this in an environment where our propellant feed is sitting next to a 1500 K radiator! Frisbee tries to get around this by converting the anti-hydrogen into antiprotons, feeding these down to the engine in the form of a particle beam.
Frisbee’s 40 light-year mission with a duration of 100 years is set up as a four-stage antimatter rocket massing millions of tons, with radiator length for the first stage climbing as high as 7500 kilometers, and computed radiator lengths for the later stages still in the hundreds of kilometers. Frisbee points out that the 123,000 TW of first-stage engine ‘jet’ power demands the dumping of 207,000 TW of 200 MeV gamma rays. Radiator technology will need an extreme upgrade.
And to drop just briefly back to antimatter production, check this out:
The full 4-stage vehicle requires a total antiproton propellant load of 39,300,000 MT. The annihilation (MC2) energy of this much antimatter (plus an equal amount of matter) corresponds to ~17.7 million years of current Human energy output. At current production efficiencies (10-9), the energy required to produce the antiprotons corresponds to ~17.7 quadrillion  years of current Human energy output. For comparison, this is “only” 590 years of the total energy output of sun. Even at the maximum predicted energy efficiency of antiproton production (0.01%), we would need 177 billion years of current Human energy output for production. In terms of production rate, we only need about 4×1021 times the current annual antiproton production rate.
Impossible to build, I’m sure. But papers like these are immensely useful. They illustrate the consequences of taking known theory into the realm of engineering to see what is demanded. We need to know where the showstoppers are to continue exploring, hoping that at some point we find ways to mitigate them. Frisbee’s paper is available online, and repays a close reading. We could use the mind of a future Hawking to attack such intractable problems.
The Les Shepherd paper cited above is “Interstellar Flight,” JBIS, Vol. 11, 149-167, July 1952. The Frisbee paper is “How to Build an Antimatter Rocket for Interstellar Missions,” 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 20-23 July 2003 (full text).
Antimatter’s staggering energy potential always catches the eye, as I mentioned in yesterday’s post. The problem is how to harness it. Eugen Sänger’s ‘photon rocket’ was an attempt to do just that, but the concept was flawed because when he was developing it early in the 1950s, the only form of antimatter known was the positron, the antimatter equivalent of the electron. The antiproton would not be confirmed until 1955. A Sänger photon rocket would rely on the annihilation of positrons and electrons, and therein lies a problem.
Sänger wanted to jack up his rocket’s exhaust velocity to the speed of light, creating a specific impulse of a mind-boggling 3 X 107 seconds. Specific impulse is a broad measure of engine efficiency, so that the higher the specific impulse, the more thrust for a given amount of propellant. Antimatter annihilation could create the exhaust velocity he needed by producing gamma rays, but positron/electron annihilation was essentially a gamma ray bomb, pumping out gamma rays in random directions.
Image: Austrian rocket scientist Eugen Sänger, whose early work on antimatter rockets identified the problems with positron/electron annihilation for propulsion.
What Sänger needed was thrust. His idea of an ‘electron gas’ to channel the gamma rays his photon rocket would produce never bore fruit; in fact, Adam Crowl has pointed out in these pages that the 0.511 MeV gamma rays generated in the antimatter annihilation would demand an electron gas involving densities seen only in white dwarf stars (see Re-thinking the Antimatter Rocket). No wonder Sänger was forced to abandon the idea.
The discovery of the antiproton opened up a different range of possibilities. When protons and antiprotons annihilate each other, they produce gamma rays and, usefully, particles called pi-mesons, or pions. I’m drawing on Greg Matloff’s The Starflight Handbook (Wiley, 1989) in citing the breakdown: Each proton/antiproton annihilation produces an average of 1.5 positively charged pions, 1.5 negatively charged pions and 2 neutral pions.
Note the charge. We can use this to deflect some of these pions, because while the neutral ones decay quickly, the charged pions take a bit longer before they decay into gamma rays and neutrinos. In this interval, Robert Forward saw, we can use a magnetic nozzle created through superconducting coils to shape a charged pion exhaust stream. The charged pions will decay, but by the time they do, they will be far behind the rocket. We thus have useful momentum from this fleeting interaction or, as Matloff points out, we could also use the pions to heat an inert propellant — hydrogen, water, methane — to produce a channeled thrust.
But while we now have a theoretical way to produce thrust with an antimatter reaction, we still have nowhere near the specific impulse Sänger hoped for, because our ‘beamed core’ antimatter rocket can’t harness all the neutral pions produced by the matter/antimatter annihilation. My friend Giovanni Vulpetti analyzed the problem in the 1980s, concluding that we can expect a pion rocket to achieve a specific impulse equivalent to 0.58c. He summed the matter up in a paper in the Journal of the British Interplanetary Society in 1999:
In the case of proton-antiproton, annihilation generates photons, massive leptons and mesons that decay by chain; some of their final products are neutrinos. In addition, a considerable fraction of the high-energy photons cannot be utilised as jet energy. Both carry off about one third of the initial hadronic mass. Thus, it is not possible to control such amount of energy.
Image: Italian physicist Giovanni Vulpetti, a major figure in antimatter studies through papers in Acta Astronautica, JBIS and elsewhere.
We’re also plagued by inefficiencies in the magnetic nozzle, a further limitation on exhaust velocity. But we do have, in the pion rocket, a way to produce thrust if we can get around antimatter’s other problems.
In the comments to yesterday’s post, several readers asked about creating anti-hydrogen (a positron orbiting an antiproton), a feat that has already been accomplished at CERN. In fact, Gerald Jackson and Steve Howe (Hbar Technologies) created an unusual storage solution for anti-hydrogen in their ‘antimatter sail’ concept for NIAC, which you can see described in their final NIAC report. In more recent work, Jackson has suggested the possibility of using anti-lithium rather than anti-hydrogen.
The idea is to store the frozen anti-hydrogen in a chip much like the integrated circuit chips we use every day in our electronic devices. A series of tunnels on the chip (think of the etching techniques we already use with electronics) lead to periodic wells where the anti-hydrogen pellets are stored, with voltage changes moving them from one well to another. The anti-hydrogen storage bottle draws on methods Robert Millikan and Harvey Fletcher used in the early 20th Century to measure the charge of the electron to produce a portable storage device.
The paramagnetism of frozen anti-hydrogen makes this possible, paramagnetism being the weak attraction of certain materials to an externally applied magnetic field. Innovative approaches like these are changing the way we look at antimatter storage. Let me quote Adam Crowl, from the Centauri Dreams essay I cited earlier:
The old concept of storing [antimatter] as plasma is presently seen as too power intensive and too low in density. Newer understanding of the stability of frozen hydrogen and its paramagnetic properties has led to the concept of magnetically levitating snowballs of anti-hydrogen at the phenomenally low 0.01 K. This should mean a near-zero vapour pressure and minimal loses to annihilation of the frozen antimatter.
But out of this comes work like that of JPL’s Robert Frisbee, who has produced an antimatter rocket design that is thousands of kilometers long, the result of the need to store antimatter as well as to maximize the surface area of the radiators needed to keep the craft functional. In Frisbee’s craft, antimatter is stored within a fraction of a degree of absolute zero (-273 C) and then levitated in a magnetic field. Imagine the refrigeration demands on the spacecraft in sustaining antimatter storage while also incorporating radiators to channel off waste heat.
Image: An antimatter rocket as examined by Robert Frisbee. This is Figure 6 from the paper cited below. Caption: Conceptual Systems for an Antimatter Propulsion System.
Radiators? I’m running out of space this morning, so we’ll return to antimatter tomorrow, when I want to acknowledge Les Shepherd’s early contributions to the antimatter rocket concept.
The paper by Giovanni Vulpetti I quoted above is “Problems and Perspectives in Interstellar Exploration,” JBIS Vol. 52, No. 9/10, available on Vulpetti’s website. For Frisbee’s work, see for example “How to Build an Antimatter Rocket for Interstellar Missions,” 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 20-23 July 2003 (full text).
Antimatter will never lose its allure when we’re talking about interstellar propulsion, even if the breakthroughs needed to harness it are legion. After all, a kilogram of antimatter, annihilating itself in contact with normal matter, yields roughly ten billion times the amount of energy released when a kilogram of TNT explodes. Per kilogram of fuel, we’re talking about 1,000 times more energy than nuclear fission, and 100 times the energy available through nuclear fusion.
Or we could put this into terms more suited for space. A single gram of antimatter, according to Frank Close’s book Antimatter (Oxford, 2010), could through its annihilation produce as much energy as the fuel from the tanks of two dozen Space Shuttles.
The catalog of energy comparisons could go on, each as marvelous as the last, but the reality is that antimatter is not only extremely difficult to produce in any quantity but even more challenging to store. Cram enough positrons or antiprotons into a magnetic bottle and the repulsive forces between them overcome the containing fields, creating a leak that in turn destroys the antimatter. How to store antimatter for propulsion remains a huge problem.
Here’s Close on the issue:
…`like charges repel’, so in order to contain the electric charge in a gram of pure antiprotons or of positrons, you would have to build a force field so powerful that were you to disrupt it, the explosive force as the charged particles flew apart would exceed anything that would have resulted from their annihilation.
As with so many issues regarding deep space, though, we tackle these things one step at a time. Thus recent news out of CERN draws my attention this morning. Bear in mind that between CERN and Fermilab we’re still talking about antimatter production levels that essentially have enough energy to light a single electric bulb for no more than a few minutes. But assuming we find ways to increase our production, perhaps through harvesting of naturally occurring antimatter, we’re learning some things about storage through a project called PUMA.
The acronym stands for ‘antiProton Unstable Matter Annihilation.’ The goal: To trap a record one billion antiprotons at CERN’s Extra Low ENergy Antiproton (ELENA) facility, a deceleration ring that works with CERN’s Antiproton Decelerator to slow antiprotons, reducing their energy by a factor of 50, from 5.3 MeV to just 0.1 MeV. ELENA should allow the number of antiprotons trapped to be increased by a factor of 10 to 100, a major increase in efficiency.
Image: The ELENA ring prior to the start of first beam in 2016. Credit: CERN.
The PUMA project aims to keep the antiprotons in storage for several weeks, allowing them to be loaded into a van and moved to a nearby ion-beam facility called ISOLDE (Isotope mass Separator On-Line), where they will be collided with radioactive ions as a way of examining exotic nuclear phenomena. The nature of the investigations is interesting — CERN has two experiments underway to study the effects of gravity on antimatter, for example — but it’s the issue of storage that draws my attention. How will CERN manage the feat?
This update from CERN lays out the essentials:
To trap the antiprotons for long enough for them to be transported and used at ISOLDE, PUMA plans to use a 70-cm-long “double-zone” trap inside a one-tonne superconducting solenoid magnet and keep it under an extremely high vacuum (10-17 mbar) and at cryogenic temperature (4 K). The so-called storage zone of the trap will confine the antiprotons, while the second zone will host collisions between the antiprotons and radioactive nuclei that are produced at ISOLDE but decay too rapidly to be transported and studied elsewhere.
Thus ELENA produces the antiprotons, while ISOLDE supplies the short-lived nuclei that CERN scientists intend to study, looking for new quantum phenomena that may emerge in the interactions between antiprotons and the nuclei. I’m taken with how Alexandre Obertelli (Darmstadt Technical University), who leads this work, describes it. “This project,” says the physicist, “might lead to the democratisation of the use of antimatter.” A striking concept, drawing on the fact that antimatter will be transported between two facilities.
Antiprotons traveling aboard a van to a separate site are welcome news. In today’s world, low-energy antiprotons are only being produced at CERN, but we’re improving our storage in ways that may make antimatter experimentation in other venues more practical. Bear in mind, too, that an experiment called BASE (Baryon Antibaryon Symmetry Experiment), also at CERN, has already proven that antiprotons can be kept in a storage reservoir for over a year.
Image: A potential future use for trapped antimatter. Here, a cloud of anti-hydrogen drifts towards a uranium-infused sail. Credit: Hbar Technologies, LLC/Elizabeth Lagana.
We’re a long way from propulsion, here, but I always point to the work of Gerald Jackson and Steve Howe (Hbar Technologies), who attack the problem from the other end. With antimatter scarce, how can we find ways to use it as a spark plug rather than a fuel, an idea the duo have explored in work for NASA’s Institute for Advanced Concepts. Here, milligrams of antimatter are released from a spacecraft onto a uranium-enriched five-meter sail. For all its challenges, antimatter’s promise is such that innovative concepts like these will continue to evolve. Have a look at Antimatter and the Sail for one of a number of my discussions of this concept.
An antimatter probe to a nearby star? The idea holds enormous appeal, given the colossal energies obtained when normal matter annihilates in contact with its antimatter equivalent. But as we’ve seen through the years on Centauri Dreams, such energies are all but impossible to engineer. Antimatter production is infinitesimal, the by-product of accelerators designed with a much different agenda. Moreover, antimatter storage is hellishly difficult, so that maintaining large quantities in a stable condition requires multiple breakthroughs.
All of which is why I became interested in the work Gerald Jackson and Steve Howe were doing at Hbar Technologies. Howe, in fact, became a key source when I put together the original book from which this site grew. This was back in 2002-2003, and I was captivated with the idea of what could be called an ‘antimatter sail.’ The idea, now part of a new Kickstarter campaign being launched by Jackson and Howe, is to work with mere milligrams of antimatter, allowing antiprotons to be released from the spacecraft into a uranium-enriched, five-meter sail.
Reacting with the uranium, the antimatter produces fission fragments that create what could be considered a nuclear-stimulated ablation blowing off the carbon-fiber sail. As to the reaction itself, Jackson and Howe would use a sheet of depleted uranium U-238 with a carbon coating on its back side. Here’s how the result is described in the Kickstarter material now online:
When antiprotons… drift onto the front surface, their negative electrical charge allows them to act like an orbiting electron, but with different quantum numbers that allow the antiprotons to cascade down into the ground orbital state. At this point it annihilates with a proton or neutron in the nucleus. This annihilation event causes the depleted uranium nucleus to fission with a probability approaching 100%, most of the time yielding two back-to-back fission daughters.
Now we get into a serious kick for the spacecraft:
A fission daughter travelling away from the sail at a kinetic energy of 1 MeV/amu has a speed of approximately 13,800 km/sec, or 4.6% of the speed of light. The other fission daughter is absorbed by the sail, depositing its momentum into the sail and causing the sail (and the rest of the ship) to accelerate.
The concept relies, as Jackson said in a recent email, on using antimatter as a spark plug rather than as a fuel, converting the energy from proton-antiproton annihilations into propulsion.
Image: The original antimatter probe concept. Credit: Gerald Jackson/Hbar Technologies.
The current work grows out of a 2002 grant from NASA’s Institute for Advanced Concepts but the plan is to develop the idea far beyond the Kuiper Belt mission Jackson and Howe initially envisioned. Going interstellar would take not milligrams but tens of grams of antimatter, far beyond today’s infinitesimal production levels. In fact, while the Fermi National Accelerator laboratory has been able to produce no more than 2 nanograms of antimatter per year, even that is high compared to CERN’s output (the only current source), which is 100 times smaller.
Even so, interest in antimatter remains high because of its specific energy — two orders of magnitude larger than fusion and ten orders of magnitude larger than chemical reactions — making further research highly desirable. If the fission reaction the antimatter produces within the sail is viable, we will be able to demonstrate a way to harness those energies, with implications for deep space exploration and the possibility of interstellar journeys.
The original NIAC work led to a sail 5-meters in diameter, with a 15-micron thick carbon layer and a uranium coating 293 microns thick. Interestingly, the study showed that the sail had sufficient area to remove any need for active cooling of the surface. Indeed, the steady-state temperature of the sail would be 570? Celsius, below the melting point of uranium.
Image: A cloud of anti-hydrogen drifts towards the uranium-infused sail. CREDIT: Hbar Technologies, LLC/Elizabeth Lagana.
The work was based around a 10 kg instrument payload to be delivered to 250 AU within 10 years. Turning to interstellar possibilities, Breakthrough Starshot has been talking about reaching 20 percent of lightspeed with a beamed laser array pushing small sails. Jackson and Howe now seek roughly 5 percent of c, making for a mission of less than a century to reach Proxima Centauri, where we already know an interesting planet awaits.
But here’s a significant difference: Unlike Breakthrough Starshot’s flyby assumptions, the antimatter sail mission concept is built around decelerating and attaining orbit around the target star. In the absence of magsail braking against Proxima’s stellar wind, this would presumably also involve antimatter, braking with the same methods to allow for long-term scientific investigation, thus avoiding the observational challenges of a probe pushing past a small and probably tidally-locked planet at 20 percent of lightspeed.
Here’s how Jackson describes deceleration in his recent email:
Our project considers deceleration and orbit about the destination star a mission requirement. There are serious implications for spacecraft velocity when the requirement of deceleration at the destination is imposed. Either drag or some other mechanism needs to be invoked at the destination, or enough extra fuel must be accelerated in order to accomplish a comparable deceleration. Because the rocket equation equates probe velocity with mass utilization, a staged spacecraft architecture is envisioned wherein a more massive booster accelerates the spacecraft and a smaller second stage decelerates into the destination solar system.
The discovery of Proxima b, that interesting planet evidently in the habitable zone around the nearest star, continues to energize the interstellar community. The Kickstarter campaign, just underway and with a goal of $200,000, hopes to upgrade earlier antimatter sail ideas into the interstellar realm. Tomorrow I want to say a few more things about the antimatter sail and the issues the Kickstarter campaign will address as it expands the original work.
Talking about antimatter, as we’ve done in the past two posts, leads to the question of why the stuff is so hard to find. When we make it on Earth, we do so by smashing protons into targets inside particle accelerators of the kind found at the Fermi National Accelerator Laboratory in Batavia, IL and CERN (the European Organization for Nuclear Research). It’s not exactly an efficient process from the antimatter production standpoint, as it produces a zoo of particles, anti-particles, x-rays and gamma rays, but it does give us enough antimatter to study.
But there is another way to find antimatter, for it occurs naturally in the outer Solar System and even closer to home. James Bickford (Draper Laboratory, Cambridge MA) has looked at how we might trap antimatter that occurs in the Earth’s radiation belts. In a report for NIAC back in 2006 (available here), Bickford laid out a strategy for using high temperature superconductors to form two pairs of RF coils with a radius of 100 meters, to be powered by nuclear or solar power. The idea is that the magnetic field created through the RF coils will concentrate and trap the incoming antiproton stream.
Now the model changes from production on Earth to harvesting natural antimatter in space. We get antimatter in the Solar System because high-energy galactic cosmic rays (GCR) bombard the upper atmosphere of the planets, causing ‘pair production,’ which is the creation of an elementary particle and its antiparticle. The kinetic energy of the cosmic ray particle is converted into mass when it collides with another particle. According to Bickford’s calculations, about a kilogram of antiprotons enter our Solar System every second, and any planet with a strong magnetic field is fair game for collection.
As the planet’s magnetic field holds the antimatter particles, they spiral along the magnetic field lines. This is a process that continually replenishes itself both for matter and antimatter. Jupiter is a source, but Saturn is even better, for a larger flux enters its atmosphere. Saturn is, in fact, the place where the largest total supply of antiprotons appears, with reactions in its rings injecting 250 micrograms per year into the planet’s magnetosphere. But we can start with the Earth, for the antimatter production process was confirmed here in 2011.
These results came from the PAMELA (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) satellite, a joint mission among scientists from Italy, Germany, Russia, and Sweden (see Antimatter Source Near the Earth). The most abundant source of antiprotons near us is found to be in a thin belt that extends from a few hundred to about 2000 kilometers above Earth, moving along Earth’s magnetic field lines and bouncing between the north and south magnetic poles.
Image: An antimatter reservoir near our planet in the form of a belt of antiprotons that lies within the innermost portion (pink) of Earth’s magnetosphere, the large bubble-like region interior to the blue arc that is controlled by the planet’s magnetic field. Credit: Aaron Kaase/NASA GSFC.
Compared to harvesting antimatter on Earth, space harvesting is five orders of magnitude more cost effective, and Bickford’s report suggests we could be collecting 25 nanograms of antimatter per day near our planet. And here’s a spectacular mission concept that can grow out of this, also drawn from the Bickford report:
The baseline concept of operations calls for a magnetic scoop to be placed in a low-inclination orbit, which cuts through the heart of the inner radiation belt where most antiprotons are trapped. Placing the vehicle in an orbit with an apogee of 3500 km and a perigee of 1500 km will enable it to intersect nearly the entire flux of the Earth’s antiproton belt. The baseline mission calls for a fraction of the total supply to be trapped over a period of days to weeks and then used to propel the vehicle to Saturn or other solar system body where there is a more plentiful supply. The vehicle then fully fills its antiproton trap and propels itself on a mission outside of our solar system.
We can imagine fuel depots in the Solar System that could support our growing infrastructure with missions to Mars and the asteroids. There is even the possibility, tantalizingly referenced in the report, of using the galactic cosmic ray flux enroute to a destination to further bulk up the fuel supply. It’s bracing stuff, and a reminder that when we talk about gathering antimatter for a mission, we aren’t necessarily limited to the sparse production from today’s colliders.
But back to the original question. Why is antimatter so hard to find? If it is truly ‘mirror matter,’ as the title of Robert Forward’s book suggests, shouldn’t there be equal amounts of it, and shouldn’t that equality have prevailed from the beginning of the universe? It seems logical to think so, but of course if that had occurred, we would not be here to contemplate the problem.
Now we’re entering the realm of charge-parity (CP) symmetry, which asserts that physics should be unchanged if we plug in antiparticles where particles currently are. Most particle interactions show this charge-parity symmetry to hold, and it carries the implication that the universe should have begun with equal amounts of matter and antimatter. Why and where CP symmetry does fail is a serious question, one that has us looking for any observable violation of the principle.
We have no definitive answer, but we do have interesting results from the T2K experiment in Japan, as reported in New Scientist following their discussion at Neutrino 2016 (the XXVII International Conference on Neutrino Physics and Astrophysics), held in London in early July. The researchers at T2K have been monitoring the oscillations that occur when neutrinos spontaneously change ‘flavors’, from electron to muon to tau. Neutrinos as well as antineutrinos each come in these three types, and all three types can undergo such oscillations.
Image: The inside of the Super-Kamiokande detector in Japan. Credit: T2K.
Observing that 32 muon neutrinos that traveled between the J-PARC accelerator in Tokai to the Super-Kamiokande neutrino detector in Kamioka had turned into electron neutrinos, the team ran the same experiment with muon antineutrinos. Charge-parity symmetry says that the rates of change should be the same, but the researchers report just four muon antineutrinos have changed into the anti-electron neutrino. The numbers are small but the possible violation of CP symmetry is provocative. Results from NoVA, a similar experiment sending neutrinos between Illinois and Minnesota, are showing roughly similar values for apparent CP violation.
More data are needed to reach any firm conclusions, but these results point to the direction of future work at both installations. Some process that violates CP symmetry has to be in place to explain the overwhelming difference between the amount of matter and antimatter in the universe. Thus we can expect any results showing deviations from this symmetry will make news. Meanwhile, from a propulsion standpoint, we have to reckon with the paucity of antimatter by imagining creative ways of creating or finding enough to use in our future experiments. Space-based antimatter harvesting may prove to be the most cost effective way to proceed.
I’ll close by quoting James Bickford in a 2014 interview, where I think he strikes just the right note about the need for small scale experiments as well as avoiding antimatter hype:
For the most part, propelling spacecraft to near the speed of light with antimatter lives in the realm of Star Trek. The technical obstacles are non-trivial and probably won’t be solved in the near future, if ever. From this perspective, the potential for antimatter probably has been overhyped. However, the small scale experiments are just the first baby steps that could help us down the long path. More importantly, research and development in this area is part of a broader framework that could help fundamental science and our understanding of the universe. Antimatter plays a central role in some of the Holy Grail problems of physics, such as the nature of dark matter and why matter dominates over antimatter.