by Paul Gilster | Aug 20, 2008 | Antimatter |
Just how hard would it be to build a true interstellar craft? I’m not talking about a spacecraft that might, in tens of thousands of years, drift past a star by happenstance, but about a true, dedicated interstellar mission. Those of you who’ve been following my bet with Tibor Pacher on Long Bets (now active, with terms available for scrutiny on the site) know that I think such a mission will happen, but not any time soon. And the proceedings of the Joint Propulsion Conference, held last month in Hartford, go a long way toward explaining why the problem is so difficult.
Wired looked at the conference results in a just published article, the most interesting part of which contained Robert Frisbee’s speculations about antimatter rocketry. Two things have been clear about antimatter for a long time. The first is that producing sufficient antimatter is a problem in and of itself, one that may keep us working with tiny amounts of the stuff for some time to come. Even so, interesting mission concepts, like Steve Howe’s antimatter-energized sail, have grown out of the studies that have been performed on possible hybrid systems.
As to antimatter itself, while the annihilation of matter and antimatter releases vast amounts of energy, controlling the result is even more difficult than producing antimatter in quantity in the first place. Proton/anti-proton annihilation is preferable to electron/positron because the gamma rays produced by the latter can’t be directed to produce thrust, a problem Eugen Sänger wrestled with fifty years ago. But the former is a possibility because the reaction products (pions) can be directed and confined electromagnetically. The idea here is to transfer some of that vast energy of annihilation to a propellant working liquid.
Even so, our rocket still has problems. Check our friend Adam Crowl’s recent piece on antimatter for several good links and some musing on the relatively poorer performance with antimatter than one might have expected (an exhaust velocity of 0.33 c may itself be a surprise, but take a look at this Frisbee presentation). Frisbee (NASA, Jet Propulsion Laboratory) has been studying the interstellar conundrum for a long time, with particular attention to antimatter. The design he presented at the conference, a stack of linked components designed to keep radiation away from crew or payload, is summed up by Wired this way:
At the rocket end, a large superconducting magnet would direct the stream of particles created by annihilating hydrogen and antihydrogen. A regular nozzle could not be used, even if made of exotic materials, because it could not withstand exposure to the high-energy particles… A heavy shield would protect the rest of the ship from the radiation produced by the reaction.
A large radiator would be placed next in line to dissipate all the heat produced by the engine, followed by the storage compartments for the hydrogen and antihydrogen. Because antihydrogen would be annihilated if it touched the walls of any vessel, Frisbee’s design stores the two components as ice at one degree above absolute zero.
So far, so good. We then include basic spacecraft systems in front of the tanks of propellant and then our payload. But theory meets a grim reality in the numbers: Frisbee is talking about an 80 million metric ton starship (the Space Shuttle weighs in at 2,000 metric tons), with another 40 million metric tons each of hydrogen and antihydrogen. The payoff is a forty year mission to Alpha Centauri.
At least it’s designed as a rendezvous mission. A forty year flyby to the Centauri stars would be moving at something better than a tenth of lightspeed once it gets up to cruise. Even if exquisitely targeted, such a probe would operate within 1 AU of the target system (let’s say Centauri B) for something less than three hours. Ponder the challenge presented by collecting imagery and data from Centauri planets in such a scenario.
What to do? These results reinforce much that we already knew about the difficulty of coming up with an interstellar mission design that is remotely affordable, and everything comes down to energy. As noted by Wired, interstellar theorist Brice Cassenti (Rensselaer Polytechnic Institute) comes up with a minimum value of the current energy output of the entire world to send a probe to the Centauri system, a figure Cassenti is quick to note could easily swell to 100 times that value.
It’s useful to ponder the size of the challenge as we continue to scout for concepts that can overcome these problems. The dual track that interstellar studies takes continues to work this way: 1) Push concepts constructed under the parameters of known physics to their utmost, to see where they might lead. Antimatter rockets, laser sails, pulsed fusion and their ilk all fall under this category. 2) Investigate potential concepts that might extend our knowledge of known physics. Here we turn to studies like those sponsored by, among others, NASA’s now defunct Breakthrough Propulsion Physics project. The Tau Zero Foundation hopes to bring philanthropic support to both approaches.
No one can say whether interstellar missions will ever be feasible. What we can insist is that studying physics from the standpoint of propulsion science may tell us a great deal about how the universe works, whether or not we ever find ways of extracting propulsive effects from such futuristic means as dark matter or dark energy. And if it turns out that our breakthroughs fail to materialize, the potential of multi-generational missions supported by human crews still exists. They will be almost inconceivably demanding, but nothing in known physics says that a thousand-year mission to Centauri is beyond the reach of human technology within a future we can still recognize.
How big would an interstellar mission be? Let me close by quoting Robert Frisbee himself, from a presentation he gave at the 2003 iteration of the Joint Propulsion Conference:
In the long term, it will represent a Solar System civilization’s defining accomplishment in much the same way we look to the past accomplishments of humanity, like the Pyramids, Stonehenge, the great medieval Cathedrals of Europe, the Great Wall of China and, not so long ago, a space program called Apollo.
by Paul Gilster | Jan 24, 2008 | Antimatter |
Those gamma rays coming out of galactic center, flagging the presence of an antimatter cloud of enormous extent, have spawned few explanations more exotic than the one we consider today: Black holes. Primordial black holes, in fact, produced in their trillions at the time of the Big Bang and left evaporating through so-called ‘Hawking radiation’ ever since. That’s the theory of Cosimo Bambi (Wayne State University) and colleagues, who are studying the same antimatter cloud we recently examined here in terms of its possible connection with low mass X-ray binary stars.
Hawking radiation offers a mechanism for small black holes to lose mass over time. But since the phenomenon has never been observed, the upcoming launch of the GLAST (Gamma-ray Large Area Space Telescope) satellite again looms large in significance. GLAST should be able to find evaporating black holes, assuming they are there, and there is even some possibility that the Pierre Auger Observatory may eventually detect tiny black holes created when high-energy cosmic rays slam into the upper atmosphere. If so, we would have a window into any evaporative effects associated with these enigmatic events.
But assuming that black holes do evaporate, the trick is to figure out how fast, and that rate depends upon mass, with more massive black holes producing fewer evaporated particles. What Bambi’s team argues is that a mass of about 1016 grams, roughly that of a fairly common asteroid, will produce the right amount of antimatter to explain the detections. Theoretically, the signature radiation from black holes of this particular size should be observable given the right equipment, but neither the GLAST mission or ESA’s INTEGRAL satellite seems well suited for that task (more on the latter problem in this New Scientist story).
All of which is interesting it itself, but the paper offers a bonus:
We have considered evaporating primordial BHs [black holes], as a possible source of positrons to generate the observed photon 511 keV line from the Galactic Bulge. The analysis of the accompanying continuous photon background produced, in particular, by the same evaporating BHs, allows to ?x the mass of the evaporating BHs near 1016 g. It is interesting that the necessary amount of BHs could be of the same order of magnitude as the amount of dark matter in the Galactic Bulge. This opens a possibility that such primordial BHs may form all cosmological dark matter. The background MeV photons created by these primordial BHs can be registered in the near future, while the neutrino ?ux may be still beyond observation. The signi?cance of this model would be difficult to overestimate, because these BHs would present a unique link connecting early universe and particle physics.
So there’s a theory for you: Primordial black holes as the explanation for dark matter itself. But bear in mind that along with the x-ray binaries so recently considered in relation to galactic antimatter, other explanations are still in play, including type Ia supernovae and a host of far more exotic possibilities outlined in the introduction to the paper. GLAST should help, but the suspicion grows that the antimatter cloud at galactic center may remain enigmatic for some time to come.
The paper is Bambi et al., “Primordial black holes and the observed Galactic 511 keV line,” available online.
by Paul Gilster | Jan 11, 2008 | Antimatter |
Although I had planned to push straight on to look at instrumentation for a true interstellar mission (using Mike Gruntman’s landmark paper on the topic), I want to revise that schedule because of the recently announced antimatter news. We’ll return to the instrumentation issue on Monday, including the tricky question of how a probe designed to reach 400 AU can make effective measurements given its speed (75 km/s in the best case scenario Gruntman looks at). Because that question just gets trickier as speeds ramp up, it’s a major one for planning.
But on to antimatter, a cloud of which has been known to exist around the galactic center since the 1970s, when balloon-based gamma-ray detectors first located it. Gamma rays are significant in terms of antimatter because electrons encountering positrons (their antimatter equivalent) annihilate each other, with their mass converted into high energy gamma rays. So the cloud’s presence is well established. The question since its detection is what could have caused it.
Now a new paper in Nature may offer an answer, noting the asymmetric distribution of the antimatter cloud, which extends further on one side of galactic center than on the other. We’re talking about a cloud some 10,000 light years across, generating the energy of 10,000 Suns. The research team used data from the European Space Agency’s Integral satellite (INTErnational Gamma-Ray Astrophysics Laboratory) to detect the asymmetry. Their paper notes that it matches the distribution of a certain type of binary star systems, the latter thought to contain neutron stars and black holes.
Image: Integral mapped the glow of 511 keV gamma rays from electron-positron annihilation. The map shows the whole sky, with the galactic center in the middle. The emission extends to the right. Credit: ESA/Integral/MPE/G. Weidenspointner.
Are these binary stars the cause of the antimatter cloud? They’re what’s known as ‘hard’ low-mass X-ray binaries. The mechanism at play is that gas from a low-mass star spirals into a black hole or neutron star nearby, with high-energy (hard) X-rays resulting. That and the relative similarity between the distributions of cloud and stars makes the case that the binaries are producing these interesting positrons. In fact, says lead author Georg Weidenspointer (Max Planck Institute for Extraterrestrial Physics), “Simple estimates suggest that about half and possibly all the antimatter is coming from X-ray binaries.”
Of course, what comes immediately to mind at this end is James Bickford’s interesting work on antimatter collection here in the Solar System. As we saw in several earlier posts, Bickford has been advocating collection strategies that would mine the antimatter being formed naturally not only near the Earth but also in abundance further out in the Solar System, especially around Saturn. So the idea of antimatter farming again comes to the front with this renewed reminder that the exotic stuff occurs as a result of astrophysical processes and not just in particle accelerators.
Not that we’re able to tap a cloud like this one, so vast and so much further from Earth. But on a theoretical level, it’s useful to learn more about antimatter production even while we’re discovering the limitations in our existing theories. For the questions the antimatter cloud poses are themselves vast. The low-mass binaries seem associated with the antimatter cloud but we lack knowledge of how they could produce enough positrons to account for it. That probably targets particle jets as the necessary area for investigation, something NASA’s GLAST (Gamma-ray Large Area Space Telescope) may be able to shed further light on. And GLAST is helpfully ready for a 2008 launch.
The paper is Weidenspointner et al., “An asymmetric distribution of positrons in the Galactic disk revealed by big gamma-rays,” Nature 451 (10 January 2008), pp. 159-162 (abstract).
by Paul Gilster | Nov 8, 2007 | Antimatter |
Robert Forward used to talk about antimatter factories in space, installations that would draw their power from the Sun. He would point out that at a distance of 1 AU, our star delivers a gigawatt of energy for each square kilometer of collector. And being Robert Forward, he thought big: Build a collector array one hundred kilometers on a side to produce a power input of ten terawatts, enough to drive several antimatter factories at full power and produce a gram of antimatter each day.
Forward saw the antimatter problem as a matter of scaling and cost (and he often talked about ‘small problems of engineering’). As we’ve seen in the last few days, James Bickford (Draper Laboratory) is more than aware of both these issues, but unlike Forward, he’s keen on mining naturally occurring sources of antimatter right here in the Solar System. Forward’s huge factories may some day be built, but for now, let’s talk about how to get our early antimatter missions into the realm of possibility by learning how to cull the needed antimatter, if only in the minute quantities that would kick a payload up to 100 kilometers per second. Along the way, we establish the expertise to go further.
What Bickford has in mind is using a plasma magnet to create a magnetic scoop that can influence the trajectory of a charged particle over large distances. In an equatorial orbit around Earth, such a scoop could trap antiparticles occurring in the planet’s radiation belt as they, obeying the Lorentz force, bounce back and forth between their mirror points in the Northern and Southern hemispheres. The antiproton belt our spacecraft is tapping is analogous to the protons in the Van Allen radiation belt, and provides a close to home source of antimatter before we look out into the Solar System, where we find further stores especially around Saturn.
We’re still talking about large structures to collect this material, but nothing like Forward’s huge collector arrays (or, for that matter, his vast Fresnel lens in the outer Solar System that would enable a laser-beamed lightsail mission). In his Phase II NIAC report, available here on Centauri Dreams, Bickford talks about using high temperature superconductors to form two pairs of RF coils with a radius of 100 meters and a weight of some 7000 kilograms combined. 200 kW is needed to operate the system, achievable through nuclear or solar power. What happens is straightforward: The magnetic field driven by the RF coils concentrates the incoming antiprotons and then traps them.
Comparing this concept with what we do today, Bickford finds that space harvesting of antimatter is five orders of magnitude more cost effective than the Earth-based alternative. Working out Earth’s antiproton flux in relation to this scheme, he talks about collecting 25 nanograms per day, with up to 110 nanograms stored in the central region between the coils. And note: If you’ve looked at Bickford’s earlier Phase I report, be aware that his plasma magnet supplants that report’s single coil loop. The Phase II paper goes through all of the options that led to this result.
And here’s a spectacular mission concept from the 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’re talking realistic missions into the Kuiper Belt and perhaps to the Sun’s gravitational focus, where so many interesting things happen to enhance the image of objects examined from there (bear in mind that the Sun’s gravity focus, unlike an optical lens, allows incoming radiation to stay on the focal axis at distances greater than 550 AU — just get to that distance and a new category of observation becomes possible). In any case, missions of greater speed and with even more ambitious targets are possible. As Bickford notes: “Future enhanced systems would be able to collect from the GCR [galactic cosmic ray] flux en route to further supplement the fuel supply.”
For that matter, how about creating an antimatter ‘fuel depot’ that could be used to support closer-in missions to Mars and the asteroids? All seem feasible provided the collection system can be made to function. But the idea is compelling: Bickford says the plasma magnet influences the trajectory of charged particles near the spacecraft, which then follow the field lines and are concentrated in the throat of the collection system. The momentum of the particles is then braked by RF and/or electrostatic methods so that the antimatter becomes trapped in the inner coil region. Long-term storage seems possible given the paucity of protons in the region between the coils, hence little loss by annihilation.
We’ll talk again soon about the technologies needed to make this come together, and discuss Bickford’s ideas on future research possibilities. As happens so often in examining space technologies, we can sketch out the shape of deeply promising concepts that rely on the development of a true space-based infrastructure to come to fruition. The major point remains: Building that infrastructure will open up possibilities denied to us now, among them the opportunity to experiment with propulsion methods that may one day power true interstellar missions.
by Paul Gilster | Nov 7, 2007 | Antimatter |
James Bickford’s antimatter work for NASA’s Institute for Advanced Concepts, a Phase II study completed just as NIAC was announcing its closure, prompted a number of comments from readers when I opened discussion of it on Monday. And I can see why. We’re used to thinking of antimatter production as an extraordinarily expensive process happening only in particle accelerators. And even when we commit the resources to make it, we get only the tiniest amounts, and at costs so high that they make propulsion concepts for antimatter seem chimerical.
But Bickford wants us to consider a naturally occurring source of antimatter, one that might offer the potential of being collected in space for a variety of missions. Key to the idea is the fact that high-energy galactic cosmic rays (GCR) continually bombard the upper atmosphere of the planets in our Solar System, as well as interacting with material in the interstellar medium. The result is ‘pair production,’ the creation of an elementary particle and its antiparticle. The kinetic energy of the GCR particle is converted into mass when it collides with another particle.
When Bickford analyzed this natural source of antimatter, he realized that about a kilogram of antiprotons enters our Solar System every second, but only a few grams reach the vicinity of the Earth in a year. That would seem to make collecting naturally produced antimatter impossible save for one thing. Planets with strong magnetic fields create properties in nearby space that can create much larger fluxes as the particles interact with both the magnetic field and the atmosphere.
Here’s Bickford on the process near the Earth:
In comparison to artificial production, natural sources of antimatter are plentiful and relatively easy to exploit for benefit. A natural antiproton radiation belt is generated in a manner analogous to the traditional Van Allen radiation belts, which surround the Earth. The high-energy portion (E>30 MeV) of the proton belt is primarily formed by the decay of neutrons in the Earth’s magnetosphere. The GCR flux interacts with the planet’s upper atmosphere to release free neutrons with a half-life of just over 10 minutes. A fraction of these neutrons travel back into space (albedo) and decay into a proton, electron, and an anti-neutrino while still within the influence of the magnetosphere.
What happens next? The magnetic field of the planet forms a ‘bottle’ that can hold the protons and electrons formed during the decay process. The Lorentz force causes the particles to spiral along the magnetic field lines in a slow drift around the planet. And the process is self-replenishing: As particles are lost through diffusion, new ones are generated so that the supply is relatively static. Bickford sees the same processes at work for the antiparticles produced from pair production. Quoting the report:
The interaction of cosmic radiation with the upper atmosphere also produces antiparticles from pair production… The produced antineutrons follow a trajectory primarily along the path of the original cosmic ray, but can be backscattered after interacting with the atmosphere. These albedo antineutrons decay in a manner similar to the regular neutrons. However, the antineutron will decay into a positron, antiproton, and neutrino and therefore acts as a source for the antiparticle radiation belts surrounding the Earth. The physics that govern the trapping and motion are identical between the particle and its antiparticle with the exception that the two will spiral and drift in opposing directions due to their opposing electric charges.
Working through the possibilities in the Solar System, Bickford examined their capabilities at generating antimatter. And here’s where the surprise comes in. You would think that Jupiter would be the gold mine of antiproton production, given its size and field strength, but it turns out that the magnetic field actually shields the Jovian atmosphere from the GCR production process, lowering its effect. If you want better antiproton production, go to Saturn, where a larger flux reaches the atmosphere and, as Bickford notes, “…the antineutrons that are copiously produced in the rings do not have to be backscattered to yield stable trapping.”
But here’s another surprise: “…the highest flux is actually found around Earth where the relatively slow radial transport in the magnetosphere produces long residence times, which allow the antiproton trap to fill over a period of years.” So Earth’s radiation belts give us our most intense localized source of antiprotons, while the greatest total supply of antiprotons is the magnetosphere of Saturn. In fact, reactions in Saturn’s rings inject almost 250 micrograms per year into the planet’s magnetosphere.
Image: Antiparticle generation in Saturn’s rings. Will the giant planet turn out to be a source for the antimatter that can drive future space missions? Credit: James Bickford.
With this in mind, we need to look at collection options. How do we get at this stuff and store it for future use? Tomorrow we’ll take a look at a baseline concept that Bickford thinks could extract large quantities of these trapped antiparticles from space. And we’ll check out mission designs that might flow out of these proposals, as well as the technologies necessary to make them happen.
If Bickford is right, available antiproton supplies in our Solar System are sufficient to power the nanogram to microgram-level missions we discussed earlier, such as the Antimatter Catalyzed Microfission/Fusion (ACMF) mission designed at Penn State and the ‘antimatter sail’ concept Steven Howe developed for NIAC. And that’s a fine start for true interstellar precursor missions, one that gets us into the antimatter game in a serious way and points to better (and faster) things to follow.
James Bickford’s Phase II report is titled “Extraction of Antiparticles Concentrated in Planetary Magnetic Fields.” Although produced for NIAC, it is not yet available on the Institute’s server. E-mail me if you are interested in seeing a copy.
by Paul Gilster | Nov 5, 2007 | Antimatter |
Great ideas fan out in unexpected directions, which is why James Bickford now looks at antimatter in a new light. Bickford (Draper Laboratory, Cambridge MA) realized that an adaptation of Robert Bussard’s interstellar ramscoop might have its uses in collecting antimatter. The concept grew out of the realization that antimatter sources were available not only near the Earth but farther out in the Solar System, where antiparticles could be collected and used to boost spacecraft initially to speeds of 100 kilometers per second. That’s sufficient for interstellar precursor missions outside the heliosphere, including the possibility of getting a payload to the Sun’s gravitational focus, where a new kind of space-based astronomy waits to be exploited.
Refine the process enough and you start talking about even greater speeds through more efficient antimatter collection, one great benefit being that instead of producing the stuff in Earth-bound particle accelerators, you’re actually mining natural supplies. Bickford was kind enough to pass along his complete final report for NASA’s Institute for Advanced Concepts (NIAC), one of the last projects funded by that agency as it encountered the kind of budgetary crises familiar to deep space researchers and closed. It’s a fascinating document that I want to discuss over several days this week (though perhaps not consecutively, because we’re about to get some interesting exoplanet news).
Before we get into collecting it, ponder the beauties of antimatter itself. The annihilation of a particle with its antiparticle liberates the entire rest mass of each as energy. Indeed, the process is so spectacular — ten orders of magnitude greater than chemical reactions, and between 102 to 103 more efficient than nuclear — that antiprotons on the order of tens of nanograms might be sufficient to reach the 100 km/sec velocities mentioned above. Clearly, larger quantities of antimatter expand the options enormously, offering higher speeds still up to the relativistic velocities needed for interstellar missions.
Bickford’s numbers on antimatter’s potential further drive the point home. The annihilation of a single kilogram of antimatter releases the energy equivalent of thirty million barrels of oil. If you work out worldwide energy production per year in these terms, you find that the total is equivalent to 2200 kg of antimatter. Contrasting sharply with antimatter’s potential, however, is the price. Using the methods available today, which rely on extracting antimatter from sub-atomic collision debris in accelerators, the worldwide output is in the low nanogram per year range. The cost: an estimated $100 trillion per gram, give or take a few megabucks.
That’s one reason that antimatter propulsion concepts have taken a sharp turn toward the realistic after heady earlier speculations. Antimatter Catalyzed Microfission/Fusion, which uses antiprotons to trigger a efficient form of nuclear fission, has been extensively studied at Pennsylvania State University, showing potential for interplanetary missions (Mars becomes reachable in about 45 days). And ACMF is, as it has to be, stingy with the antimatter, requiring only nanogram quantities. Steven Howe examined a variant of this approach in an earlier NIAC study, while Gerald Jackson produced a NIAC study on harvesting antimatter that would factor into Bickford’s later analysis.
Can we think about getting antimatter up to the microgram level? With ten micrograms of antiprotons, we can envision a 100-ton payload on a one-year round-trip mission to Jupiter. But how do we go about producing antimatter at this level, and where is the best place to produce it? In my next post on Bickford’s work, I want to contrast current production methods with the antimatter ‘harvesting’ option and explain why our own Solar System may serve as a renewable source of the fuel we need once we can build the infrastructure necessary to collect and deploy it. We’ll talk about these and other issues soon, and also discuss what technologies will have to reach an appropriate readiness level before we can put an efficient antimatter collector into operation.
