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.
The Centauri Dreams blog is where I first learned that the rings of Saturn were antimatter production “factories”, so that may be one place to harvest them:
https://centauri-dreams.org/?p=22934
Much closer there is antimatter in Earth’s Van Allen radiation belts:
https://centauri-dreams.org/?p=36068
The main reason so little antimatter is produced on Earth and costs so insanely much is that those who make the particles are not doing so for commercial purposes. Otherwise we could make more and for much less. As with most things, once an idea becomes viable, the money will follow. Please note I said most.
The way this is described is that each anti-hydrogen fissions 1 uranium atom. Doesn’t that mean that the device has to generate as many anti-hydrogen atoms as uranium atoms in the sail? What is the energy needed to do that and what would be the mass of the device needed to generate these anti-atoms?
Is the carbon layer sufficient to absorb the fragment that cancels the thrust? If so, that seems to suggest that the carbon rods used in reactors were unnecessarily thick. Is that the case?
Finally, is this a better way to generate thrust than a fission fragment rocket? What are the benefits of a sail by comparison?
Anti-hydrogen is generated on the ground, the ship only stores it. Currently at CERN it takes around 1 billion times more energy to produce antimatter than the energy contained on it.
Control rods in nuclear reactors are used to absorb neutrons, not nuclei, and they usually are not made of carbon but boron, silver, indium, cadmium, cobalt, high-boron steel, etc.
Probably not, but this design is much simpler.
Carbon is used to slow neutrons in some reactors (around 20% of reactors). The rest of reactors use (regular or heavy) water.
On the nuclear fusion front, MIT “set a new world record for plasma pressure in the Institute’s Alcator C-Mod tokamak nuclear fusion reactor. Plasma pressure is the key ingredient to producing energy from nuclear fusion, and MIT’s new result achieves over 2 atmospheres of pressure for the first time.”
The details are here:
http://www.nextbigfuture.com/2016/10/mit-set-nuclear-fusion-plasma-record.html
No, it’s not “the key ingredient to producing energy from nuclear fusion.” Three ingredients are needed: density (not pressure, that usually is constant for a given reactor design), temperature, and time. You need the product of the three to be high in order to produce energy from fusion (search for “triple product” on Wikipedia). It’s easy to increase one of them providing you decrease the other two.
Also, Alcator C-Mod will probably be shut down this year, since the DOE will stop funding it: https://www.scientificamerican.com/article/flagship-u-s-fusion-reactor-breaks-down/
The primary advantage of this concept is the low mass of the system to convert the released energy to directed thrust.The available energy per unit of fuel is similar to a conventional fission system. This system will not capture any energy or thrust from the neutrons which will pass through the sail. It also needs significant mass forward of the fission site to capture that forward momentum from high energy penetrating fission fragments. The fraction of releases energy converted to directed thrust is likely to be less than a conventioal fusion design.
Neverthe less this is a very interesting low mass propulsion concept.
Jim Early
is cooling a problem with this type of sail ?
To quote from the article:
“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.”
Alex Tolley. The carbon layer is not absorbing the fission fragment. The protons in the carbon atoms are being annihilated by their contact with the anti-protons which release p mesons and excess kinetic energy which must be converted into high energy gamma rays, down or strange quarks. The gamma radiation is high enough energy to fission the Uranium 238 nucleus which splits into two daughter nucleus which have a velocity and kinetic energy. Due to Newton’s third law the speed of these fission fragments leaving the sail push it and the rocket forwards.
Since the carbon atom has six protons, I assume than the carbon nucleus changes into another element when it looses some protons through annihilation so the carbon layer is slowly being changed into another element due to loss of carbon atoms. https://en.wikipedia.org/wiki/Annihilation
I think this is all wrong. If it were as you say, the fission fragments would not emerge on one side of the sail, only, which they must in order to provide thrust.
In my understanding, the antiprotons react with the Uranium nuclei directly, at the surface, causing them to fission. One of the fission products emerges at the back, the other is absorbed. Either by the Uranium itself, or, if that is all used up, by the carbon. The carbon has two functions: structural, and to absorb forward fission fragments that make it through the Uranium layer.
There are no rods only a layer of carbon on the sail. The control rods are in the old design of a nuclear thermal rocket in a book called Space Frontier by Werner Von Braun. S.F. main library has it A good book. A nuclear thermal rocket has a graphite reactor where the graphite is mixed with U 235 and the reaction heat I recall is slowed down with control rods. The whole carbon chamber gets hot and fuel flows through the chamber and is heated up and expelled through a combustion chamber. The cold fuel flows around the combustion chamber to keep it cool from the heat of the hot exhaust which would easily melt through it’s walls but now days we could use an electromagnetic field like in VASIMR but a nuclear thermal rocket only has a specific impulse was not very high though less than 1000. It is also dangerous since it takes a week to cool down and is highly radioactive.
I suppose that the tethers have to be negatively charged wires to prevent any annihilation by the antiproton plasma? How is the rest of the spacecraft shielded from the reaction by-products?
Those who will work with the probe and its descendants will need one of these warning symbols:
https://astrowright.wordpress.com/2010/05/06/proposing-an-antimatter-hazard-symbol/
Would it not be better to use a U239/U235/U233 coated sail and a neutron source, much less dangerous and doable near term.
The neutrons would go right through the sail. The few that react will do so in the interior, sending as many fission fragments to the front as to the back, with no net thrust.
Cold neutrons would be slow enough to interact with the fission material except maybe plutonium which needs a faster neutron.
I’m still not clear about the amount of anti-hydrogen needed. If 1 anti-hydrogen fissions 1 uranium atom, then doesn’t the craft need as much anti-hydrogen as uranium to accelerate the to a final velocity equal to the velocity of the fission fragments (actually more to account for the carbon backing and storage container.) This seems more than a spark-plug analogy. The fissioning must release enough neutrons to further fission teh uranium, with quite a large multiplier to make this work. Yet the thin sail seems at odds with this requirement.
Is there a better reference (academic paper?) explaining this concept in more detail?
Alex, I don’t have the references, but Jackson and Howe are putting their website together — the plan is to incorporate a lot of their research, so I would assume that would include any relevant papers. For now, though, you can check the decade-old NIAC report:
http://www.niac.usra.edu/files/studies/final_report/740Howe.pdf
which at least covers the basics.
Exactly what I was looking for. The fission causes more neutral U238 atoms to be ejected, lowering Isp, but increasing the mass ejected. The authors estimate 15000 atoms/fission as optimal. No neutron fission occurs, just ejected mass from the high velocity fission fragment.
This cannot work. The 5% light speed that is desired requires the full Isp of individual fission fragments. Lower Isp means proportionally lower terminal velocity.
So, as you say, it has to be one antiproton per Uranium nucleus. If they really “estimate 15000 atoms/fission as optimal”, something is very wrong here.
After looking at the paper, things are more clear to me: They describe a 250 AU mission, with 15,000 U atoms ejected per annihilation, yielding a modest Isp of 7500 s. That is not anywhere near 5% light speed and well below what is needed for a true interstellar mission.
An interstellar mission at ~5% c would require one antiproton per Uranium atom, as you have said. It is not clear that that is even feasible, as it stands to reason that there will always be extra atoms emitted.
Moebius the tethers are far enough away from the target center to not have to worry about any fission fragments.
The problem with U239/U235/U233 is that they are isotopes of Uranium with shorter half-lifes than U238 which is the longest or 4.5 billion years. The shorter half life’s make them more unstable so it is easier to cause them to fission. It takes a higher energy photon, neutron or particle to fission U238 since there there is more neutrons in the nucleus the protons are spaced farther apart so it takes more energy to fission it. I am not an expert in atomic physics but I know that you don’t want to use these isotopes especially U235 for this rocket design because a run away fission through neutrons will use up all your fuel too quickly and you get too much radiation.
You can’t possibly get a chain reaction in a thin film of material such as this sail. No way to achieve critical mass. Almost all neutrons will simply leave the foil before they have a chance to split another atom.
You want the fission to be controlled as much as possible otherwise you get too much heat as well. The fission in atomic weapons with neutrons is like the Domino effect.
I assume the carbon layer is placed over the U235 to give a more controlled fission to avoid the chain reaction of the domino effect where a few neutrons split many atoms into daughters creating more neutrons and splitting more atoms
I meant carbon placed over U238.
Actually it is the U 238 that gives the controlled reaction and the carbon I guess is just to protect the sail from the heat.
In a comment on a recent post at NBF last week, I suggested that the use of a laser to push foils, small sails, made of anti-matter (because anti-matter is sooo easy to come by) towards an outbound Starship, could be used for acceleration by impacting a “mist” of matter released by the craft, the resulting plasma pushing against a magnetic nozzle.
Pushing a companion sail of fissionable matter to intersect with the AM sail at the craft, though, would mean acceleration could be continuous for the entire first half of a journey to Alpha Centauri, and then continuous for the deceleration period without the ship needing to carry any antimatter or the associated confinement apparatus, nor reaction mass.
A larger craft would be best due to the relatively large amount of AM required to produce a viable sail.
If the craft were a modified Oneill cylinder colony, the laser pushed sails could push through its center to collide at the leading end of the ship and act to decelerate the craft at its destination.
A fusion (pardoned the pun) of laser propelled sails, Orion and an antimatter mag-nozzle type propulsion.
That image of the antimatter probe, it’s Cassini attached to the sail! And not very tightly, either.
Positron Dynamics near term work to proving out antimatter catalyzed deuterium fusion propulsion with over 100,000 ISP
Interesting parallel idea. I hope one of these exotic ideas comes to fruition.
In order for this to work (at least for the 5% c figure), the payload mass has to be much less than the sail mass. The illustrated configuration (with the Cassini probe and the mini-sail) grotesquely fails in this requirement.
Curium 250 decays mainly by spontaneous fission, and has a half life of ~8300 years. It would make quite a decent fission fragment sail, with no antimatter needed at all.
Unfortunately, the next more active nuclide that decays mainly by spontaneous fission is Californium 254, with a half life of 60 days. A bit short. Nothing in between 60 days and 8300 years that I could find, so Curium 250 will have to do. Not very powerful, but long-lasting….
Their NIAC slide presentation is here:
http://www.niac.usra.edu/files/library/meetings/fellows/oct02/740Howe.pdf