In 2002 they were funded by the NASA Institute for Advanced Concepts to perform preliminary work on an antimatter-based propulsion system. This campaign seeks to fund a continuation of the design and experimental validation of an antimatter thruster capable of reaching (or exceeding) 5% of the speed of light. The goal is to enable interstellar travel, with the initial requirements of accelerating, decelerating, and studying a nearby solar system all within a human lifetime. It is now time to advance this concept from an idea and onto the drawing board.
The spacecraft has a sail that is composed of two layers- a carbon backing (sail) and a uranium coating (fuel). The entire sail is 5 meters in diameter. The carbon layer is 15 microns thick and the uranium is 293 microns thick. The total mass of uranium is 109 kg. The large area of the sail removes the requirement for active cooling. The sail is assumed to dissipate waste heat passively via black-body emission. We assumed an emissivity of 0.3 for both the carbon and the uranium. The steady state temperature of the sail would be 570C, well below the melting point of uranium.
A second round of funding was granted by NIAC to experimentally investigate one of the open issues that was raised. Unfortunately, due to the abbreviated funding from NIAC, this investigation was never completed. It is one of the goals of this campaign to complete the experimental apparatus and prepare it for exposure to an antiproton beam.
Lab Scale Propulsion Test of Antimatter and Uranium sail
Antimatter has the highest specific energy of any source known to man. At nearly 10^17 J/kg, it is two orders of magnitude larger than fusion and ten orders of magnitude larger than chemical reactions. Although currently produced and stored in small quantities using Penning traps, antimatter must be stored in much higher densities to be applicable for interstellar missions. Currently, research is underway around the world to develop high-capacity storage of antimatter in smaller, lighter electrostatic traps or in the form of antihydrogen. However, even if proven successful, no propulsion system has been demonstrated that would efficiently convert the antimatter into usable thrust.
Fission is the process by which an atomic nucleus splits into two or more daughters, multiple neutrons, alpha particles, electrons, and gamma rays. Most of the energy released by fission is in the kinetic energy of two daughters. Invoking conservation of momentum, a nucleus at rest produces two daughters of equal momentum travelling in opposite directions. The kinetic energy of each daughter is approximately 1 MeV/amu (1 million electron volts in each proton and neutron).
Assume a thin sheet of depleted uranium U238 upon which a carbon coating is applied on the back side. When antiprotons (physics symbol P with an overhead bar) drift onto the front surface, their negative electrical charge allow 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. 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.
Instead of creating a standard nuclear rocket in which hydrogen gas is heated and expelled as thrust, we have created a lightweight nuclear rocket wherein the uranium is the fuel and the antimatter is the spark plug. This dramatically reduces the needed amount of antimatter, making this the first proposed antimatter-based propulsion system that is within the near-term ability of the human race to produce.
At present physicists like us know of no shortcuts (like warp drive, wormholes, or star gates) to traverse the immense distances between stars. Nor are there any tangible theories that could provide such shortcuts. Therefore, we are going to have to use good old Newton’s Laws and Einsteins Special Relativity to get us to these destinations.
There are two propulsion technologies that are within the current or near-future technical ability of humans that can generate velocities capable of reaching the nearest candidate solar system (4.3 light years) in a single life span. A spacecraft travelling 5% of the speed of light would require 90 years to make the trip. Note that our fastest spaceships currently leaving the solar system would take over 80,000 years to cover that distance.
Of course, one technology is antimatter. The other approach that is currently garnering widespread press reports is Breakthrough Starshot (http://www.breakthroughinitiatives.org/). They envision using powerful lasers to accelerate tiny solar sails to 20% of the speed of light. Their mission profile calls for a 20 year trip and a high-speed fly-by of the target solar system.
While we encourage a diversity of approaches for reaching our mutual goal of reaching the stars, there are many problems inherent in accomplishing mission goals with a fly-by at 20% of the speed of light. For example, such a high velocity coupled with the expected light levels at Proxima b (their stated destination) requires that any reasonable camera shutter speed would produced smeared images, The same is also true of almost any current sensor technology, such as magnetic field measurements. Also, at this velocity the protons in solar wind near Proxima b would have an equivalent incident energy of 21 MeV, more than enough to transmute the probe materials, generating radioactive isotopes that would mask any radiation measurements.
Therefore, one of our mission requirement is to decelerate the spacecraft just before entering the target solar system and going into orbit about that star. 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 and stored in order to accomplish a comparable deceleration. 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. It is the design of this new spacecraft architecture that is another of the goals of this campaign.
The first portable storage bottle for antimatter currently resides at their headquarters. Pictured below, it can store either positrons (the antimatter partner of electrons) or antiprotons. Bottle, which operates at liquid helium temperatures and a very hard vacuum, can store up to 1 billion particles.
For the purpose of interstellar missions, the quantity of antimatter is too large to store as charged elementary particles. Therefore, they propose to transport the antimatter initially in the form of antihydrogen (an antiproton with an orbiting positron). Eventually, an optimum form of antimatter might be antilithium. During this campaign we will develop an detailed design report describing the construction and performance of a long-lived antihydrogen storage bottle suitable for spacecraft. The storage mechanism will be similar to that employed in the classic Millikan Oil Drop experiment. They will also devise a storage test to be funded in a subsequent campaign in which the storage of cryogenic solid hydrogen will be maintained for years at a time, demonstrating the reliability of such a bottle.
SOURCES- Kickstarter, NASA NIAC, Antimatter drive