Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Headed by the Tau Zero Foundation and British Interplanetary Society, a non-profit group of scientists dedicated to interstellar spaceflight, Icarus is working to develop a spacecraft that can travel to a nearby star.
Adam Crowl, Module Lead for Fuel and Fuel Acquisition for Project Icarus, investigates the pros and cons of various fusion fuels required to accelerate an interstellar vehicle to a nearby star.
One might think that fusion propulsion requires some exotic fuel to propel a rocket a million-or-so-times more energetically than standard chemical fuels. However, one fusion fuel option isn't so exotic.
In fact, by drinking the recommended 8 glasses of water per day you've ingested about half a pound of the stuff: hydrogen. One-ninth of all water on Earth is hydrogen. But there's a snag in its widespread adoption as a fusion fuel.
Regular hydrogen fuses very, very slowly even in a place as unimaginably hot as the center of the sun. That's fortunate for all life on Earth -- because that's what allows stars to shine for billions of years -- but it does make it a very difficult fusion fuel to utilize.
But there's an answer: Add a neutron to the single proton in the heart of every hydrogen atom and you have deuterium, also known as "heavy hydrogen."
Deuterium is incredibly easy to fuse compared to hydrogen and most of the sun's energy actually comes from fusing it. Inside the sun, deuterium is continuously made by banging two protons (hydrogen nuclei) together fast enough for one to become a neutron and stick to the other, and once made it fuses with another deuterium in less than a second.
Thus, no deuterium accumulates in the sun and in the rest of the natural world it's relatively rare -- 1 in every 6,500 atoms of the hydrogen we drink is deuterium. However, because deuterium, in so-called "Heavy-Water," is used to moderate neutrons in some nuclear reactor designs, it is separated from regular water on a large scale.
Pure deuterium can already be fused by technological means and was used in the first hydrogen bomb detonated in 1952, but fusing it with tritium (hydrogen with two neutrons, so it's heavier than deuterium) is even easier and this is the preferred reaction used by fusion research today.
Unfortunately, if this method was used to fuel a starship -- such as the Icarus interstellar vehicle -- the deuterium-tritium (D-T) reaction produces high-energy neutrons that transfer heat from the reaction directly to the engine's structure. About 80 percent of the fusion energy released is in the form of those neutrons, so the reaction isn't very healthy (or useful) for a starship.
Pure deuterium reactions also produce neutrons, though only about 1/3 of the fusion energy is released as such. That's better than the D-T reaction, but when we're talking about engine powers in the hundreds of gigawatts to terawatts, then such percentages mean gigawatts of heat that must be gotten rid of, adding to the mass of the engines and degrading the overall performance.
Fusion physics knows of other reactions. The reaction of boron-11 (an isotope of boron) and plain hydrogen produces all its energy in the form of charged particles which can be directed by a magnetic field, but the reaction is very difficult to sustain and many fusion physicists doubt it will ever prove practical. If it was successfully demonstrated as a viable fuel option, then the fuel mixture could be stored in solid form as decaborane, which remains solid below 100 degrees Celsius.
However, there is a very attractive reaction between deuterium and a light isotope of helium known as helium-3. Helium-3 has one less neutron than regular helium (helium-4) and is also produced in the sun and almost as quickly consumed in fusion reactions as deuterium.
Like deuterium, it is rare relative to helium-4, but, unlike hydrogen, helium doesn't form chemical compounds as abundant as water. Almost all Earth's helium has long since blown away and only small amounts are available on the planet -- much of it can be found in the gas mines of North America. What helium is available is depleted in helium-3 relative to what we see in the sun, because most of Earth's helium-4 is freshly made via natural radioactive decay of the elements uranium and thorium.
We know the sun contains lots of helium, and as the solar wind has been depositing helium into the rocky surface of the moon, perhaps we can extract it. Just how much is available can presently only be estimated at somewhere between 1 million and 2.5 million tons.
To extract it would require digging up much of the moon's upper few feet of soil and baking the soil to release the solar wind-implanted gases. Project Icarus Consultant, Bob Parkinson, has examined this resource and, surprisingly, concluded it might take more energy to extract than would be produced by fusing the helium-3 liberated.
The Gas Mines of Uranus
However, there is a surprising amount of helium-3 in the gas giant planets of the outer solar system, and in the original 1978 "Project Daedalus" report Bob Parkinson suggested mining it via floating robotic factories in the atmosphere of Jupiter. Since then a different planet has moved to the forefront of gas-mining plans because it lacks Jupiter's intense gravity, Saturn's gigantic rings of orbital debris and is closer than distant Neptune.
You guessed it; the best helium-3 supply in the solar system is from the "Gas Mines" of Uranus.
That the planet which is the butt of so many poor jokes should be relatively rich in methane as well is purely coincidental, but as a mining site it has several advantages. The surface gravity, which is defined from the 1 bar pressure level in a gas giant's atmosphere, is 90 percent that of Earth's and the speed needed to reach low orbit is lowest of all the gas planets. Uranus's rings are also high, thin and not showering the atmosphere below with a hail of meteors, unlike Saturn's.
Accessing the gas riches of Uranus will require nuclear power, however. Designs exist for nuclear powered ramjets that could fly indefinitely in the atmospheres of the gas giants -- this might prove a viable means of keeping an extraction factory aloft. Else we'll be back to using balloons like "Project Daedalus," serviced by nuclear ramjets.
An atmosphere composed of a cold gas mix that is lighter than helium and not much heavier than hydrogen, means that hot-air ballooning will need to be used. That the oldest technology of flight will find a role supporting the latest, fusion propulsion, has a certain poetic justice.
Getting the fuel home, where it can be used domestically as well as for tanking-up starships, could provide an early pay-off for developing a fusion propelled starship.
A Helium Market
The original "Daedalus" starprobe design had two stages. A Stage Two, by itself, would be well suited to being a deep space freighter, able to carry payloads of up to 500 tons at very high speed. Uranus is nearly three billion kilometers from the sun and Earth, thus traveling there, and back, requires a high-speed vehicle.
A Stage Two freighter could carry itself, with an empty mass of 500 tons, to Uranus in 70 days for just 114 tons of fuel, and then bring back a load of 614 tons using about 254 tons of fuel. Of the return load, 114 tons would be used to return the empty tanker to Uranus, while 500 tons would be used for starships and the terrestrial energy market.
A starprobe might launch by the year 2100 and if world energy demands continue to increase at their historic rate of 2.5 percent, then by 2100 about 14,000 tons of deuterium/helium-3 fuel-mix would supply the world's energy demand per year, adding an incentive to develop the gas-mines of Uranus.
Alternatively, a means might be found to put the neutrons from pure deuterium fusion to good use. Some fusion ignition designs can confine the fusion neutrons in the dense plasma formed by the reaction, sharing their energy with the rest of the fusion plasma, thus reducing the damage to the reactor walls. If such a design can be successfully used for a starship engine, then a source of deuterium can be sought closer to home.
Unlike helium-3 we know the moon has large amounts of hydrogen, as ice, and a significant fraction of it will be deuterium. The moon's low gravity also means that water composed of regular hydrogen and oxygen will escape quicker than heavy water, perhaps leading to a concentration of deuterium in the water of the moon. We won't know until we return to the moon for a closer look.
WIDE ANGLE: Read more about Project Icarus, its predecessor "Project Daedalus" and the motivation behind interstellar spaceflight in the exclusive Discovery News Wide Angle: "Reaching for Interstellar Space."