Project Icarus: Gas Mining on Uranus

Project Icarus is a 21st century theoretical study of a mission to another star. Icarus aims to build on the work of the celebrated Daedalus project. Between the period 1973-1978 members of the BIS undertook a theoretical study of a flyby mission to Barnard's star 5.9 light years away. This was Project Daedalus and remains one of the most complete studies of an interstellar probe to date. The 54,000 ton two-stage vehicle was powered by inertial confinement fusion using electron beams to compress the D/He3 fusion capsules to ignition. It would obtain an eventual cruise velocity of 36,000km/s or 12% of light speed from over 700kN of thrust, burning at a specific impulse of 1 million seconds, reaching its destination in approximately 50 years.

The terms of reference for Project Icarus are:

• 1.     To design an unmanned probe that is capable of delivering useful scientific data about the target star, associated planetary bodies, solar environment and the interstellar medium.
• 2.     The spacecraft must use current or near future technology and be designed to be launched as soon as is credibly determined.
• 3.     The spacecraft must reach its stellar destination within as fast a time as possible, not exceeding a century and ideally much sooner.
• 4.     The spacecraft must be designed to allow for a variety of target stars.
• 5.     The spacecraft propulsion must be mainly fusion based (i.e. Daedalus).
• 6.     The spacecraft mission must be designed so as to allow some deceleration for increased encounter time at the destination.

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.
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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.

Seeking Helium-3

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.
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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.

Icarus Construction

The spacecraft construction would necessarily take place in space. If constructed in low-Earth orbit (LEO), we would most likely need a high thrust chemical propulsion system, and ideally a reliable single-stage-to-orbit solution, such as SKYLON or perhaps a revised program for mass production of launch vehicles.

A large scale space-based infrastructure would then be needed, which would most likely use a wide range of solar powered systems.

This orbiting shipyard might look like a fabrication ring capable of moving around the spacecraft (pictured above), with a number of remote manipulators working on sections of it at a time.


The engineers may be operating the manipulators safely from Earth, working in shifts around the clock and from around the world. The sheer size of the orbiting shipyard would make it suitable for visits, effectively making the Icarus construction a tourist attraction:

Navigation and Trajectory Insertion

Attitude control and navigation are of particular importance inside our solar system, where the initial Icarus interstellar trajectory insertion would take place.
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An orbital insertion trajectory called the Oberth maneuver, has the spacecraft dip towards the sun and then quickly power outwards, in a move reminiscent to an orbital slingshot. In this case however, the benefit comes from using up as much of the fuel as possible deep inside the gravitational field of the sun, which saves energy because that same amount doesn't need to be carried it up and out of the gravity well.

This move also brings the spacecraft very close to the sun, which gives us an opportunity to use a solar sail to either supplement the main engine, or to assist in trajectory insertion.

Interstellar Operations

The Icarus may suffer malfunctions that would need to be repaired from time to time.

The Daedalus team decided to deal with on-board repairs by using a type of mobile repair robot, they called "Wardens," as they were made to survey and patrol the spacecraft.

If Team Icarus makes the decision to use something similar, these robots will also require a means of propulsion for locomotion. This would most likely be a form of electro-thermal pulsed plasma thruster, which is capable of micro-newton performance, giving the bots the ability to position themselves very accurately.

An alternative is to use a "repair rail" with robotic manipulators, inspired from the Canadarm which has served on board the Space Shuttle (Canadarm 1) and ISS (Canadarm 2) for almost 30 years (without single event failure).

While the Icarus is on route to the target star, there are still numerous tasks required of the secondary propulsion systems. The fuel tanks, for instance, would be dropped as they are depleted, after which trajectory corrections are to be expected.

Fuel Tanks as Relays

The use of these dropped fuel tanks as communications relays has been proposed to assist in maintaining a robust communications link with Earth. These parabolic, clam-shelled fuel tanks could split open to reveal two telecommunications dishes, one pointing at the Icarus, while the other back at Earth.
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These relays would be, for the most part, free-flying with whatever velocity and direction they had when they were released. To allow them to point towards the Icarus and the Earth, they would require some accurate method of orientation. In this particular case, a Radioisotope Thermal Generator (RTG) powered flywheel fits the bill quite well, as both of these systems can be used for long term operation.

Deceleration Options on Arrival at Target Star

There is a current push in observational astronomy to determine if there are any Earth-like planets in our solar neighborhood.

The Icarus target definition has remained open for precisely this reason, where on discovery of an Earth-like planet within 15 light-years, it will almost certainly become our target star.

In order to carefully study the planets and moons of the target system, the Icarus would need to explore several deceleration options, which may allow the spacecraft to at least spend more time in the target system, if not fully decelerate.

We essentially have two alternatives. We can either decelerate the entire Icarus spacecraft mainly using reverse main engine thrust, or we can decelerate the individual probes using a combination of magsails, solar sails and perhaps nuclear rockets. This task is complicated by the fact that we would need to first survey the target solar system and acquire reliable orbital information for the planets or moons of interest.

What are then the requirements for maximizing the scientific data return from the probes, and what demands does this place on the probe propulsion systems?

Planetary Science Probes

Let's assume full deceleration at the target star has been achieved and planetary orbit information has been processed and assigned to a number of science probes.

By that time, near-Earth telescopes would be sufficiently advanced to verify and inform the Icarus computers on which scientific objectives are most desirable. As might be expected, there will be an incredible amount of "feature creep" at this stage, where scientists will be arguing over which objectives should take priority over others, while the probe power and propulsion systems last.

Essentially this means a solar powered propulsion system would be needed. Solar thermal engines require a reaction mass (usually Hydrogen) for propellant that, however, will have to be carried along and stored safely for the duration of the interstellar journey.

Solar electric engines could be used, especially if combined with a stable and inert ion source like teflon. The best overall option for a reliable and abundant propulsion method would appear to be the use of solar sails, particularly for planetary surveyors and orbiters with prolonged remote sensing agendas.

If we were to take it a step further, we might consider landers with rovers or even submarines. RTG power sources, combined with aerobreaking, parachutes and airbags require the least amount of propellant, but would probably only be chosen for use in relatively dense atmospheres.

Engineering Proofing

Keeping these systems operational after 100 years of storage, sends everyone back to the drawing board, as the entire history of human spaceflight isn't that long. It is however possible with current engineering practices to construct and test many of these systems.

Perhaps testing these advanced systems in dual-role planetary exploration missions, as concurrent precursor and engineering proofing platforms for interstellar missions is the best way to both explore our solar system and prepare for interstellar exploration.

New Evidence For Seas of Liquid Diamond On Neptune

Remote gas giant planets Neptune and Uranus could be covered in vast seas of liquid diamond, dotted with solid diamond chunks like icebergs. A new experiment revealed such oceans are plausible, and would explain some oddities about Neptune's magnetic field.

Published recently in Nature Physics, the study is an effort to explain two things: What causes the magnetic poles of Neptune and Uranus to be so far off their geographic poles; and what would cause the planets to have a 10 percent carbon makeup. Diamond seas are the answer, and their experiment suggested that these seas would behave a lot like water oceans.
cientists have produced liquid diamond before (you can see a picture of the "z machine" above, which liquifies diamond using electricity and pressure), but mostly for industrial applications. The researchers in this study, however, wondered what would happen to diamond in an ultra-hot, high-pressure atmosphere like Neptune's. Once they had liquified the diamond at extreme pressures, the researchers brought the pressure back up to Neptune levels (about 11 million times Earth's), while bringing the up to the planet's usual 50,000 degrees. As the atmospheric mix stabilized, they discovered that chunks of solid diamond appeared in the liquid, floating atop it like sheets of ice.

Vast seas of diamond could also explain how the giant planets' magnetic fields got warped too, with the magnetic pole as much as 60 degrees off from the geologic one.