Thursday, September 22, 2011

Mining Gas Giants for Helium-3

For terrestrial populations as well as habitats in and near terrestrial orbits, sunlight and perhaps even Lunar 3He might provide clean, efficient power for the foreseeable future.  However, habitats will eventually expand into extraterrestrial orbits, too far from Sol for sunlight to be a viable energy source.  These habitats will depend on 3He mined from the gas giants.  This is even more true for habitats bound for interstellar destinations.
Mining gas giants  for helium-3 has been widely studied for many years. For example, more than 50 years ago, the British Interplanetary Society proposed a Project Daedalus interstellar probe to be fueled by Helium-3 (3He) from the atmosphere of Jupiter. Further study has caused many scientists to also consider extracting 3He from the other gas giants because Jovian gravity might be problematic. Bryan Palaszewski, Glenn Research Center, has written an excellent paper on the subject (Atmospheric Mining in the Outer Solar System).

In-Situ Resources
Thus, mining in the outer solar system will present great opportunities for many enterprises. Launching and transporting required materials from Earth will always be an expensive activity and would make such enterprises untenable. Thus, “in situ” resources will be leveraged to the max. Large reserves of atmospheric gases in the outer planets are an excellent resource for fuels and other life sustaining or colony building gases. The moons of the outer planets can provide essential resources for life support and construction.
(Note to self:  add list of moons and perhaps materials).

Many decades of research have focused on using the natural resources of the solar system to allow sustainable human exploration and exploitation of the environments of the planets and the Sun. Everything from preliminary experiments in propellant production to creation of human colonies in space has been proposed. Studies have shown the outer planets can provide the rich resources for interstellar exploration
As human exploration starts in the outer solar system, the travel time and other natural hazards (planetary radiation belts, solar coronal mass ejections, etc.) will create new challenges for the explorers. In-situ resources will likely be a great asset in this exploration. Shielding from radiation can be created with rock from the moons or with hydrogen and other liquefied gases from the planetary atmospheres.

Atmospheres of the Outer Planets
TABLE I. Atmospheric Gases
by volume percentage (%)
H2 HeMethaneTrace
Uranus 82.5%15.2%2.3%1.0%
In the outer planet atmospheres, highly energetic materials include: hydrogen, helium and small amounts of other gases.

These atmospheric gases can fuel both chemical and nuclear propulsion systems. Hydrogen and methane are excellent fuels for chemical rockets that can be used for the ascent from and descent to the local moons’ surfaces (i.e. Callisto, Jupiter’s best way-station candidate). Also, hydrogen can be the fuel of choice for nuclear fission and fusion rockets.  3He is another future fusion reactor fuel and can be found in these atmospheres.
Some studies propose that hydrogen, helium, and other fusion forms can be recovered in their solid form (i.e., “ices”) deeper within atmospheres of Uranus and Neptune; these ices may be crucial to the exploration beyond the solar system.
Why Mine There?
Sheer abundance of gases and their locations make the gas giants good way stations to travel beyond the solar system. A stopover could be used to gather many tonnes of fusion fuel for the long journey to a neighboring star.  3He is an especially attractive fusion fuel. The  3He-3He fusion process emits no neutrons (aneutronic); thus, the only neutron radiation comes from side reactions from inevitable presence of Hydrogen isotopes which (2H, deuterium, and 3H, tritium) which cause neutronic reactions. This greatly reduced neutron radiation will extend the life of the vessel's power reactors.  However, the gas giants present challenges.
  • Access Energy.  "Delta V" required to closely approach any gas giant for atmospheric access and return can be quite high.
  • Radiation.  These planets typically have powerful magnetic fields; thus, the vehicle will need heavy shielding to block radiation.  This added mass will further increase energy requirements.
  • Atmospheric Wind Speeds. Gas giant atmospheres have very high wind speeds and wind shears. Given the typical lightweight nature of aerospace vehicles, the predictability of wind speed in the atmosphere will affect factory design and the related aeronautical vehicles.
    • Jupiter and Saturn have intense wind shears;  thus, stresses may be too much for a conventional aerostat ("balloon"). Jupiter and Saturn will likely need an aggressive aerodynamic option such as high speed cruisers. 
    • Uranus and Neptune probably have calmer wind profiles. Due to more predictable atmospheric conditions and wind speeds, mining with aerostat borne stations seems more practical.
Mining Methods
Mining stations may be a series of buoyant stations in the planetary atmosphere; aerodynamic scoopers that dive into the atmosphere or cruisers that ply the atmosphere, gathering the needed gases and liquefying them. The vehicle types are described below.
Aerostat stations
Aerostat vehicles are buoyant stations which persist in the atmosphere and continually process atmospheric gases for the needed final products—3He , 3He , and H2, etc. Orbital vehicles will frequently visit the aerostat to transfer the final product to orbit or another destination (a moon of the planet, transfer vehicle back to Earth, etc.). Aerostats have long been considered for mining Jupiter’s atmosphere.
A typical aerostat scenario has the orbiting vehicle transferring propellants and other products to the vicinity of Jupiter’s moon, Callisto. Callisto is an attractive way station as it can provide materials for vehicle construction from its surface and is outside Jupiter’s major radiation belts; thus, making it safer for human explorers. Such a scenario may be the most attractive of the three atmospheric mining options.
Scoopers “scoop” a portion of the planet’s atmosphere inside the vehicle for later processing. This method provides a short term option for fuel capturing, and enables final processing to take place in orbit.
For really large amounts of gas, scoopers perform hundreds of missions in the atmosphere. The scooper’s useable life may be limited by the heating and other stresses from multiple entries/exits. Also, the atmospheric composition may impose new stresses on materials. Flying through icy particles and potentially reactive materials (hydrogen, methane, etc.) will no doubt create new challenges for long lived vehicles.

A cruiser gathers atmospheric gases as it flies in the atmosphere for an extended period. Smaller vehicles visit the cruiser to transfer the liquefied gases to orbit. Many launch vehicle and trans-atmospheric vehicle designs have been investigated using a liquid air cycle engine (LACE) technology. A similar design for liquefying hydrogen, helium and 3He will work for the outer planet atmospheres. Since delivering cruiser fleets from may be a prohibitive task, in-situ construction and maintenance of such vehicles will prove essential.

Daedalus project called for an unmanned spacecraft  to travel one-way to Barnard's Star. At an average speed of 0.15c (15% light speed), the vessel would take about 50 years to travel the 6 light years distance.

 The Daedalus interstellar concept was planned in detail by the British Interplanetary Society in the 1970's. Powered by nuclear fusion, vessel's gross weight would be 49,000 metric Tonnes (mTs) and would require 27,000 mTs of 3He  for fuel (not available on Earth but readily available in Jupiter's atmosphere).

Before starting the long journey, the vessel would need a lengthy stay in orbit around a gas giant. The Daedalus Project suggested a harvest rate of 1,500 MT per year (about 4 mT/day) for a 20 year period. Robotic systems, tended by humans, will likely be needed for effective construction and fuel production.

Mining the atmospheres of the outer planets will prove essential. Using specialized factories, the energy of hydrogen, helium, and helium 3 gases can power nuclear fusion propulsion systems.
Many issues complicate the harvesting of these valuable resources. The dynamics of the atmosphere, radiation, and the energy for orbital transfer all call for very energetic and reliable propulsion systems to allow for rapid, reliable, and repetitive visits to the planets and their moons. For example, more predictable atmospheric conditions at Uranus make mining with aerostat borne stations more applicable for use there, while more dynamic winds at Jupiter require more aggressive aerodynamic options (high speed cruiser aircraft).
The numerous maneuvers and extended vehicle lifetimes required for wresting the resources from the outer planets will likely lead through a learning curve. Mining of outer planet moons, especially if water is detected there, will be an excellent option. Initial experiments to prove the efficacy and lifetime of atmospheric scoopers need to be conducted before committing to such ambitious vehicles.
These are great possibilities for human exploration throughout and beyond the solar system. Effective harvesting of these resources will control the timetables for human exploration. Indeed, the outer planets' resources will lead to the stars.


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