EMMI - Electric Solar Wind Sail Facilitated Manned Mars Initiative
EEMMI – Electric Solar Wind Sail Facilitated MannedMars Initiative
Pekka Janhunen, Sini Merikallio and Mark Paton
Corresponding author: P. Janhunen, Finnish Meteorological Institute, Erik Palm´eninaukio 1, FI–00560 Helsinki, Finland. (pekka.janhunen@fmi.fi)
Abstract
The novel propellantless electric solar wind sail concept promises efficient lowthrust transportation in the Solar System outside Earth’s magnetosphere.Combined with asteroid mining to provide water and synthetic cryogenicrocket fuel in orbits of Earth and Mars, possibilities for affordable contin-uous manned presence on Mars open up. Orbital fuel and water enablereusable bidirectional Earth-Mars vehicles for continuous manned presenceon Mars and allow smaller fuel fraction of spacecraft than what is achiev-able by traditional means. Water can also be used as radiation shieldingof the manned compartment, thus reducing the launch mass further. Inaddition, the presence of fuel in the orbit of Mars provides the option foran all-propulsive landing, thus potentially eliminating issues of heavy heatshields and augmenting the capability of pinpoint landing. With this E-sailenabled scheme, the recurrent cost of continuous bidirectional traffic betweenEarth and Mars might ultimately approach the recurrent cost of running theInternational Space Station, ISS.
Keywords:
E-sail, Mars, manned spaceflight, asteroid mining,propellantless propulsion
1. Introduction
Manned missions to Mars have been in the planning stage since the onsetof space age [1, 2]. Current proposals include national space agency plans aswell as private sector efforts. The European Agency (ESA) as well as the Na-tional Aeronautics and Space Administration (NASA) have both expressedinterest in manned Mars flights. Private sector ventures, such as MarsOne or
Preprint submitted to Acta Astronautica August 20, 2018 a r X i v : . [ a s t r o - ph . I M ] F e b ars Direct have more ambitious schedules, but are struggling with resourcelimitations and technical feasibility issues [3]. All of the above base the trans-portation on traditional propulsion including heavy launchers. Moreover, ithas been estimated that MarsOne would require 15 Falcon Heavy launchesto initiate the project even before manned flight phase [3].The electric solar wind sail (E-sail) [4] is a novel propellantless propulsionconcept utilizing the solar wind. It is estimated to be very efficient in termsof impulse versus propulsion system mass [5]. As its propulsion system islightweight and does not consume any propellant, the E-sail can transportcargo payloads in the solar system with reasonable costs and flight times.The alternative technological path that we propose in this paper couldprovide means for affordable and continuous trafficking of cargo and pas-sengers between the Earth and Mars and thus continuous presence of humanbeings on the surface of Mars. Because of the exponential nature of the rocketequation, being able to refill the fuel tanks on the way one could decreasethe transportation cost tremendously. This can be achieved by hauling wa-ter from the asteroids and converting it to LOX/LH2 fuel in orbital fuelingstations. An outline of this scheme is shown in Fig. 1. Other assets requiredby our proposed E-sail facilitated manned Mars presence include the possi-bility to use the water as a radiation shield, potable water and a source ofbreathable oxygen. We call the scheme presented on this paper the E-sailfacilitated Manned Mars Initiative (EMMI).The purpose of this paper is to analyze EMMI schemes at high level ofabstraction by attempting to identify the leading terms (drivers) of their massand risk budget. In particular cases, we shall also dwell on some details, butfull engineering design of the needed space assets is outside the scope of thepaper.
2. Electric solar wind sail
The electric solar wind sail (E-sail) provides thrust in the solar windwithout consuming fuel [6, 4]. The E-sail uses charged tethers to extract mo-mentum from the solar wind ions (mainly protons) by electrostatic Coulombinteraction with the plasma flow to produce thrust. According to current es-timates, the E-sail is 2-3 orders of magnitude more efficient than traditionalpropulsion methods (chemical rockets and ion engines) in terms of producedlifetime-integrated impulse per propulsion system mass, if the mission dura-tion is 10 years and the mission does not proceed too far into the outer solar2 ater from asteroids to Mars orbit M a nn e d fl i g h t s EarthMars Asteroid
E-sailHox/Lox
F F F
Fuel station C a r g o W a t e r f r o m a s t e r o i d s t o E a r t h o r b i t M i n i n g e q u i p m e n t Figure 1: Outline of EMMI, showing the locations of the orbital fuel stations where thewater is split and condensed into LOX/LH2 fuel. On these fuel stations manned vehiclestraveling between Earth and Mars can be fueled, dramatically reducing the overall missionfuel ratio at launch. Cargo transport and mining activities take place propellantlessly usingE-sails. F ∼ /r [7]. In comparison, the thrust producedby a photonic solar sail decays faster as F ∼ /r . Thus the E-sail has par-ticular potential for long-lasting solar system missions. The E-sail requiresno fuel and the charging of the wires can be accomplished by an electron gunpowered by modest-sized solar panels. The electric power consumption perproduced thrust is 1 kW/N at 1 au from the Sun [5]. For fixed tether voltage,the power consumption is proportional to the electron current gathered bythe tethers, which is in turn proportional to solar wind density, behavingas ∼ /r . Because this is the same scaling as the illumination of the solarpanels, it implies that if the solar panels are dimensioned to power the E-sailat a specific distance such as 1 au, they also suffice to power the sail at otherdistances as well if one assumes constant panel efficiency.To enable maneuvering and trajectory control, the E-sail thrust can bevectored in a cone of ∼ ◦ around the solar wind velocity vector [10]. It ispossible to adjust the magnitude of the thrust between 0-100 % by modify-ing the current and voltage of the electron gun. Turning E-sail propulsionoff is possible at any time by turning off the electron gun. A strategy ofmaximizing available thrust by matching the electron gun current with thecurrent gathered by the tethers leads by certain natural negative feedbackmechanisms to a situation where the thrust varies much less than the solarwind dynamic pressure [11]. Even with simple trajectory control law, themaneuverability of the E-sail is sufficient to allow navigation to, for example,Mars [11].Applications of the E-sail have been extensively researched [12], outlin-ing for example outer planet exploration [13, 14, 15], asteroid flyby [16],asteroid rendezvous [17], NEO sample return [18], and hazardous asteroiddeflection [19].
3. Asteroid mining
At the core of the E-sail facilitated Manned Mars Initiative (EMMI) is theutilization of the propellantless thrust provided by the E-sail to haul waterfrom asteroids into orbits of Earth and Mars. A large E-sail, that can provide4 N of thrust at 1 au from Sun, can travel from the Earth to the asteroid beltin a year, and bring back three tonnes of water in three years [17, 18]. OneE-sail based spacecraft is capable of repeating the journey multiple timeswithin its estimated lifetime of at least ten years.
We outline here a baseline scheme for extracting water from the asteroids.An alternative method will be discussed in the following section. In thissection we introduce a twin container, of which the other part is heated withelectric power generated by solar panels. The evaporating water vapor iscondensed in the other container.The extractor apparatus consists of two container parts connected witha pipe (Fig. 2). The asteroid material is gathered into the other container,which we call an oven from here on. The oven is closed up and tightly sealed,after which it is heated up to about +50 o C for the water contained in theasteroid material to evaporate (at +50 o C, water boils at 0.13 bar). The watervapor thus fills the oven and starts flowing through the connecting pipe tothe other container (the tank). Temperature of the tank is held slightlyabove freezing, say at +5 o C, which causes the water vapor to condense onthe walls of the tank, but keeps it from freezing and blocking the connectingpipe. After the flow of the vapor recedes, the valve in the connecting pipeis closed and the asteroid material of the oven can be exchanged for anotherbatch of asteroid material. Once the tank is filled up to the desired level,it can be disconnected from the connecting pipe and transported to its finaldestination. A new, empty tank can be connected to the oven and the processrepeated for as long as there are empty tank modules available. These canbe transported en masse from Earth to the asteroids with an E-sail.A semipermeable membrane that only lets the water pass in vapor formcould be installed on the connecting pipe between the oven and the tank toensure that the water stays in the tank. This membrane could be heated tokeep any larger water mass from forming on top of it (and thus blocking thevapor transfer). Alternatively, the gravity of the asteroid might be enough tohelp guiding the condensing water towards the end of the tank. The gravitycould also be of an artificial (rotational) origin, which could be arranged ifthe whole extractor apparatus is lifted from the surface of the asteroid andmade to spin in space, as is discussed in the section 3.2.To get a handle on the mass and cost of asteroid mining, let us estimateits energy consumption. The ice-fraction of the asteroid requires 2900 kJ/kg5 ieve valve +50 °C +5 °Crubble c o n d e n s a t i o n i n t o w a t e r h a t c h evaporation dockingsystembaking unit condensation bladdercentrifugal force and pressure diffusion gradientbladder cooling by heat pumpSun heating by solar panels or direct absorption. Figure 2: A schematic presentation of the water extractor unit. to melt and evaporate into +50 o C steam, if an initial temperature of -50 o Cfor the asteroid ice is assumed. Using a heat pump [assuming coefficient ofpower (COP) of 3] for transferring the condensation heat from the tank in tothe oven, we could reduce the energy demand down to 1300 kJ/kg. However,asteroids mainly consist of rocky material, which has to be heated in theprocess as well, here from the initial -50 o C up to the +50 o C. Assuming thetarget asteroid to have water content of 10%, rest 90% being basalt (c p =0.84 kg kJ − K − ), the total process would require 2000 kJ/kg. In the caseof a dry asteroid with the water content of barely 2%, the energy requiredto extract a kilogram of water would raise up to 5400 kJ/kg. Moreover, onthe surface of Mars the combination of only 2% of soil water content and ofthat water being released only when the soil is heated up to around 400 o Cincreases the energy requirement up to 20 MJ/kg [20].We assume that we can find and choose an asteroid with the desired high(10%) water content, which leads into baseline electric energy requirementfor water extraction of 2 MJ/kg.
Another, perhaps more elegant way to extract water from the asteroidmaterial would be to adjust the container temperature by alternating the6ontainers surface albedo and thus by using only direct solar energy. Herethe oven would need to be coated by a material whose optical absorptivity ismuch higher than the infrared emissivity, such as gold or copper [21], and thetank cooled down by a cold coating, e.g. white paint. The oven would then befilled with asteroid material, or a small asteroid could even be enveloped inits entirety (in the way of recently proposed NASA Asteroid Initiative [22]),lifted from the surface and set into a rotating motion. Rotation would ensurethat the tank does not shadow the oven for prolonged periods, and moreimportantly, it creates an artificial gravity keeping the condensed water atthe outer wall of the tank. Once the extraction is complete, the oven andthe tank would be separated, the oven discarded and the now ice-filled tankleft to wait for its carrier E-sail, constantly beaconing to ensure it will notget lost.
Transporting the liquefied water from the asteroids first to the heliocen-tric orbits of the Earth and Mars and then to bound orbits could be donein lightweight, thin walled container tanks, each weighing a few tonnes whenfilled. The container has to be specially designed to endure the space environ-ment. The main requirements are containment ability, low mass, resistanceto micrometeoroids and tolerance of possible freezing/thawing cycles of thecargo. For these purposes, we propose a layered membrane structure por-trayed in Figure 3.The cargo could be brought from a high orbit down to LEO without fuelconsumption by using gradual aerobraking. The thin walled container tankconducts heat to the water mass it is carrying, thus making the tank heatresistant. During aerobraking, the tank could be maneuvered to be in thefront so that all the control electronics could be protected behind it.
The water, once transported to orbits of Mars and Earth, is split intoits constituent hydrogen and oxygen by electrolysis or by photocatalyticmeans [23]. After liquefaction, one obtains cryogenic rocket fuel. EMMIis based on manufacturing this LOX/LH2 fuel from the water transportedfrom the asteroids. With 20 kW of electric power one can produce 30 000 kgof LOX/LH2 from water in a year.However, if liquefying H in temperatures prevalent close to asteroidsproves difficult, producing an methane-oxygen fuel could be an alternative7 Figure 3: A schematic drawing showing the layers of the tank wall. Multiple layers and aspace filler in-between them are necessary preparations for micrometeorite impacts. CO .The carbon dioxide would then be combined with hydrogen in a Sabatierreaction to produce methane and water ( CO + 4 H → CH + 2 H O ).Water to fuel conversion plants would reside in high orbits, i.e. barelycaptured in the gravity well of Earth or Mars. Storages of fuel would be ac-cumulated on these strategic places well in time before they are needed. Theredundancy can be built into the system, having a number of smaller tankingstations with combined storage capacity exceeding the required mission fuelconsumption. Thus malfunctioning of any one fuel production station wouldnot be mission critical. Fuel would also be produced so that it would beready and waiting before the first humans are sent to space.
4. Operations on Mars
It has been estimated from the Curiosity Mars rovers measurements thataround 1 . −
3% of the mass of Martian surface soil is water that can bereleased by heating the material up to 200 ◦ C − ◦ C [20]. This water, onceextracted, could be split into same kind of fuel to that produced from thewater mined from the asteroids, see sec. 3.4, thus allowing to use the samerocket engines (and thus the same vehicles) for lift-off from Mars as for otherparts of the mission. This could dramatically further reduce the costs of themission, albeit also posing additional design restrictions for the spacecraft.Landing humans on Mars could be circumvented altogether by using re-mote controlled robotics as proposed in HERRO concept (Human Explo-ration using Real-time Robotic Operations, [24]): instead of landing, hu-mans would orbit around Mars and teleoperate robots on its surface. Thisapproach would significantly reduce the costs and risks and alleviate the issueof forward contamination of Mars by human-carried microbes.
5. Mission outline
EMMI proposes means for continuous habitation on the Martian surface.Recurrent trips with LOX/LH2 powered transport vehicle between Earth andMars would transport the exchange crews and their food in regular intervals.Additional support equipment, food and accessories could be taxied fromthe Earth propellantlessly with E-sails. The crew transport vehicles would9e fueled up on the fuel stations situated in high planetary orbits and/orplanet-Sun Lagrange points. The ∆V required for LEO – Lagrange pointtransfer and for the Lagrange point – Mars transfer is in the scale of 2.5-3.3km/s, meaning the fuel requirement is about 50% - 60% of the wet mass ofthe spacecraft if LOX/LH2 is used, moderate Isp of 420 s is assumed and10% fuel margin is used.The mission could thus be done on separate stages, filling up the fuel tankin-between each leg. First, a launcher is used to lift the passengers/payloadto LEO where the first tanking occurs. Alternatively, the lift is continuedwith electric propulsion engines through the magnetosphere. The craft islifted to L1, L2 or a high orbit, where again fuelling from the awaiting fuelreservoirs occurs. The last leg from Earth orbit to Mars and capture to Marsorbit would again consume the fuel.The target asteroids would be sought from the vicinity of the Martianorbit, at around 1.5 au, as this is where majority of the mining products areheaded. Yearly water extraction pace of roughly 50 000 kg would suffice for amanned bidirectional trip between Earth and Mars taking place every otheryear. To achieve this, 3.2 kW of electric power on the asteroid is required. Atthe distance of 1.5 au from the Sun, this translates into 230 kg of solar panelsassuming a horizontal panel on the equatorial surface of a rotating asteroidand a characteristic mass for the solar panels of 100 W/kg at 1 au (Joel Ponzy,private communication). As power is also needed for other purposes, such asmoving, communications and countering power system aging, we will assumea 50% higher power consumption, thus arriving at the whole extractor powersystem weighing 340 kg. We estimate that the whole extraction vessel wouldin this case weigh around 2000 kg, which is much lower than the expectedmass of the orbital fuel factories. This and other key mass figures of EMMIare listed in Table 1.For the spacecraft carrying the astronauts there is a need for radiationshielding, which increases the mass of the manned vehicle considerably. Thewater stored on fuel stations can be used as a radiation shield, thus removingthe need to launch a heavy shielded manned module all the way from theEarth’s surface. Potable water for the needs of the crew and oxygen forbreathing could also be manufactured from this water.
The vehicle transporting the crew between Earth and Mars orbits wouldhave a mass of 50 000 kg, including radiation shielding but not including fuel.10 able 1: Some key mass figures of EMMI.
Production and transportation of water per year 50 000 kgextractor vehicle weight 2000 kgTransporter E-sail 500 kgManned vehicle 50 000 kgPayload transferred from Earth to Mars 10 000 kg yearlyAs sturdy radiation protection is mandatory on manned Mars flights [25, 26,27], 40% of the mass (20 000 kg) would be radiation shielding water, gottenon-board from Earth’s orbital tanking station. Only the crew and their foodis needed to be launched from Earth by traditional means; the transfer vehiclewould be waiting on a high orbit around Earth with a full tank of LOX/LH2.The manned transfer vehicle would shuttle in-between orbits of Earth andMars, getting fueled from the LOX/LH2 produced from the asteroid waterat the tanking stations residing on these locations.
The normal way of landing on Mars requires the use of a heat shield tocombat the excessive temperatures generated during a high-speed entry intothe atmosphere. Given low cost fuel in Mars orbit, however, one could alsoconsider an all-propulsive landing reducing the speed of the spacecraft beforeit enters the atmosphere. The craft would fill its propellant tanks once morein orbit around Mars, make a large burn with its engine to almost cancel itsorbital speed and then drop into the atmosphere with a speed much lowerthan that experienced when following traditional entry trajectories. Thistogether with a propulsive insertion into orbit around Mars would diminishthe mass fraction of the vehicle’s thermal protection system close to zero.An all propulsive descent would also make it easier to land precisely as itallows greater control over the trajectory. Only a short trajectory throughthe atmosphere is necessary which reduces the possibility of navigation errorspropagating. In a partly similar concept, an idea of reducing heat shielddemands by using propulsive landing to Mars was introduced by [28].Control over the landing point is an essential aspect once a colony hasbeen built, as landing too near or too far from it could be hazardous. A pos-sibility exists to design the crew transport vehicle, which transports humansbetween Earth and Mars, to also function on transfers in-between Martian11urface and orbit. This would, however, increase the design requirements ofthe spacecraft.
EMMI would first start with launching of asteroid mapping E-sails toscout for suitable water and/or carbon containing asteroids. After the map-ping phase (duration approximately four years), the mining spacecraft wouldbe sent to selected target asteroid and the mining would start (1 year totransport, 1.5 years to set up operations and to produce the first 50 000 kgof water). A third of the mined water would be transferred to Earth’s andtwo thirds to Mars orbit. After 8 years from the first launch, one would have30 000 kg of fuel on the orbit of Earth.At the first stage of operations, humans would stay on the orbit of theMars, letting HERRO [24] take care of the surface operations. This wouldgreatly reduce the costs at this stage as the astronaut habitat, life supportand return launch from the surface of Mars would not need to be considered.However, HERRO systems would facilitate the mapping of the surface androbotically prepare the base for the future crews that could then land on thesurface more prepared.As EMMI continues, mining operations are expanded to additional as-teroids to provide a more continuous and reliable fuel supply. Redundancycan be built up by additional fuel stations on strategic locations. Continuoushuman presence on the surface and/or on orbit of the Mars would be possiblewith affordable costs and symmetrically bidirectional traffic.Once the asteroid mining and fuel transportation system is built, its main-tenance would be relatively low cost as all of the craft included are reusableand all the advantages from serial production (lower production and design-ing costs per unit) could be taken advantage of. Thus only crew, spare parts,and life support material has to be put into the system and launched into or-bit analogously to the maintenance of the International Space Station (ISS),currently orbiting the Earth with continuous manned presence [29]. Thismakes the maintenance effort and thus also the costs between the EMMIand ISS roughly comparable with each other for each astronaut-year.
6. Development risks
EMMI is based on providing water in Earth and Mars orbits, which inturn is based on E-sail propulsion and on asteroid water mining. These12echnologies have not yet been demonstrated at high TRL and therefore theEMMI scheme as a whole must be considered at least somewhat risky atthe present time. However, demonstration of asteroid water mining and itstransportation by E-sail propulsion to planetary orbit is an easily scalableprocess. This means that it can be first demonstrated in a small scale at lowcost (many orders of magnitude lower than full EMMI) and then scaled up.Manufacturing cryogenic fuel in orbit is also something that has not yet beendone, but we think that few would doubt its feasibility. Thus, demonstrationof the E-sail and small-scale asteroid water mining would strongly reduceuncertainty about EMMI’s feasibility, and the investment needed would bevanishingly small compared to the cost of manned space activity.
7. Summary and conclusions
We analyzed how E-sails could act as critical enabling technology forsetting up continuous manned bidirectional traffic to Mars, using asteroidwater mining and orbital fuel manufacturing. The most massive and there-fore the most costly part of the fuel supply chain are the orbital fuel factories,because splitting water into hydrogen and oxygen takes much more energythan liberating it from asteroid soil. The E-sails are a relatively minor partof the total mass budget. The efficiency of the overall concept stems from theintermediate tankings that, due to the exponential nature of the rocket equa-tion, reduce the fuel consumption considerably compared to the traditionalapproach without refueling opportunities.It is also a significant cost benefit that the orbital water can be usedas crew radiation shield. We think that with EMMI continuous mannedpresence on Mars (with bidirectional traffic) would be possible at a cost levelwhich is comparable to that of maintaining the International Space Station,ISS.
8. Acknowledgements
The work was partly funded by Academy of Finland, grant number 250591.
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