Near Earth Asteroids:The Celestial Chariots
Marc Green, Justin Hess, Tom Lacroix, Jordan Marchetto, Erik McCaffrey, Erik Scougal, Mayer Humi
aa r X i v : . [ phy s i c s . pop - ph ] J un Near Earth Asteroids:The Celestial Chariots
Marc Green, Justin Hess, Tom Lacroix, Jordan Marchetto,Erik McCaffrey, Erik Scougal and Mayer HumiWorcester Polytechnic Institute,Worcester, MA 01609 ∗ November 2, 2018
Abstract
In this paper we put forward a proposal to use Near Earth Objects as radiationshield for deep space exploration. In principle these objects can provide also a spa-cious habitat for the astronauts and their supplies on their journeys. We undertakealso a detailed assessment of this proposal for a mission from Earth to Mars. ∗ e-mail: [email protected]. Introduction
In the last half-century, humans have set foot on the moon and, as a result, deep spacetravel seems to be within Humanity’s reach. However, there are many issues that mustbe resolved before space travel to other objects in the solar system can become a reality.Examples of these issues include radiation in space, physiological effects of zero gravity,power generation, and propulsion methods.In this paper we put forward a novel proposal that can mitigate the dangers thatradiation can pose to space travel. The basic idea behind this proposal is the use of NearEarth Objects (NEOs) as a temporary shelter from radiation for astronauts on deep spacemissions, like a voyage to Mars.Just as fire can be both dangerous and beneficial, the NEOs that pose a danger toHumanity on Earth can be used as ”Celestial chariots.” By protecting the astronauts fromradiation, and providing spacious housing and storage facilities, long journeys into deepspace become possible.We begin with a general overview of radiation in space and its effects on exposedhumans. Various strategies to protect astronauts from these harmful effects are thendescribed. We concentrate on the use of NEOs a radiation shields, and we put forth aproposal using them on a trip to Mars and the feasibility of such a proposal. We concludeby discussing ideas that, if realized, would greatly increase the likelihood and the benefitof NEOs as radiation shields.To examine this idea in detail, one has to choose the most suitable NEO out of theten thousand known that could be utilized to travel between Earth and Mars. Thechosen NEO must be of sufficient size and material to properly shield its cargo of humansand supplies from the radiation in space. In addition, we examine the possibility of”domesticating” an asteroid by manipulating its orbit. To protect the astronauts on theirjourney, a habitat can be built in the NEO in which a manned crew would ride safelyfrom Earth to Mars. The crew would then exit the NEO upon approaching Mars.
Short wavelength radiation, such as X-rays and Gamma rays, is harmful to livingorganisms. On Earth, protection is provided from these harmful effects by the Van-Allen2elts, Earth’s magnetic field, and the atmosphere. In space, however, there is no suchshielding.Radiation in space is made up of Galactic Cosmic Rays (GCRs) and Solar ParticleEvents (SPEs), which are atomic and subatomic particles accelerated to (very) high en-ergy levels. Another source of radiation, though usually ignored, is known as secondaryparticles [1]. These high-energy radiation sources pose a major threat to astronauts asthey can cause cancer and genetic mutations. Humans outside the Earth’s protectiveshield must use other means to protect themselves from radiation’s ill effects.
GCRs originate from outside our solar system and have particle energies known topeak around 1 GeV within the solar system. Approximately 98 percent of these particlesare protons or heavy ions which possess a high linear energy. This linear energy enablesthese particles to deeply penetrate almost any shielding. The collision of the radiationwith a shielding material is likely to generate secondary particles, increasing the totalamount of shielding needed to maintain a safe environment for life.GCR radiation levels within the solar system vary with the natural solar cycle. Thefluctuation of the Sun’s magnetosphere and material output changes the amount of GCRsdeflected away from the core of the solar system. [2]
SPEs are made of solar flares and coronal mass ejections (CMEs), both of which spewionized gases and radiation from the Sun. Large events of this nature are rare, howevertheir frequency depends upon the sun’s 11-year activity cycle.Impulsive flares are short lived, on the order of hours, but release large amounts ofelectron radiation. They are not overly dangerous to space travel to other planets as theyare emitted in a range from 30 to 45 angles in solar longitude and leave the ecliptic planequickly.CMEs are much longer lived than flares, lasting days, and are characterized by ahigh proton flux on the order of 10 particles per square centimeter. They can eruptanywhere from the Sun’s surface and reach Earth and other planets, impacting spacecraft3nd space-based resources en route to or in orbit around these planets.[3] Radiation in the International System of Units (SI) can be measured in Sieverts (Sv).The Sievert is the equivalent absorption of one joule of energy by one kilogram of matter.This means that it represents the biological effects of ionizing radiation by taking intoaccount the biological context.A human exposed to more than 1 Sv will suffer varying illnesses including leukopeniaand other immune system-impairing conditions. A radiation dose greater than 8 Sv willbe fatal to humans within days, as they succumb to organ failure and severe burns. Anastronaut on board a spacecraft exposed to a solar flare or CME would likely be killed.Even on spacecrafts without humans, radiation can pose a hazard to the mission. Spaceradiation can interact with the spacecraft electronics, damaging or destroying them. Itaffects computer operations and stored data by flipping the charge of bits between positiveand negative. This happens when a charged particle impacts a bit, it flipping the chargeand corrupting the data. Under extreme radiation, like the particle flux of CMEs, theinformation can be completely corrupted.Currently, there are two main ways to internally shield electronics. By using older chipdesigns with larger gaps between bits, it is less likely that a particle will flip multiple bits,minimizing the damage. Additionally, the CPU equivalent of lightning rods can be builtinto the vulnerable circuitry to redirect the incoming radiation into less-critical areas. Anexternal method to shield the electronics involves physically encasing the computers withprotective material, but at the cost of adding mass to the mission. It is desirable to findthe lightest and most cost effective shielding.
Radiation shielding is measured in ”half thickness”, which is the amount of materialneeded to reduce the incoming radiation in half. By overlapping several half thicknesses,incoming radiation can be further reduced and a safe environment can be established. Ifwe know the expected radiation levels and the material properties of our shield, we candetermine how much shielding is needed. The following equation can be used to determine4he half thickness, where µ is the material’s bulk mass absorption coefficient and ρ is thematerial density. t / = − ln(0 . µρ Determining the shielding thickness needed is simple in concept, but requires knowl-edge of High Energy Particle Physics and Material Science. The interaction between theincoming radiation with the shielding materials needs to be accurately known.To keep the astronauts healthy throughout their travel in space, it is desirable tokeep the radiation levels in the spacecraft as close to Earth-level as possible. There areestablished guidelines published by the NCRP (National Council on Radiation Protection)which NASA currently uses to determine what the astronauts on the ISS can safelyexperience.[4]
Shielding materials should be selected based on several criteria including half thicknessand total mass. The ideal material would be both lightweight and have good deflectionproperties against radiation particles. Lead is the de-facto shielding material on Earth,but it does not meet our criteria due to it’s large mass. Even though it will not be usedfor spacecraft shielding, it will be used as a basis for comparison with the other materialsbecause it is a common shielding material.[4] Below, in Table 1, we provide a list ofmaterials that could be used to protect against radiation in future space missions. In thistable we explain what kind of material it is, how it can be used against radiation, andhow much of the material is required. 5able 1: A Sample of Radiation Protective Materials
Protective Material What is it How does it work How much is requiredagainst radiationLead Chemical element with atomic number It’s extremely high density provides 10cm for a reduction of 1000x82 shielding from radiation particlesPolyethylene Most commonly used plastic for Demron is lightweight, flexible and A thickness of 2.7 cm (72 layers) and 29(Demron) commercial products contains proprietary materials that cm (240 layers) of Demron would beA chemically synthesized polymer block radiation.It can be treated like required for a two factor and ten factorwith high amounts of hydrogen a fabric for cleaning, storage, and reduction in transmission-Demron is polyethylene between two disposal purposeslayers of fabricBoron Nitride An equal chemical combination of Due to its light nucleus, can Approximately 1.5m is needed to reduceNanotubes both Boron and Nitrogen nanotube successfully absorb harmful neutrons the effective dose rate (Eiso) by 45-containing Boron Nitride in secondary radiation. Additionally 48%if used in the development of a spaceshuttle, this can further decrease theharm of radiation exposure forastronauts.Electrostatic A material capable of blocking the The use of conductive materials such Several feet of carbon a nanotube areShielding & Carbon effects of an electric field, while also as carbon-nanotubes (CNTs) can required to shield against radiationNanotubes allowing the passage to magnetic conduct enough energy to generatefields. an electrostatic shield capable ofblocking all incoming ion particlesC60 (Buckminster- A spherical fullerene molecule with Provides potential benefits against TBD, more research needs to be done tofullerene) the formula C60 with a cage-like fused radiation if used as an antioxidant determine appropriate dosesring structure drug
Near Earth Objects (NEOs) are defined as any object that passes within 0.3 AU ofEarth at some point in their orbit around the Sun. These include, but are not limitedto, asteroids and dead comets. NEOs pose a danger to humanity on Earth as even the6mallest NEO impact can release as much energy as a nuclear weapon or volcanic eruption.The Chelyabinsk Meteor that struck Russia on February 15th, 2013, was approximately18 meters in diameter and generated nearly 440 kilotons of TNT. The threat of NEOs tohumanity is very real, but what if they could be tamed and used for space travel?
We propose to use NEOs for transportation within the solar system. With properasteroid selection and preparation, it will be possible to take advantage of their materialproperties to protect the astronauts from space radiation. Other benefits of using NEOswill be spacious living quarters and expanded storage facilities for food, water and medicaland scientific supplies. Larger crews can thus be supported, which will allow for socialinteraction during the long voyage and help the psychological well-being of the crew. Forthe purposes of explaining our proposal, we shall use a mission to Mars as a case study.
1. Identify NEOs that meet orbital requirements to pass Mars and Earth.2. Determine NEO composition through physical landing, practicing orbital maneu-vers.3. Prepare NEO for habitation through robotic missions.4. Land a crew on board NEO when habitation is completed and the asteroid is passingEarth en-route to Mars.5. Travel to Mars inside NEO and leave when at Mars.The implementation of this program is not simple. Although we have the technologyto precisely calculate the orbits of NEOs, at present, it is quite difficult to determine if aparticular NEO will be suitable as a radiation shield.Furthermore, efficient methods have to be developed to prepare an asteroid for habi-tation. Below, we propose some ideas to reduce these complications, including changingthe trajectory of asteroids. While these are all obstacles to the success of our vision, webelieve that they are not insurmountable. 7hese preparations will need to be done ahead of time through robotic missions. Likely,a single launch or quick series of launches will transfer all the materials needed for theconstruction. Until the asteroid passes close to Earth again, the robots will build thehabitable and radiation shielded volume. This allows for astronauts to simply land onthe asteroid and have everything prepared. Additionally, it will be needed to plan for thereturn trip.
One of the consequences of using a conventionally shielded spacecraft is the largeamount of mass that must be put into space. The cost of doing this behooves missionplanners to design the interplanetary spacecraft with longevity in mind to reduce the needfor expensive launches. However, long-term operation in high-radiation environments isdangerous, as radiation embrittles metals over time. This can best be seen in nuclearsubmarine reactors, where thirty years of service takes its toll on the reactor walls. Metalembrittlement and fatigue are a major concern for conventional spacecrafts where theinterior has to be pressurized. However, this issue becomes moot if NEOs are used. Whilealuminum and other materials might be needed to construct the living quarters, they willbe shielded by the bulk of the asteroid, and not weaken from radiation. Additionally,building inside the NEO allows for pressurization stresses to be spread into the NEO,reducing the stresses in the construction material.Furthermore, one of the most dangerous physical threats to spacecraft, micro impacts,can be all but eliminated using NEOs. Small, high velocity debris generated in collisionscan rupture conventional spacecraft. They are extremely hard to detect due to their size,but can vent the spacecraft’s atmosphere into space, killing its crew. Nestled deep insidean NEO, micro impacts will not be able to penetrate far enough to be a threat to thecrew.
There is a lot to consider when constructing a mission to Mars using NEOs as trans-portation and radiation protection. Due to the large amount of unknown data, we cannotbe certain that a given solution will work. However, we believe that the solution we have8rovided is flexible enough to accommodate the unexpected.
Selecting the right NEO out of the tens of thousands in orbit around the Sun is adaunting challenge. We shall limit the search to asteroids, as they are more likely toprovide the radiation shielding we require compared to the other objects. From the morethan 9,500 asteroids presently identified by NASA and other Space Agencies, our asteroidwill have to meet the following criteria.1. Must approach Earth and Mars’ orbits and meet other requirements to improvecrew transfer and safety.2. Must be large enough and of the right composition to provide adequate shielding.3. Must be small enough to be sufficiently movable to fine-tune the orbit. Our targetsize for such an asteroid is approximately 100 meters in diameter.Our target size for such an asteroid is approximately 100 meters in diameter. Thisoffers us the highest likelihood that we will have enough shielding, living space, and itwill be small enough to move. Unfortunately, current detection methods are not accurateenough to detect asteroids 100 meters in diameter of less, and future satellite asteroidobservation missions should be conducted to increase the likelihood of finding ideal aster-oids. Infrared telescopes would be ideally suited for our purposes as they can sometimesidentify the composition of the asteroid using spectrology.
Even if we find an asteroid with a perfect material composition for radiation shielding,it means nothing if that asteroid does not fly by Earth and Mars on a regular basis.Therefore orbital criteria are an essential part of selecting asteroids. To reduce the amountof orbital manipulation required for each potential asteroid, those closest to Earth, whichalso intercept Mars’ orbit, will be examined. Their perihelion should be within 0.05AU ofEarth’s orbit and their aphelion should extend to between Mars perihelion and aphelion,a range of 1.4 to 1.7 AU. The aphelion of their orbits should not extend beyond 1.7 AU to9void a collision with another asteroid in the Asteroid Belt, thus drastically altering theirorbit and making them useless for our purpose. These parameters reduce the number ofNear Earth Object asteroids from over 9500 to around 200.A final orbital consideration is the inclination of the asteroids orbits relative to Earthand Mars. In an Ecliptic plane reference frame, which passes through the Sun and Earthorbit, Mars orbit has an inclination of -1.8 degrees. In order for Asteroids to approach bothEarth and Mars, they must orbit in between the planets inclination angle. For simplicity,we shall assume that we can manipulate the inclination angle of those asteroids nearestto the ideal orbit in such a way that they will precisely intercept the planets. By limitingour search to +/-10 degrees from the Ecliptic plane, the number of potential asteroids isrefined to 43 (these are the highlighted asteroids in the Appendices). A complete list ofknown asteroids and their orbital parameters is available in Ref 17. Unfortunately, notmuch, if anything is known about their physical properties and currently we can onlyidentify potential asteroids by their orbit.An artistic conceptual trip to Mars using NEOs is attached at the end of this paper
Asteroids that are classified as NEOs are broken into several categories based ontheir orbital path and material composition. One particular class of NEOs, known asChondrites, are made of iron, water and carbon. Based upon infrared spectrology, NASAestimates that these asteroids are approximately 88 percent iron, making them an idealradiation shields from a material standpoint. However, since iron is difficult to dig into,other asteroids that have a looser, or less-dense composition might be more suitable.With reduced density, they would have to be bigger to account for the difference in halfthickness, but in space size doesn’t matter. If the asteroid’s mass is lower it will requireless work to change its orbit.Iron is the second best metallic element used to protect against gamma radiation beingonly second to lead. It only takes 4 inches of Iron to reduce gamma radiation damageby a factor of 10, whereas it takes 24 inches of water to reduce the damage by the sameamount. Drilling down 340 feet of iron would reduce radiation damage by a factor of 1million. The optimal choice of an NEO for mission purposes will need to take into accountother properties of NEOs, such as their temperatures.10 .4.4 The Unknowns
While an asteroid may fit the orbital requirements for use in a manned mission, ifit is incapable of shielding astronauts from radiation, it is useless. Not much is knownabout the material and physical makeup of the vast majority of asteroids. Therefore thenecessary shielding thickness cannot be computed to construct safe living spaces on board.Experiments performed on samples of each asteroid, can provide the necessary data.To precisely identify how much shielding is needed, samples of asteroids need to becollected and analyze. Using instruments similar to the Curiosity Rover x-ray spectrome-ter and Laser Induced Breakdown Spectroscopy system, a probe exploring each potentialasteroid can relay the exact material composition and makeup to Earth.Attempting to rendezvous with the most promising asteroids for sample analysis isnot a new idea. In 2000, Japan launched the Hayabusa mission which landed on 25143Itokawa and was able to retrieve samples and return them to Earth.[7] Complications inthe attempts to collect the samples leads us to believe that a spacecraft designed to staywith the asteroid and analyse it would be better. It would allow for more information to becollected about the asteroid and possibly begin mapping the surface aid in construction.
Making feasible use of asteroids for transportation will often require that the aster-oid’s current orbit be modified in order to make the transfer between Earth, Mars, andthe asteroid more convenient. Unfortunately, this process requires extensive use of im-pulsive thrusting over the course of several orbits. The thrusting will have to be doneover perigee and apogee centered burn-arcs, possibly at constantly changing thrust an-gles. Furthermore, concurrent changes in the asteroids apogee, perigee, eccentricity, andinclination angle will have to be made in order to affect the orbit change in a reasonableamount of time and minimum amount of resources.[8] Due to the difficulty in analyzingsuch a scenario the specifics will not be covered in this paper. However the nature of thepropulsion that should be used in this process can be analyzed. The basis for analyzingdifferent propulsion methods lies in an equation of motion derived from the Reynolds11ransport theorem. This equation is: (cid:18) M − dmdt (cid:19) A = dmdt V Where: • M is the total mass of the craft (the asteroid) • A is the acceleration of the craft • t is thrust time • dmdt is the mass flow rate of the propulsion system. • V is the propellant velocity of the propulsion systemThe focus will be on the right half of the equation, dmdt V , which is equal in magnitudeto the thrust force on the spacecraft. These two parameters, the mass flow rate dmdt , andthe propellant velocity (V), can be used to evaluate the advantages and disadvantagesof propulsion methods. The advantage of a high V is that more energy is imparted perpropellant mass, meaning that in a situation where propellant mass is limited, such aslong distance missions, more total energy can be imparted. High V systems tend to havevery low dmdt , but also very low thrust. However many high thrust propulsion technologieshave low V, making them mass inefficient and suitable only for short missions.Since propellant will be brought to the asteroid very infrequently due to the rarity ofclose approaches and the difficulty of coordinating long distance shipments, the propulsionmethod used on the asteroid will have to be as propellant mass efficient as possible. Tothis end a high propellant velocity is necessary, which can be found in ion engines.An ion engine generates thrust by accelerating ions to high velocity and expellingthem out of the rear of the spacecraft. The gas is typically an inert gas such as Xenonto avoid unwanted reactions. [10] There are two ways to accelerate the gas. First isthe electron bombardment method where the engine bombards the propellant with high-energy electrons, knocking Xenon’s valence electrons free. [11] Second is the electroncyclotron resonance method, which excites the electrons in the gas via microwaves andmagnetic fields. [11] Once ionized, the gas is accelerated out of the engine by electrostatic12orces generated by a positively charged grid at the beginning of the flow chamber and anegatively charged grid at the end. [12]Some high velocity ion engines expel their propellant at speeds in excess of 90,000 m/s,but produce minimal thrust and expel very little propellant. [13] Atypical modern ionengine thrust is a mere 0.5 N, but fortunately higher thrust engines are in development.[16]One such project, the High Power Electric Propulsion project has developed a 40 kWengine, more than 19 times more powerful than the engine used by Deep Space. [17] [18].Furthermore, 200 kW configurations have been suggested, which would boost thrustsup to 18 N, propellant velocities of 40,000 m/s or higher, and with a low mass flow ratebetween 100 and 1200 mg/s.[19] Despite the power increase, they retain a power efficiencyof upwards of 60% and a calculated thrust to mass flow ratio of up to 50000 N*s/kg.If several of these higher thrust ion engines are placed on the asteroid they will allowfor the asteroid’s orbit to be modified with low propellant and energy costs. The mainobjective of these orbit manipulations will be to place the asteroid into a mildly ellipticalorbit which will travel from Earth to Mars in about 200-250 days. This will require theasteroid’s velocity to be approximately 25 kilometers per second, an easily attainablefigure. If the engines are thrusting for years on end. The solution to the radiation problem that we are proposing in this paper relies onhaving a collection of easily accessible domesticated NEOs, which we have implicitlyassumed to exist in our presentation. However, this is where the complexity of our solutionlies, and this is what will determine the likeliness of using NEOs as transportation vehicleswithin the solar system.Domesticating a single NEO involves modifying the NEO orbit and excavating aradiation-protected area in its body. As previously discussed, it is plausible to modify anNEO orbit with ion thrusters. Further research will need to be done in order to deter-mine the best methods to create a radiation-protected habitat, but we can put forth herethe essential requirements. Using advanced robots that are physically and electronicallyhardened to survive in deep space, the selected asteroid will be prepared by drilling awaymaterial to construct the shielded living quarters for the crew. Being essentially strandedon the asteroid with no material support and little communication, the robots will have13o be rugged, capable of repairing themselves, and completing the complex operationsdemanded of them with minimal error.The vision we have is not just one domesticated asteroid. We believe that a few dozenswill be necessary in order to make frequent trips to and from Mars due to their long orbitalperiods. Great care should be taken when creating this group so that the asteroids willnot be on a collision course with Earth and survive for centuries to make the most of ourinvestment in converting them.
When dealing with radiation and space travel, time is in the essence. It dictates howmuch shielding, food, water and fuel is needed for the mission, and therefore somethingthat we should be aware of during mission planning. Trying to calculate the close ap-proaches between the asteroid and each planet is difficult when the different orbit periodsand angle differences are considered. In some cases it might be necessary for the crew tostay on board the asteroid for several years while the planets move into position. On theother hand, if everything went according to plan, an asteroid could take a direct routebetween Earth and Mars.With orbit modification, this direct route could become more routinely possible ifenough asteroids were domesticated and enough orbit modifications done. In this manner,the travel times between the planets would be around 200-400 days. Most of the asteroidsthat met our orbital criteria typically orbited the Sun in 500 to 650 days and they intersecteach orbit in two places, creating two transfer points from planet to asteroid or vise-versa.Getting to and from the asteroid is another concern for astronauts, as their exposureto radiation is greatest during this transfer. Given that the asteroid cannot come intoEarth’s gravitational influence, the astronauts must exceed Earth’s escape velocity of11920 m/s. From launch, it should only take a few days to leave Earth’s gravity well andrendezvous with the asteroid. The total time of flight for this portion of the mission issolely dependent on how close the asteroid is to Earth. With precise orbit modificationsthe asteroids can be put a safe distance from Earth’s gravity, but not too far away thatit risks the health of the astronauts. Leaving from Mars should be very similar to thisprocess, but quicker as Mars gravity is weaker than Earth’sLeaving the asteroid will be different than landing on it, as the escape velocities are14housands of times lower than any planet. In theory this transfer should be a simplereverse of leaving a planet, but this time, the astronauts have to enter the atmosphereand precisely land at their target. With proper care and planning, this can be done, butit will not be simple.
Domesticating a fleet of asteroids will not be cheap, nor perhaps cost effective in theshort term, but it is the long-range returns that make NEO transportation worthwhile.The international Space Station has orbited for more than 15 years and will continuedoing so for many years to come, but at some point the space station will reach theend of it’s life and be retired. The data gathered onboard the ISS cannot be collectedanywhere else, making it well worth the 150 billion dollars it has cost so far. Thus, thedomesticated asteroid orbiting around the Sun will be worth the trillions invested in themas they serve humanity. The benefits of such a mission extend much further beyond thesimple monetary, scientific, and humanitarian returns it will generate. It offers a way toprotect Earth and humanity. To date, Earth has no defense against asteroids and cometsand we bear the scars of massive impacts in the past. The likelihood of such a collisionis relatively admittedly small, but using the orbit modification techniques developed todomesticate the asteroids, humans can remove the most dangerous NEOs that threatenEarth and humanity. The safety of our planet and continuation of our species is worthany price.
We believe humanity is on verge of new era in which humans will expand their habitatto other celestial bodies. The advent of this new era is driven by the combination ofhuman curiosity and drive to einsure human survival. Radiation poses one of the greatestthreats to the successful expansion of our race throughout the solar system and beyond.Without proper protection from this hazard, future space travelers may either die or begenetically altered. This paper put forward the idea that the use of NEOs is a possiblesolution for protecting humans against radiation en route to a new planet or celestialobject. This idea should be investigated further in the future as a supplement to other15adiation protection efforts.Although NEOs have primarily been viewed as hazardous to Human survival on Earth,we have developed a different perspective allowing us to take advantage of the propertiesthat make them dangerous. In the same way that fire, an incredibly destructive force,was mastered in order to advance and expand human society on Earth, we believe thatNEOs can be used are needed to expand human society across space.