IAC-04-IAA.3.8.1.02 EXPLORATION: A VISION FOR THE NEXT HUNDRED YEARS

R. L. McNutt, Jr. Johns Hopkins University Applied Physics Laboratory Laurel, Maryland, USA [email protected]

ABSTRACT

The current challenge of space travel is multi-tiered. It includes continuing the robotic assay of the solar system while pressing the human frontier beyond cislunar space, with as an ob- vious destination. The primary challenge is propulsion. For human voyages beyond Mars (and perhaps to Mars), the refinement of as a power source and propulsive means will likely set the limits to optimal deep space propulsion for the foreseeable future. Costs, driven largely by access to space, continue to stall significant advances for both manned and unmanned missions. While there continues to be a hope that commercialization will lead to lower launch costs, the needed technology, initial capital investments, and markets have con- tinued to fail to materialize. Hence, initial development in deep space will likely remain govern- ment sponsored and driven by scientific goals linked to national prestige and perceived security issues. Against this backdrop, we consider linkage of scientific goals, current efforts, expecta- tions, current technical capabilities, and requirements for the detailed exploration of the solar system and consolidation of off-Earth outposts. Over the next century, distances of 50 AU could be reached by human crews but only if resources are brought to bear by international consortia.

INTRODUCTION years hence, if that much3, usually – and rightly – that policy goals and technologies "Where there is no vision the people perish.” will change so radically on longer time scales – Proverbs, 29:181 that further extrapolation must be relegated to the realm of science fiction – or fantasy. There is a great deal of new interest in However, as we have now entered the sec- space and space travel with the recent (Feb. ond century of aeronautics, we have a base- 2004) publication of the Aldridge Commis- line to look back on. We can look at what sion Report2, efforts on developing current people thought might happen in the previous , and the ongoing series of hundred years, what really did happen – and missions to provide scientific exploration why, and, perhaps more to the point, what it (notably of Mercury, Mars, , and cost. ). NASA undergoes a “road mapping” Especially following the technical gains in activity on three-year centers that feed into rocketry and atomic power brought about by the overall Agency Strategic Plans, and this the second world war, there was a great deal type of planning has been extended in the of speculation of what we could, and would, recent work of the Aldridge Commission. do in space. Historically, Nonetheless, these exercises typically has always been aligned with an outward project only into a “far future” of typically 20 look. In the beginning, the vision was of

1 manned flights to earth and beyond. formance required of the Crew Exploration While that outlook gave way to robotic Vehicle (CEV) if it is to be up to that task. probes always going to a destination first, it has always, at least to the majority of people, Key Mission Elements been with the idea that eventually humans would follow. For any mission there are four key ele- ments: the case for going, the means to go, The Art of Prediction an agreement to go by all stakeholders, and a source of funds sufficient to finance the Future predictions can, and many times, expedition. are misleading. An example is the prediction The reason for going for the mis- from the early days of the automobile that it sions was the geopolitical situation while the would be soon supplanted by personal air- reason for going for robotic missions has planes for local travel, a scenario yet to be been a combination of supporting Apollo borne out. At the same time, such predic- goals (early on), science mixed with geopoli- tions, if grounded in limits imposed by phys- tics (the U.S. and Soviet Union were both ics and chemistry and for which there is a sending missions to the , , and technology path, can work out. Usually, the Mars early on), and science with some level implementation is not as easy as originally of lingering international competition (Mars thought because the Devil always is in the Express and Mars Global Surveyor) and in- details. An example is Von Braun’s 6400- ternational cooperation (Cassini/Huygens). metric ton ferry vehicles for carrying 39.4 The means for going, , metric tons to a 102-km orbit for building has a long history6 but significantly grew from Mars transfer vehicles4, 5. The propellants for the V-2 work of World War II and later inter- three stages were nitric acid and hydrazine. continental ballistic missile (ICBM) develop- The is a single vehicle with ment programs during the Cold War. The detachable solid boosters (using solid end of that geopolitical competition combined propellant) and a throwaway main tank car- with the current lack of deep-space commer- rying liquid oxygen (LOX) and liquid hydro- cial activity and limited Earth-orbital gen (LH2). It is a 2040-metric ton vehicle that markets has led to slower development in can carry 24.4 metric tons to a 204-km orbit. many areas, especially where radiation The Shuttle has better performance but with hardness of electronics is an issue. less cargo because a realistic altitude is Stakeholders include advocates of a mis- higher than Von Braun envisioned. Von sion, non-advocate reviewers (whose job is Braun also envisioned the driving cost for his to see that invested monies are well spent), Mars mission being the $500M for 5,303,850 those who are forwarding funds, and those metric tons of propellant for the ferry ships to who can be impacted positively by success transport propellant and supplies to Earth and negatively by failure. A well thought out orbit (this translates to ~$3.6B FY96$) during strategy includes all stakeholders and pro- the 950 flights by 46 vessels in eight months vides both framework and substance of an (one ferry flight every 10 days, 6 ferries as- agreement to do the mission. sumed to be out of commission at any given Finally, there must be a source of funds time). Given the historical costs of building sufficient and available in the needed time- and operating four Space Shuttles, these are phasing to support the endeavor. Such pro- obviously not the resilient, cost-effective ve- grammatic support has typically come from hicles Von Braun had in mind. However, his the government exclusively for non- calculations may give some into some communications . Typically new of the top level requirements of a human missions have significant new technologies mission to Mars that may translate into per- that require implementation (and sometimes

2 DRIVERS Science Questions Mission Science Objectives Distance / Objective Questions Time = Science Measurement Objectives Speed Required Instruments/Crew Instrument/Crew Resources and Data link Requirements Mission and Requirements Operational Data Product lifetime Analysis Product Science Result

FIG. 1: Answering solar system science questions is a “system of systems” problem. unanticipated development) and involve the the Earth-moon system will be driven by, next step in a non-routine, non-recurring and/or framed in terms of, science return scenario of exploration. Costing of such en- from various solar system bodies. While be- deavors is tricky at best and requires some ginning with the Moon and Mars, the es- “adequate” level of reserves that can be poused vision in principal reaches throughout drawn upon to support the resolution of un- the solar system. The science objectives are anticipated development problems. At the thus the generic drivers that trace down same time, some members of the through the hardware requirements. Appro- stakeholder community will be pressing to priate analyses should then be capable of keep outlays as low as possible. While such providing results that answer the posed tensions are “obvious”, space missions, as questions. Demonstration of such closure (at with other large engineering endeavors, in- least in principle) is part of the strategy that volve large sums of money that are not easily must be developed up front (Figure 1). The turned off mid-project if the budget is ex- global requirements that drive the mission ceeded. Overruns and missed schedules design, implementing technologies, and es- are, and have been, tolerated on many indi- pecially cost include: (1) how far are we go- vidual programs, because the termination ing, how long can we take, is this a flyby, cost can be too high if sufficient funds have rendezvous, or return mission, i.e. what total already been spent. However, such overages velocity change capability is required, (2) can put a programmatic end to follow-on what are the data link requirements, band- missions. width, coverage continuity, and (3) what is the required operational lifetime – driven by System of Systems both point (1) and reserve requirements – and including hardware, autonomy, sparing Currently, and for much of the foresee- philosophy, and consumables, both for the able future, deep-space missions (defined as mission and for a human crew, if there is non-Earth orbit), and certainly those beyond one.

3 THE PAST Those who seen to think that the Moon is the goal of interplanetary travel should "Those who cannot remember the past are remember that the Solar system contains condemned to repeat it.” – G. Santayana7 eight other , at least thirty and some thousands of asteroids. The Beginnings total area of the major bodies is about 250 times that of the Earth, though the The conquest of the solar system has four giant planets probably do not pos- been a favorite theme of science fiction writ- sess stable surfaces on which landings ers for well over a hundred years. Modern could be made. Nonetheless, that still interest in Mars was kindled by Percival leaves an area ten times as great as all Lowell’s suggestion of intelligent beings on the continents of the Earth – without that , a theme taken up in the early counting the asteroids, which comprise a writings of Edgar Rice Burroughs8 and oth- sort of irregular infinite series I do not ers. Contemporaneous technical work by propose to try to sum. Tsiolkovskii, Oberth, and Goddard laid foun- dations for the technical realization of travel Clarke9 notes that the practical uses for enabled off Earth. Realization of space travel (manned) space stations in Earth orbit would has been more difficult. As with many large- be as meteorological observation stations scale technical innovations, national interests and as relays for worldwide television broad- provided the main means of development, in casting. this case the Second World War and the en- suing Cold War. The first activity supported The Launch Record13, 14 the development of the V-2, and the second enhanced and consolidated those technical Beginning with Sputnik I in October 1957 efforts, first in the development of Interconti- and going through calendar year 1998, there nental Ballistic Missiles (ICBMs), and later in have been 3973 successful space launches, the development of the launch vehicles cur- the majority of which by the United States rently in use for manned, commercial, and (1161) and the former Soviet Union and scientific exploration purposes. Russia (2573). However, most of these mis- Immediately following World War II, sions have been to Earth orbit. Even ex- 9 10-12 Clarke and others discussed the appli- tending the database of lunar and planetary cation of fission power to to enable missions through the current calendar year, new capabilities. Clarke notes: there have been only 139, or which 112 have been successful. Thus less than 3% of all Before the year 2000 most of the major space missions launched to date have gone bodies in the Solar system will probably beyond Earth orbit (Figure 2). have been reached, but it will take centu- ries to examine them all in any detail.

4 Launches 1957-1998

140

120 US successful launches 100

USSR/CIS successful launches 80

60 Worldwide successful launches Launches

40 Successful lunar and planetary launches x 10 20

0 1950 1960 1970 1980 1990 2000 2010 Year

FIG. 2: Historical launches from 1957 to 1998. Successful interplanetary launches through 2004 have been added from NASA websites. All launches, including the percentage that were beyond Earth orbit continued to grow until about 1965 when the number of U.S. launches began to taper off. The downturn in Russian launches coincides with the end of the Cold War era. Lunar and interplanetary launches include all countries. Following a peak in the mid-1960s, they have tapered off to 10% or less of the U.S. only launches.

Interplanetary Robotic Probes The Outer Planets. Following the success, NASA focus turned toward Discussion of Earth orbiting satellites ac- the outer solar system. and 11 celerated during the planning for the Interna- were the first probes to reach and tional Geophysical Year (IGY)4 and culmi- Saturn, respectively, and showed that nated with the Sputnik launch of October spacecraft could traverse the asteroid belt 1957. The problematic U.S. Vanguard project with a minimum number of dust hits. Voyager was moved aside and the successful U.S. 1 and 2, spacecraft that were descoped from Explorer I satellite was place in orbit in Janu- the original Grand Tour mission concept15, ary 1958. In the U.S. the National Aeronau- followed up with more intensive detailed tics and Space Administration (NASA) was studies of the Jupiter and Saturn systems. established in 1958 to coordinate all non- followed the original Grand Tour military, scientific space activities. A combi- trajectory, providing the first in depth studies nation of scientific and national goals led to of the Uranus and systems. Al- an of satellite launches with an though the Pioneers have fallen silent, the early interest in lunar and later Venus and two Voyagers continue to broadcast back to Mars probes. Trends are shown in Figure Earth data on the interplanetary medium – 213,14. Historically (in the U.S. at least), focus and perhaps its interaction with the very local on the Moon (Ranger, Surveyor, Lunar Or- (VLISM). biter) and early interplanetary probes (Pio- Voyager results motivated a desire for neer 5-9 to measure interplanetary radiation) more in-depth study of the outer solar sys- supported the manned lunar landing effort . Detailed study of the Jovian system was announced by President Kennedy in May provided by the recently completed 1961. Both the U.S. and Soviet Union sent mission. Such studies of the Saturnian sys- scientific probes to Venus and Mars. tem are just now commencing with the Cas-

5 sini/Huygens mission, a joint effort by NASA landers, the life-finding experiments provided and the (ESA). disappointing and contradictory results, now Venus. Venus exploration can be split thought due to the highly ionizing conditions into orbital missions that probed the space in the top layer of Martian regolith at the environment of the planet and used radar to landing sites. penetrate the permanent clod deck and A hiatus in robotic Mars exploration en- landers that gave brief spot measurements of sued for 20 years, including the failed Soviet conditions at the surface. From the mid- Phobos missions of 1988 and the failed 1960’s to the mid-1980’s the then-Soviet NASA Mars Observer mission of 1992. How- Union built up a capability culminating in ever, Mars exploration is back as a mainstay photographs and composition data from the of world scientific investigation. Starting with surface of Venus. the Mars Global Surveyor (NASA – Flybys of Venus by the U.S. probes Mari- still operating in extended mission), suc- ner 2, 5, and 10 measured the global tem- cessful mission include the NASA Mars perature of the planet, showed the lack of a Pathfinder rover mission (second of NASA’s global magnetic field, and provided informa- Discovery line of missions), NASA’s Mars tion on the interaction of the planet with the Odyssey orbiter (operating), ESA’s Mars Ex- solar wind and the global cloud circulation press Orbiter (operating), and NASA’s pattern. (Mariner 2 also provided conclusive and rovers (operating in ex- evidence for the solar wind, and Mariner 10 tended missions). Problems have continued went on to provide the first – and only to date as well as evinced by the failures of Nozomi – observations off the planet Mercury). (Japanese Space Agency), Mars Climate Orbital studies by Pioneer 12 (also Pio- Observer, Mars Polar , Deep Space 2 neer Venus Orbiter or PVO), atmosphere (all NASA), and Beagle 2 (ESA/UK lander). studies by Pioneer 13 (also Pioneer Venus Multiprobes), and radar studies by PVO and Capacity for Future Growth then the spacecraft in the 1990s have completed Venus investigations to Whether financed by governments or by date. Venus Express, an ESA mission based private interests, space exploration is an ex- upon the successful Mars Express space- pensive undertaking. Unless and until com- craft, is to return to Venus in the near future. mercial advantages can be identified for Mars. While Mars first evoked public in- deep-space exploration, missions beyond terest in solar system travel, initial results Earth, and almost certainly beyond the Earth- proved to be discouraging. Mariner 4’s flyby Moon system will remain financed by individ- provided 21 pictures showing a cratered ter- ual governments or international government rain and measurements of a very low atmos- consortia. pheric pressure. More data were accumu- Capacity for such growth is, therefore, lated by the U.S. Mariner 6,7, and 9 space- tied to economic activity and levels capable craft and by the Soviet Mars 2-7 probes. of supporting such continued expansion. Figure 3 shows records of U.S. economic An all-out effort was made by the U.S. to 16 determine whether life existed on Mars with indicators from 1789 through 2002 . During the and 2 missions that consisted of this time, this reconstruction of financial ac- orbiters that mapped the planet and its satel- tivity shows a gross domestic product (GDP) lites Phobos and Deimos and landers. The in fixed-year (1996) dollars increasing from landers carried sophisticated atmospheric ~$4 billion to ~$9,400B as the population and chemical instruments for measuring at- grew from ~3.8 million to ~280 million peo- mospheric properties and detecting life and ple, yielding a GDP per capita increase of a were powered by radioisotope thermoelectric factor ~30 over ~200 years. The economy generators (RTGs). While the changing of exhibits an e-folding time of ~30 years that the Martian seasons were studied by the has yet to show signs of decelerating.

6 Gross Domestic Product (U.S.) y = 4E-28e0.0361x 2 100,000 R = 0.9976

Nominal GDP (billions of dollars) 10,000 Real GDP (billions of 1996 dollars) 1,000 GDP Deflator (index 1996=100) 100 Population (in millions)

10 Nominal GDP per Capita (current Economic Indicators dollars) 1 Real GDP per Capita (1996 dollars)

0 Expon. (Real GDP (billions of 1996 1750 1800 1850 1900 1950 2000 2050 dollars)) Calendar Year

FIG. 3: U.S. economic and population trends 1789-200217. The GDP in fixed-year dollars shows an e-folding time of ~28 years.

While corresponding figures for developed LEO is ~20 tons). While acknowledging the and developing countries were not as readily possibility of advances leading to nuclear available for this study, the continued eco- rockets, von Braun’s scheme was based nomic capacity for human exploration ap- upon nearer term chemical technology in pears to be present. which case there was a need of 5,320,000 tons of propellant to assemble (and fuel) the Manned Planetary Mission Studies Mars ships. The author noted that “about 10 per cent of an equivalent quantity of high There have been no human excursions octane aviation gasoline was burned during beyond Earth orbit except to the Moon with the six months’ operation of the Berlin Airlift” the Apollo missions. These expeditions in- in 1948-1949. Estimated total cost for clude the lunar flybys of , 10, and 13, launching the mission into Earth orbit as and the landings of , 12, and 14 $500 million (now over $3B as noted in the through 17 (Apollo 7 and 9 were checkout introduction). missions in Earth orbit). In discussing the many Mars mission Das Marsprojekt. The first detailed study plans, Portree4 notes that von Braun’s think- for manned planetary mission requirements ing was influenced by the then-recent Ant- 5 was carried out by Von Braun for a manned arctic Operation High Jump, a U.S. naval ex- Mars expedition (in the appendix to a novel pedition in 1946-1947 consisting of 4000 4 that was never published ). The plan pro- men, 13 ships, and 23 aircraft. Von Braun vided for ten 4000-ton ships with a total crew explicitly stated that such a mission would of 70 assembled in Earth orbit with ten crew tend to resemble Magellan’s expedition per manned ship and three cargo ships. Nine around the world due to the need for self- hundred and fifty flights of three-stage ferry reliance of the mission. rockets would be required to assemble the Von Braun’s assertion in the late 1940’s flotilla in low-Earth orbit (LEO), each ferry that any realistic mission to Mars with hu- using 5583 tons of nitric acid and alcohol mans aboard would tend to resemble Ma- (later modified to hydrazine) to put 40 tons gellan’s expedition and have the same cost into orbit (the Space Shuttle capability to as a “small war” rapidly evolved with a vari-

7 ety of NASA studies (Early Manned Plane- three remaining ships reached the Pacific tary-Interplanetary Roundtrip Expeditions or Ocean through the Straits of Magellan. Fol- EMPIRE17, 18) and, after President Kennedy’s lowing arrival at the Spice Islands on 6 No- 1961 speech, with considerations of archi- vember 1521 with 115 crew left, Magellan tecture elements for reaching the Moon by was killed by locals before the remnant of the the end of the 1960’s. The focus was on expedition left for home. On 6 May 1522, Mars, but many of the EMPIRE concepts in- Victoria rounded the Cape of Good Hope un- cluded a manned Venus flyby on the return der command of Juan Sebastian de Elcano; leg to minimize propellant requirements. 20 of the crew died of starvation before Flyby-only missions were also advocated to reaching the Cape Verde Islands; another 13 drop probes, based upon the (then) notion crew were abandoned there on 9 July (Cape that robotic probes were too unreliable to Verde was a Portuguese holding). Eighteen drop probes successfully to Mars surface (a men returned on the Victoria to Seville in notion borne out by the track record of ro- 1522; four of the original 55 on the Trinidad botic probes up to that time). reached Spain in 1525. The voyage had Magellan’s Voyage.19, 20 It is worth re- covered 14,460 leagues (69,000 km) calling that while this first circumnavigation of The Victoria reached Spain one day early the world is ushered in as one of the defining despite maintaining a careful log; they had moments of exploration, it was entirely moti- gained a day by traveling west. In spite of the vated by combined geopolitical and eco- loss of four of five ships, the return of 26 tons nomic goals of determining the location of of cloves along with nutmeg, mace, cinna- the Space Islands and breaking the Portu- mon, and sandalwood covered the expedi- guese monopoly solidified only some ten tion cost. years earlier. The expedition began 10 The cargoes were of sufficient value per August 1519 as five ships left Seville for San weight that voyages continued involving Lucar de Barrameda; five weeks later on 20 Portuguese, Dutch, British, and American September 1519 they sailed (Table 1). traders. Fortunes and empires were made and lost, but the stakeholders of the next TABLE 1: Magellan’s Expedition three centuries saw the potential gains as Ship Ton- Crew Disposition outweighing the risk and upfront costs. nage “Small Wars”. With respect to the other Trinidad 110 55 Captured by the initial assertion of the cost being comparable Portuguese while trying to return via to that of a “small war”, comparison of finan- the Pacific cial costs of U.S. major wars allows such an San Antonio 120 60 Turned back dur- assertion to be put into perspective21. ing passage through the Straits TABLE 2: U.S. War Costs in Fixed 1990 Doillars. Conception 90 45 Abandoned in Fall, Conflict Cost in Cost per 1521 in the Spice 1990 B$ capita Islands (Moluccas) (1990$) Victoria 85 42 Reaches Spain 6 1.2 $342.86 September 1522 Revolutionary War Santiago 75 32 Wrecked on Ar- War of 1812 0.7 $92.11 gentine coast, Mexican War 1.1 $52.13 winter during Civil War 44.4 $1,294.46 southern winter Spanish American 6.3 $84.45 TOTALS 480 234 War World War I 196.5 $1,911.48 2091.3 $15,665.17 There were two mutinies early on, one World War II Korea 263.9 $1,739.62 before and one after reaching the South Vietnam 346.7 $1,692.04 American coast. On 28 November 1520, Gulf War 61.1 $235.00

8 Budgets Per Capita FY96$

100,000.000

10,000.000 Federal Budget (per capita U.S. per yr 1996 fixed 1,000.000 $)

100.000 Defense Budget (per capita U.S. 10.000 per yr 1996 fixed $) Dollars 1.000 GDP per capita (FY96$) 0.100

0.010 NASA Budget (per capita U.S. 0.001 per yr 1996 fixed 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 $) Year

FIG. 4: Comparison of fixed year (1996) GDP, U.S. budget, U. S. Defense budget (1940 on), and combined National Advisory Committee on Aeronautics (NACA) and NASA budgets on a per capita basis. The NACA budget is for the years 1915-1959 when NASA was formed. All budgets are based upon actuals from 1901 or later until 2002 and projections from 2003-2008.

Von Braun’s initial assertion of the cost of the extent that any such components would a Mars expedition lies between the financial be needed for a , outlays of the U.S. government for the Mexi- their development would have to come anew can War (1846-1848) and the Spanish- from a Mars-specific program. American War of 1898. These events can be Work continued on Mars mission con- seen more clearly in Figure 422, 23. Spikes in cepts based upon Apollo hardware, and, in the Federal and Defense budgets indicate particular, multiple launch vehicles the two World Wars; smaller localized in- for moving the hardware into orbit. With the creases indicate the Korean and Vietnam Mariner 4 flyby of Mars and the measure- conflicts. The large relative peak in the NASA ments of the atmospheric pressure, all notion budget is from the Apollo program (and the of aerodynamic landings such as used in the other programs supporting the manned lunar Von Braun scheme vanished. All notions of missions). the possible presence of advanced life on Downsizing the Architecture. On 11 July Mars vanished as well4. 1962, NASA announced the lunar orbit ren- In January 1966 the National Academy of dezvous (LOR) approach for getting men to Sciences (NAS) Space Studies Board (SSB) the surface of the Moon. This had the risk of issued their report : Direc- rendezvous and docking in lunar orbit but tions for the Future targeting “the near plan- required the lowest lunar spacecraft weight ets” for scientific focus following the manned and only one Saturn was lunar landing. A significant robotic mission needed per flight. Hence, this was the named Voyager, first proposed at JPL in cheapest, fastest way to get to the Moon. It 1960 as a follow-on to the Mariner flyby mis- eliminated the need for the larger Nova sions and designed to make use of the Sat- launch vehicle, for any Earth orbital assem- urn V launch vehicle was reassessed. The bly, and for a man-rated nuclear rocket. To new atmosphere data drove a redesign and

9 increased program costs, but launches were ploration of Antarctica and establishment of scheduled for 1971 and 1973.4 scientific bases there may have more rele- Within two years, the Mars Voyager mis- vance. The Antarctic continent was only dis- sions were cancelled and were replaced with covered relatively recently in 1820. Initial ex- the Viking orbiters and landers that subse- peditions were all privately funded, including quently flew to Mars. By 1973, the post- Amundsen’s successful traverse to the south Apollo program for manned exploration be- pole in 1911 and Byrd’s establishment of his yond Earth orbit had collapsed along with the Little America bases up through Little Amer- nuclear rocket development program. Plans ica III during his 1939-1941 expedition. for any future uses of the Saturn V were Highjump. While there had been growing shelved, and the Space Shuttle development commercial whaling and fishing competition was begun as a new initiative. off the shores of the continent, the modern Over the next 30 years, plans for manned history of Antarctica began for political rea- Mars missions have continued to be studied, sons following the end of World War II. The notably including the National Commission U.S. commenced operations of Task Force on Space Report of 1986, with priorities reset 68 in 1946-1947. The context was disputed by the following the loss of the claims of Argentines and British during World , the Space Explo- War II and growing interests in the possible ration Initiative of 1989, and the Design Ref- exploitation of natural resources following the erence Mission of 19974. Almost all conceiv- war by a variety of governments. able architectures were advocated in these With now Admiral Byrd in command, Op- and other studies. In these studies, required eration Highjump established the Little masses in low-Earth orbit (sometimes re- America IV base and involved 13 ships, 33 ferred to as “initial mass in low-Earth orbit,” aircraft and 4700 men. Larger than all previ- or IMLEO) are hundreds of tons and the es- ous expeditions put together, the operations timated costs are from tens to hundreds of surveyed ~350,000 square miles of the con- billions of dollars4. tinent and photographed ~60 per cent of the In each case the combined cost of infra- coast. structure and implementation were seen as Deep Freeze. High Jump was followed by prohibitive, and the programs were not Operation Deep Freeze I in 1955 that laid started. Whether the new exploration initia- much of the groundwork for U.S. participation tive24 will fare better remains to be seen2, 25. in the International Geophysical Year (IGY). While there have been some reports on That effort involved 12 countries establishing what a manned mission to destinations fur- some 40 scientific bases, and led to the de- ther from Earth than Mars would require – velopment of the Antarctic treaty governing notably very advanced propulsion systems26 treatment of the continent and its uses as a – the goal of Mars has been the focus of all multinational scientific exploration site. This “near-term” thinking. Here we take a more work continues to this day. The IGY marked global view of the drivers for solar system the start of the first permanent bases on the exploration, the requirements that follow, and continent, this action took some 137 years how the overall task can be divided up into from the first discovery. coherent, synergistic robotic and human components. Logistics. U.S. costs for sustaining the Ant- arctic research bases, infrastructure and fa- An Antarctic Paradigm?27,28 cilities have grown from ~$150M per year at the time of the IGY up to a current spending Discovery and Exploration. While the level of ~$200M per year28. drivers behind Magellan’s voyage do not ap- To support Antarctic operations, both pear to be strictly applicable to Mars, the ex- people and cargo must be moved in ant out.

10 In the 1990/1991 Deep Freeze operation, relatively large amounts of cargo that must ~7200 metric tons of cargo were transported be shipped in, the “breakeven” point for utili- (ship and air, intercontinental and intraconti- zation of local resources in Antarctica has nental) and 6200 personnel were transported not been reached. by air (intercontinental and intracontinen- tal)29. Extreme Engineering Limits? If we extrapolate over a 46-year period, the total mass is ~330,000 tons, the mass of Before contemplating large masses of the Empire State building, a rough estimate materials in space for eventual use, e.g. for of resources transported into and out of the deep-space human exploration and our es- continent. Looking at large masses that have tablishment of permanently crewed bases, it been transported by air, the Berlin Airlift is worth inquiring what the scale is of large transported 2.1 million metric tons (slightly engineering projects. To give an idea of less half the mass of the Great Pyramid of scale, Table 3 lists projects and items that Giza) over 14 months with ~280,000 airplane have been built and Table 4 others that have flights. This was a major undertaking, but the been contemplated. The largest deep space point is that it was not impossible27. vehicle built and flown is the ~30 MT Apollo With all of the constraints of costs and system, less than 1/1000 the mass of Von the terms of the Antarctic Treaty, in situ re- Braun’s original Mars expedition concept. In these examples human capabilities and source utilization (ISRU) has not been a vi- imagination span some nine orders of mag- able approach for providing needed re- nitude for “extreme engineering.” sources for the bases, e.g. for fuel for the transport and electrical power. Even at these

TABLE 3. Extreme Engineering: What We Have Built Item Type Mass (tons) Great Wall of China30 Structure 730,000,000 (est) Hoover Dam31 Structure 6,600,000 Golden Gate Bridge32 Bridge 811,500 Jahre Viking33 Ship (ULCC) 647,955 Empire State Building34 Building 365,000 Nimitz-class aircraft carriers35 Ship 102,000 Ohio-class submarines36 Ship 16,600 Eiffel Tower37 Structure 10,000 Saturn V at launch38 Rocket 3,038.5 An-255 Cossack39 Cargo plane 600 Statue of Liberty40 Structure 225 Apollo 1738 Crewed space 30.34 Cassini41 Robotic space 5.655 Notes: Mass of the Great Wall is estimated from a density of 2.0 g cm-3, length of 7300 km, average height of 10 meters, and average width of 5 meters; the Jahre Viking is an the largest Ultra- Large Crude Carrier (tanker) currently in maritime service; the Saturn V at launch is the mass as fully fueled prior to launch; is the mass injected from Earth to the Moon; the Cassini mass is the total mass launched to Saturn prior to any trajectory correction maneuvers or injec- tion into Saturn orbit.

11 TABLE 4. Extreme Engineering: What We Have Thought About Building Item Type Mass (tons) Interstellar Photon Rocket42 Crewed space 2,301,000,000 L5 (10,000 pop.)43 Structure 10,618,000 Das Marsprojekt propellant5 Propellant 5,303,850 Das Marsprojekt vehicles5 Crewed space 37,200 ROOST post-Nova LV44 Launch vehicle 16,980 Saturn V-D concept45 Launch vehicle 9,882.1 Jupiter/Saturn fusion ship26 Crewed space 1,690 Completed ISS46 Structure 453.6 TAU-probe (interstellar)47 Robotic space 61.5 RISE probe (interstellar)48 Robotic space 1.1 Notes: The photon rocket was based upon a round trip at one “g” acceleration to using total matter annihilation for thrust; ROOST - Reusable One Stage Orbital Space Truck; the Saturn V-D was from a Marshall Space Flight Center study of 1968; the ISS is the Interna- tional ; TAU is the Thousand AU mission; RISE is the Realistic .

orbit, to almost ten years for the Huygens STRATEGY AND REQUIREMENTS probe into ’s atmosphere, to even multi- ple decades for the Voyager spacecraft trav- “If you can fill the unforgiving minute / With eling to the interstellar medium. Scientists sixty seconds worth of distance run, / Yours can be patient; “reasonable” timescales are is the Earth and everything that’s in it…” set by design lifetime for spacecraft compo- – R. Kipling49 nents and budgets for mission operations and for the science teams as they await the The Previous NASA Strategic Plan50 data.

The NASA Strategic Plan of 2003 posed “The Vision for Space Exploration” three compelling questions that drive explo- ration: (1) How did we get here, (2) Where On 14 January 2004, President Bush es- are we going, and (3) Are we alone? As an poused a new vision for NASA24. There are actionable goal, these questions broadly map four points: into Mission II: To explore the Universe and (1) Implement a sustained and affordable search for life, and, in turn to Goal 5: Explore human and robotic program to explore the the solar system and the universe beyond, solar system and beyond; understand the origin and evolution of life, (2) Extend human presence across the and search for evidence of life elsewhere. solar system, starting with a human return to This approach, largely endorsed by the the Moon by the year 2020, in preparation for scientific community in the recent Decadal human and other desti- Study on solar system exploration goals, fo- nations; cuses within the solar system on under- (3) Develop the innovative technologies, standing the origin of the conditions for life knowledge, and infrastructures both to ex- on Earth, and possible origins of life else- plore and to support decisions about the where in the solar system. Focus has been destinations for human exploration; and, on sites with water and suspected pre-biotic (4) Promote international and commercial chemistry, namely Mars, , and Titan. participation in exploration to further U.S. For all of these primary targets, as well scientific, security, and economic interests. as other targets of exploration, the issue is If the logical extension of point (2) is to always one of timely return of conclusive sci- be considered serious, and if all entific data. In the case of robotic missions, stakeholders, both those of the new vision “timely” can be from a year for reaching Mars and those of the old plan are to come to-

12 gether to further space exploration, then a take 1 to 5 million lbs to LEO (454 to 2270 comprehensive plan for the exploration of the Metric tons). Such EHLLVs would be very solar system – or at least a framework for large and hence inherently expensive, so one – is needed. reusability would be required to be economic, Key issues are the sustainability and an idea studied in, e.g. the “Reusable Orbital public engagement of the effort over a space Module-Booster and Utility Shuttle" (ROM- of decades, especially where this is an act of BUS). public policy and not a war for survival. Un- Development costs would also be very derlying these issues are the assumptions large, but such vehicles could solve the ac- that will motivate the expenditure of funds cess-to-space problem for transporting infra- required for this activity. structure to the Moon and beyond. Recalling the Antarctic example, suppose that ~7200 Access to Space metric tons of cargo was required to be shipped to Mars each year. To transport 600 “Once you get to earth orbit, you're halfway MT of cargo to Mars orbit, Von Braun needed to anywhere in the solar system.” 37,000 MT tons in LEO. Getting 7200 MT of – R. A. Heinlein51 supplies (whatever they are) to Mars orbit (and still not to the surface), would likely If the new initiative is to be sustained as need ~300,000 MT in LEO. With a 2000 MT the Antarctic program and not a short-term to LEO EHLLV, the transport could be done program like Apollo – and humans will be a with 150 flights, about a year at three flights major player in deep space – then one thing a week. Such an undertaking would be nei- is clear: we will need to be able to move a lot ther cheap nor easy, but it could be done. of mass reliably, efficiently, and cheaply from Both in leaving Earth and traveling to other the Earth’s surface into orbit. Once free of destinations, propulsion is the key issue. Earth’s gravity, appropriate vehicles can be deployed – like Apollo – or built – like Von “Difficult” Robotic Missions53 Braun’s Mars vehicles, but we will need ca- pabilities well past those of the Space Shuttle The same missions that were “difficult” without the high costs. in the NASA 1976 Outlook for Space Study54 Costs of access to space can already be remain difficult today, primarily, but not ex- decreased if sufficient up front orders are clusively, due to the propulsion requirements. placed. If the architecture requires 20,000 A sample return mission from Mars is just metric tons in low-Earth orbit, an order for doable with current technology, providing 1000 heavy lift launch vehicles (HLLVs) with one is willing to pay the price. Sample re- a 20-MT to LEO capability will come in at a turns from Mercury and Venus need high lower cost per unit than an order for 10. thrust to deal with descent (at Mercury) and If to LEO will come in units of ascent (at both). In addition, round trip travel 1000 metric tons, then a new class oflaunch to Mercury is prohibitively long if chemical vehicle should be considered. Before the Lu- systems are used. nar Orbit Rendezvous technique was Titan orbiters, Uranus and Neptune sys- adopted for Apollo, “million pound to LEO” tem probes (similar to Galileo at Jupiter and vehicles (generically called Nova) were Cassini at Saturn) have also “been on the studied in the early 1960’s44,52. If we are seri- books” for almost 30 years. These have been ous about taking humans to Mars and be- joined by newer mission concepts such as yond, this old idea should be considered in Europa landers and submarines and Io orbit- the guise of a new class of launch vehicle. ers that also lie at or beyond current (chemi- Extremely heavy lift launch vehicles cal) propulsive capabilities. (EHLLVs) could extend the Nova-concept: The propulsive difficulties are generic. To

13 go to the outer solar system to any body and The Role of Humans (Do We Need Astro- into orbit with a capable science re- nauts?) quires much higher to be implemented, but can also be low thrust (but Robotic missions can be summarized as: not too low). To land on any solid – and air- Launch a probe to a target and the probe less - body from Mercury to Pluto requires returns knowledge. The problem is that with- high thrust at the target as well – and hence out return of all the data, there are always more mass. The problem is further exacer- lingering doubts about how the knowledge is bated if a sample is to be returned to Earth. to be validated (“Compression” leaves Depending upon the body, potential des- doubts of data fidelity). Return of all data re- tinations can be arranged into three groups quires large bandwidth that, in turn, drives as a function of their , indica- receiver and transmitter sizes and transmitter tive of the amount of high-thrust propulsive power. Thus data return drives power and, capacity required (Table 5) 53. hence, mass, that, in turn, drives propulsion. Even with all the data returned, there is al- TABLE 5. Escape Velocities for Some Objects ways still the lingering doubt of whether we Group Object Escape speed really – and correctly – thought through all of (km/s) the contingencies and possibilities ahead of 1 Earth 11.18 Venus 10.36 time. 2 Mars 5.02 Having humans in the loop can make a Mercury 4.25 large difference. An obvious example is the 3 Moon 2.38 hand selected moon rocks returned by astro- Io 2.56 Europa 2.02 naut/geologist Schmidt on Apollo 17 versus 2.74 the ~100 grams of lunar material returned by 2.44 the robotic Luna 24 probe. However, if one 4 Rhea 0.635 also takes into account the cost differential of 3 Titan 2.64 4 Titania 0.773 the two missions, one immediately reaches Triton 1.45 two conclusions: (1) sending be- Pluto ~1.2 yond Earth orbit is really expensive and (2) one gets what one pays for. So there is a NASA’s is a first step role, but it will not come cheaply. While toward trying to solve these transportation eventual human missions to Mars are an as- difficulties. The first planned mission, the Ju- sumption of the exploration vision, it is, rele- piter Icy Moons Orbiter (JIMO)55 will use a vant to ask whether it is desirable – or even nuclear electric propulsion system powered doable – to take human missions beyond by a fission reactor to address science goals Mars, farther into the solar system26. in the Jovian system that cannot be ad- dressed with a single mission otherwise. Sci- The Requirements ence goals include the deduction of internal structure and the possible presence of liquid Sample return missions, or human mis- water beneath the ice crusts of these worlds. sions (which also involve a return) require A June 2003 NASA workshop identified 8 either double the time or quadrupling the science areas, 33 objectives, and 102 inves- propulsive requirements of a simple, fast tigations requiring 188 different types of flyby mission. If we take ~50 AU as the char- measurements. The main question will be acteristic outer size of the solar system (the cost: reactor + certification! + spacecraft + heliocentric distance to Pluto from the instruments/science. near its aphelion in ~2113 as well as the distance to the most distant Ob- jects seen to date), then the required mission

14 time sets the required propulsive capability. respectively. In contract, the Apollo space- With a human crew, a maximum mission craft at translunar injection had a capability duration of four years (longer than Magel- for a mass of ~48,000 kg, about 10 times as lan’s voyage!) would still be long, and per- much for the sample return missions. The haps unacceptable from both psychological Viking 1 and 2 spacecraft had masses of as well as radiation exposure aspects. In ad- 3399 kg. This can be contrasted with NASA’s dition, in the absence of a true 100% en- crewed Design Reference Mission (DRM, closed ecosystem, the mass of food, oxygen, manned Mars) of 1997 that required an initial and water for the crew increase with the mis- 303 tons4. This was a scrubbed version of sion time. To traverse the solar system in the DRM of 1993 that took 6 astronauts to four years (roundtrip), a two-year voyage to Mars for a 2.5 year mission including a 600- Pluto would require accelerating to ~25 day stay and requiring ~380 tons (including AU/yr and then decelerating by the same the propellant, but with some offset from the amount. The return voyage would require the manufacture of some fuel stock on Mars). In same performance, for a total onboard capa- any case, about 100 times the mass initially bility of 100 AU/yr or ~500 km/s of delta V. in low-Earth orbit is required (for a Mars mis- The corresponding specific impulse is sion at least) for a crewed round-trip mission ~51,000 s. This is significantly in excess of versus a robotic lander (the twin Spirit and even capabilities, and, Opportunity rovers had masses of ~830 kg). in the absence of practical controlled fusion, These numbers are also significantly less means that such a voyage could only be than the 40,000 tons in orbit required for the contemplated with a nuclear electric propul- initial von Braun concept. sion (NEP) system of a very advanced de- sign. Power Consumption. This approach can provide for a quick estimate of what the implied systems must Suppose the total DV capability is 50 provide. Shorter distances or longer times km/s with a total “burn time” of 4 x 150 days, yield less needed capability. A sample return i.e. ~1.6 years, at a constant mass flow rate. mission to Pluto near aphelion that took 40 Suppose also that the initial propellant mass years to bring a sample back would require a fraction is 60%, translating into a mass ratio delta V capability of 4 x (50 AU/20 years) = of 2.5 and a required exhaust speed of ~55 10 AU/yr ~50 km/s. Similarly, a crewed mis- km/s (specific impulse of ~5600s). For a sion to the Jupiter system at 5 AU with a 3000 kg spacecraft, the initial propellant load roundtrip time of 4 years would also require a would by 1800 kg. If this amount were to be delta V capability of 4 x (5 AU/2 years) = 10 processed over 600 days, the required mass AU/yr ~50 km/s, assuming that the accelera- flow rate is 3 kg/day or ~35 milligrams per tion time is short compared to the cruise second. The implied exhaust power is 51.7 time. For example, a constant acceleration of kW, and acceleration is in the 10s to 100s of 0.001 g would build up to 12.5 km/s in about micro-gs. For a prime power conversion effi- 15 days. Reaching 125 km/s would require ciency of 20% using an NEP system, the re- 150 days at the same acceleration. actor thermal power would need to be ~260 The primary difference between crewed kWth. These parameters are close to current and robotic missions is mass. Even compar- specifications for the reactor concepts being ing with a sample return mission with the studied for the Jupiter Icy Moons Orbiter same time scale for mission completion, (JIMO) spacecraft under Project Prome- mass, and its associated cost, remains the theus. principal driver. For example, the Soviet lu- For a crewed expedition of ~300 metric nar sample return mission Luna 16, 20, and tons, and the same scalings, the power con- 24, had masses of 5600, 5600, and 4800 kg, sumption required would be up by the in-

15 creased mass flow rate, a factor of 100 to dred years this newly won knowledge will ~5.2 MWe (megawatts electric), with a ther- pay huge and unexpected dividends.” mal reactor power output of ~26 MWth, con- – W. von Braun59 sistent with Stuhlinger’s NEP system de- signs56,57 of the 1960s. For either of these To discuss future possibilities, one has to cases, an efficient vehicle should have a make many trials, with the knowledge that 2 power plant specific mass a of ~t /vexhaust , in most – if not all – of them will miss the mark. this case ~17.5 kg/kWe, implying a power Where we currently are includes a robust ro- supply of ~905 kg (out of 1200 kg dry) and botic program at Mars, a new probe to Mer- 90 MT out of 120 MT, respectively58. cury, an ongoing program to robotically reach For a far ranging mission with the same Pluto, and a stalled human program. Future propellant burn time (300 days out of a cruise plans now being acted upon show promise leg of 2 years) and an initial mass of 300 for enhancing robotic missions with nuclear tons but with a total DV of 500 km/s, the ex- electric propulsion and a Vision for taking the haust speed would need to increase by a human race beyond Earth orbit, and staying factor of ~10 for efficient propellant usage. there this time. The past has shown us that But in keeping the power plant working effi- the robots will come first. If we combine this ciently, the power plant specific mass would principle with the goal of expanding the hu- need to decrease by a factor of 100 for the man presence throughout the solar system same “burn” time. during the next hundred years, then a struc- Human missions to the Jovian system ture for doing so can be imagined that starts may not be totally crazy, but certainly not with sample returns and ends with perma- easy, and far more difficult than a human nent bases and expansion into the outer so- mission to Mars. Human missions to more lar system (Table 6). Underlying all of these distant destinations will be even more difficult possibilities is the assumption that easy ac- – and certainly more massive. cess to space for large masses is available to develop self-sustaining infrastructure off- A PLAN FOR THE FUTURE Earth. In the meantime, we need to keep ex- “If you can keep your head when all about ploring with a diversity of missions to enable you / Are losing theirs and blaming it on you.” all levels of scientific enquiry. Pluto will be at – R. Kipling49 aphelion about 2113; we decide whether we will be there as well. It really is up to us. “The greatest gain from space travel consists in the extension of our knowledge. In a hun-

TABLE 6. A Future Exploration Plan – Selected Milestones Year Goal 2013 Mars Sample Return 2015 Interstellar Precursor Mission launched (“Interstellar Probe”) 2015 Launch autonomous robotic tour of Jupiter system (JIMO) 2020 Human launch to L2 for servicing of James Webb 2023 JIMO arrives at Jupiter 2025 Crewed expedition to a Near Earth Object – demonstrate hazard mitigation 2025 Launch robotic tour to Neptune system 2030 Permanently staffed lunar base 2030 Human mission to Mars 2035 Commence robotic sample-return missions to the outer solar system 2050 Human mission to Callisto (Jupiter system) 2075 Human mission to Enceladus (Saturn system) 2080 Permanent Mars base established 2090 Human mission to Triton (Neptune system)

16 CONCLUSION AND PERSPECTIVE questions of our origins and the origins of life in this system, but there will always be a “And pluck till time and times are done / The need for a clear, yet evolving concept of silver apples of the moon, / The golden ap- where we can ultimately go, if we want to… ples of the sun.” – W. B. Yeats60 ACKNOWLDEGEMENTS Although crewed missions beyond the main asteroid belt may remain problematic (if Portions of this work have been sup- even desirable) throughout this century, ported under the NASA Institute for Ad- there are still major scientific questions that vanced Concepts. The views expressed are can be answered only by going there, a entirely those of the author and not neces- situation that has not changed during the last 6, 54 sarily endorsed by the sponsor nor by the 30 years . Enigmatic Europa may contain Applied Physics Laboratory. even more clues than Mars about the evolu- tion of life in the Universe, yet actually re- motely probing beneath that moon’s icy crust REFERENCES will likely be as difficult as landing a field ge- 1. Proverbs, Ch. 29, v. 18, The Holy Bible, ologist on the Martian surface. All of the solar Authorised King James Version, Collins’ system must be considered as part of the Clear-Type Press, New York, 1952. ultimate Vision, if it is to take fire and go for- 2. Aldridge, Jr., E. C., C. S. Fiorina, M. P. ward. Jackson, L. A. Leshin, L. L. Lyles, P. D. A new perspective is required. Here we Spudis, N. deGrasse Tyson, R. S. Walker, are talking about a program that will last for M. T. Zuber, A Journey to Inspire, Innovate, decades and will – at some point – require and Discover, Report of the President’s large capital pieces of equipment. Again, the Commission on Implementation of United analogy of permanent science stations in States Space Exploration Policy, U. S. Antarctica comes to mind. For the solar sys- Government Printing Office, Washington, D.C. June 2004. tem to become our extended home, the effort 3. Belton, M. J. S., et al., New Frontiers in the involved must become “obvious” to public Solar System: An Integrated Exploration and politicians alike as a long-term goal in Strategy The National Academies Press, the national interest. Only in this way can Washington, D. C., 2003. progress be maintained toward the consoli- 4. Portree, David S. F., Humans to Mars: Fifty dation of the solar system during the next Years of Mission Planning, 1950-2000, century. Monographs in Aerospace History #21, Top-level elements and requirements in- NASA SP-2001-4521, NASA History clude communications infrastructure, modu- Division, Office of Policy and Plans, NASA lar hardware that is evolvable and scalable Headquarters, Washington DC 20546, February 2001. (not-unlike Airbus or Boeing airliners), role 5. Von Braun, W., , (English and characteristics of a crew exploration ve- trans.) The Univ. of Illinois Press, Urbana, IL, hicle (CEV) for multiple uses, minimum buys 1953. of large numbers of EELVs to keep unit costs 6. Gruntman, M., Blazing the Trail: The Early – and the cost of access to space – down, History of Spacecraft and Rocketry, need for cargo-capable HLLVs (or larger American Institute of Aeronautics and EHLLVs) to push past Apollo tasks on the Astronautics, Inc., Reston, VA, 2004. Moon, and prepare the way for Mars, roles 7. Santayana, G., from Chap 10. Flux and for L2 as a way-station and observatory for constancy in human nature, The Life of other systems, and roles for automated Reason or the Phases of Human Progress 82, Charles” Scribner’s Sons, New York, NY, CEVs wed to Prometheus vehicles for outer 1954 [ solar system exploration and sample returns. http://members.aol.com/santyana/gb/cgs.html The current goal is to answer fundamental Readers’ Comments on “A George

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19