contract N.: 12756/98/NL/JG(SC)
contractor(s):DLR (D) with Alenia Aerospazio (I) and Technicatome (F)

System Concepts, Architectures and Technologies for Space Exploration and Utilisation (SE&U Study) Executive Summary
 
 

The study "System Concepts, Architectures and Technologies for Space Exploration and Utilisation" (SE&U) has been carried out on behalf of ESA in 1998/99 by an European consortium under the lead of the German Aerospace Center (DLR).
Other contractors included Alenia Aerospazio (ALS), Technicatome (TA) and several individual consultants.
The goal was to identify promising opportunities, key capabilities and technologies for the exploration and utilisation of space within the next 30 years.
The study focused on the region between Venus and Mars including, specifically, the Earth-Moon system, Mars,
near-Earth asteroids and comets.
Scientific motivations were an important aspect in all potential SE&U activities under study, but economic/commercial and technological aspects were paramount in addressing central questions, such as, where should we go and what are the requirements and potential benefits.

Initially, a large variety of viable SE&U applications describing the "what to do" in a coherent space exploration effort was reviewed.
Applications considered of high priority were examined by employing a qualitative and quantitative evaluation procedure. The approach taken involved a structured decision process that provided a reproducible basis to determine the value of the applications relative to each other.
The evaluation criteria were:
credibility/feasibility, benefit/ justification, affordability and benefit for other applications. Combining the most promising applications with respect to the question of "how to do something" in order to achieve important SE&U objectives resulted in four distinct SE&U reference scenarios (Fig.1).
These scenarios were analysed in detail and a technology roadmap was developed. Finally, programmatic aspects of the scenarios were examined.
 
 

The attractiveness of Mars as a target of space missions has dramatically increased with recent scientific discoveries and considerable public interest was raised.
Mars exploration gradually develops into a multi-national cornerstone of SE&U.
Currently planned missions involving almost a dozen US, European and Japanese probes will lead to the expansion of our knowledge about Mars by several orders of magnitude within the next 10 years.
These missions include orbiters, landers, rovers, sample return vehicles and in situ resources utilisation (ISRU) experiments. In view of manned missions proposed to be launched, perhaps, around 2020, a second preparatory phase of robotic Mars exploration could commence after the first set of samples has been retrieved.
This subsequent phase should provide an integrated network of robotic Mars stations, advanced exploration equipment including drills and unmanned reconnaissance planes as well as provisions (cargo) and reliable ISRU plants for the manned landings. Assuming that robotic missions to Mars will cost in the order of several hundred million dollar each, the total costs of all intended missions over the next 17 years would remain below
$ 5 billion, which is equivalent to less than $ 300 million per year.

Resources to commence with the manned exploration of Mars may become available following the end of the build-up phase of the International Space Station (ISS) around 2005.
NASA’s Reference Mission scenario [1] represents the latest specific concept for a manned Mars exploration endeavour.
In its 3.0 version (RM3.0; NASA) this scenario provides a split mission concept in which three manned missions each including a crew of six and involving two unmanned cargo flights are launched over a period of about 10 years.
The proposed Trans Mars Injection (TMI) stages feature nuclear propulsion systems. Reduced launch costs were assumed to be achievable by the development of the Magnum launcher (payload capacity: 75 mt) which is an inline core vehicle with two
fly-back boosters based on Shuttle technology.
This scenario together with five other modified sub-scenarios was analysed in this study with DLR’s software tool FAST
(Fast Assessment of Space Technologies) to provide detailed information about the costs, masses and technologies and to reach further optimisation:

1. NASA RM3.0 with nuclear thermal TMI stages and ISRU plant on Mars
2. Modified RM3.0 with chemical TMI stages and ISRU plant on Mars
3. Modified RM3.0 with chemical stages without ISRU plant on Mars
4. Single Stage To Mars (SSTM) with ISRU plant on Mars and Phobos
5. Modified RM3.0 with solar electric TMI stage (using 304 ESA-XX engines)
6. Modified RM3.0 with solar electric TMI stage (using 21 RIT 100 engines)

The main results of these analyses are shown in figure 2, indicating, for example, that the lowest masses at LEO
(Low Earth Orbit) departure appear to be achievable with solar electric TMI stages.
A life cycle cost reduction of several billion dollars may be possible in various sub-scenarios with respect to the $ 42.2 billion dollars calculated for the first manned mission within NASA’s reference scenario.
Especially, the use of nuclear propulsion proposed by NASA appears to be problematic in view of technical and political issues.
Although the solar electric design has the strongest potential for mass and cost improvements it also requires the strongest developmental effort with respect to new technologies.
However, an intensified research effort in the near and medium future may dramatically improve the feasibility of this option. Presently, the second sub-scenario with chemical TMI stages and propellant production on Mars appears to be the most promising option and is recommended as a new baseline mission.
The fourth scenario involving a single-stage spacecraft, which is re-fuelled on Mars and Phobos suffers, for example, from the lack of direct evidence for accessible water on Phobos rendering this scenario rather hypothetical.
Despite the uncertainties still afflicting manned Mars mission concepts, estimates of the total program costs for three manned missions of about $ 52 billion correspond to relatively modest annual costs of $ 2.6 billion for a 20 year program.
This amount, for example, is roughly equivalent to the annual budget for Space Shuttle operations.
Thus it appears that robotic preparatory missions can lead to an affordable manned mission scenario which could result in the first manned Mars landing fifty years after Apollo.
 
 

"Science of the Moon" addressing the origin and evolution of the Earth-Moon system and "science on/from the Moon" involving a large variety of experiments and observations on/from the Lunar surface have long superseded national prestige as the main driver of Lunar exploration.
This SE&U scenario also provides numerous opportunities and challenges as a technological testbed especially in view of future manned missions to Mars and other planetary bodies.
The current level of knowledge about Lunar materials and the potential presence of water ice in the polar regions have resulted, for example, in increased interest in ISRU applications.
A stepwise approach towards Moon exploration as proposed in the resolution of the International Lunar Workshop in Beatenberg, Switzerland, in 1994 [2] involves as a first step the unmanned exploration of the Moon, aiming to fill significant gaps in our knowledge.
This goal may be achieved within the first decade of the new century and would require orbiters, landers and rovers with ranges of several ten to hundred kilometres.
Proposed European missions such as LUNARSAT and SMART-1 may play a crucial role in this phase but are jeopardised by ESA’s current level of funding.
Subsequent steps reaching their peak of activity after 2010 would involve more sophisticated systems of great diversity, such as automated observatories, autonomous rovers and sample return devices as well as pilot plants for oxygen, power, fuel and eventually food and construction material production.

In the case that it becomes obvious during these exploratory missions that a human presence on the Moon is mandatory for scientific and other reasons, a forth phase with manned missions to the Moon can be envisioned to resume around 2020 thus establishing an initial lunar outpost that may ultimately evolve into a permanent Lunar base.
This phase would require an increase in crew size from about two up to eight and would also feature gradually increasing stay times on the Moon.
However, a manned mission scenario is strongly dependent on improvements in international co-operation, efficient and innovative use of Lunar resources (mainly lunar oxygen bound in silicates and oxides) and on developments towards adequate transportation vehicles, several of which can be derived from elements of Apollo, Shuttle and conventional launchers (Fig.3). Based on the assumption of a ten year development period, the projected costs for building and operating for nine years a growing manned Lunar infrastructure are in the range of about $ 85 billion.
An alternative and more optimistic estimate with respect to advances in cost saving technologies, such as a cheap reusable transportation infrastructure, results in costs of about $ 57 billion.
Extensive ISRU use has the potential for additional savings of at least 10%.
In general, an increased European effort in Moon exploration appears to promise valuable technological and scientific returns at moderate costs during the robotic phases.
Applying an efficient use of cost saving strategies, a manned return to the Moon appears to be affordable and may be realised prior to the first manned landing on Mars, if the necessity emerges and can be demonstrated to the public.
 
 

The growing world energy demand and the environmental and safety problems associated with conventional power plants warrant the consideration of solar energy as a major future power source.
The collection of solar energy in space and the subsequent microwave transmission to Earth is one proposed option, involving Solar Power Satellites (SPS) which feature very large structures with photovoltaic cells in space.
Such SPS systems would be favoured with respect to insolation by a factor of almost five compared to terrestrial Solar Thermal Power Plants (TPP) (1350 W/m2 vs. 300 W/m2).
Complementary, Power Relay Satellites (PRS) could be employed to transmit energy produced, for example, by TPPs located in regions of high insolation, such as deserts.
PRS systems inherently show a simpler and smaller design than corresponding SPS systems, but the required microwave power transmission technologies would be similar.

In order to constrain future technological requirements and to compare terrestrial and space based solar power applications, two terrestrial solar energy concepts are discussed in the analyses.
The Solar Thermal European-African Power Link (STEP) consists of a 33640 km2 (183 km edge length) sized TPP in the Sahara desert and would produce 500 GW of electricity.
The energy is converted to high-voltage direct current (HVDC) which is transmitted to Europe via 2700 km of 800 kV power lines.
Hydrogen generation plants and storage facilities are also included to ensure a continuous power supply.
An alternative to H2 storage is the extension of STEP into a Global Energy Network (GLEN) system.
This system, consisting of interconnected TPPs placed in deserts and distributed all over the world, is dimensioned to cover the projected global electricity demand in 2020 at any time of the year (~2.6 TW).
The total TPP area required for this system is about 94000 km2 (300 km edge length).
Although characterised by huge total costs of $ 28000 billion (STEP) and $ 198000 billion (GLEN), respectively, these terrestrial systems could provide electricity at a price of $ 0.20-0.30 per kWh.
Such systems appear to be technologically feasible and the land use seems acceptable.
All technologies required are or will soon be available and an incremental approach could be pursued.

To be competitive with these terrestrial systems a mass specific power production of, at least, 200 W/kg, transportation costs of 1500 $/kg and hardware costs of 5000 $/kg are required for SPS systems.
A configuration with 500 GW output would cost about $ 44000 billion (0.40 $/kWh).
Much more advanced SPS systems may be possible in the distant future at a total cost of $ 5600 billion (0.05 $/kWh) [3]. However, the projected costs for all these SPS systems do not include the required ground infrastructure.
Even if such SPS systems can be designed to be cost competitive with terrestrial solutions, the technological demands and operational uncertainties would be immense.
One of the critical technologies are light-weight deployable or inflatable structures based, for example, on elements of NASA’s Sun Tower Concept [3] or the Sail Tower Euro SPS concept(Fig.4) derived from Solar Sail technologies currently under development at DLR [3].
A cost competitive European PRS system with a mass specific power output of 10000 W/kg can be more easily envisioned.
Such a system would be technologically much less problematic, especially because most of the development and testing can be carried out in a stepwise approach at relatively moderate costs. In general, the attractiveness of PRS and, to a much larger extent, SPS is strongly dependent on future launch costs as well as on the rate at which innovations in the fields of e.g. microwave power transmission and large deployable structures will become available.
 
 

Subscenario SSP-1

Wireless power transmission experiment on the International Space Station (ISS) to a free-flying subsatellite with on-board scientific experiments (material sciences, imaging, servicing)

• t.b.d. kW transmitted power at 2.4/5.8/35GHz (microwaves)
• study of microwave-plasma interaction (interaction of space plasma and microwave beam)
• beam-steering experiments, phase control
• optional: satellite-to-satellite power transmission to multiple receiving satellites in ISS vicinity (e.g. for future satellite constellations)

Subscenario SSP-2

Wireless power transmission from one terrestrial place to another via a geostationary Power Relay Satellite (PRS)

• large reflector satellites in GEO
• 10 GW transmitted power at 2.45 GHz (microwaves)
• increasing industrial involvement and commercial operations (ecologically driven)

Subscenario SSP-3

Solar Power Satellites (SPS) for terrestrial power supply

• "Solar Power Towers" in modular design, gravity gradient-stabilised in GEO
• power generation: modular from 7 MW to GW
• CFRP structure spreads sail-like solar array of thin-film solar cells
• microwave power transmission to ground at 2.45 GHz
• optional: fabrication of satellites from lunar or NEO materials

d) Space Tourism

The huge annual market volume of tourism on Earth ($ 3400 billion) has raised interest in the possibilities of Space Tourism.
Polls indicate that millions of people would be willing to spend up to $ 50000 for a trip into space.
The main obstacles are currently high transportation costs in the order of several ten million dollars per trip and safety aspects.
However, to make this huge market accessible a phased sequence of subscenarios (Fig.5) beginning with suborbital flights on rocket planes followed by short stays and extended Earth orbital tourism can be envisioned.
 
 

The diversity of the four SE&U scenarios requires the development of a large variety of capabilities and technologies.
Therefore, key capabilities which are crucial to secure competitiveness and/or leadership must be identified.
Capabilities required in each SE&U scenario were quantitatively evaluated with respect to commercial, scientific and transfer potential as well as with respect to public profile and status in Europe.
 
 

The space industry and space agencies are facing major changes which will have a strong impact on the programmatic aspects of SE&U.
For example, the establishment of novel management and funding schemes and emerging trends including increasing globalisation, a rising demand for deregulation and privatisation, new opportunities for commercialisation as well as the rich
dual- and multiple-use potential of space technologies are requiring new modes of interaction between the industry, governmental institutions and the public.
A significant effort should be directed towards improving the public perception of the benefits of SE&U and of the role of the agencies and companies involved.
Recent polls indicate that the level of public awareness with respect to NASA is dramatically higher compared with the European players?
 
 

The exploration of planetary bodies including the Moon, Mars and near-Earth asteroids has tremendous potential for short term scientific, educational and "cultural" benefits, but will certainly remain a largely non-commercial enterprise within the next decades (Fig.7).
Currently, commercial activities are mainly hampered by relatively high costs for "Access to Space" and thus launch cost reduction is of highest priority to make progress in this direction.
For Space Tourism and Space Solar Power a considerable commercial potential has been identified for the medium-term with Space Tourism being more promising than SSP scenarios. This is mostly due to the fact that terrestrial competitors for power generation including regenerative options significantly affect the affordability and justification of SSP.
In the long run the build-up of complex planetary infrastructures appears to be possible and may lead to a plethora of new markets and commercial activities.
This should be a central consideration in the layout and development of capabilities during earlier missions.
In a first step info-/entertainment could represent an initial bridge between science and markets that may be followed by a large number of commercially attractive space products.

[1] NASA (1997) Human exploration of Mars: The reference mission of the NASA Mars exploration study team, NASA, SP 6107, March.
[2] ESA (1994) International Lunar Workshop. Doc. No. ESA, SP-1170, November 1994.
[3] ESA (1999) Space Exploration and Resources Exploitation (EXPLOSPACE) Workshop.
20-22 October 1998, Cagliari, Sardinia, Italy, WPP-151.