Projects: Aim for Mars
Stepping into the Future
A Workshop in Memory of the Columbia Seven
On April 29-30, 2003, The Planetary Society,
the Association of
Space Explorers, and the American
Astronautical Society held a workshop at the George Washington University's Space
Policy Institute about the future of human space transportation. The following
was presented as a background paper to the workshop.
Transportation Concepts for Human Space Exploration Beyond Low-Earth Orbit
by Douglas Stetson
Introduction
Purpose
This paper is intended to provide background material and to stimulate
discussion of transportation techniques and propulsion technologies
that may be relevant to future human exploration of the solar system.
It is submitted to the "Stepping into the Future" workshop
on future launch vehicle issues, to be held in April 2003. It does
not comprise an exhaustive technical review of the literature or
of expert opinion in the subject areas; rather, it is intended
to provide a broad overview of realistic options and to present
some strawman architectures for discussion and possible further
study.
Scope
This paper, and the workshop to which it is submitted, will attempt
to take a realistic view of the issues, timeframe, and investments
facing human space exploration. Accordingly, not every conceivable
propulsion technology will be discussed here. Rather, in this paper
we will focus on those technologies that fall into one of three
categories: 1.) The technology exists today in some form and can
be extended to meet human exploration requirements; 2.) Some meaningful
technology development investments are either being made now or
are at least being seriously considered, so that the near-term
investment pathway is clear; or 3.) The technology is of sufficiently
broad benefit, and sufficiently well understood in concept, that
there is a reasonable likelihood that it could be brought to readiness
by the end of the next decade. Based on these criteria we will
give only passing mention to a number of highly speculative advanced
propulsion concepts which we judge to be too immature, given any
reasonable development funding, to be relevant to this discussion.
Overview of Imperatives and Top-Level Requirements
Cost-effective human exploration of deep space is clearly a tremendously
challenging problem, with an enormous spectrum of technologies
and implementation trades that must be considered. At the risk
of over-simplifying, we can try to write down some fundamental
requirements that govern our discussion of transportation beyond
low-Earth orbit. These requirements will provide a context for
our qualitative assessment of the relative merits of various options.
The requirements to minimize flight time and maximize mass delivery
represent the classical tradeoff faced by all planetary missions,
and there is in general an inverse relationship between those two
quantities. For human missions into the solar system the need to
minimize flight time will be even more important, since long exposures
to low-gravity environment and to deep-space radiation are known
to be extremely hazardous. Ongoing research aboard the International
Space Station is providing us with better insight into these issues,
which is the first step toward possible mitigation of them, but at
present these are very serious problems. Even if there is found to
be some reasonable means of ensuring crew physical health on long
missions, issues of mental health, happiness, and crew effectiveness
will always cause us to place a premium on flight time. So for the
purposes of our discussion we will assume that minimizing flight
time is at least "first among equals" as we search for
discriminators among the various transportation options.
Mission Imperative Derived Requirement on Transportation
System
- Crew health
- Minimize flight time in deep space
- Provide
sufficient mass for life support and countermeasures
- Crew safety
- Provide propulsive capability for abort modes/return
to Earth
- Mission success
- Provide large mass delivery capability
- Provide
access to a variety of solar system destinations
- Mission affordability
- Systems should be of broad benefit so
costs can be shared
- Leverage current investments and utilize
proven systems
- Do not require major new test facilities
Destinations
There are a number of solar system destinations that have been discussed
as suitable and important for human explorers. A companion paper
at this workshop describes in depth the scientific goals of human
presence at these locations. They are summarized below, with particular
emphasis on the transportation challenges imposed by each.
Mars
For our purposes, the ultimate destination for human explorers is
Mars. It may someday make sense to seriously discuss the reasons
and methods of sending humans farther into the solar system, but
for the time being Mars is far enough. Human exploration of Mars
will be science-driven and will help to fulfill an overall Mars
exploration strategy focused on understanding the history and evolution
of the planet, its biological potential, and the possibility that
life actually developed there. Sending a human to Mars to plant
a flag and return home will not be justifiable. This is important
because it implies that human explorers must have available at
Mars the tools and infrastructure they need to conduct intensive
scientific studies; that translates to mass that must be delivered
from Earth, and this will have important implications for our debate
of transportation techniques. This also implies that the human
exploration of Mars will be a continuing process, and we must plan
for multiple missions over a period of several decades. This too
may also affect our transportation technology decisions.
Mars has important assets which we may eventually exploit to the
benefit of human exploration. One is its atmosphere, which is well
suited to the use of atmospheric friction to assist in orbit capture
or orbital energy reduction. This can provide significant mass savings.
Another asset is martian resources, which may be suitable for propellant
production for the return trip to Earth. It should be noted that
both of these technologies would affect mission-critical events and
probably will not be fully utilized until they have been thoroughly
demonstrated on robotic missions.
A nuance to our Mars exploration strategy is that it may make sense
for the first human mission to actually go to one of Mars' moons,
most likely Phobos, instead of to the surface of the planet itself.
This would be done mostly for crew safety reasons as a validation
of end-to-end mission techniques and procedures, prior to undertaking
the challenging Mars landing and ascent phases. This scenario is
analogous to the Apollo program's phased approach to the lunar exploration.
Phobos may also have important intrinsic science value, and it may
provide a platform from which humans could remotely interact with
robotic explorers on the martian surface. Since Phobos is essentially
a small asteroid in Mars orbit, with very low gravity and no atmosphere,
access to its surface is much simpler than is access to Mars itself.
Near-Earth Asteroids
An intermediate destination for human explorers may be near-Earth
asteroids. There are compelling reasons for robotic exploration
of this class of bodies, and as our understanding progresses there
may become evident reasons for human exploration as well. Since
asteroids are the remnants of solar system formation, their structure
and composition can provide us with important clues to our origins
and evolution. Near-Earth asteroids also comprise the most serious
Earth impact hazard, and so an understanding of their diversity
and physical characteristics is a key to our ability to predict
and perhaps mitigate any impact threat. Near-Earth asteroids are
also frequently cited as a potential resource that could be exploited
for economic reasons, or as a potential source of propellant to
support human exploration of the solar system.
In addition to their scientific and resource value, near-Earth asteroids
provide an important stepping-stone to Mars. They are accessible
with relatively short flight times and provide us with an opportunity
to exercise many of the required transportation elements in a relatively
low-risk manner. If the first human mission to the martian system
is actually to Phobos, as described above, a precursor mission to
a near-Earth asteroid would allow us to demonstrate almost the entire
mission at a destination closer to Earth, with ample solar power
availability, high communications rates, and relatively short return-to-Earth
flight times that provide an extra measure of safety.
Libration points
The first destination for humans beyond low-Earth orbit may well
be one of the Earth-Moon or Earth-Sun libration points. The Sun-Earth
L2 point (SEL2) may be the preferred location for many of the large
telescopes that have been conceived for future observations of
the universe. Depending on their characteristics, it may be important
for humans to be able to travel to SEL2 to construct and/or service
those telescopes; the alternative would be to assemble and service
the telescopes closer to Earth, perhaps at an Earth-Moon libration
point, and robotically transfer them to SEL2. In addition to their
value as a scientific destination, the libration points may be
the appropriate "gateways" for staging of vehicles and
cargo prior to departure for more distant solar system destinations.
This can provide mass advantages due to the orbital mechanics of
libration points and the transfers between them. The distance of
libration points from Earth, and hence the flight time to reach
them, will be an important consideration in these decisions. It
is possible to enter "halo" orbits about the Earth-Moon
libration points within about a week, while the transfer to SEL2
takes a month or more.
The Moon
Among the destinations we're considering, the Moon always seems to
be the most controversial. On the one hand it is close by, so flight
time and communications issues are less severe. We've also already
been there, which can either be an advantage or a disadvantage,
depending on your point of view. There are important scientific
activities to be conducted on the Moon, but it is not clear that
they can't all be satisfactorily done robotically. One frequently
cited reason for going back to the Moon is for validation of techniques
for exploration of Mars; however, the environments are so different
that there may be less commonality among the required tools than
one might think, and the costs of going to the Moon may outweigh
the benefits in this regard.
Fortunately, however, lunar exploration probably doesn't impose
many unique requirements on transportation systems. We will assert
that the systems we develop to enable human presence at libration
points, near-Earth asteroids, and Mars will also enable lunar exploration,
if a decision is made that the Moon is an important destination.
The only exception may be in the development of large chemical propulsion
systems for lunar descent; since it is a large airless body, soft-landing
on the Moon is actually a fairly demanding propulsion challenge.
However, chemical propulsion technology will certainly be used elsewhere
in our exploration architecture, so no fundamentally new investment
would be needed to support lunar exploration.
Candidate Transportation Technologies
Given our understanding of the top-level requirements and destinations
for human exploration, we can assess the characteristics of candidate
transportation technologies. This will be done in a mostly qualitative
sense; detailed engineering trade studies are required to define
the technical parameters. However, we will make reference to on important
quantity: specific impulse (Isp), which can generally be thought
of as a measure of the efficiency of a propulsion system. In simple
terms, a higher Isp (measured in seconds) indicates that greater
DV can be provided for a given amount of propellant.
Chemical Propulsion
Chemical propulsion (CP) has been used on all previous planetary
missions, and so it offers the huge advantage of decades of refinement
and flight experience. It provides a relatively high thrust level,
which helps to keep flight times low; it can be started and stopped
numerous times during a mission; and it has a long lifetime in
deep space. However, even with potential technology advances that
have been identified (and are not presently funded), the maximum
Isp that can be anticipated for our purposes is only about 350-400
sec. Given this relatively low efficiency, a correspondingly large
propellant mass is required in order to deliver the dry mass needed
for a human exploration mission within a reasonable flight time.
If no other steps were taken, this large propellant load would
greatly increase the total mass that must be lifted from the surface
of Earth, which is in turn a major driver of launch vehicle size
and total mission cost.
Notwithstanding this handicap, the reliability and affordability
of CP make it very attractive, and it will almost certainly play
a major role in future human exploration beyond low Earth orbit.
The challenge is to select the overall architecture so that the mission
impact of the CP mass disadvantage is minimized. One option is to
use CP only for specific focused tasks, e.g. for Mars orbit insertion
or descent burns, for which its high thrust and reliability are of
paramount importance. Other, more efficient technologies could then
be used for primary interplanetary propulsion.
If the mass disadvantage can somehow be circumvented, however, CP
may also be viable as the primary propulsion technology to take humans
and/or cargo from LEO to their destinations. In general this requires
minimizing the total mass to be delivered by CP vehicles. One option
is to split the mission into two or more parts; for example, cargo
and crew could be sent on separate vehicles so the mass of any one
is not too large and the propellant mass penalty is not too high.
Another option is to use staging scenarios, whereby in-space assembly
or fueling are used to reduce the total mass that must be transported
by CP at any given time. Through the rocket equation this will reduce
somewhat the total mass to be lifted from Earth's surface; it will
also allow it to be done in smaller increments, thus reducing the
overall launch vehicle requirements. As will be seen, our preferred
architecture makes use of both of these options. A third possibility
is to use propellants derived from extraterrestrial resources for
return trips from asteroids or Mars, so that the return-trip fuel
would not need to be lifted from Earth at all (although the equipment
to produce it would). This will be discussed further in a later section
of this paper.
Electric Propulsion
There are several types of electric propulsion, but for our purposes
the class also known as ion propulsion is of greatest interest.
Ion propulsion systems use an electrically charged grid to accelerate
ions of a propellant (xenon, for example) to very high velocities.
Although the thrust produced is very low, when acting over long
periods of time in the vacuum of space this technique can provide
a large DV for a small amount of propellant. Specific impulses
over 3000 sec are possible with present technology, and plans are
underway to extend this up to 5000 -8000 sec within a decade. As
a gauge of the benefit of this technique, using present electric
propulsion technology a DV of 5 km/s can be imparted to a 1000
kg spacecraft using about 180 kg of propellant. It would require
over 2500 kg of chemical propellant to do the same job.
While the mass advantages of electric propulsion can be enormous,
the downside for human exploration is the low thrust level of these
systems. This means that for the destinations of interest to us,
flight times may be long compared to chemical propulsion. EP systems
do not provide sufficient thrust for rapid departures from LEO or
capture into Mars orbit; rather, they must gradually spiral into
or out of planetary orbit. They are also not useful for de-orbit
prior to descent to the martian surface, nor for the terminal braking
required for soft landing. They are, however, very well suited to
rendezvous with low-gravity bodies such as near-Earth asteroids.
For high-energy robotic missions to the outer solar system, EP systems
can far out-perform chemical propulsion, and EP enables missions
that would (for all practical purposes) be otherwise impossible.
Solar Electric
Solar Electric Propulsion (SEP) refers to ion propulsion using electricity
derived from solar power. One of the most significant advances
in solar system transportation in the past four decades occurred
in 2001, when NASA's Deep Space 1 spacecraft completed the space
flight validation of SEP. Our next use of SEP will be on the Dawn
main-belt asteroid orbiter, to be launched in 2006. That system
is being designed to provide an Isp of about 3100 sec and a total
propellant through-put of about 410 kg during the mission lifetime,
for a total DV of about 11 km/s. Thus SEP is considered an existing
technology, and the improvements that would be required to make
it useful for human exploration missions are well understood.
The capability of SEP is limited by the available solar power. Advances
in the specific power of solar array technology are expected during
this decade, and SEP power levels up to 50kW with Isp of 5000 sec
or more are forseeable. These systems could be an important asset
for human exploration, but flight times to Mars would still be relatively
long. In addition, solar array degradation due to radiation or micrometeoroid
impacts must be considered during the design phase. SEP is likely
to be used in a supporting role in an overall human exploration architecture;
for example, it can enable comprehensive robotic precursor missions,
and it can propel cargo vehicles for the delivery of large masses
to libration points and near-Earth asteroids.
Nuclear
Electric
By applying nuclear power to electric propulsion systems we can greatly
improve their utility. Nuclear Electric Propulsion (NEP) refers to
the use of fission-derived power instead of solar power for electric
propulsion. NEP is presently under development within NASA's space
science program as a means of enabling high-priority robotic missions
to the outer solar system, and thus any future human exploration
program may benefit substantially from these investments. A fission
reactor capable of producing up to 100 kW is planned, and this could
be pushed an order of magnitude higher without a fundamental shift
in technology. Such a system would last for many years and would
provide a large NEP propulsive capability (Isp's of 7000-9000 sec)
as well as a large available power source at its final destination.
Readiness of the initial robotic flight system is planned for about
2010. This system is being designed so that the fission reaction
does not start until the vehicle has been boosted into a 1000 km
orbit, so there is no risk of inadvertent re-entry of an active reactor
or radioactive fission products.
As with SEP, the main benefit of NEP is its very high fuel efficiency.
And while NEP can provide a somewhat higher thrust level, it is still
by definition a low-thrust system with the flight time disadvantages
described earlier. Nonetheless, any prudent human exploration architecture
should anticipate the availability of this technology and make good
use of it. We assert that the primary role for NEP will be to enable
massive cargo vehicles to travel from Earth to Mars, perhaps to enter
Mars orbit well ahead of a crewed vehicle. This cargo vehicle would
carry most of the supplies required for the exploration mission and
will serve as an orbiting resource in Mars orbit. This strategy would
free up the mass capability of a chemically-propelled crew vehicle
to concentrate on crew safety and life support with a minimum flight
time.
Thermal Propulsion
As a middle ground between the high thrust/low Isp of chemical systems,
and the low thrust/high Isp of electric propulsion, there exists
a class known as thermal propulsion. As with EP there are both
solar and nuclear variants, but for our purposes we will concentrate
on Nuclear Thermal Propulsion (NTP). This concept uses direct heating
of propellant gas within a nuclear reactor core to provide high
thrust, comparable to that of chemical engines, at an Isp of 800-1000
sec or more - which is two to three times that of the best current
chemical technology. NTP engines were developed and demonstrated
on the ground in the 1960's, though none have yet been flown. More
recently, design and concept development work continued in the
late '80s and early '90s with an eye toward strategic defense applications
as well as future NASA missions. NASA engineers have continued
to study and develop these concepts for consideration in potential
next generation in-space transportation architectures. As part
of this work they have also evolved the idea of a "bimodal" reactor
design that could be used to produce electrical power when not
being operated for propulsion.
From the point of view of mass and flight time, NTP may well represent
the best technology for human exploration beyond the Earth-Moon system.
However, although it is well understood in concept, there is no program
currently developing NTP flight systems (in contrast to chemical,
SEP, and NEP). Thus NTP is a technology for which the entire burden
of investment and advocacy would need to be borne by the human exploration
program. In addition, there are serious environmental issues and
infrastructure investments that would need to be addressed to enable
development and testing of NTP technology. Ground tests of NTP rockets
would produce effluent gases for which new handling and cleaning
facilities would be required. These investments and political concerns
are a significant hurdle, and so we assert that the preferred solution
is to establish a workable first-generation human exploration architecture
without relying on NTP. In the future, once a human exploration program
is underway and well accepted, the trade space could be re-opened
and NTP could be added as a means of expanding our exploration capabilities
and enabling more frequent missions.
Solar Sails
Solar sailing has been in existence in concept for many decades,
and there have been numerous attempts to develop and fly solar
sails. The Planetary Society has long been an advocate of this
technology and is currently planning to fly a solar sail test mission
in the near future. This concept relies on very lightweight films,
deployed over large areas, to develop thrust from the constant
impingement of solar radiation. Solar sails represent the height
of propulsion efficiency because they require no propellant at
all, just lightweight sails, booms, and deployment systems, along
with sophisticated flight control techniques.
Solar sail spacecraft can reach very high velocities and can provide
the shortest flight times for certain robotic missions, possibly
including interstellar missions. However, for the relatively nearby
destinations of interest to us, they carry a significant flight time
penalty compared to higher-thrust systems. Even accepting long flight
times, transportation of the large masses required for human exploration
would imply very large sails, for which deployment and control are
serious engineering issues. For the time being we assert that solar
sails would be most useful for purely robotic exploration missions,
and that, if developed, they may in the future be considered for
cargo transportation in relation to human missions.
Aeroassist
Atmospheric drag can be used very effectively to modify the orbit
of a spacecraft, at a greatly reduced cost in propellant. There
are two primary forms of this technique, one of which has already
been utilized on robotic missions and one of which has been studied
extensively but not yet put into practice. Aerobraking, which has
been used at Venus and Mars, involves repeated passes through the
upper part of a planet's atmosphere to gradually reduce orbital
energy. Over time this can bring a spacecraft all the way from
a highly elliptical orbit to a low circular orbit, while using
only a fraction of the propellant that would be required to make
the same change propulsively. In aerocapture, which has not been
demonstrated, a spacecraft on approach to a planet would make a
high-speed entry into the planet's atmosphere. This encounter would
be targeted to remove sufficient energy from the spacecraft's trajectory
so that it is captured into orbit with no engine burn required.
In both aerobraking and aerocapture the spacecraft must still carry
a propulsion system for orbital adjustments after the atmospheric
pass, to ensure that subsequent periapses are at safe altitudes.
Under some conditions, aeroassist may be useful as a weight-saving
measure for chemically-propelled spacecraft going to Mars or returning
to Earth. A trade study that considers vehicle parameters and mission
objectives must be conducted to determine if there is in fact a net
savings; in some cases the additional shielding mass or packaging
constraints may outweigh the benefit of the fuel saved.
Aerobraking at Mars or Earth probably offers some mass savings for
missions of interest to us and is relatively simple, but the additional
flight time required for repeated atmospheric passes may make it
undesirable for crewed missions. Aerocapture would offer much more
significant mass and flight time savings, but safety concerns for
human missions may hinder its application in the near term. In order
to derive the benefits of aerocapture a mission would essentially
have to be completely reliant on its success. This is probably not
realistic for human missions, at least until aerocapture has been
thoroughly developed and extensively used on robotic missions.
There are several aeroassist concepts that may be useful at some
point in the more distant future. One method that may enhance the
effectiveness and safety of aerocapture would use an inflatable drag
device known as a ballute. This could increase the effective drag
area of a spacecraft and allow an aeropass to be conducted at a higher,
safer altitude while imparting the same DV. Another concept, known
as aero-gravity assist, can be used to increase the trajectory "bending" from
a standard gravity-assist planetary flyby by dipping into the planet's
atmosphere. This could be used, for example, to increase mission
design flexibility and safety by enabling return-to-Earth trajectories
and abort modes that would otherwise be unavailable.
For the purposes of this discussion, we conclude that aeroassist
should be considered a staple mission design tool in the event that
chemical propulsion is used to send cargo vehicles to Mars. Aeroassist
benefits would probably be negligible, if any, for electric propulsion
vehicles. Mass trade studies must be conducted on a case-by-case
basis. For the first generation of crewed missions, aerobraking may
be of some benefit if the added flight time is not too large. Aerocapture
is likely to be considered too risky for the early crewed missions,
but in the longer term it should probably be considered as a means
to enhance mission performance.
In Situ Propellant Production
The use of extraterrestrial resources has often been proposed as
a means to provide fuels, oxidants, and/or propellants for travel
in the solar system. Such approaches offer an attractive alternative
to lifting resources from Earth and carrying them along for the
round trip. In most cases, of course, it remains uncertain whether
the required resources actually exist in forms and amounts that
would make their utilization cost effective. Such approaches would
undoubtedly first be utilized on robotic missions, to demonstrate
and validate the technologies and techniques and thereby reduce
the risks to human explorers.
While the emphasis here is on propulsion, it should be noted that
an approach for obtaining and utilizing certain extraterrestrial
resources, especially water, would have concomitant fundamental benefits
in the area of life support for human missions.
Earth's Neighborhood
We know that oxygen is abundant on the Moon. The Lunar Prospector
mission also revealed substantial amounts of hydrogen near the lunar
poles, possibly in the form of water. Hydrogen and oxygen together
are, of course, tremendously useful. H2/O2 is a good propellant for
a variety of applications and is a simple, reliable, monopropellant.
Cryogenic O2 and H2 can be used to fuel high-performance bi-propellant
systems. Water electrolysis has also been proposed as a direct propeller.
Some electric propulsion concepts can use oxygen as primary fuel,
possibly complemented by readily available sodium. Finally, H2 is
the fuel of choice for nuclear thermal rockets.
There are a number of reasons why production of these propellants
on the Moon might be interesting, even though the Moon may not be
one of our primary destinations. Energetically speaking, the lunar
surface is actually closer to low Earth orbit than is Earth's surface.
Thus fueling spacecraft with lunar-derived propellants may be attractive.
The use of lunar propellants is particularly interesting if used
in combination with staging at an Earth-Moon libration point, since
these points are also energetically much closer to the Moon's surface
than to the Earth's surface. The use of lunar-derived propellants
and libration point staging can significantly reduce the Earth-launch
mass of human and cargo missions to asteroids and Mars. The cost
benefit of this technique is, of course, yet to be determined.
Mars
Many investigators have speculated on the use of martian resources
to support human exploration, both for life support and propulsion.
For example, oxygen produced from martian atmospheric CO2 has been
suggested for use in conjunction with hydrogen brought from Earth
(or possibly methane for ease of storage) to lift vehicles off the
martian surface. Martian water could also be used to provide not
only oxidant but also fuel for travel from Mars.
In addition to such chemical propulsion scenarios, various methods
might be proposed for provision of propellants for nuclear thermal
or nuclear electric spacecraft. For our purposes, however, we assert
that in the reasonably near term the only viable use of martian resources
is for propellant for ascent from Mars' surface to orbit, and this
would depend on a pre-emplaced propellant factory operating prior
to the arrival of human explorers. As is always the case, the feasibility
and cost-effectiveness of such scenarios depends on the existence,
abundance, and accessibility of the resources in question. Moreover,
the additional life support benefits associated with some extraterrestrial
resources - H2O in particular - mean that we probably should not
assess the benefits of such techniques from the standpoint of propulsion
only, but rather from a total mission perspective.
Near-Earth Asteroids
Near-Earth asteroids might prove to offer a number of advantages
over the Moon for resource utilization. Although more distant, their
low gravity reduces the DV for repeated landings and lift-offs, and
SEP could be used to advantage to ferry raw materials back to Earth's
vicinity. The water content and abundance of asteroid materials is
thought by some to be much more significant than that of lunar material,
offering the potential for more efficient production of H2/O2 and
hydrocarbon fuels. They are also rich in a number of pure metals
(Al, Ni, Co, Ca, Fe, etc.) of potential interest for other uses in
space or on Earth.
Novel Trajectory Concepts
Research into the orbital mechanics of libration points has shown
that they may be used to advantage in an overall robotic-human
exploration architecture. Not only is SEL2 an important science
destination in itself, it can also serve as a "gateway" for
travel elsewhere in the solar system. The Earth-Moon libration
points can also be used in this way, for certain destinations.
The concept of an "interplanetary superhighway" has been
developed to describe low-energy trajectories between solar system
destinations that capitalize on the gravitational balances at libration
points and the complex trajectories that connect them with other
solar system bodies. The trade-off of DV vs. flight time is still
an important consideration; whereas some of these novel trajectories
can reduce energy requirements, they may incur a flight time penalty.
Thus they may be more important for cargo transportation than for
crewed missions.
While the technical details are outside the scope of this paper,
we assert that the effective utilization of libration points and
the trajectories between them will be an important part of a human
exploration architecture. One concept that we favor would involve
preparing large cargo vehicles (powered by SEP or NEP) at SEL2, and
then using that gateway to send them to near-Earth asteroids or Mars.
The crew itself might depart from EML1, to avoid the flight time
penalty associated with travel all the way to SEL2. Also important
would be the capability to regularly transport humans and cargo between
EML1 and SEL2, both to prepare and service telescopes and to assemble
and fuel the large cargo vehicles that will be departing to solar
system destinations. The development of a reliable transportation
network connecting Earth's surface with the Earth-Moon and Sun-Earth
libration points may be an important first step in development of
a long-term human exploration program.
Interplanetary Staging and Fuel Depots
An extension of the notion of staging in Earth orbit or at libration
points is staging in deep space. It is possible to consider caching
supplies and propellant in a variety of orbits, such as in "cycling" orbits
that regularly approach near Earth and Mars. A crew vehicle on
its way to Mars could rendezvous with such an orbiting cache to
re-fuel prior to completing the journey. This could help to reduce
requirements for lifting mass from Earth's surface. Cache locations
could be selected to optimize their utility for an overall human
exploration architecture. While such a strategy could pay some
dividends, it is not clear that the savings are worth the added
complexity, at least until the human exploration program is mature
enough to take advantage of economies of scale.
Advanced Concepts
A large number of advanced propulsion concepts have been proposed
over the years. These include, for example, systems that use controlled
fusion or the energy from antimatter annihilation to accelerate
propellants to extremely high velocities. Also proposed have been
laser "light sails", which would use laser beams from
Earth to power spacecraft carrying large solar sail-like reflectors,
and many other ideas. Many of these may have some merit, but none
are anywhere near full-scale development, and the investments required
would be substantial. We believe that the most prudent path toward
a human exploration capability in the relatively near term lies
in the more mature technologies of chemical, SEP, NEP, and aeroassist.
Integration and Quality Assessment
The first step toward readiness for any of the transportation technologies
described here is the setting of priorities for a technology investment
program. In our case, this is especially difficult because there
is no universally agreed-upon mission concept or mission objective
against which to assess technology benefits. For the time being,
though, we can identify some qualitative discriminators to frame
the discussion; this must be followed up with technology and implementation
roadmaps and investment strategies once actual mission scenarios
are developed.
Based on the technology assessments provided earlier, we define
two qualitative measures for comparison of propulsion options. Utility
will be defined as a combined measure of the capability of a given
technology in terms of mass delivery, flight time, safety, and applicability
across the destinations of interest. Readiness is defined as a function
of maturity and cost to develop for application to human exploration
missions. Based on our analysis and on informal discussions with
several technology experts, we can plot the various options as shown
in the figure. Using this admittedly subjective analysis, we assert
that a combination of chemical and electric propulsion, possibly
enhanced by aeroassist techniques, is the most likely to provide
a usable transportation infrastructure that meets our mission imperatives
at the earliest possible date. Other propulsion technologies, and
other techniques such as in situ propellant production and the use
of orbiting fuel depots, should be considered as second-generation
capabilities to be added to our transportation "toolkit" as
the human exploration architecture expands.
Synthesis and Recommendations
As discussed previously, chemical and electric propulsion
are best suited to different parts of an overall human exploration
architecture. The relatively high thrust and reliability of chemical
propulsion makes it a good choice as primary propulsion for the crew
vehicle, while the high efficiency of electric propulsion makes it
ideal for transportation of massive equipment and infrastructure
elements for which short flight times are less critical. Thus we
recommend that strong consideration be given to splitting the human
exploration mission transportation requirements along those lines.
Following such a strategy would allow us to offload onto an electric
propulsion cargo vehicle much of the mass required to meet the mission's
science and exploration objectives, thereby minimizing the mass (and
thus the flight time) for the crew vehicle. The crew vehicle would
need to carry only the absolutely necessary operational and life
support equipment and supplies, sufficient for the planned trip to
the destination and for any emergency return scenario. Although not
required, this could also allow us to plan to launch the crew vehicle
only after the cargo vehicle has safely reached the destination.
Such "pre-emplacement" of assets can be considered for
all potential human exploration missions as a means of reducing risk
and improving overall mass performance.
In this "split mission" scenario, the type of electric
propulsion selected for cargo vehicles may vary by destination. For
libration point and near-Earth asteroid missions, SEP would probably
provide sufficient propulsive capability at minimum cost. For Mars
missions, the increased solar distance, larger mission mass requirements,
and the need to avoid power reductions during eclipses in Mars orbit
would argue for the use of NEP. This would provide a substantial
power and propulsion resource in Mars orbit with which the crew would
rendezvous. It is even possible that this orbiting resource could
be designed to support multiple crewed missions over a period of
several years.
Aerocapture is unlikely to be of substantial benefit for an EP cargo
vehicle at Mars, since its large size would make packaging into an
aeroshell difficult. Aerobraking for modest DV reduction may be useful.
For the chemically propelled crew vehicle, aerocapture could provide
a substantial mass benefit, but it is unlikely to be used in such
a mission-critical role early in the human exploration architecture.
The use of aerobraking to take the crew vehicle from a large capture
orbit into a low Mars orbit could also result in significant mass
savings and may be perceived to entail less risk, provided the added
flight time isn't too great.
Another major recommended architectural element is the use of staging
locations at libration points, or perhaps in Earth orbit, for final
preparation and/or assembly of spacecraft elements prior to launch.
This can provide a significant mass performance improvement and allows
us to take full advantage of the split mission architecture. Initial
launch from Earth's surface can be used to emplace an empty cargo
vehicle at a staging location, for example, to be followed by actual
cargo and equipment that would be loaded and installed by astronauts.
Doing so will dramatically reduce the need for heavy-lift launch
from Earth's surface, since multiple launches of smaller launch vehicles
could do the job. As an enhancement, if lunar resources are found
to be valuable as propellants, they could actually be transferred
to a libration point for less energy than would be required to lift
them from Earth's surface. Both cargo and crew vehicles can depart
from such staging locations for their final destinations, and some
of the unique orbital mechanics associated with libration points
can be used to advantage. This argues for a robust capability to
service, fuel, and supply vehicles in space prior to their departures
for their destinations. It also implies the need to routinely transport
crew members to Earth orbit, and thereafter to a staging location
at a libration point.
Increased knowledge of the human health effects of long-duration
space travel, and the possible development of countermeasures to
mitigate these effects, could change our conclusions relative to
the split mission architecture. If we find a way for humans to live
and work safely in space for many months, it could be decided to
merge the crew and cargo functions onto a single megawatt-class NEP
vehicle, possibly with an artificial gravity capability. Likewise,
if Nuclear Thermal Propulsion is developed in the future, it could
provide a greater capability to take both humans and cargo together,
in this case without paying the flight time penalty associated with
electric propulsion. Given the current state of technology, however,
we believe that the best path to a human exploration capability is
to utilize the propulsive capabilities that either already exist
or are under development, and not to rely on fundamentally new and
speculative technologies.
Conclusions
A human exploration architecture utilizing both chemical and electric
propulsion will best meet the driving requirements of safety, mass
delivery, flight time, and mission success. These should be applied
in a manner that matches the characteristics of the propulsion system
to the needs of individual mission elements. This results in the
recommendation to separate cargo and crew delivery functions, with
the former being sent in advance using electric propulsion, and the
latter being sent later on the fastest possible trajectory using
highly reliable chemical propulsion.
From the standpoint of transportation technology, a "stepping
stone" approach whereby human exploration proceeds gradually
into the solar system will provide the best pathway to safe and cost-effective
capabilities. The first step may be a libration point, both for telescope
servicing and as a staging location for future missions. Near-Earth
asteroids provide an intermediate destination of high science and
programmatic value. Mars is the ultimate destination for the foreseeable
future, with possible use of Phobos/Deimos as the first targets in
the martian system. Human exploration of the Moon is not on the critical
path to Mars from the standpoint of propulsion technology development,
although it may provide a platform for the validation of other mission
elements and certainly has some intrinsic science value.
To most effectively advance the cause of human exploration, we should
leverage existing investments to the maximum extent possible. Relying
on fundamentally new transportation technologies, highly capable
though they may eventually be, is not required and will in all likelihood
delay the date at which we are ready to move forward. By using Earth
orbit or libration points as staging locations for human exploration
missions we can greatly reduce the requirement for heavy lift launch
from Earth's surface, and can rely instead on multiple launches of
smaller masses with assembly in orbit. The next-generation launch
system should focus on routine and minimum-cost access to low Earth
orbit for both cargo and crew. There should also be developed a companion
system to transport cargo and crew to a mission staging location,
perhaps also in Earth orbit but more likely at a libration point.
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