Third International Workshop on Satelllite Constellations and Formation
Flying, Pisa, Italy, 24-26, 2003.

# USE OF SWARM INTELLIGENCE IN SPACECRAFT CONSTELLATIONS FOR THE RESOURCE EXPLORATION OF THE ASTEROID BELT

S. A. Curtis^1^, M. L. Rilee^2^, P.E. Clark^2^ , and G. C. Marr^1^

 

^1^*NASA Goddard Space Flight Center*

^2^*L-3 Communications EER*

*Laboratory for Extraterrestrial Physics, Code 695, Greenbelt, MD 20771,
USA*

*e-mail: <Steven.A.Curtis@gsfc.nasa.gov>*

*e-mail: <Michael.Rilee@L-3Com.com>*

*e-mail: <Pamela.Clark@gsfc.nasa.gov>*

*e-mail: <Greg.Marr@gsfc.nasa.gov>*

 

 

# ABSTRACT

We describe the Prospecting ANTS Mission (PAM) whose object is to
explore the resource potential the Solar System\'s Asteroid Belt. The
mission is consistent with the present NASA strategic plan for the HEDS
(Human Exploration and Development of Space) enterprise. In this plan,
the automated discovery of space resources is envisioned as a building
block for expanding the human presence in space. The Asteroid Belt
consists of thousands of widely separated bodies. Targeting thousands of
individual asteroids, which will require hovering in a highly irregular
gravity field, demands a very large autonomous constellation of
specialized workers. The ANTS (Autonomous Nano-Technology Swarm) concept
has a number of possible applications of which the PAM mission is one.
ANTS, based on a hierarchical insect social order, use an evolvable,
self-similar, hierarchical neural system in which individual spacecraft
represent the highest-level nodes. ANTS uses swarm intelligence attained
through collective, cooperative interactions of the nodes at all levels
of the system. At the highest levels this can take the form of
cooperative, collective behavior among the individual spacecraft in a
very large constellation. The ANTS neural architecture is designed for
totally autonomous operation of complex systems including spacecraft
constellations.

The PAM mission is envisioned to fly in 20 - 30 years. It assumes a
Lagrange point launch, consistent with the HEDS roadmap that projects
human occupation during this time. The spacecraft are solar sail powered
and reach, flying as a swarm, the Asteroid Belt in a little over 3
years. The swarm would consist of 1000 pico- (1 kg) spacecraft with 9
types of radio-linked, specialized worker spacecraft defined by the
remote sensing instrument that they each carry. These instruments would
include magnetometers, mass spectrometers, x-ray, UV, visible, and
infrared sensors. There would be 100 of each worker spacecraft type,
allowing multiple asteroids to be simultaneously explored and allowing
any given worker to be expendable, consistent with the social insect
analog. In addition to the workers, there are 100 ruler and messenger
spacecraft. The messengers return amassed data to the terrestrial
Lagrange point and the rulers coordinate the activities of the
instrument worker spacecraft at each asteroid encounter. The last data
would be returned to the terrestrial Lagrange point about 10 years after
departure.

We discuss the operational characteristics of the PAM mission and
possible applications of the technology employed in nearer term
missions.

![](media/image1.png){width="4.860416666666667in"
height="3.548611111111111in"}

Fig. 1. An artist's concept of Prospecting ANTS Mission during Science
Operations.

INTRODUCTION

### Prospecting the Main Belt

The Main Belt of Asteroids between the orbits of Mars and Jupiter
represents the last great accessible surface to be explored within the
orbit of Pluto. The mass of the thousands of objects in the Main Belt is
perhaps a fraction of that of Mars, but their combined surface area may
dwarf that of the Earth. The outer reaches of the Belt are rich in
frozen volatiles including water and other ices. The inner reaches are
more refractory. Some asteroids are largely the metallic shards from the
cores of larger differentiated bodies that have since been destroyed by
collisions with other asteroids. There has been much interest in the
exploration and development of asteroids as a resource enabling the
continuous human presence in space. Interest driven partly because,
compared to other planetary objects, asteroids have weak, but irregular,
gravity and a wide variety of resources.

The Prospecting ANTS Mission (PAM) is an advanced mission concept for
the 2020s that seeks to perform a first general exploration of the belt
to survey and map the surface characteristics and other properties of
the Main Belt (Ref. \[1\], ANTS will be discussed below). Within the
time frame of a single mission (5-10 years), thousands of asteroids are
to be visited to provide an extensive sample of what exists in the Main
Belt. With a goal of characterizing in detail at least one thousand
asteroids a year, PAM forces us to reconsider our fundamental approaches
to mission implementation. However, the rewards for taking up this
challenge are great. The atlas of surface and internal properties that
PAM produces will be critical for the future exploration and development
of Main Belt resources.

For such advanced missions, a key theme in the long-term roadmap of
NASA\'s Office for the Human Exploration and Development of Space (HEDS)
is the close interaction of humans and robots to achieve mission goals.
The HEDS roadmap foresees the synthesis of integrated human and robotic
technologies enabling advanced missions that involve a continuous human
presence beyond low Earth orbit. Robotic autonomy is an important
component of the HEDS approach, both for health and safety of systems
and subsystems, but also for the reliable interaction of human and
robotic components (see Ref. \[2\] for the ANTS perspective).
Multi-element missions must be no more difficult to operate than
single-spacecraft missions are today. Furthermore, no cost-savings are
to be gained by implementing human-based mission operations locally on
deep space missions. Hence, sub-system and component automation and
autonomy enable missions. Furthermore, partial autonomy, in the sense of
having an external (human) controller in the loop, gains little, for
even small deviations from total autonomy will drive human interaction
concerns and hence mission costs.

The PAM mission is a case in point. If about a month of observation is
required to characterize an asteroid, the requirement to visit a
thousand a year implies that about one hundred asteroids are to be
visited at any given time. Using current implementation techniques and
reasonable budgetary expectations, it is infeasible to field one hundred
unmanned deep space missions. Therefore, the requirements of the PAM
mission drive us to consider approaches that are highly distributed and
fully autonomous and yet are still reliable and cohesive enough to meet
mission goals efficiently.

## ANTS: The Autonomous Nano-Technology Swarm

ANTS is a mission architecture drawn in analogy to biological social
insects (see Refs. \[1,2\] and references therein). In nature,
biological ants are one of the most successful species to appear.
Elements of their success lie in their division of labor, their
collaboration, and their numbers. Their numbers provide reliability
through redundancy for many functions of the swarm. However, individual
ants are themselves highly autonomous and capable creatures, however
they do depend on other members of their society to perform tasks
necessary for their survival and procreation. Because individual ants
are specialized to a given task, they are much more likely to succeed in
that task than a non-social generalist. The ants and their swarm benefit
from these successes, and the colony continues on with its competitive
advantage. In some sense, we aspire to produce systems with the
robustness and adaptability of ants. Biological ants are capable of
relatively sophisticated behaviors with relatively simple neural
structures and communication mechanisms, we seek to emulate both the
individual and swarm capabilities in the ANTS mission architecture.

The ANTS mission architecture posits highly autonomous spacecraft each
specialized to a particular mission function, e.g. data collecting,
communications, or control. ANTS spacecraft all are able to perform
autonomously tasks that all spacecraft must perform to survive, these
include standard functions such as Guidance Navigation & Control,
Attitude Control, etc. However, they are experts within their specialty.
The physical makeup of the spacecraft themselves, and their operations,
including trajectories, can be optimized for their tasks. This greatly
enhances mission science return, simplifies science analysis, and leads
to a better overall results. Reliability and tolerance to fault and
failure is enhanced redundancy at the spacecraft level. Furthermore, the
risk vs. payoff trade is significantly transformed: ANTS spacecraft,
although already optimized for their task, could occasionally engage in
risky maneuvers that are nonetheless scientifically merited that
single-spacecraft missions could not even consider. The architecture can
be applied to a broad range of spacecraft platforms and constellation
concepts.

Applying ANTS to the PAM mission goals leads to the division of mission
functions across a thousand or more spacecraft (see Figure 1 for an
artist's concept). As the mission concept is set in the 2020s, the
spacecraft are to be developed at human habitat at a Lagrange point as
aimed for in the NASA/HEDS roadmap. PAM spacecraft are to use solar
sails and other technologies that obviate consumable resources. Once
released from the habitat, they travel for about two years on their own
to the Main Belt. ANTS spacecraft involved in data collection and
processing for asteroid characterization are called Workers. There are
about 8-10 types of Workers each specialized on a particular kind of
scientific instrument and measurement program. ANTS spacecraft involved
in swarm cohesion and communications are called Messengers. ANTS
spacecraft involved in command are called Rulers. Briefly, Workers
working in about 100 teams produce data and some higher-level data
products that are then communicated and archived within a network of
Messengers. Rulers oversee the data flow and steer the progress of the
swarm. At times when enough data has been accumulated a Messenger is
dispatched to a terrestrial Lagrange point or other communication node.
We believe the preceding scenario is a feasible alternative to the one
hundred spacecraft mission alluded previously. The level of
communications and interaction between ANTS spacecraft has yet to be
determined, but most detailed control must be performed locally, i.e.
within Worker teams in the vicinity of asteroid targets. The
communications layer formed by Messengers provides long haul (\~0.1 AU)
communications across the swarm.

## Autonomous Formation Flying in Deep Space

The above description of PAM goals and overall mission architecture
presents requirements for several kinds of formation flying. Optimally
configured and operated science instruments drive one set of
requirements: these can be very demanding, e.g. interferometers or other
observations that require coordinated sensor operations. Another set of
requirements is driven by teaming and resource allocation requirements:
for example, spacecraft loss may require reshuffling team membership
that places constraints on spacecraft trajectories. Indeed, a key
attribute of a team is their relative positioning or formation and how
this formation adapts to mission phase: inter-encounter cruise,
orbit-insertion, science operations, and de-orbit. Furthermore, within
the PAM framework there are several levels of such cooperation.
Messengers and Rulers, in the present scenario, do not suffer encounter
operations, yet their communications network forms a widely spread team
that is critical to mission success: the parameters and requirements for
their formation are quite different from the Workers.

#### Transfer from Earth's Lagrange Point to the Main Belt

The preceding formation flying tasks are difficult enough in free space
or while orbiting a major planet. But the requirements for PAM are more
severe and fluid to boot. The first phase of a PAM spacecraft's mission
is to travel from Earth's Lagrange point along with the rest of the
swarm. The PAM spacecraft might be released over some period of time,
possibly months, so the PAM swarm might extend over a large region of
space. The current PAM mission design calls for this first leg to take
between two and three years under solar sail. This allows the spacecraft
to operate in the Main Belt for a greater fraction of its mission life.
Spacecraft capable of such acceleration should be able to travel quickly
between asteroids during science operations, but this remains to be
verified. In fact, most aspects of solar sail propulsion for deep space
operations require significant technological development.

#### Operations in the Main Belt: Infrastructure, Teams, and the Unknown

Next, after having raised their orbits by an AU or more, the Messengers
and Rulers position themselves to provide communications and control to
the swarm. Workers set about their jobs of detecting and obtaining
information about Main Belt asteroids. Some Workers work alone, others
are continually forming teams to perform science encounters including
orbital operations. Workers equipped with large, wide field imagers
should detect vastly more asteroids than the tens of thousands we
currently have on catalog. From these asteroids, target selection
according to mission goals is a critical activity. However, these
mission goals change as more is learned about the Main Belt.

The teams of spacecraft will have to construct and use gravity models
for irregularly shaped, rotating, and a priori unknown asteroids that
may have moons. Orbit planning for coverage and instrument placement
will have to occur at the spacecraft or team level. Logistics is also a
key activity, for mission goals require the right mix of instruments
being on-site for the right length of time during the appropriate phase
of an asteroid's year. An asteroid may receive multiple visits during
the mission to get different viewing conditions or for a variety of
other reasons. And even though the swarm may have many copies of any
given kind of spacecraft, none are to be wasted. On occasion the PAM
swarm will send a representative back to Earth or another communication
node to report on swarm findings.

All of this is to occur without requiring any human intervention in the
affairs of a swarm of kilogram-class spacecraft operating under solar
sail between 2 and 3.5 AU from the Sun over the course of a decade.

## Plan of paper 

In the following sections, we present some of the context of the ANTS
Prospecting Asteroid Mission. These considerations of asteroid science,
instrumentation, and optimal operations drive requirements for formation
flying technology and, more broadly, cooperation. Three important kinds
of formation flying are delineated from the structure of PAM. The
importance of fully autonomous operations in very uncertain environments
is discussed.

## ASTEROID EXPLORATION CONTEXT

Though much progress has been made, there exists little detailed data to
constrain our ideas concerning asteroids. Much has been learned through
the analysis of terrestrial meteorite (e.g. Ref. \[3\]). Most data come
from ground-based telescopes and the very successful Near Earth Asteroid
Rendezvous (NEAR) mission that landed a spacecraft on an asteroid (for
more on asteroid research see Refs. \[3,4,5,6,7,8\]). A major
breakthrough in our understanding of Solar System origin will require
survey of a representative cross-section of the Main Belt asteroid
population, whereby the following questions can be addressed:

1\) What is the nature of smaller, darker, more remote asteroids more
difficult to observe from Earth? Do they contain the undifferentiated
material that is found prevalently in meteorites but is missing in
asteroid spectral evidence?

2\) How are elements, minerals, rocks distributed in asteroids and
asteroid parent bodies in space and time? Where are the parent bodies?

> 3\) How is regolith formed from the original parent body material on
> asteroids? What is the role of \'space weathering\' in determining the
> nature of an asteroids surface?

4\) What is the relationship between dynamical and compositional
properties? What are the distributions and effective limits in size,
shape, spin, mass, density, and composition?

These are some of the main issues concerning Solar System and planetary
origins. To address them requires detailed observation by an entire
suite of scientific instruments including imagers, spectrometers, radio
frequency metrology, radars, and magnetometers among others (Ref. \[7\]
provides an example of such a suite). Furthermore, the particular mix of
instruments and their optimal operation varies according to the
particular scientific issue being addressed. To make progress, then,
current space missions must negotiate a compromise that allows mission
goals to be met in spite of the necessity of sub-optimal operations.

## Multi Spacecraft Mission Applications

What is the nature of the contribution that multi-spacecraft missions,
as compared to single spacecraft missions or near Earth observation
programs, can provide? Multi-spacecraft missions are uniquely capable of
providing measurements for a comprehensive asteroid survey.

1\) Ground based or even Earth orbiting observatories, even with
projected improvements in sensitivity, will be not be able to provide
measurements for more remote, smaller, or darker asteroids, which must
be observed by spacecraft. We must have observations from these
more-difficult-to-observe objects in order to understand the formation
and evolution of the asteroid belt.

2\) Single spacecraft missions are useful in providing extensive
documentation for one to a handful of previously observed asteroids, but
are not designed for surveying a wide range of unexplored asteroids, an
application for which multi-spacecraft missions are ideally suited.

3\) Multi-sensor spacecraft have flown before (NEAR) or are flying now
to solar system bodies such as asteroids. Essential sensors, such as
imagers, spectrometers, and altimeters, have very different optimal
operational requirements for a) illumination conditions, b) pointing
geometry, c) distance to target, and d) orbital configuration. This
translates into constant compromising to meet sensor requirements and
results in less efficient collection of high quality data, a problem
that is magnified when a small, irregularly shaped object, such as an
asteroid, is being explored. As mentioned above, the PAM concept calls
for single sensor spacecraft, working individually or as teams.
Individual spacecraft can be flown to meet optimal instrument
operational as well as science requirements simultaneously, reducing the
time required to obtain a comprehensive of observations as well as
increasing the quality of those observations, for each target.

## Flying PAM: The team and \'virtual science instrument\' concepts

The first two drivers for PAM formation flying involve teams and the
more tightly coupled \`virtual science instrument' concepts. Although
individual ANTS spacecraft can be flown to optimize their observing
capabilities, they can also, using built in reactivity and response
capability, acquire simultaneous coverage of the same target, thus
providing a comprehensive measurement set required to solve a particular
scientific problem. Identical sensor or multiple sensor \'virtual
science\' teams could be formed. These teams would be chosen to achieve
mission goals and their structure is thus determined by the mission
science requirements. Particularly useful teams, and the approximate
order of their deployment during science operations, are:

1\) Asteroid Detector/StereoMapper consisting of two wide field imaging
spectrometers with enhanced navigational (location and pointing
awareness) capability separated by distances varying from hundreds of
kilometers to kilometers which would be used to detect and determine the
orbit of potential targets at a distance, or move to within kilometers
of a target to obtain astronomical classification, rough shape, and
dynamic properties of more likely candidates for detailed studies. These
are the \'point men\'.

2\) Dynamic Modeler consisting of an enhanced radio science instrument,
altimeter, and wide field imager separated by tens of kilometers to
kilometers which would be used to acquire detailed figure parameters
(including shape model) and dynamic properties (spin, density, mass
distribution).

3\) Petrologist consisting of X-ray, Near Infrared, Gamma-ray, Thermal
IR, and wide field imager separated by tens of kilometers to kilometers
which would be used to determine the abundances and distribution of
elements, minerals, and rocks present, from which the nature of
geochemical differentiation, origin, and history of the object, and its
relationship to a \'parent body\' could be inferred.

4\) Photogeologist consisting of Narrow Field and Wide Field Imagers and
Altimeter separated by tens of kilometers to kilometers which would be
used to determine the nature and distribution of geological units based
on texture, albedo, color, and apparent stratigraphy as expressed on the
surface, from which the nature of the dynamic history and origin of the
object could be inferred.

5\) Prospector consisting of altimeter, magnetometer, Near Infrared,
Infrared, and X-ray spectrometers separated by tens of kilometers to
kilometers which could be used to determine the distribution of
\'resources\', including Fe/Ni and volatiles on pre-selected candidates
for \'mining\'.

Note these roles or functions are performed by groups of individual
specialized spacecraft. An advantage of working with individual
spacecraft is that they may be reassigned as the needs of the mission
change. Mission priorities will change as more data is gathered about
the Main Belt asteroids, and ANTS provides the necessary adaptability.
Finally, using individual specialists provides flexibility as the swarm
decays and loses spacecraft over time.

## Flying PAM: The Swarm

The preceding has emphasized science-driven requirements and
configurations revolving around the needs of the asteroid science
encounter operations. Viewed hierarchically, the PAM swarm is a team of
teams, some of which exist longer than others, each of which is
dedicated to a mission task or function. At the level of the swarm, the
global structure of the constellation itself drives its own set of
requirements. Whereas science teams of Workers are building up a
database of a particular asteroid, the Messengers and Rulers are
building up a database in which that asteroid is a single entry.

The communications layer that is implemented on the network of
Messengers and Rulers must maintain position and trajectory information
on every spacecraft in the swarm as a prerequisite for any
communications. This is due to the large distance between asteroids and
the great size of the PAM swarm itself. As Workers band and disband into
their various science teams and virtual instruments, the Swarm must
maintain its own global cohesiveness, but must also be able to adapt to
changing mission priorities. The requirements for this rather loosely
coupled formation are in some ways less stringent than for the virtual
instruments and coordinated observations. The time scales are determined
the dynamics of solar sail spacecraft in the Main Belt, and these are
fairly benign. However, the requirements on communications links are
mission critical and their maintenance challenging.

As an aside, the ANTS architectural requirement for no single-point
failures implies that there is no single monolithic Ruler whose commands
the swarm depend upon. Clearly the details of managing the orbital
insertions of a thousand spacecraft around a hundred asteroids are too
great for a single Ruler to handle across deep space. Indeed, the nature
of the ruling element itself is an open issue. Indeed, Rulers simply
might be Messengers with particular responsibilities for decision making
and command, or Rulers may be more akin to software agents that travel
within the communications network provided by the Messengers (for more
on agent technology, see Ref. \[9\]). However these highest-level
decision makers are implemented, how their directives should affect the
actual operations of Workers is an open topic.

## CONCLUSION

There is much work to be done to realize the ANTS mission architecture,
but not all of the problems must be solved before elements of the ANTS
concept might be applied to missions. The most immediate work to be done
involves the development of subsystem, and beyond that, full system
autonomy for spacecraft. However, we make a distinction between
subsystem autonomy and partial autonomy: here, partial autonomy implies
that external control is required for reliable operations, while
subsystem autonomy implies that the subsystem performs its duties
without detailed external command and control. As mentioned previously,
while partial autonomy drives cost, full autonomy backed up by
redundancy, particularly at the spacecraft level, is the keystone of the
ANTS architecture.

In the near-term, the construction of spacecraft that have some fraction
of the reliability, capability, and autonomy of biological ants would
bring us a long way to realizing advanced missions such as PAM. The ANTS
architecture posits that autonomous subsystems of a spacecraft would
interact to provide a reliable platform for mission functions, in
essence a swarm within the box. The specific methods used to achieve the
required behaviors depend on the subsystem and are not limited by the
architecture. We note that low-level, neural methods of control suit
most needs of biological ants. On the other hand, for social
interaction, namely teaming, forming virtual science instruments,
swarm/constellation cohesiveness, and mission success, higher-level
discourse definitely seems warranted.

Another relevant area that requires groundbreaking is in the field of
autonomous deep space rendezvous/orbital insertion with spacecraft using
continuous low-thrust propulsion, e.g. ion propulsion or solar sails.
Solar sail propulsion is attractive for pico-spacecraft operations in
the Main Belt, because it adds no consumable resources to the system and
is nearly always available. Hovering trajectories may be possible.
However, their dynamical and control properties are not well understood.
Autonomous attitude control and navigation for individual solar
propelled pico-spacecraft, let alone formation flying, are open
problems. The dynamic, irregular gravitational fields that surround
asteroids compound these problems.

Once autonomous spacecraft platforms are available, there yet exists the
question of how they seek to achieve mission goals. Are the encounters
with asteroids in PAM to be scripted? If so, to what level of detail?
May the scripts be rewritten? Certainly as the secrets of the Main Belt
asteroids are uncovered the mission priorities should change, the
operational methods should adapt, however this will have to occur
without human micro-management.

These techniques might find their first implementation in near-term
missions that have special requirements involving multiple sensors. Some
measurements in planetary science missions involve identical sensors
moving in formation to separate resolve 3-dimensional-space and time
effects in phenomena (Ref. \[10\]). Measurements involving electric and
magnetic field measurements, radio science (range and velocity),
spectrometry, and imaging all benefit from the improved signal-to-noise,
spatial and temporal resolution, or extended coverage that multiple
sensors provide. Data from different kinds of instruments may be
combined to form higher-level products such as surface geology, changes
over seasonal cycles, volatile budgets and circulation, and indicators
of resources such as water/ice, Fe/Ni, volcanic material, and Aluminum.
If such studies are done, say, on the Moon (as described in Ref.
\[10\]), they would provide an opportunity for the real-time training
and development of human/machine exploration ranging from human-led to
human-interactive to full machine autonomy.

# ACKNOWLEDGMENTS

#  

#  Support from NASA/GSFC IR&D and Code 588, Advanced Architectures and Automation Branch is acknowledged.

 

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\[3\] The Planetary System Group at the Uppsala Astronomical
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\[5\] Ostro, S.J.: Asteroid Radar Astronomy, Jet Propulsion Laboratory,
[http://echo.jpl.nasa.gov](http://echo.jpl.nasa.gov/)

\[6\] Near Earth Asteroid Rendezvous (NEAR) mission website:
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\[7\] NEAR mission first results: Science, Vol. 289 (2000)

\[8\] Santo, A.G. et al.: NEAR Spacecraft and Instrumentation, J. of
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\[9\] Truszkowski, W.F. and H. Hallock: Agent Technology from a NASA
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\[10\] Clark, P.E, et al.: ANTS: Applying a new paradigm for Lunar and
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