# IAA-00-IAA.Q.5.08

## 

ANTS (Autonomous Nano Technology Swarm): An Artificial Intelligence
Approach To Asteroid Belt Resource Exploration

## S. Curtis, J. Mica, J. Nuth, and G. Marr 

## NASA's Goddard Space Flight Center (GSFC)

## Greenbelt, Maryland

###### M. Rilee and M. Bhat/Raytheon ITSS

**51^st^ International Astronautical Congress**

**2-6 Oct 2000/Rio de Janeiro, Brazil**

##### For permission to copy or republish, contact the International Astronautical Federation 

**3-5 Rue Mario-Nikis, 75015 Paris, France**

**ANTS (AUTONOMOUS NANO TECHNOLOGY SWARM):**

**AN ARTIFICIAL INTELLIGENCE APPROACH TO ASTEROID BELT RESOURCE
EXPLORATION**

Steven A. Curtis

NASA's Goddard Space Flight Center (GSFC)

Greenbelt, Maryland

Tel: 301-286-9188/fax 301-286-1683

Steven.A.Curtis.1@gsfc.nasa.gov

J. Mica, J. Nuth, and G. Marr /GSFC

M. Rilee and M. Bhat/Raytheon ITSS

The final frontier of solar system exploration after the flyby of Pluto
will be asteroid belt between the orbits of Mars and Jupiter. Although
there has been much speculation concerning the resource potential of the
asteroid belt for the exploration and industrialization of space, we are
still very far from the required detailed and quantitative data to make
informed decisions. The ANTS NASA advanced concept envisions the use of
a large SWARM of pico-class (1kg) totally autonomous spacecraft to
prospect the asteroid belt. The SWARM using a social insect type of
artificial intelligence would individually fly using solar sails
directly from the outer edge of Earth\'s gravity well to the targets in
the asteroid belt of 1 kilometer or greater diameter. Many asteroids
(\>1000) would be visited by the SWARM. Data would be transmitted to
Earth via returning SWARM members. Replacement workers would join the
ongoing SWARM from Earth as needed. Each SWARM worker would have a
specialized instrument capability (magnetometer, x-ray sensor, gamma-ray
sensor, Visible/IR sensor, neutral mass spectrometer) needed to evaluate
the resource potential of each target asteroid. We discuss the technical
requirements for such a mission and the role that near term precursor
missions could play.

Q. Space Exploration Symposium; Q.5. Small Bodies Missions and
Technologies.

## INTRODUCTION[^1]

After the exploration of Pluto and full mapping of Mercury, there is
another major planetary frontier: the Asteroid Belt with thousands of
asteroids with diameters of greater than 1 kilometer (km). Exploration
of this region may offer great insights about the origin and evolution
of the solar system and its potential resource value both for space
exploration and Earth. This region offers major challenges for would‑be
explorers. Visiting many bodies at reasonable cost demands extremely
high autonomy, minimal communication requirements to Earth, and very
small explorers with few consumables. These requirements far exceed
current capabilities, but may be feasible within twenty years.[^2] The
Autonomous Nano Technology SWARM (ANTS) would provide such a means. ANTS
would allow the totally autonomous exploration of the asteroid belt with
the goals:

(1) Scientifically categorize all asteroids greater than 1 km in
    diameter;

(2) Perform initial prospecting expedition for resources becoming
    depleted on Earth and that are of use in space exploration and
    development.

A SWARM of 1000 picospacecraft (mass \<1 kg each) would fly from Earth
orbit to the Asteroid Belt using solar sails, Figure 1. As an insect
colony analog, ANTS is composed of specialized workers using remote
sensors including imagers, spectrometers, radiation and particle
detectors involving active and passive techniques. Exciting progress in
Robotics and Artificial Intelligence (AI) has been made using
insect-analogs, and we believe their application to space based systems
will be profitable.[^3]

The individual elements of ANTS, one of which we call an ANT, are to be
autonomous, but highly heuristic with a hierarchical intelligence shared
by all members of ANTS. ANTS will have the capability of dividing into a
number of groups, each with a full complement of workers to study many
asteroids simultaneously. Data will be returned to Earth by sending
multiple workers back after using the on‑board computational and
heuristic capabilities of ANTS workers to minimize data volume and to
incorporate knowledge gained into their efforts to prospect more
efficiently. Individual worker lifetimes are less of an architectural
driver since new workers could be sent from Earth as needed.

The heuristic systems enabling technology for ANTS would profoundly
affect all later space missions in allowing complicated many platform
missions to be executed with far less continuous terrestrial monitoring
and guidance than even simple single platform missions today.

Presently, Goddard Space Flight Center (GSFC), is vigorously pursuing
technologies for nanospacecraft (10kg or less)[^4]. GSFC is pioneering
studies into the design, manufacturing, and operations of large
Constellations of nanospacecraft (100 or more). ANTS represents a direct
extension of GSFC's present efforts to evolve multiplatform missions
toward greater numbers of increasingly smaller platforms. This
evolutionary sequence includes NASA's Sun Earth Connections' (SEC)
5-miniplatform Magnetospheric Multi Scale (MMS) Mission, the ST-5
nanospacecraft technology demonstration mission using 3 microplatforms,
and the 60--100 nanoplatform SEC Magnetospheric Constellation
Mission.[^5] This research provides a pathway toward meeting the
computing performance requirements implicit in the autonomy goals of
ANTS.

The purpose of this paper is to outline the ANTS concept. We present a
variety of technical aspects about the ANTS system including
architectural possibilities, instrumentation, and the demands on
spacecraft subsystems. To illustrate the level of autonomy required and
the possible ways that ANTS may achieve mission goals, an operational
scenario is presented. This paper opens a discussion on the requirements
for a mission to survey thousands of Main Belt asteroids and the
technologies that must be developed to enable ANTS.

## TECHNICAL ASPECTS

#### System Architectures

The system architectures include: 1) the overall mission design for
ANTS; 2) the design of the individual workers for ANTS; 3) the mission
operations scenario for ANTS as a near totally autonomous undertaking;
4) the evolutionary scenario to ANTS from plans for near‑term
nanospacecraft Constellations; 5) use of artificial intelligence,
distributed artificial intelligence, and the holistic integration of
knowledge shared among ANTS in some predefined social hierarchy; 6) use
the highly homogeneous integration of benefits of artificial
intelligence, software, and hardware as one in the ANTS spacecraft
system architectures. Together, these represent a dramatic departure
from traditional, reductionist system concepts. The highly homogeneous
spacecraft integration, artificial intelligence on-board, and the
holistic integration of shared knowledge make artificial intelligence a
part of all systems, instruments, ANTS spacecraft, on-board operations,
and mission.

As an integrated development of the artificial intelligence ANTS
on-board system, the study of system architectures for ANTS includes
looking at the possibility of assuming the knowledge and expertise of
the following agents.

**[Scientist]{.underline}** For ANTS to make local deter-minations of
what is interesting to keep or observe and also what to seek requires
the functions that are now the

Figure 1. A schematic ANT under full sail at 2.8 AU. A large 100 m^2^
sail is required to reflect enough light to achieve acceptable
accelerations. Solar radiation flux is 7.8 times less at 2.8 AU than at
Earth.

domain of the scientist. The on-board scientist is needed for resolving
conflicts for what science or search has the most value, based on
current prevailing conditions, and what resource risks are worthwhile.
The scientist's knowledge will be used to set to priority, assignments,
and actions of that ANTS mission.

**[Navigator]{.underline}** Like a sailor of old, an ANT will need to
account for changes during the flight caused by events like solar winds
due to solar storms. The navigator will need to account for variations
in the performance character of the sails. The navigator will need to
deal with the dangers and opportunities that foreign objects present.

**[Operator]{.underline}** Like the operations person that normally
would be on the Earth making decisions for the spacecraft, ANTS will
need to make its own decisions locally. These decisions are based on
knowledge the ANT was given initially, but it may be possible for an
ANT's knowledge and ability to grow in time via machine learning
techniques. To be studied are aspects of operations within the ANT to
handle locally the assignments of ANTS, route planning, optimize search
methods, communications, organization science data, resource management,
and house keeping.

#### System Elements

The major system elements are the major spacecraft subsystems, including
instruments, and owing to the highly integrated nature of the mission,
the mission designs itself. As an example, a predefined social hierarchy
for study could include a "ruler" class ANT and a "worker" class ANT.
The ruler class ANT may consist of one ANT or some fraction of the total
colony. The worker class ANT may consist of one or several types of
specialized spacecraft with each focusing on a particular science goal.
The SWARM's social structure may be determined by a particular set of
science and mission requirements. Cooperation amongst peers,
coordination by an oligarchy, and even competition in a market are among
the possible ways that desirable emergent behaviors might be elicited
from the SWARM. Representative system elements may include:

**[ANTS (General Functions):]{.underline}**

-   Distributed Intelligence Operations

-   Communications (SWARM, Messenger)

-   Resource management

-   Navigation

-   Collision-Avoidance, Rendezvous

-   Local status, housekeeping

-   Local conflict resolutions

**[ANTS (Ruler):]{.underline}**

\- Includes general ANT functions

-   Ruler of SWARM Heuristics Operations Planner

-   Overall mission objectives

-   Assignments for worker ANTS

-   Maintain shared SWARM statistics

-   Sharing science discovery data

-   Mission conflict resolutions

-   Resource management for SWARM

-   Messenger delivery of science, status

**[ANTS (Worker):]{.underline}**

\- Includes general ANT functions

-   Worker Heuristics Operations Planer

-   Possible ascension for ruler replacement

-   Science data acquisition processing

# **[Spacecraft Subsystems]{.underline}**: 

-   Artificial intelligence heuristic systems involve every spacecraft
    function.

-   Attitude Determination and Control

-   Communications

-   Command and Data Handling

-   Power

-   Thermal

-   Structures and Mechanisms

-   Guidance and Navigation

-   Solar Sail Propulsion System

#### Pico spacecraft mission architecture 

Present planning for nanospacecraft will be extended to the
picospacecraft regime. Attention will be given to solar sail
accommodation and the specialized instruments for each type of ANTS
worker. The highly integrated individual spacecraft system architecture
is envisioned, and may drive a "Spacecraft on a Chip" paradigm.
Implementing ANTS will require technology expansion in radiation hard
technology, memory density, package size, power, and computer
processing. Planning for the prospective picospacecraft architecture
will promote a more homogeneous approach at the chip structure level to
support the mission function level of integration. Holistic
relationships will be sought between specialized worker ANTS and the
common mission. A guiding theme of this work is the use of heuristic
systems to integrate the varied elements of the mission architecture to
achieve mission goals. These varied architectural elements range from
the individual spacecraft to the distribution of the SWARM in mission
operations, planning, autonomy, parallel processes, and fault tolerance.

#### Miniaturized instruments

It is assumed in the following section that the initial goal of a
mission employing the ANTS design scenario will be a general survey of
some type. In such a mission it will be important to catalog the mass,
density, morphology and chemical composition, including any anomalous
concentrations of specific minerals, of a large number of relatively
small bodies. Basic instrumentation for such a SWARM would most likely
include the following:

\- IR/Visible/UV imagers and spectrometers,

-   X‑ray and Gamma Ray detectors,

-   magnetometers,

-   accelerometer and laser rangers,

-   neutral mass spectrometers, and

-   Radio sounders and rangers.

Each of these instruments has unique viewing requirements for optimal
data collection: in general, a survey mission incorporating many
different instrument types on a single spacecraft makes a large number
of compromises in order to accommodate the needs of each specific
instrument for at least a portion of the mission. In the ANTS concept,
each instrument collects data in an optimal mode for the entire time it
remains in the vicinity of its target. Brief descriptions of instruments
and their data collection modes follow.[^6]

**[Magnetometers]{.underline}** The multipoint character-ization of the
magnetic signature of planetary bodies has proven key to our better
understanding of not only surface characteristics of planetary bodies,
but also their bulk properties and internal structure. The use of
magnetometers on Mariner 10 at Mercury, on the NEAR Shoemaker asteroid
flybys, the MGS orbiter, Voyager encounters at Jupiter, Saturn, Uranus,
and Neptune, as well as the Galileo Jupiter orbiter. Provide extensive
examples of this.[^7] Simultaneous observations by several ANTS would
provide a detailed characterization of the magnetic properties of the
asteroid.

**[Radiosounders]{.underline}** A new emerging technology for the
sensing of the interiors of planetary bodies to a depth of several
kilometers is radiosounding. Using radio waves to actively probe below
the planetary surface is being actively explored for Mars missions as
well as the Europa Orbiter. A radiosounder specialized Ant could map
conductivity variations over the entire surface to several kilometers
depth.

**[X-ray Fluorescence Spectrometers]{.underline}** In our solar system,
x-rays from the sun excite fluorescent x-rays from elements on the
surfaces of airless bodies such as asteroids, the moon and quiescent
comets.[^8] The x-ray emission comes from material within a few tens of
Angstroms of the surface of the body and is therefore highly sensitive
to both the excitation and viewing geometry: low angles of incidence
lead to high attenuation of the signal. The best analytical attitude for
an ANT carrying an x-ray spectrometer is to look straight down on the
target with the sun at its back. In this attitude the sun directly
illuminates the target and the fluorescent x-rays emerge at normal
incidence to the surface.

**[Gamma-ray Spectrometers]{.underline}** Galactic Cosmic Rays permeate
our solar system and excite characteristic gamma rays from as deep as
several meters beneath an airless body. In addition, natural radioactive
elements such as Potassium, Thorium and Uranium also emit characteristic
gamma rays. Since the gamma rays from both cosmic ray induced emission
as well as from natural radioactivity emerge independent of the sun
angle, the ANTS carrying gamma-ray spectrometers can be positioned
anywhere around the body under examination.

**[Accelerometer/Camera]{.underline}** The mass, density and morphology
of the target body are intimately related. An ANT optimized to determine
these important parameters would carry an accelerometer and a relatively
high-speed, moderate-resolution, monochromatic camera. Initial passes
over the body would photograph the sunward-facing side of the target as
it rotated: simultaneously recording variations in the gravitational
acceleration measured at the ANT. Once an initial digital model of the
surface was constructed from these images, close passes over selected
areas would be made to measure the variation in gravitational
acceleration from specific portions of the target and thus derive a
measure of local density. Passes over the terminator could be used to
derive a measure of the surface roughness by recording the contrast
variations per unit area due to variable illumination of craters,
crevices and other surface irregularities.

**[Radio Ranger]{.underline}** Another instrument that can obtain good
measurements of asteroid density and mass distribution involve radio
ranging and accelerometry.[^9] The method consists of measuring small
shifts in the frequency of radio transmissions from spacecraft being
accelerated by bodies of mass. This technique has been used to develop
mass distribution models of the Earth and other planets. Because around
ten ANTS may encounter an asteroid at a time, the potential amount of
acceleration data is great, and it may be possible to rapidly develop a
asteroid mass distribution model.

**[Visible/Near-Infrared Spectrometers]{.underline}** The mineralogy of
the surface can be determined to some extent by subtle absorption
features in the visible through infrared spectral regions. For the
visible and Near-IR regions, the best viewing geometry is looking
directly down on the target with the sun at zenith. This yields maximum
signal in the reflected sunlight. Mineralogy can be very important
indicator of the processing history of the body under study and of the
resources it might contain.

**[Thermal-IR Spectrometers]{.underline}** The best viewing geometry for
Thermal-IR instruments is just beyond the terminator as the body under
study rotates out of the sun. The thermal infrared spectrum of the body
can be another very useful indicator of the mineralogy of the body,
while the time-dependent rate of change in the thermal IR emission of a
particular region can indicate the thermal inertia or internal
heat-flux. The former can be a measure of the density or porosity of an
area while the latter might indicate a localized radioactive heat source
or a local "cold-sink". Such a cold sink could be due to a large mass of
ice kept at constant temperature via sublimation.

**[UV/Visible Spectrometers]{.underline}** Volatile emissions from the
surface of a small body could be an extremely important indicator of the
presence of water or hydrocarbons -- both very valuable resources
necessary for the future exploration and utilization of space. The best
position for an ANT carrying an UV-Visible spectrometer is either just
beyond the terminator. It would view the sun on the "horizon" through
the maximum extent of any residual atmosphere and search for atmospheric
absorption features. Or view along the terminator searching for
fluorescent atomic. Or search for molecular emissions from atmospheric
constituents escaping from the interior of the body.

**[Mass Spectrometers]{.underline}** Confirmation of the presence of
large deposits of volatile materials such as hydrocarbons or ice might
require the use of a mass-spectrometer optimized for the detection of
low mass, volatile molecules. The measurements would be optimized with
close fly-bys over suspected surficial vents following maximal heating
by the sun. Depending on the geometry of the body, such passes would be
made from \~ local noon to \~ local sunset. In extreme cases, there
might be some advantage in flying the ANT into a suspected vent system
to confirm the presence and obtain a detailed analysis of the volatiles
present. Unless an onboard propulsion system was available, the ANT thus
deployed would be lost from the SWARM.

For each of the instrument types enumerated above, very lightweight
detector systems are already available for laboratory use. NASA's PIDDP
Program, the SBIR Program, and various interagency agreements are now
financing several studies of new, lower-mass, more-capable sensors with
fewer restrictive operational parameters. These smaller, more capable
instruments are needed for the next generation of Lander and Rover
missions to both Mars and to minor planets. Terrestrial applications of
such smaller sensors (such as in Forensic Investigations -- currently
being funded jointly by the National Institute for Justice and NASA)
could spur increased economic incentives for the improvement of existing
sensors or the development of new technologies. In particular, for each
of the instrument types that are most useful in a resources survey
mission, sensors already exist with the required sensitivity and
operational capabilities at masses that are less than half a kilogram.
Future developments should push the masses of most such systems down
near the 100-gram range within a decade or so. The onboard computational
capabilities of the ANT must be sufficient to operate the instrument,
analyze the data, and optimize the data-gathering phase of an encounter.
Then only portion of a traditional space instrument need be incorporated
into the individual ANT version of the low-mass sensors now under
development.

#### Propulsion 

Both inflatable and other types of solar sail designs will be considered
and optimized for the picospacecraft of the ANTS. We will be studying
the control of the solar sails with heuristic systems including the
navigator, attitude control, and power management. The solar sail may be
useful to concentrate the light but needs the heuristic systems to
balance the need for power with navigation and attitude control.

**[Heuristic systems]{.underline}**

The architecture for the ANTS heuristic system must be developed with an
aim toward near total autonomy. The hierarchical structure among the
ANTS workers will be defined in the developed mission design context.
The mission design context will refine the details of the hierarchical
architecture, specialized responsibilities, the goals of the collective,
and the social structure of the ANTS. As the mission is developed, rules
will be defined that use the locally available, intermediate, partial
knowledge and allow ANTS to efficiently search its set of actions for
mission planning and scheduling. The developed heuristics are to enhance
many important mission parameters, including the science return, mission
operations efficiency, and the efficiency in the spacecraft systems.

The layers of social structure of the SWARM, relationships between
spacecraft, sets of rules, the layers and methods of artificial
intelligence will together determine how autonomous planning and
execution will be achieved. The top layer of the heuristic topology may
be a planning system that makes the top-level decisions. These layers
may include genetic algorithms to help in the creation of new strategies
or the elimination nonproductive strategies, a virtual blackboard system
for multiple agent management and support management of the mission.
Approaches to be studied include: 1) Neural Nets, a model free, or
implicit model, discipline that would have the training supervised
autonomously by higher level system components, used to learn about
empirical system function, patterns, and control requirements; 2) Fuzzy
Logic, also a model free discipline useful in control and conflict
resolution applications arising in ANTS: navigation, attitude control,
collision avoidance, systems and subsystems; 3) distributed artificial
intelligence to support, e.g.: mission objectives, SWARM collective
intelligence, SWARM system architecture, management of system,
navigation, attitude, command and data handling; 4) genetic algorithms
to support, e.g.: effective search and navigation; 5) on-board
operations planner supporting, e.g.: mission objectives, navigator,
encounters for science, data management, ANTS systems architecture
management.

Mission development will define the planning system mission objectives
with respect to current status of the SWARM, control of individual SWARM
spacecraft operations, navigation planning, orbit determination,
maintenance communications with the virtual network of ANTS spacecraft,
maintenance the leadership responsibilities of the swarm of ANTS.

The mission development will define goals, roles and responsibilities
among the social structure of ANTS. This development will define how
autonomous commands will be generated, passed, assignments made,
coordination of SWARM and the sharing of science or status data.

[**Advanced on‑board computation**]{.underline}

Advanced on‑board computation will be examined from the perspective of
limiting the volume of data to be returned to Earth, coupled with the
heuristic systems to increase autonomy and worker productivity by
optimizing the prospecting strategy at each asteroid and across numbers
of asteroids. We need to investigate the concept of a "spacecraft on a
chip", where a generic highly configurable processing chip could be set
up to perform the unique support required for Worker ANT, Ruler ANT,
spacecraft artificial intelligence system, individual instruments, and
subsystem support. This investigation will include benefits for
reduction of mission complexity, reduction of interfaces, reduction of
cost, improved performance, and improved survivability.

#### Attitude control 

As all the individual ANTS will have solar sails they all will require
an attitude control system capable of at least rough three-axis
pointing. A sun sensor can provide one direction reference, but at least
two are needed. In deep space, each individual will need to able to
perform stellar navigation unless some other reference is provided
externally. A star camera can provide the reference information but may
be expensive to put on every spacecraft. Another approach to study would
be to place the star camera on only a few ants and they would broadcast
an ANTS network GPS like signal that the others could receive. A relay
network communications should be studied to avoid a limitation of this
GPS approach to avoid the ANTS needing to stay close together, i.e.; the
slowest member would limit the progress of the group. This may be merged
navigator and communications as part of the various heuristic systems
and disciplines such as distributed artificial intelligence, fuzzy
logic, on-board operations planner, to be studied could come into play
to support this relay network communication and attitude.

##### Power

Power for the ANTS project will have unique problems to study. The
sun\'s intensity falls of with the square of the distance reducing the
effectiveness of solar cells. Areas for study include the use of a solar
sail as a concentrator for solar cells. The study of the heuristic
systems necessary to support power control and management is also
required.

#### Thermal

Thermal design for the ANTS project has unique problems. The spacecraft
will be cold and power for the spacecraft is likely to be low.

###### [Navigation]{.underline}

Decisions that are made in-route will need to anticipate navigation
changes with enough time for the solar sail to respond to the changes.
The Navigator is an artificial intelligence mission spacecraft system,
functioning like the sailor of old.

Once in the vicinity of an asteroid target, the target will have to be
imaged to obtain fine scale navigation data to enable close approach.
The limited detail data we have on small astronomical objects suggests
that a significant percentage will be complex objects. They may be
non-uniform and rotating or binary objects creating tidal forces. They
may have moonlets that create collision hazards. Or they may be emitting
gasses or particles that could be hazardous. All of these will need to
be assessed locally by the ANTS to determine if it is safe before a
close approach to the target can commence.

#### Communications

The communication design will need to part of the highly autonomous
nature of ANTS and the need to share knowledge, data between the Ruler
and Worker ANT, and assignments issued to the Worker ANTS.
Communications is an artificial intelligence mission and spacecraft
system. A communications method such as a network relay is a candidate
for all communications within ANTS. Also communication relays may be
used for messengers that are assigned to report back the science and
SWARM house keeping status to an earth orbiter such as Space Station.

#### Command and Data Handling

The highly autonomous nature of ANTS and the ANTS predefined social
hierarchy will cause much of the command and data handling control to be
contained within the heuristic systems of the ANTS and addressed also in
the Communications. Data processing will also need to be addressed to
support the instruments. Command and Data Handling is an artificial
intelligence mission and spacecraft system.

#### Mission design 

A mission design is required for a highly autonomous expedition, which
will optimize the prospecting strategy in terms of time and propulsion
requirements. In order to make efficient use of the individual ants;
target assignments will need to be made while in-route. This will permit
attrition and sailing performance to be accounted for.

#### Operational Strategies 

A detailed scenario for the ANTS Asteroid Belt prospecting mission is
required. This will include the social structure of ANTS: the division
of workers in groups and the size of the groups and the number of
asteroids to be investigated simultaneously. Once deployed ANTS on-board
execution of operational strategies will need to be autonomous and an
artificial intelligence mission and spacecraft system.

**[Evolutionary Scenarios]{.underline}**

Evolutionary scenarios will be examined starting with a
nanospacecraft‑class ANTS mission to near‑Earth asteroids with a smaller
number of ANTS workers, more limited autonomy and on‑board computation
and alternative propulsion systems such as hydrazine.

####### AN OPERATIONAL SCENARIO

To explore the way individual ANTs might work together we develop an
operational scenario involving an ANTS encounter with an asteroid. In
this scenario, ANTS detects an asteroid, tracks it, passes information
about the asteroid\'s trajectory to the SWARM. Rulers, workers, and
messengers each act on this information accordingly. The preceding
discussion has revolved around the possibilities associated with
ANTS-like architecture. The scenario presented here briefly sketches one
such possibility.

**[The Asteroids]{.underline}**

An order of magnitude estimate for a mean distance between asteroids may
be found by dividing the mass of Mars into 1 km diameter chunks and
distributing them evenly in a ring of thickness 0.1 AU with and inner
and outer radii of 2 and 4 AU respectively. This yields a mean distance
of about 3×10^4^ km. The ANTs as configured above can change their
semimajor axis by about 2×10^5^ km in a day. Therefore ANTs can spend
most of their time in the vicinity of asteroids and very little time in
transit between them. Spacecraft communication range then drives the
physical size of ANTS.

**[The Spacecraft]{.underline}**

Before discussing the asteroid encounter we briefly describe the
spacecraft that make up the SWARM. First, we presume that individual
ANTs are complete spacecraft capable of orienting themselves, operating
their sensors, monitoring themselves and their surroundings, maintaining
health and safety, and pursuing goals such as trajectories or orbits.
This already implies a highly advanced level of autonomy, but a level of
autonomy that can be developed in the single spacecraft context. In
addition, individuals will have at least a low and a high bandwidth
(LBW, HBW) communication capability. The LBW communication link is to be
used for collective operations and behaviors, while the HBW link is
mainly for close range ANT to ANT transfers. We suppose that these
communication links use omni directional antennae. For example, the
bandwidth and range of the LBW link might be on the order of ten bits
per second at 210^5^ km, respectively. The HBW link may deliver
megabits per second at a few hundred kilometers. Avoidance of collisions
with asteroids or spacecraft is within the domain of system health and
safety and must be a fairly low level autonomous behavior. Thus each ANT
has a package of software and hardware for traversing space and
encountering asteroids while maintaining their place in the SWARM.

**[The SWARM]{.underline}**

The number of asteroids within ANTS depends on its physical length
scale, *\~l~swarm~ l~lbw~* where the LBW range is *l~lbw~*. A rough
estimate for the number of asteroids is:

$$\left( \frac{l_{\text{swarm}}}{1} \right)^{3}\left( \frac{l_{\text{lbw}}}{1\text{.}5 \times \text{10}^{5}\text{km}} \right)^{3}\left( \frac{l_{\text{asteroid}}}{3 \times \text{10}^{4}\text{km}} \right)^{- 3} \approx \text{125}\text{.}\ $$

For *l~swarm~ \~* 1, all members of the SWARM are within LBW contact of
each other. ANTS's optimal operating mode for studying individual
asteroids has yet to be determined. For example, one could have roving
teams of ANTs or teams could form within the SWARM as needed. In either
case, we can crudely estimate the mean number of ANTS required to study
an asteroid as a number of Workers supported by an overhead of
Ruler/Messengers. From the discussion of diagnostic instrumentation
above, we see that there are perhaps 8-12 types of instruments, and
hence Workers, that would be useful. The optimal ratio of Workers to
Ruler/Messengers depends on the reliability of the Ruling/Messaging
support, as well as how well information travels through the SWARM.
Recall that Messengers are responsible for high volume data transfers
through the SWARM and to Earth. We may try:

10 Workers + 2 Ruler/Messengers = 12 ANTs/asteroid

as a first guess. A more detailed analysis modeling the information
transfer, command, and control strategies is required for further
progress. We note that, unlike terrestrial ants, robotic or natural^1^,
ANT spacecraft do not readily interfere with each other's operations;
there is plenty of room for ANTs to avoid collisions while seeking to
achieve their goals. Therefore limits on system scalability and
cooperation for ANTs differ from those readily realizable in terrestrial
labs.

With these estimates, we see that ANTS can be reasonably sized at about
one thousand spacecraft, which could study about 80 asteroids at a time.
Within this scheme, ANTS would study all of the asteroids within one
"coordinate cube" and then move *en masse* to the next. If it takes
about two weeks to study an asteroid, then perhaps 2000 asteroids per
year could be studied, before taking into account inefficiencies due to
conflict resolution, hazard avoidance, etc.

For the sake of discussion, we consider a SWARM of 1000 spacecraft
composed of about 12 types with about 80 spacecraft of each type. The
three broad types of spacecraft have already been mentioned: Ruler,
Messenger, and Worker. Most ANTs fall within the Worker class. Each type
of Worker is specialized with a given sensor, in addition to those
systems required for space faring. The Ruler and Messenger have similar
requirements: both must communicate with possibly large numbers of ANTs
and both must store or process data from the SWARM. This points to the
Ruler and Messenger having good information handling capabilities, i.e.
memory, processor, and communication (including physical speed).
However, Ruler and Messenger do have slight differences, mainly in their
handling of data and responsibility for directing ANTS. An intriguing
possibility is to use a single Ruler/Messenger type, in which the
qualities required for one role are latent in the other. For example,
processing capability available to a Ruler may be latent, i.e. turned
off, in a Messenger. The behavior of a Worker depends a great deal on
the particular sensor it carries, making such polymorphism more
difficult.

Therefore, we consider a SWARM with about 160 Ruler/Messengers that are
central to the goal achieving behavior of the SWARM. The remaining 840
are divided among the various sensor platforms required for the survey.
These spacecraft may be grouped according to the kind of spacecraft
trajectory required by their role. Possible categories include: (1)
Cruisers, (2) Orbiters, and (3) Hoverers.

**[Cruising]{.underline}** Cruisers include those spacecraft that do not
need to drop very close to asteroids. Such spacecraft include Rulers and
Messengers as well as those that have certain remote sensing
capabilities. For example, Multispectral imagers, particularly those
using IR telescopes are useful for detecting and tracking asteroids.
Furthermore, having dozens of these in a SWARM opens up interesting
possibilities for obtaining asteroid parallax and proper motion
information, i.e. asteroid range, speed, and bearing information. ANTs
not requiring proximity to asteroids for resolution or signal strength
can be cruisers.

**[Orbiting]{.underline}** Orbiters require proximity to the asteroid of
interest. This is the case when an instrument such as a spectrometer
needs to be brought close to the asteroid because a map is to be
constructed or because the phenomena to be measured is weak. Imaging
spectrometers are an example of an instrument that could produce
resource maps of asteroid surfaces. Gamma ray spectrometers are an
example where a weak signal is to be maximized. Some spacecraft may only
require one pass to make their observations, but for our purposes we may
consider these orbiters as well.

**[Hovering]{.underline}** There is the possibility of matching the
gravity of an asteroid with the force generated by an ANT\'s solar sail,
in which case trajectories in which the spacecraft hovers over the
asteroid may be possible. Such a trajectory may be useful for asteroid
mapping or SWARM logistical roles, e.g. Rulers and Messengers may hover
at an asteroid to wait for and communicate with the Workers there. As
described previously for Radio Rangers, another possibility is that a
Hoverer, by monitoring its radio contact with Orbiters, could obtain
acceleration data from which information about an asteroid\'s mass
distribution could be extracted.

**[Asteroid Encounter]{.underline}** The operational scenario is as
follows. The spacecraft of ANTS are traveling through the asteroid belt
as a number of irregular clumps all within LBW communication of each
other. The size and distribution of the individual clumps will depend on
the history of the SWARM and the range of HBW communications.

**[Detection and Tracking]{.underline}** Workers that have IR/Visible/UV
(IVU) imaging capability are constantly keeping track of the asteroids.
As new asteroids are detected the information is propagated through the
array. The IVU data is used to catalog the asteroids and to determine
asteroid orbits. Ruler ANTS maintain HBW communication with the IVU
Workers and decide when nearby asteroids are interesting.

**[Rulers React]{.underline}** The Rulers put information about
important asteroids onto the LBW link; individual ANTs receive this
information and respond according to their role and the nature of the
Rulers' communication. Rulers may designate important asteroids or may
assign degrees of importance to asteroids. These degree assignments
could act as inhibitory or excitory signals in analogy with
neurophysiology or subsumption techniques[^10]. Cruising imagers
maintain their watch on the asteroid as Hoverers and Orbiters head for
important asteroids. The decision of an individual ANT to drop towards
an asteroid should depend on the excitement level of the Rulers towards
that asteroid and its own ability to perform its task. Because all
spacecraft will have resource limits, e.g. memory for data storage, ANTs
may not always be able to perform detailed observations of asteroids.

**[Arrival at the Asteroid]{.underline}** A Messenger, being faster than
most Workers arrives in the vicinity of the asteroid, perhaps at the
hover point. The Messenger also has somewhat better than average
communication range and can act as a communication node for other
spacecraft. Furthermore, the Messenger attempts to take data from the
Workers for eventual transmission to Earth. Rulers and Messengers might
also act as timing beacons for positioning and asteroid gravity studies.
Simple models of the asteroid may be created and transmitted to the ANTs
starting to SWARM about the asteroid; these models may help individual
ANTs plan trajectories about the asteroid.

**[Workers Acquire Data]{.underline}** Workers arrive and begin seeking
trajectories that will allow efficient operation of their instruments.
As the spacecraft drop towards the asteroid the aforementioned collision
avoidance functions gain importance. To reduce the possibility of single
point failures, we believe that trajectory determination should be
distributed to the individual spacecraft rather than handled through a
central controller.

Workers with Gamma Ray detectors seek orbits with low periasters on the
dark side of the asteroids; those with X-ray spectrometers seek high
integration times on the sunlit side. Workers with magnetometers seek
orbits with a wide and deep coverage of the space around the asteroid.
As mentioned previously, a hovering spacecraft that can measure the
Doppler shifts of radio beacons of the Workers could fairly rapidly
build a model of the mass distribution of the asteroid. ANTs should be
able to adapt their observing plans to take advantage of interesting
features as they are detected.

**[Workers Complete Observations]{.underline}** As the spacecraft
complete their measurements or fill their data buffers, they start to
move away from the asteroid. Moving away from the asteroid creates the
opportunity for other spacecraft to approach the asteroid. Workers may
also require time to digest or reduce their raw data into models and
statistics that are more suitable for transport. Workers may call
Messengers via the LBW link and then transfer the reduced data via HBW
links to Messengers who move from Worker to Worker as needed. Once done
transferring data to the Messengers, the Worker proceeds to the next
important asteroid for more data.

**[Messengers Consolidate Results]{.underline}** The Messengers respond
to the calls of Workers and Rulers for data transfers, and also share
data amongst themselves. When a Messenger reaches the limit of its
memory store, it uses its superior mobility to move rapidly to Earth
where it downloads its information to an appropriate communications
point.

**[Scenario Summary]{.underline}** In this operational scenario we have
described the role that Rulers play in assessing information provided by
Imaging Workers that detect and track asteroids. The Rulers propagate
signals throughout the SWARM that cause Workers to drop towards
important asteroids where the Workers collect their data. Workers
collect their data and, when full, process the data into more
transportable forms. Messengers work to alleviate Workers of their data
load so they may return to work. Messengers transfer reduced data and
other information throughout the SWARM and eventually to Earth.

**[CONCLUSION]{.underline}**

There are several areas that require further study for a realistic
assessment of the feasibility of the ANTS concept. First there should be
a careful analysis of the utility of each individual instrument in terms
of potential mission goals, the current state-of-the-art in terms of the
sensor technology employed in the measurement and the likelihood that
the technology would be significantly improved by the time frame of the
mission. Second, the parameters for the automated survey of the target
by an individually instrumented ANT would need to be specified in terms
of realistic objectives achievable with the sensors expected to be
available at the time of the mission. Finally, an optimal mission
strategy that specifies the primary mission objectives is required.
Several key requirements must be elucidated: 1) the optimal number of
each of the specific instruments deployed within each SWARM; 2) the time
requirements to survey each target object to a specific degree of
accuracy; 3) the utility of each instrument in achieving the primary
mission goals; 4) a straw man exploration strategy to be followed by the
SWARM after arrival at each target.

Mission requirements and cost will be affected by the technology
available during mission implementation. A goal of this work is to point
out areas in which technology development should be fostered. A
multispacecraft mission was advocated to increase SWARM robustness to
hazards and faults, to decrease mission implementation and operational
costs, and to enable the survey of thousands of asteroids. A simple
operational scenario was presented to help illustrate how ANTS might
function. Necessary and desirable system traits and functions within the
analogy of social insect behavior point towards the importance of
single-spacecraft autonomy operating within a system that achieves goals
via emergent, collective behavior. Single spacecraft autonomy is an
existing important technological thrust for NASA, while understanding
and utilizing emergent, collective behaviors is a fundamental problem
driving current research in Computer Science. In addition to the
technologies that deal with self-directed and collective operations, a
broad range of sensors and other spacecraft subsystems were mentioned
with their needs for technological advance in mind.

[^1]: ^^ ^ ^Copyright  2000 by the American Institute of Aeronautics
    and Astronautics, Inc. No copyright is asserted in the United States
    under Title 17, U.S. Code. The U.S. Government has a royalty-free
    license to exercise all rights under the copyright claimed herein
    for Governmental purposes. All other rights are reserved by the
    copyright owner.

[^2]: For a discussion of the state-of-the-art in small spacecraft and
    sensor technology, including technology roadmaps, see the
    *Proceedings of the Second International Conference on Integrated
    Micro/Nanotechnology for Space Applications*, E. Robinson ed., 1999,
    *et seq*.

[^3]: M. Krieger, J.-B. Billeter, & L. Keller, "Ant-like task allocation
    and recruitment in cooperative robots", *Nature*, 406, 992, 31
    August 2000 provides experimental evidence for the utility of the
    insect analogs. Another use of an insect-analogy in the development
    of a distributed autonomous control system is C. Ferrel, "Global
    Behavior via Cooperative Local Control", *Autonomous Robots*, 2:2,
    105-125, 1995. A somewhat more conventional approach, but within the
    context of spacecraft autonomy is N. Muscettola et al., "Remote
    Agent: To Boldly Go Where No AI System Has Gone Before", *Artificial
    Intelligence* 103(1-2):5-48, August 1998.

[^4]: P. Panetta et al., "NASA-GSFC Nano-Satellite Technology
    Development", in *Proceedings of the 12^th^ AIAA/USU Conference on
    Small Satellites*, 1998.

[^5]: NASA Sun-Earth Connections: "sec.gsfc.nasa.gov".

[^6]: The Near Earth Asteroid Rendezvous (NEAR) mission is the current
    state-of-the-art in asteroid exploration. A. Santo, S. Lee, and R.
    Gold, "NEAR Spacecraft and Instrumentation", *J. Astronomical
    Sciences*, Vol. 43, No. 4, 373-397, 1995. A sample of NEAR science
    results appears in the 22 September 2000 issue of *Science*.

[^7]: M. Acuña, C. Russell, L. Zanetti, and B. Anderson "The NEAR
    magnetic field investigation: Science objectives at asteroid Eros
    433 and experimental approach", *J. Geophys. Res.,* 102, E10, 23751,
    1997.

[^8]: J. Trombka, et al., "The Elemental Composition of Asteroid 433
    Eros: Results of the NEAR-Shoemaker X-ray Spectrometer", *Science*,
    289, 2101, 22 September 2000.

[^9]: D. Yeomans, et al., "Radio Science Results During the
    NEAR-Shoemaker Spacecraft Rendezvous with Eros", *Science*, 289,
    2085, 22 September 2000.

[^10]: R. Brooks, "A Robust Layered Control System for a Mobile Robot",
    *IEEE Journal on Robotics and Automation*, RA-2, 1, March 1986.