AsterAnts: A Concept for Large-Scale Meteoroid Return and Processing
Al Globus, MRJ Technology Solutions, Inc. at NASA Ames Research
Bryan A. Biegel, MRJ Technology Solutions, Inc. at NASA Ames Research
Steve Traugott, Sterling Software, Inc. at NASA Ames Research Center
AsterAnts is a concept calling for a fleet of solar sail powered
spacecraft to retrieve large numbers of small (1/2-1 meter diameter) Near
Earth Objects (NEOs) for orbital processing. AsterAnts could use
the International Space Station (ISS) for NEO processing, solar sail construction,
and to test NEO capture hardware. Solar sails constructed on orbit are
expected to have substantially better performance than their ground built
counterparts [Wright 1992]. Furthermore, solar sails may be used to hold
geosynchronous communication satellites out-of-plane [Forward 1981] increasing
the total number of slots by at least a factor of three, potentially generating
$2 billion worth of orbital real estate over North America alone. NEOs
are believed to contain large quantities of water, carbon, other life-support
materials and metals. Thus, with proper processing, NEO materials could
in principle be used to resupply the ISS, produce rocket propellant, manufacture
tools, and build additional ISS working space.
Unlike proposals requiring massive facilities, such as lunar bases,
before returning any extraterrestrial materials, AsterAnts could conceivably
begin operation with a single spacecraft whose payload is no larger than
a typical inter-planetary mission. Furthermore, AsterAnts could be
scaled up to deliver large amounts of material by building many copies
of the same spacecraft, thereby achieving manufacturing economies of scale.
Because AsterAnts would capture NEOs whole, NEO composition details, which
are generally poorly characterized, are relatively unimportant and no complex
extraction equipment is necessary. In combination with a materials processing
facility at the ISS, AsterAnts might inaugurate an era of large-scale orbital
construction using extraterrestrial materials.
Meteoroids are defined to be solid objects in space with diameters between
100 microns and 10 meters [Beech 1995]. We focus on the return to Low Earth
Orbit (LEO) of meteoroids with diameters between 1/2 and 1 meter presently
in orbits similar to that of the Earth around the Sun. Such meteoroids
might be captured by relatively small spacecraft. Such NEOs should
have a mass roughly equal to the dry mass of the Deep Space 1 and NEAR
spacecraft (very roughly, 500 kg). Thus, they may be returned by
current or near term propulsion systems such as solar electric. Capturing
whole meteoroids and returning them may be simpler than returning part
of a larger asteroid. Developing an automatic system to capture a
meteoroid whole should be substantially easier than digging up a sample
from a surface of unknown composition. For safety reasons, asteroids
with diameters greater than about three meters should not be returned to
LEO [Globus 1998].
There have been no direct measurements of the number of 1/2-1 meter
diameter NEOs. However, approximately seven such objects enter the
Earth's atmosphere each day [Ceplecha 1988] assuming densities similar
to those calculated for asteroids. [Rabinowitz 97] estimates that
there are approximately one billion ten-meter-diameter NEOs and there should
be far more smaller objects. The distribution of NEOs with diameters greater
than 10 meters roughly fits an inverse diameter power law with a coefficient
of about 2.5. Finally, a substantial fraction of NEOs may be returned with
lower round trip delta-v requirements than a round trip to the surface
of the Moon (9.4 km/seconds) [Davis 1993]. (Delta-v is the change in velocity
needed to move from one orbit to another). It is clear that the number
of accessible objects in the desired size range is huge. Launch opportunities
may be nearly continuous, although detection would be difficult.
The detection problem is discussed below when we consider Earth-based telescopes
for detection and radar for orbit/size/rotation characterization.
Not only are small NEOs numerous, they almost certainly have a very
diverse composition. Laboratory studies of meteorite composition
and the spectra of NEOs do not provide a consistent picture of NEO composition.
Meteorite data reflect the internal composition of NEOs that survive atmospheric
entry, while the spectra provide data on surfaces exposed to sunlight and
deep space for millions of years. However, both measures strongly
suggest that NEOs contain a wide variety of materials which probably include
water, volatiles, and metals in large quantities [Nelson 1993], [Lewis
1993], and [Nichols 1993].
Solar sails are a promising propulsion system for returning meteoroids
because no reaction mass is necessary. (Reaction mass is the material
thrown backward by a chemical rocket or solar electric propulsion system
to generate forward thrust, taking advantage of Newton's third law). Thus,
if the mass of a captured meteoroid has been underestimated, a solar sail
could still return the meteoroid given sufficient time, whereas a reaction
mass dependent propulsion system may become stranded in an orbit far from
Earth. Solar sails are large sheets of thin reflective material that reflect
photons to produce thrust. In the mid-1970s JPL designed, but never built
or tested, a 820x820 meter solar sail to rendezvous with Halley's Comet
[Wright 1992]. ([Wright 1992] provides the data for most of the following
discussion). Solar sailing was also used by the Mariner 10 mission to Mercury.
This was done by differentially twisting the solar panels so solar pressure
would create torque around the spacecraft's roll axis for attitude control.
The JPL study determined that an unloaded characteristic acceleration
of approximately 1-2 mm/s2 was probably achievable using 1970s
materials. Much lower mass sails could be constructed in orbit [Drexler
1979]. (The characteristic acceleration is the acceleration of a
sail directly facing the Sun at a distance of one astronomical unit (AU)).
Actual acceleration is less because the sail must be oriented at an angle
to the Sun to produce thrust in most directions. Thrust is generated approximately
normal to the sail in the leeward direction. While 1-2 mm/s2 may
seem small, it is continuous. Given a characteristic acceleration of 1
mm/s2, a solar sail produces a delta-v of approximately 1.3
km/s per month with the sail set at a 45 degree angle to the Sun. Such
a sail could reach a large fraction of all NEOs within one year. Table
1 contains the size of square sails necessary to return a 500 kg meteoroid
with two different desired characteristic accelerations and two different
sail masses per unit area. Even with a very low characteristic acceleration,
the sails need to be nearly 200 meters on a side. Although space manufacture
does not reduce the size of sails much, the unloaded outbound leg would
be much faster with the high-performance space manufactured sail because
the sail itself has much less mass.
|sail mass per unit area: g/m2
||side length (m) to achieve desired characteristic acceleration (including
payload) = 1 mm/s2
||side length (m) to achieve desired characteristic acceleration (including
payload) = 0.25 mm/s2
|5.27 (ground manufacture)
Table 1: The size of square solar sails necessary
to achieve a particular characteristic acceleration when moving a 500 kg
meteoroid. Data are given for two potential values for sail mass
per unit area taken from [Wright 1992].
|1.17 (space manufacture)
It should be noted that solar sails cannot operate below about 1000
km since atmospheric drag exceeds the acceleration due to sunlight. Orbits
between approximately 1000 km and 20,000 km are subject to high radiation
[Wright 1992]. Thus, solar sails built at the ISS would probably need to
be moved to a 1,000+ km orbit by chemical, tether, or solar electric propulsion
or construction must take place in a 20,000+ km orbit. Worse, aerodynamic
pressure on large sails may pose a hazard to the ISS. Detailed engineering
would be required to choose a proper site for sail construction, but a
design where spars and rigging are assembled at the ISS then moved along
with rolled sail material to a teleoperated facility in high orbit for
final assembly may be advantageous.
Admittedly, the financial return of delivering thousands of meteoroids
to low earth orbit is a long-term prospect, making the development of the
key technology, solar sails, a tough sell on that basis alone. However,
solar sails could probably be employed to increase the number of geosynchronous
satellite orbital slots by a factor of three, providing a compelling case
for solar sail development in the short-term. Geosynchronous communication
satellites are a thriving business generating substantial profits today.
Geosynchronous satellites must be spaced approximately 2-3 degrees apart
to avoid radio interference. Thus, only 120-180 satellites may be
accommodated for a particular frequency band. This has led to substantial
congestion, particularly in desirable locations. The last geosynchronous
slot with a view of all of North America, a particularly crowded area,
allocated for direct broadcast satellites was auctioned for in excess of
$800 million. There are at least eight slots with a view of all of
North America. A sufficiently capable solar sail could hold a geosynchronous
communication satellite out-of-plane [Forward 1981]. Unfortunately,
space manufactured sails are probably required to achieve 2-3 degree separation
[Forward 1981]. However, solar sails might increase the number of
geosynchronous slots with a view of North America by at least a factor
of two. Assuming $250 million per slot [Van Bloom 1999], solar sails might
create approximately $2 billion worth of North American direct broadcast
slots alone, not including slots created south of the equator or over Eurasia,
Africa, or Australia.
In this section, we describe some of the steps that could be taken in the
next few years to make AsterAnts a reality. Ground facilities could
be used to develop computational models of solar sailing, meteorite processing,
and orbital operations. A ground based telescope facility could be
developed to detect and characterize 1/2-1 meter diameter NEOs. An orbital
mission to demonstrate solar sailing should be fairly inexpensive.
Finally, meteorites could be used to test meteoroid processing techniques
on the ISS, small scale experiments to develop ISS techniques for thin
aluminum film development could be started, solar sail assembly techniques
could be developed, and meteorite capture experiments could be conducted
with "artificial meteoroids" released from the ISS.
Computational Problem Solving Environment
Many detailed questions regarding AsterAnts development could be answered
by a sufficiently well developed computational facility. Solar sail
performance, navigation, optimal trajectory determination, and autonomous
operations could all be investigated using simulation. Orbital materials
processing might also be investigated using computational chemistry and
materials techniques. While such investigations would require substantial
computational resources, their cost is so much lower than orbital operations
that only a few orbital improvements are necessary to justify the computational
cost. The NAS facility at NASA Ames Research Center is developing
the Information Power Grid (IPG) to provide large distributed computational
resources for solving aerospace problems. A Problem Solving Environment
using IPG resources could be developed to address AsterAnts development
Ground Based Telescope Facility
Recognition that NEO impacts have played a very destructive role in Earth's
history [Lewis 1996], highlighted by the spectacular collision of Comet
Shoemaker-Levy with Jupiter, has spurred development of several successful
NEO search programs. These include the Planet Crossing Asteroid Survey
[Helin 1979][Helin 1985], Spacewatch [Gehrels 1991], and the Near-Earth
Asteroid Tracking program [Helin 1997]. Spacewatch alone had discovered
189 NEOs as of February 1999. These systems are designed to find NEOs somewhat
larger than those that AsterAnts targets. For example, Spacewatch
is most efficient at detecting asteroids with a diameter of approximately
300 meters [McMillan 1999]. One of the smallest NEOs ever discovered (1991
BA), with a diameter of approximately 5-10 meters, was discovered when
only 0.0053 AU from the Earth. 1991 BA was discovered by the 0.91 meter
Spacewatch Telescope [Scotti 1991]. An optical telescope capable of reliably
detecting 1/2-1 meter diameter objects would probably require substantially
greater light gathering capability and therefore be quite a bit larger,
perhaps six meters diameter or greater. Six such telescopes currently exist.
Although optical telescopes do a good job of finding NEOs, radar is
orders of magnitude more accurate for position determination [Ostro 1997]
and can determine the size, shape, and rotation of NEOs. Radar telescopes
cannot find unknown NEOs because their beam is narrow, so a facility to
find and characterize one-meter-diameter meteoroids accurately would require
both an optical and a radar telescope. Extrapolating from table 2 in [Ostro
1997], existing radar facilities could image one-meter-diameter meteoroids
out to a distance of 0.006-0.018 AU. For comparison, the Moon is
approximately 0.0026 AU from Earth. [Ostro 1997] proposes a dedicated radar
telescope for NEO imaging. This facility is proposed to characterize
large NEOs that threatened Earth. Presumably, only minor adjustments would
be necessary for such a facility to be part of an AsterAnts ground infrastructure.
The search for appropriate meteoroids should begin at least a few years
before AsterAnts spacecraft become available. This should provide
numerous targets when the spacecraft are ready. If AsterAnts does not pan
out, the telescope facility would be useful for a wide variety of scientific
observations and for detecting Earth threatening NEOs.
Solar Sail Demonstration
In the 1980s, the World Space Foundation developed a solar sail engineering
test article and demonstrated deployment on the ground. Development
was halted when no affordable launch opportunity could be found.
This development was accomplished without government funds, only voluntary
contributions. Given the relative success of a shoe-string operation,
a modestly funded effort could be reasonably expected to build, launch,
and operate an orbital solar sail mission. The major risk is deployment
failure. A series of small ground built solar sails might be used to develop
an experience base for AsterAnts solar sails.
Meteoroid processing experiments need not wait for the return of meteoroids.
Hundreds of meteorites have been collected on Earth and some of these could
be used to develop on-orbit processing facilities. Meteorites represent
that subset of meteoroids that collide with Earth and survive atmospheric
entry. Therefore, there is no guarantee that the first meteoroids
returned by AsterAnts would be similar to the meteorites used to develop
on-orbit processing. However, if a sufficiently large number of meteoroids
are returned by AsterAnts, some would undoubtedly have characteristics
similar to meteorites in existing collections. Full-scale meteoroid processing
is expected to be very energy intensive, possibly requiring a solar furnace.
However, initial experiments should be possible with projected ISS energy
Solar Sail Construction
Although full-size solar sails would probably be built only in high orbits,
small sails could be built at the ISS to develop construction techniques.
Spars and rigging compressed into small packages could be brought to the
ISS by the shuttle and assembled on orbit. Rolls of ground-built
sail material could also be delivered along with machinery to unroll the
sail material onto the spars and rigging. A human presence would be of
great value in understanding and fixing deployment and construction problems.
Once the procedures have been thoroughly debugged, the facility could be
moved to high orbit for full-size sail construction.
Solar Sail Materials Manufacturing
To build solar sails on-orbit, tens of thousands of square meters of thin-film
aluminum must be produced. Clearly, this would require a large, dedicated
facility outside of the pressurized ISS volume. However, experiments
to understand the behavior of thin-films in weightlessness and to develop
manufacturing techniques could be conducted in the pressurized volume.
For sail making, one approach is an electroplating technique, where a large
drum is continuously plated with evaporated, charged aluminum on one side,
and the solidified sheet is peeled off the back side of the roller. A more
elaborate mechanism, credited to Eric Drexler, appears in [Wright 1992].
"Artificial Meteoroid" Capture
A critical portion of each AsterAnts return mission is meteoroid capture
and control. These operations could be tested at the ISS by developing
a controllable "artificial meteoroid" that mimics the characteristics of
the real thing to test the capture and control hardware and software. A
human presence would allow a much quicker try-and-fix cycle.
The AsterAnts concept uses the ISS to help develop a fleet of small spacecraft,
propelled by very large solar sails, to capture and return meteoroids from
near Earth orbits. These meteoroids could be used for ISS resupply,
hydrogen/oxygen propellant production from water, and metals for orbital
construction. While the overall project is large, a number of small steps
could be taken in the near future that have near-term value while contributing
to the long-term goal. Although developing solar sails, meteoroid capture
hardware, and meteoroid processing is probably too risky for the private
sector, once the technologies have been demonstrated and one or two meteoroids
returned, it may be possible for the government simply to pay for meteoroids
delivered to the ISS by the kilogram, with a price conceivably competitive
with materials boosted from Earth. This would provide a market for
private companies to develop fleets of AsterAnts providing extraterrestrial
materials for the massive expansion of human activities in space.
We would like to thank Bonnie Klein, Chris Henze, David Kenwright, and
T. R. Govindan for reviewing this document. This work was funded by NASA
Ames contract NAS 2-14303.
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