The document proposes a design called MAPPER (Micro Asteroid Prospector Powered by Energetic Radioisotopes) to catalogue resources in the Main Asteroid Belt using a fleet of small, inexpensive spacecraft over 20 years. Key aspects discussed include using 232U radioisotope power systems, microwave thrusters for propulsion and extracting water from asteroids, neutron and gamma detectors to analyze composition, tagging asteroids with radioisotopes, and preliminary optimization findings. Overall conclusions are that a host of MAPPER platforms could survey a significant fraction of asteroids and their land area could equal a major fraction of Earth's surface area.

1. Micro Asteroid Prospector Powered by
Energetic Radioisotopes: MAPPER
Steven D. Howe
Gerald P. Jackson
Presented at the NIAC Review Meeting
March 23, 2004

2. Topics Presented
zz Goals and objectives
zz Target characterization characterization- Main Belt Asteroids
zz Sub Sub-Systems
zz Preliminary optimization calculations
zz Issues
zz Conclusions

3. Goals and Objectives
zz Purpose: Catalogue the resources of the Main Belt
asteroids for future exploration
zz Goal: Design a light light-weight, in in-expensive spacecraft
so that a fleet can cruise autonomously through the
Belt for 20 years
zz Objectives:
zz Determine mass, power, and number of spacecraft for
characterizing a significant fraction of the Main Belt
asteroids
zz Identify major issues and future goals

4. Main Belt Asteroids
zzThe Asteroid Belt may be the most valuable future resource in the solar systemin system zzThe asteroid belt is estimated to contain 100 billion objects. zzThe total mass is calculated to be around 1/1000 of Earth’s mass --equivalent to breaking the top 1.3 miles of the Earth’s crust into piecescrust pieces zzDiameters range from a few kilometers to a few metersDiameters meters zzMost of the asteroids are silicon dioxide, some are iron and other metals, a lesser number are carbonaceous chondriteschondriteswith up to 10 % water by weight, and a few may be heavy with precious metals. zzAverage separation distance is 2.3x10Average 2.3x1044km for km DminDmin=20m=20m

5. The Main Belt spans the range between 2.2 and 3.7 AU

6. The main belt asteroid size size-frequency
distribution approximately follows a power law
Ntot=2.88e06 D-2.27
(Adapted from Farinella and Davis, 1994)

7. The asteroids in the belt are not uniformly distributed but form bandsuniformly bands

8. Subsystems

9. The MAPPER concept requires the
integration of many subsystems
zz A power supply to provide electrical power to a platform that
can propulsively cruise between targets
zz A radiation detector system that can measure elemental
constituents at a standoff distance,
zz A heating system that can extract and store volatiles for
propulsion,
zz A tagging method to label the target for future generations.
zz A method of absolute coordinate location determination via
star tracking or inter inter-communication with Earth or a fixed
beacon.
zz Radar system sufficient to provide guidance to the next
target and avoidance of smaller objects
zz A communication link to Earth

10. POWER

11. 232 232U offers high specific power
U zz The decay sequence of 232 232U U
releases 25% of the energy per
atom of a nuclear reactor but
requires no critical mass
zz 5 W(th)/g peak is 10 times
Pu238
zz Power scales linearly with mass
zz Relatively flat over 20 year
period
zz Production:
zz Available in spent fuel
zz Fast neutron irradiate U U-233
zz Accelerator irradiate Th Th-232
zz Uses tuned PV conversion
zz High gamma activity requires
standoff from platform
zz ISSUE: lifetime of tuned PV

12. Preliminary estimates of power source activity
(3e04 Ci Ci) are well below past launch history for
) the US

13. Power levels and mass of the power system vary
strongly with average cruise velocity and Isp

14. PROPULSION

15. Microwave driven thrusters offer the potential
of long life due to low surface interactions
(courtesy of http://www.islandone.org/APC/Electric/04.html)

16. Microwave driven electric thrusters
provide high Isp and good efficiency
zz NASA GRC recently
demonstrated a 6000s
Isp using a microwave
driven thruster with Xe
zz GRC and Penn State
have demonstrated lower
power microwave
thrusters using water
zz Significant advances in
reducing mass and
increasing power density
are predicted

17. Refueling

18. Refueling sequence
Cruise vector
Neutron probe
Water = Yes then decelerate
ΔV = cruise velocity
T = TaccelReturn to asteroid in 1.4 TacceldaysAccelerate for ½ distance and decelerate for ½ distance
Accelerate up to Vcruisein Tacceldays
Water = NO
Then radar search and course correct

19. Microwaves couple to regolith
zz In 1985, Dr. T. Meek of the
Los Alamos National
Laboratory (LANL)
demonstrated the ability to
extract water from simulated
lunar soil using 2.45 GHZ
microwaves.
zz According to Meek, using
conventional means to heat
the soil would require 10,000
times more energy than
using the microwaves to
selectively heat the water in
the soil.
2.45 GHz microwaves for 15 s
onto lunar soil sample

20. The microwave generator for the thruster will also
serve as the heating source to extract water from
CC asteroids
zz 2.45 GHz microwaves will penetrate roughly 30 cm into 10%
bearing regolith
zz A 50 cm diameter bell will extract 60 kg of water from a 10% CC
zz extraction rate is around .025 kg/w w-day
zz Slight pressure buildup will be countered by operating the
thruster at low power
zz Water in the fuel tanks will be maintained as liquid using the
waste heat transported from the power source, ~160 w required
zz Condensed water is transported from the extraction unit to the
fuel tank via capillary action

21. DETECTION

22. Neutron Source
zz Original concept used 232 232U plus Beryllium to make
U neutrons via ( αα,n ,n) reaction
) zz 4ππ source
zz Created too much background
zz Irradiated components
zz Required close approach, i.e. < one diameter
zz Propose H( 6Li,n) mini accelerator
Li,zz 13.2 MeV 6Li Li
zz 10 10o cone of emission
zz Pulsed, intense source fired during flyby closest approach
zz Optimum standoff distance is 3 X diameter

23. Flight qualified neutron detector has
been flown to find water on the Moon
zz Designed and built at
LANL by W. Feldman
zz Weight: 8 lbs
zz Electrical power req’d req’d: 3
: w
zz 3He tubes
He zz Returned neutron
spectrum depends
strongly on hydrogen
content
zz Issue: lifetimeHowe problem1.0E-091.0E-081.0E-071.0E-061.0E-051.0E-091.0E-081.0E-071.0E-061.0E-051.0E-041.0E-03neutron energy flux 3%waterwater10%
lifetime

24. Flight qualified gamma gamma-ray detector has
been flown measure gammas on the Moon
zz Designed and built at
LANL by W. Feldman
zz Weight: 3 lbs
zz Electrical power req’d req’d: 2
: w
zz BGO crystals
zz Gamma rays identify
heavy metals
zz CdZnTe or HgI may
provide better resolution

25. TAGGING and COMMUNICATION

26. Radioisotope tagging
zz Carry 100g samples of 5 isotopes
zz T1/2 greater than 30 yrs
zz Auto Auto-mixer to randomly mix different amounts of each
zz Gives passive gamma ray signal with unique spectrum
zz Pro –– no power needed
zz Pro –– long lived
zz Pro -- secure
zz Con -- Natural background may interfere
zz Con -- Must be in line of sight to register

27. ““Smart Dust” sensors developed at Berkeley low mass
and long life
zz Pro ––
zz Low mass
zz Broadcast widely, i.e.
4ππ
zz Reasonable range
zz Con Con-
zz Power needed
zz Less secure
zz Limited life
zz Radiation may
degrade
(courtesy of
http:// robotics.eecs.berkeley.edu/~pister/SmartDust SmartDust/) /)

28. Communications back to Earth rely on
demonstrated technologies
zz Galileo transmissions required 400 w on
board power
zz Platform will determine location from “split
field of view star sensors ( StarNav II)
[http://www.jsc.nasa.gov/aiaa/lal/lljun01/] ““
zz Information burst will include location,
measured gamma gamma-ray and neutron
spectra, and ship status data

29. GUIDANCE

30. The architecture depends on the average
velocity of the craft –– i.e. cone of acceptability

31. A light weight radar is required but does not
yet exist
zz Needs to see 156,000 km out
zz Correlate distance with return signal strength (RSS) and
target size
zz Indicate water content (i.e. albedo albedo) with RSS
) zz Determine relative velocity with multiple returns
zz May require power accumulator to operate in high high-power
but pulsed mode
zz Determines required delta delta-V
zz Enables target choice
zz Issues:
zz baseline needed for Synthetic Aperture Radar ?
zz power level?

32. Optimization

33. Deployment –– Nuclear Thermal Rocket
zz Current NASA program, Prometheus, is
interested in recovering the NTR to fulfill the new
Presidential space exploration direction
zz NTR is considered to be a demonstrated
technology that could be recovered within a
decade
zz Assumed operating parameters
zz 5000 lb thrust engine
zz T/W = 3
zz Isp Isp= 950 s
= zz Hohmann transfer orbit –– sum of ΔΔVs Vs= 9.52 km/s
= zz Reusable for N missions

34. Preliminary optimization calculations
are positive
zz Power level is dictated by the propulsion systems need to
accelerate over a short period (variable)
zz The same power level is sufficient for communications back
to Earth, water extraction, neutron generation, and the radar
systems.
zz The average cruise velocity, Isp Isp, time of acceleration, and
, interception velocity are independent variables
zz Propellant requirements for initial acceleration to cruise,
intercepts, and refueling are determined
zz Power system mass including waste heat radiator is found for
the tuned PV method

35. Ship characteristics depend strongly on
the average cruise velocity

36. Significant surface area can be scanned
by reasonable cruise velocity

37. Optimum specific impulse lies within
achievable values

38. Preliminary cost optimization indicates
lower than expected cruise velocity

39. Phase II will strive to resolve several
issues
zz Lifetimes Lifetimes-
zz Power conversion scintillator
zz RF taggers
zz Microwave generator
zz detectors
zz Need time dependent simulation to develop
guidance, control algorithms, and Vintercept intercept
zz Radar –– mass and power
zz Earth communications
zz Packaging Packaging- is the platform volume limited or
mass limited

40. Conclusions
zz A host of small platforms can survey a significant
fraction of the asteroid population in a 20 year
interval
zz The land area surveyed could be equal to a
major fraction of the Earth’s surface area
zz The energetic radioisotope power supply
appears adequate for the mission needs
zz Most subsystem hardware has current current-day
examples that can be improved
zz A time time-dependent simulation is needed for flight
profile demonstration and targeting algorithm
development