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Interstellar Propulsion Concepts Assessment

Final Report
on

Purchase Order H-30839D
placed by

NASA/Marshall Space Flight Center
MSFC, AL 35812

with

Dr. Robert L. Forward
Forward Unlimited
81 14 Pebble Court
Clinton, WA 98236

Covering the Reporting Period From
19 March 1999 through 5 October 1999

Report Date
17 February 2000

Interstellar Propulsion Concepts Assessment

Objectives

Activities During Contract Period

During the contract period, running from the start of the contract on 19 March 1999 thorough
5 October 1999, the Principal Investigator (PI) participated in four meetings covering the subject
of the contract effort. In addition, he communicated with a number of the meeting participants
before and after the meetings via email, and carried out some independent analyses of his own,
which will be summarized separately.

Interstellar Probe STD Team Meeting #1 - 15-17 February 1999.

Prior to the start of the contract, the PI had previously participated, at his own cost, in
Meeting #1 of the Interstellar Probe Science and Technology Definition Team (ISP-STDT),
which was held at JPL from 15-17 February 1999. The two propulsion concepts that had been
downselected by the Interstellar Probe Propulsion Team, lead by Les Johnson of MSFC, were
Nuclear Electric Propulsion and Solar Sail Propulsion. The PI joined the Solar Sail Propulsion
subteam and participated in a number of telecons between the first and second meeting of the
ISP-STDT. At the first ISP-STDT meeting, P. Weissman evaluated the possibility of imaging
one of the 500 km diameter or smaller ice-covered "Kuiper Belt Objects" (KBOs) on the way
through the Edgeworth-Kuiper Belt. Weissman assumed that science value would only be
obtained if a well-resolved image of greater than 200 pixels across the image diameter could be
obtained. He showed that this event was highly unlikely, unless a specific object were targeted,
which would be difficult to do. As a result of his negative report, no visible imaging telescope
was included on the strawman list of payload instruments. The PI objected, and pointed out that
the density distribution as a function of diameter of the KBOs at different distances from the Sun
was unknown. He was backed by Renu Malhotra, a KBO theorist, who pointed out that
gathering such knowledge was important in developing models for of the growth of planets from
the primordial dust disk. Using data estimates from Malhotra, the PI estimated that dozens or
hundreds of KBOs could be detected by a small visible spectrum telescope during a flythrough
of the Belt. Only one, or at most a few, pixels in the camera would be activated by each object,
so that no "image" could be obtained, but there would be more than enough photons in that one
pixel to ensure positive detection and enable tracking of the KBO as the probe passed it by.
Because the probe was moving, the distance to the objects could be estimated by the rapidity at
which the photon intensity rose and fell. The color could give an estimate of the albedo. Using
those two pieces of information, an estimate of the diameter of the object could be obtained.
This would result in a "census" of the KBOs as a function of object diameter and its distance
from the Sun. The PI then attempted over the next few months to get someone in the visible
space imaging field to work with Renu Malhotra, since she was a theorist, not an experimentalist,

and had no interest herself in proposing a visible spectrum telescope to add to the strawman list
of instruments. The PI also suggested that the KBO census could alternatively be obtained by a
bi-static radar method. The Belt in front of the Probe trajectory would be illuminated by
microwave energy, perhaps from the Aricebo radar transmitter system, with the weak return
signals from the KBOs detected by a receiving dish on the Interstellar Probe. This idea has since
been taken up by Richard Dickinson at JPL, who has since submitted a proposal to study the
feasibility of the concept.

Interstellar Probe STD Team Meeting #2 - 28-31 March 1999

The first meeting that the PI attended while funded by the contract was Meeting #2 of the
Interstellar Probe Science and Technology Definition Team (ISP-STDT), held at JPL in
Pasadena, California from 28-31 March 1999. At the meeting, Renu Malhotra estimated the
space density of KBOs in the region from 30-50 AU radius from the Sun and 8 AU above and
below the ecliptic plane. She estimated that KBOs with diameters around 100 km are spaced at
distances of 1.5 AU. There would be 10-15 objects of this size passing within 1.5 AU of the
Interstellar Probe as it traversed the Belt from 30 to 50 AU. KBOs with diameters around 5 km
would be spaced at 0.1 AU. There would be 200 objects of this size passing within 0.1 AU of
the Probe. KBOs with diameters around 1 km would be spaced at 0.02 AU and 1000 of these
objects would pass within 0.02 AU of the Probe. Malhotra estimated that a 10-cm-diameter lens
could see a 1 km object at 0.1 AU, thus there should be many thousands of counts of 1 km size
objects in a census of the Belt. A high-speed replay of this multitude of objects streaming by the
Probe during its journey through the Belt would produce a remarkable visual image for
presentation to the general public. The most recent estimate of the number of KBO objects that
would be counted by a typical small telescope is presented in Appendix A.

At this meeting, Charles Gamer of JPL showed a number of candidate substrates for the
aluminum film solar sail propulsion options. Most of these were the typical very thin polymer
films, but one was a very low mass-per-unit-area carbon fiber mat. The solar sail trajectory
selected in order to obtain the very high solar system exit velocities needed (>100 km/s) in order
for the probe to reach 250 AU in 10-15 years, involves a close passage to the Sun. The high
thermal and radiation environment near the Sun raised concerns about the survivability of the
thin polymer films and even the aluminum film, since thin aluminum films agglomerate (form
droplets) at temperatures above 800 K. This limited the closest distance of approach of the solar
sail to the Sun, and therefore the solar system exit velocity. Out of this meeting, the PI revived
an idea he had thought of earlier. Carbon Sails. It is described in more detail in Appendix C.

AIAA/MSFC/JPL Advanced Space Propulsion Workshop. Huntsville. AL - 5-8 April 1999
The second meeting that the PI attended while funded by the contract was the AIAA/MSFC/JPL
Advanced Space Propulsion Workshop held in Huntsville, AL from 5-8 April 1999. The PI was
primarily attending the Workshop to present papers on space tethers, so his travel expenses were
paid by Tethers Unlimited, Inc. At the Solar Sails Session of the Workshop, the PI gave a
presentation on Carbon Sails concept using the viewgraph charts in Appendix C.

Interstellar Probe SDT Team Meeting #3 - 17-19 Mav 1999

The third meeting that the PI attended while funded by the contract, was Meeting #3 of the
Interstellar Probe Science and Technology Definition Team (ISP-STDT), held at JPL in

Pasadena, California from 17-19 May 1999. George Danielson of JPL presented a first cut
design for a visible spectrum telescope suitable for a KBO census, using an all-beryllium
Cassegrain telescope with an f/8 20 cm aperture. It would use a 1024 x 1024 HIT CCD detector
which would use time delay integration, where the pixels are shifted across the array, to increase
the effective exposure time on a spinning spacecraft. It would be a "smart" camera that would
process three consecutive frames to remove star images, cosmic rays, hot pixels, and pixel to
pixel sensitivity variation to produce only a count of detected KBOs. Unfortunately, Renu
Malhotra was not in attendance to push for the science, and George Danielson was not able to
come up with a good estimate for mass. As a result, the visible spectrum telescope for obtaining
a KBO census was not placed on the "Strawman Scientific Payload" list, but was listed (with a
question mark) on the "Additional Candidates" listing. In the opinion of the PI, when there are
images to be obtained, then for maintaining public interest in supporting science missions, an
imaging telescope should be manifested on every science mission.

The Team Chairman, Richard Mewaldt then brought up the topic of "What next?" What should
be the next mission if this mission is successful? The mission selected was to "Break Out of the
Local Bubble" at 3000 AU, with a goal of reaching 10,000 AU in 20 years. This would require
an exit velocity from the solar system of 2400 km/s. In the opinion of the PI, it is conceivable
that this might be done using solar photons with a carbon sail concept that starts very close to the
Sun. Although carbon maintains its strength at temperatures well above 3000 K, it does start to
sublimate with time in vacuum at much lower temperatures than 3000 K. Such a demanding
mission scenario may require a sacrificial shield that will shade the carbon sail from the heat of
the Sun until the sail is in its outward trajectory. Even then, the carbon sail may have to be
designed to "gracefully degrade" as the carbon sublimates from the surfaces of the sail film and
support structures, with the sail getting lighter and lighter with time. If the solar-photon-powered
carbon sail is unable to obtain the necessary exit velocity for the 10,000 AU mission, then the
mission can be accomplished using laser-pushed reflective lightsails. This laser-pushed-lightsail
route would have the advantage of moving advanced propulsion right along the Roadmap toward
true interstellar exploration — missions to nearby star systems.

50th IAF Congress. Amsterdam. The Netherlands - 4-8 October 1999

The fourth and last meeting that the PI attended while funded by the contract effort was the 50th
International Astronautical Congress held in Amsterdam, The Netherlands from 4-8 October
1999. This involved only a short trip across the English Channel from the Pi's residence in
Scotland, UK. Since the PI was primarily attending the Congress to give papers on space tethers,
his travel expenses were paid by Tethers Unlimited, Inc. At the IAF Congress the PI participated
as Acting Secretary in a Saturday meeting of the Interstellar Exploration Committee of the
International Academy of Astronautics, as the Committee elected new members and officers,
finalized the plans and agenda for the Interstellar Exploration Sessions at the present Congress
and the next Congress. On Tuesday, the PI attended the Interstellar Exploration Session and
participated in the discussion.

Kuiper Belt Object In-Situ Sampling Mission

In May 1999 the MSFC Contract Technical Representative requested the PI to be on the
Technology Development Subteam of the Kuiper Belt Object In-Situ Sampling Mission Team
led by Emma Bakes. This would be a mission that would first fly by and drop a sampling probe

on one or more Centaur objects. The Centaurs are large reddish objects with orbits between
Saturn and Neptune that are felt to be similar to KBOs. The mission would then continue on to
the Kuiper Belt to rendezvous with, land on, then drill for, obtain, and analyze in place, a deep
core sample from a KBO object.

This mission, because of the rendezvous and landing requirement, cannot be done with a solar
sail. It can be done using nuclear electric propulsion (NEP), and the Team-X group at JPL
carried out a first cut design for the mission based on using NEP. This nuclear propulsion
option, however, was categorically rejected by the Origins Theme science review panel at NASA
Headquarters. The PI has invented an alternate non-nuclear propulsion concept, which if
detailed analysis proves is feasible, will allow the mission to proceed without the use of any
radioactive power sources on the mission. This new propulsion concept. Solar Concentrator
Heat-to-Electric Ion Propulsion, is discussed in Appendix B.

APPENDIX A

Kuiper Belt Object (KBO) Census

At the second meeting of the Interstellar Probe Science and Technology Definition Team (ISP-
STDT), held at JPL in Pasadena, California from 28-31 March 1999, Renu Malhotra gave an
estimate of the space density of Kuiper Belt Objects (KBOs) in the donut-shaped region with a
radius of 30-50 AU from the Sun and 8 AU above and below the ecliptic plane. She estimated
that KBOs with diameters around 100 km would be spaced at distances of 1.5 AU, KBOs with
diameters around 5 km would be spaced at 0. 1 AU, and KBOs with diameters around 1 km
would be spaced at 0.022 AU. Although imaging a KBO at high resolution was quickly
determined not to be possible, the PI suggested a simple count, or "census" of the objects in the
Kuiper Belt as a function of distance from the Sun might be useful. At the third meeting of the
ISP-STDT, George Danielson of JPL presented a first cut design for a visible spectrum telescope
suitable for a KBO census, using an all-beryllium Cassegrain telescope with an f/8 20 cm
diameter aperture. It would use a 1024 x 1024 HIT CCD detector which would use time delay
integration, where the pixels are shifted across the array, to increase the effective exposure time
on a spinning spacecraft. It would be a "smart" camera that would process three consecutive
frames to remove star images, cosmic rays, hot pixels, and pixel to pixel sensitivity variation to
produce only a count of detected KBOs. The estimated number of objects that would be detected
by a 20 cm diameter telescope during the Kuiper Belt flythrough as a function of object diameter
was calculated by the PI and is given in the following table:

Kuiper Belt Objects Counted During a Kuiper Belt Flythrough

KBO radius

Number In View

Total Number

(m)

(km)

At Any Instant

in Flythrough

(100 km) 100,000

(3AU) 14,000

(1.5AU) 7,000

( 10 km) 10,000

1,400

780

400

( 1 km) 1,000

140

100

1,300

100

6,600

_1P

34,000

It is not expected that any of the objects would be close enough to activate more than a few
pixels, so no "images" would be obtained. What would be obtained is a sample "census" of the
Kuiper Belt, or a count of objects as a function of brightness and 3-D position in the Kuiper Belt.
The range to the object can be obtained independently of the brightness of the object, since small
nearby objects will move rapidly with respect to the background stars, while larger distant
objects with the same observational brightness will stay in view much longer.

A high-speed replay of this multitude of objects streaming by the Interstellar Probe during its
journey through the Belt would produce a remarkable visual image for presentation to the
general public. If the year-long passage were compressed into a one-minute video, it would look
like you were driving through a blizzard.

At present, the visible spectrum telescope for a KBO census is not on the primary list of
instruments for the Interstellar Probe mission since a 20-cm diameter telescope aperture requires
a 5 kg instrument. The total science payload mass available is only 25 kg and there are a dozen
instruments selected ahead of the KBO telescope. Since there is a plethora of data expected for
this experiment, consideration should be given to cutting the size and mass of the telescope for
the KBO Census experiment and accepting the resulting cut in number of objects counted.

APPENDIX B

Solar Concentrator Heat-to-EIectric Ion Propulsion

In May 1999 the MSFC Contract Technical Representative requested the PI to be on the
Technology Development Subteam of the Kuiper Belt Object In-Situ Sampling Mission Team let
by Emma Bakes. This would be a mission that would first fly by and drop a sampling probe on
one or more Centaur objects. The Centaurs are large reddish objects with orbits between Saturn
and Neptune that are felt to be similar to KBOs. The mission would then continue on to the
Kuiper Belt to rendezvous with, land on, then drill for, obtain, and analyze in place, a deep core
sample from a KBO object.

This mission, because of the rendezvous and landing requirement, cannot be done with a solar
sail. It can be done using nuclear electric propulsion (NEP), and the Team-X group at JPL
carried out a first cut design for the mission based on using NEP. This nuclear propulsion
option, however, was categorically rejected by the Origins Theme science review panel at NASA
Headquarters. The PI has invented an alternate non-nuclear propulsion concept, which if
detailed analysis proves to show is feasible, will allow the mission to proceed without the use of
any radioactive power sources on the mission. This new propulsion concept is called: Solar
Concentrator Heat-to-Electric Ion Propulsion.

NEP (nuclear electric propulsion) and RTG (radioactive thermal generator) propulsion use
radioactive materials to produce heat, which in turn is used to generate electricity, which in turn
is used to operate the spacecraft instruments and electric propulsion system. The PI proposes
that we use the same heat-to-electricity subsystem, the same radiator system, and the same
electric ion propulsion system that is in the NEP design. The heat source, however, would be
solar photons collected by a very large solar-sail-like structure that has sufficient curvature to act
as a light concentrator. The PI admits that such a sail is going to be large, and difficult to build,
aim and use. It may or many not be heavier than the nuclear reactor and its shield. Fortunately,
a great deal of design and hardware development has gone into DoD and NASA studies of the
solar concentrator heated hydrogen rocket (Solar Thermal Rocket). Large inflatable solar
concentrators have been built and have achieved concentration ratios of 10,000 to 1. With the
new sail materials now available at JPL, and with this applicable solar concentrator technology
available, the design problems of a solar concentrator heat-to-electric ion propulsion system
should be solvable.

The NEP reactor in the ISP design has an output of 600 kWt (kilowatts thermal) to achieve
180 kWe (kilowatts electric). The full 180 kWe is needed by the propulsion system in order to
bring the spacecraft to a halt at a Kuiper Belt Object out at 40 AU in a reasonable period of time.
The solar flux at 40 AU, however, is only 0.84 W/m 2 . To collect 600 kWt of solar heat at 40 AU
will require a solar light concentrator with a diameter of about 1 km. Although large, this is not
impossible, since solar sails of this diameter are under consideration for the Interstellar Probe
Mission. Once rendezvous has been accomplished, the 180 kWe of electrical power on the
mother ship will be more than enough to transmit the data back to Earth at high data rates. That
amount of power available is also enough to consider the possibility of beaming power from the

spacecraft to the drilling rig, thus eliminating the need for a nuclear RTG to supply the drilling
power. Actually, since the drilling portion of the effort requires only the mechanical rotation of a
shaft, that rotary power can probably be supplied by a storable propellant version of a chain-saw
engine. Beamed power may still be needed to run the down-hole sample analysis equipment and
the data transmission subsystems, although batteries may also be sufficient for this short duration
power requirement. Eliminating all nuclear components from the mission will make it much
easier to sell.

The PI has since learned that others have looked at similar concepts. A decade ago, in the paper,
"Optics and Materials Considerations for a Laser-Propelled Lightsail", Paper LAA-89-664, 40th
Congress of the International Astronautical Federation, Malaga, Spain, 1-12 October 1989, Dr.
Geoffrey A. Landis proposed a laser light beam concentrator photoelectric ion propulsion
system. More recently, Dr. Robert H. Frisbee at NASA/JPL did a first cut analysis of a solar
concentrator photoelectric electric ion propulsion system that uses an inflatable-structure or
solar-sail type of sunlight collector/mirror to intercept and focus the weak sunlight at 40 AU onto
conventional solar photoelectric cells. These then produce electricity for an electric ion
propulsion system. As Frisbee points out, the real issue here is the required areal density (grams
per square meter) of the solar concentrator mirror. In order to keep the mass low enough that
there is minimal impact on the overall propulsion system mass (and specific mass, or kg per kW
of electricity), the mirror areal density needs to be on the order of 1 gram per square meter for a
1-km diameter mirror. This makes the concentrator mirror close to being a larger-diameter
version of the Interstellar Probe solar sail.

On the next page is a printout of the Excel spreadsheet generated by Frisbee, which shows a
comparison of a Solar Concentrator Photoelectric Electric Ion Propulsion system with a Nuclear
Thermoelectric Ion Propulsion System. The Solar Concentrator Photoelectric option is assumed
to have a solar array power system specific mass of 15 kg/kWe and a solar power to electrical
power conversion efficiency of 20%. The concentrator mirror areal density is 1 gram per square
meter, or a specific mass of 6.6 kg/kWe. The total specific mass of the entire system is thus
21.6 kg/kWe. This is compared with the Nuclear Electric Propulsion system, which Frisbee
estimated as 16 kg/kWe at 100 kWe, assuming a 30% efficiency in conversion of heat to
electricity.

There are two ways for the Solar Electric Propulsion system to match or better the Nuclear
Electric Propulsion system in specific power. It should be possible to make the photoelectric
array quite a bit smaller and lighter because it doesn't have to intercept the sunlight directly, and
most solar cells work well with moderately concentrated sunlight. Also, the inflatable-structure
or solar-sail mirror isn't load bearing, as it is in a solar sail, so it may be possible to reduce its
areal density and thus its specific mass.

The PI strongly recommends that a high-specific-impulse, low-thrust Solar Concentrator Electric
Ion Propulsion system, either a Thermoelectric or a Photoelectric version, be studied as a
replacement for Nuclear Thermoelectric Ion Propulsion for difficult deep space missions. Such
studies could be carried out by the Advanced Propulsion Group at JPL, who have analyzed
similar systems in the past.

KBO RENDEZVOUS SEP MISSION

Robert H. Frisbee, JPL

Assume use of Inflatable Optics or a Solar Sail as a mirror to collect
and focus the required amount of sunlight intensity on the solar array

Solar Power (kW/m A 2 at 1 AU) =

1.35

Typical SEP Array

Specific Mass (kg/kWe)

15.00

Specific Power (W/kg)

66.67

Efficiency

0.20

Specific Area (m A 2/kWe)

3.70

Areal Density (kg/m A 2)

4.05

For Electrical Power (kWe) of:

100.00.

Enterthis .value

Sunlight Power @ 20% eff. (kW)

500.00

Array Area (m A 2)

370.37

Array Mass (kg)

1,500.00

Mission. Parameters

End of Initial

KBO

Acceleration Phase

Rendezvous Phase

Distance (AU)

10.00

40.00

R A 2

100.00

1,600.00

1/R A 2

1 .0000%

0.0625%

For a 1 00 kWe System:

Area Required at 1 AU (m A 2)

370.37

Circle Diameter (m)

21.72

Area Required at Distance (m A 2)"

37,037

592,593

Circle Diameter (m)

217.16

868.63

Assume Inflatable Optics or Solar Sail Mirror (Sunlight Collector/Reflector)

Areal Density (grams/m A 2)

1.00.

<- Enter this value

Reflectivity

90%.

.<- Enter this value

Collector/Reflector Area (m A 2)

41,152

658,436

Collector/Reflector Diameter (m)

228.90

915.61

Collector/Reflector Mass (kg)

41.15

658.44

Coll/Refl Specific Mass (kg/kWe)

0.41

6.58

Total Power System Specific Masses (kg/kWe)

Solar Arrays

15.00

Mirror

0.41

6.58

Total KBO SEP Power Specific Mass

15.41

21.58

Total KBO NEP Power Specific Mass

16.00

APPENDIX C

Carbon Sails

The PI has invented a sail structure design that could greatly improve the performance of the
solar sail option for a deep space probe such as the Interstellar Probe Mission, where the solar
sail passes close by the Sun to obtain the necessary Solar System exit speed. Instead of making
the solar sail with a light- reflecting surface of low-melting-temperature aluminum, he proposes
to make the sail with a light- absorbing surface of high-sublimation-temperature carbon film,
backed by the carbon fiber substrate being developed by Charles Gamer at JPL. The PI carried
out a brief analysis of this "Carbon Sail" concept and showed that such a structure has the
potential to achieve a solar system exit velocity of greater than 1000 km/s. This is 10 times
better than the estimated solar system exit velocity for an aluminum film sail. The details of the
results are summarized in the charts that follow. The material in this Appendix was presented at
the AIAA/MSFC/JPL Advanced Space Propulsion Workshop held in Huntsville, Alabama from
5-8 April 1999.

It should be emphasized that absorbing sails are not a replacement for reflective sails except
under certain conditions. Absorbing sails work best where the desired sail trajectory is directly
away from the light source, and the desired accelerations and velocities are so high that the sail
has to be operated at its thermal limit. These conditions, however, are exactly those that apply to
the solar sail propulsion method for high-speed extra-solar missions such as that of the
Interstellar Probe Mission. Absorbing sails may also pay off in the interstellar propulsion
scenario, which would use high power lasers to push a lightsail to the velocities required for
rapid interstellar flight. By using a carbon lightsail capable of operating at high temperatures, a
given amount of laser light power can be concentrated on a smaller sail, allowing higher sail
acceleration levels, shorter laser operational times, smaller transmitter apertures, lower sail mass,
and lower laser power and cost. If rendezvous at the target star is desired, magnetic sails could
then be used to stop in the target star system.

Because the generic lightsail propulsion method can be extrapolated from extra-solar missions
driven by solar photons to interstellar missions using laser photons, the PI recommends that the
lightsail propulsion option for the Interstellar Probe Mission be chosen over the nuclear electric
propulsion option. In addition, the PI recommends that further study be made of the pros and
cons of using a carbon sail instead of an aluminum sail, for both the Interstellar Probe Mission
beyond the Heliosphere and for true interstellar missions to the nearer star systems.

CARBON SAILS

8114 Pebble Court, Clinton, WA 98236
Phone/Fax: 360-579-1340
Email: FUn@whidbey.com

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OMB No. 0704-0186

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data needed, and completing and reviewing the colecbon of information Send comments regarding this burden estimate or any other aspect of this colection of information, Induing suggestions for reducing
this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway. Suite 1204. Arlington, VA 22202-002, and to the Office of Management
and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503,

1 . AGENCY USE ONLY (Leave Blank)

2. REPORT DATE

17 February 2000

3. REPORT TYPE AND DATES COVERED

Final Report for period 19 March 1999 through 5 October 1999

4. TITLE AND SUBTITLE

Interstellar Propulsion Concepts Assessment

5. FUNDING NUMBERS
Purchase Order H-30839D

6. AUTHOR(S)

Dr. Robert L. Forward

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESSES)
Forward Unlimited, 81 14 Pebble Court, Clinton, WA 98236

8. PERFORMING ORGANIZATION
REPORT NUMBERS

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESSES)
NASA/Marshall Space Flight Center, MSFC, AL 35812

10. SPONSORING/MONITORING
AGENCY REPORT NUMBER

11. SUPPLEMENTARY NOTES

12a. DISTRlBUnON/AVAl LABILITY STATEMENT

12b. DISTRIBUTION CODE

Unlimited

13. ABSTRACT (Maximum 200 words)

NASA is investigating the feasibility of conducting extra-solar and interstellar missions over the next 10 to 50 years. An assessment
of technologies supporting these near and far term objectives is required. To help meet these objectives the Principal Investigator
assessed the feasibility of candidate propulsion systems for the proposed "Interstellar Probe", a mission to send a spacecraft to the
Heliopause at 250 AU and beyond. During the contract period, the Principal Investigator (PI) participated in four meetings covering
the subject of the contract effort. In addition, he communicated with a number of the meeting participants before and after the
meetings via email, and carried out some independent analyses of his own on: 1) The use of high temperature absorptive carbon light
sails instead of low temperature reflecting aluminum light sails to obtain high solar system escape velocity trajectories for extra-solar
and interstellar probe missions. 2) The count rates for a Kuiper Belt Object census using a small visible light telescope during a
flythrough of the Kuiper Belt by the Interstellar Probe. 3) A Solar Concentrator Heat-to-Electricity Ion Propulsion system as a non-
nuclear replacement for a Nuclear Reactor Heat-to-EIectricity Ion Propulsion system for a Kuiper Belt Object rendezvous mission.

14. SUBJECT TERMS

Interstellar Flight, Solar Sails, Carbon Sails, Kuiper Belt, Interstellar Probe

15. NUMBER OF
PAGES

16. PRICE CODE

17. SECURITY CLASSIFICATION

18. SECURITY CLASSIFICATION

1 19. SECURITY CLASSIFICATION |

1 20. LIMITATION OF ABSTRACT

OF REPORT

OF THIS PAGE

OF ABSTRACT

Unclassified

Unlimited

Standard Form 298 (Rev Feb 89) (MS Word Mar 97)

Prescribed by ANSI Std. 239-18

298-102

NSN 7540-01-280-5500

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