Barry Solomon Scholarly Paper

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An EVA Mobility System for NEO Surface Exploration, and an Assessment of

Human Locomotion in a Simulation Environment

Barry Nicholas Solomon, Master of Science: Aerospace Engineering 2012

Advised by Professor David L. Akin, Department of Aerospace Engineering

AbstractNASAs next stated frontier of human space exploration, is a mission to a Near Earth Asteroid. In

order to accomplish such an extraordinary feat, many new technologies will need to be created

and paired with existing NASA expertise. Proposed herein is a technology that provides mobility

to astronauts while preforming EVAs on the surface of an asteroid. Unlike the Earth, Moon, and

Mars which have high enough levels of gravity as to enable people to walk on their surfaces,

most asteroids, specifically those near Earth (NEOs), have very low levels of gravity. Therefore

technology needs to be employed to make human exploration of an asteroids surface a

possibility. To that end the author of this report is proposing a system which secures an astronaut

to the surface of an asteroid and provides them with a perceived level of gravity which enables

them to explore the surface. The design driver of this study was to create a system which

provides the most utility to the mission for the lease amount of system mass. In addition to the

top level development of this technology, an experimental study was done to help determine one

aspect of the detailed design. The goal of the experiment was to investigate human locomotion,

in a weightless environment, where an external load is applied to the body center of mass. Such

is the environment provided by the mobility system designed herein, and thus understanding

human locomotion in this environment will help to decide what level of downforce should be

provided to the user. From this investigation, it was concluded that a downforce level equal to

13.3% of the subjects body mass would be the most advantages choice. However as is

discussed, more scenarios need to be studied to prove this result, especially locomotion while

wearing a spacesuit.


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An EVA Mobility System for NEO Surface Exploration, and an Assessment of

Human Locomotion in a Simulation Environment

By

Barry Solomon

Scholarly Paper submitted to the Faculty of the Graduate School of the University of

Maryland, College Park, in partial fulfillment of the requirements for the degree of Master

of Science in Aerospace Engineering

August, 2012

Advisor: Professor David L. Akin


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Copyright

Barry Solomon2012


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Table of ContentsAbstract ........................................................................................................................................... 1

List of Tables .................................................................................................................................. 7

List of Figures ................................................................................................................................. 7

Objectives ....................................................................................................................................... 8

Part One: Asteroid mobility System Design ................................................................................... 8

Introduction ................................................................................................................................. 8

Possible Destinations ............................................................................................................... 8

The Overall Technology.............................................................................................................. 9

The Suit Ring ........................................................................................................................... 9

Utilization on Solid Asteroids ................................................................................................... 10

The Anchors .......................................................................................................................... 10

The Tethers ............................................................................................................................ 11

Anchor Formations ................................................................................................................ 12

Transitioning Between Anchors ............................................................................................ 13

Alternative: Parallel Deployment: ......................................................................................... 14

Anchor Deployment .............................................................................................................. 15

Utilization on Soft Asteroids ..................................................................................................... 18

Anchors .................................................................................................................................. 18Tethers and Tension Rigs ...................................................................................................... 18

Exploration ............................................................................................................................ 19

Anchor Deployment .............................................................................................................. 19

Surface Operation Considerations ............................................................................................. 20

Astronaut Surface Descent and Ascent Strategy ....................................................................... 20

Power Requirements ................................................................................................................. 21

Possible and Proposed alternatives ........................................................................................... 22

Boom Arm ............................................................................................................................. 22

Astronaut Maneuvering Unit (AMU) .................................................................................... 24

Alternative Anchor systems................................................................................................... 24

Mission with no EVA ............................................................................................................ 27

Downside to Anchors ............................................................................................................ 27


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Comparison of alternatives........................................................................................................ 27

Other Items to discuss ............................................................................................................... 28

Emergency Astronaut Retrieval ............................................................................................. 28

Locomotion in the System ..................................................................................................... 29

Further Study Required ............................................................................................................. 30

Appendix One ........................................................................................................................... 31

A - Harpoon Anchor System for Solid Asteroids a Detailed Description ............................. 31

B - Anchor Mass Estimate for Solid Asteroids ..................................................................... 35

C - Mass Calculations for Anchor Formations on Solid Asteroids ....................................... 38

D- Mass specific area of exploration for proposed systems and alternatives ........................ 40

E- Activity Estimation ........................................................................................................... 44

F: Mathematica Code to determine hovering V costs per day......................................... 46

Part Two: Experiment to help with design requirements.............................................................. 48

Introduction ............................................................................................................................... 48

Background ............................................................................................................................... 48

Possible Testing Environments ............................................................................................. 49

Previous Findings on Gait analysis ....................................................................................... 50

Test Hardware and Equipment .................................................................................................. 55

Treadmill ............................................................................................................................... 55Simulated Mobility System ................................................................................................... 56

Tower ..................................................................................................................................... 56

Motion Capture System ......................................................................................................... 56

Air Supply.............................................................................................................................. 57

Experiment Protocols ................................................................................................................ 58

Subjects .................................................................................................................................. 58

Dive Crew .............................................................................................................................. 58

Test Procedure ....................................................................................................................... 59

Comments made by the participants ...................................................................................... 60

Comments of the Investigator................................................................................................ 60

Sources of Measurement Error .............................................................................................. 61

Motion Capture Data processing ............................................................................................... 62


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Motion Capture Data ................................................................................................................. 65

Data Analysis ............................................................................................................................ 68

Discussion of Results ................................................................................................................ 69

Number of Steps .................................................................................................................... 70

Speed ..................................................................................................................................... 71

Metabolic Rate ....................................................................................................................... 72

Cost of Transport ................................................................................................................... 73

Conclusions ........................................................................................................................... 73

Acronyms ...................................................................................................................................... 75

Appendix Part Two ................................................................................................................... 76

MatLab Code ......................................................................................................................... 76

Works Cited .................................................................................................................................. 90


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List of TablesTable 1: Power requirements. ....................................................................................................... 21

Table 2: Mass Specific Area of Exploration. ................................................................................ 28

Table 3: Maximum motor torque and power required. ................................................................. 36

Table 4: Mass estimate of anchor by component.......................................................................... 36Table 5: Anchor mass optimization. ............................................................................................. 38

Table 6: System mass required and the explorable area provided by different configurations. ... 39

Table 7: Calculation of Mass Specific Area of Exploration. ........................................................ 41

Table 8: Mass estimating relationships ......................................................................................... 42

Table 9: Summary of the EVA protocol activities for Apollo 14-16. .......................................... 45

Table 10: Calculation of the non-dimensional parameters.. ......................................................... 63

Table 11: Metrics from all 3 subjects ........................................................................................... 65



Table 14: Number of steps per distance traveled, and per time spent walking. ........................... 71Table 15: Sensitivity of the determination of whether a step is running. ..................................... 71

Table 16: Metabolic rate ratio. ...................................................................................................... 72

Table 17: Froude numbers and cost ratios .................................................................................... 73

List of FiguresFigure 1: Astronaut using the mobility system on a solid asteroid ............................................... 10

Figure 2: A rendering of the anchor .............................................................................................. 11

Figure 3: Area formation shapes. .................................................................................................. 12

Figure 4: Hybrid formation. .......................................................................................................... 13

Figure 5: An Astronaut using the Parallel Deployment scheme ................................................... 15

Figure 6: Depictions of the way the anchors would connect to one another during deployment . 15

Figure 7: Depiction on an astronaut using the soft asteroid mobility system. .............................. 19

Figure 8: A depiction of the PMU on top of a stack of ASPs. ...................................................... 20

Figure 9: A depiction of the harpoon in its launch tube. .............................................................. 31

Figure 10: Depiction of the cable reel.. ......................................................................................... 33

Figure 11: Harpoon, cable reel, anchor bottom plate, and anchor base attach together. .............. 34

Figure 12: Tension rig mounted to the anchor bottom plate. ........................................................ 34

Figure 13: Approximate locations of the motion capture markers ............................................... 57Figure 14: Subject walking in the test environment. .................................................................... 58

Figure 15: Graph of Downforce and speed. .................................................................................. 71

Figure 16: Graph of Froude number at each downforce level.. .................................................... 72

Figure 17: Graph of downforce and metabolic cost ratio ............................................................. 72
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ObjectivesThe overarching goal of this paper was to design a mobility system that provides astronauts the

ability to walk on the surface of small asteroid. This included top level design of the hardware

and plans for deployment and usage, as well as a comparison of the system to other proposed

methodologies for human asteroid exploration. This paper also begins to explore in depth aspecific aspect of the mobility system, which is the determination of the downforce level to be

applied to the astronaut via the mobility system

Part One: Asteroid mobility System Design

Introduction

NASA has long talked about sending astronauts to visit an asteroid, and more recently the

President of the United States has made this goal part of the current roadmap for human

exploration. In order to accomplish such an extraordinary feat, many new technologies will needto be created and paired with existing NASA expertise. Proposed within the following report is

one such technology. The hurdle this technology addresses is that of providing mobility to

astronauts while preforming EVAs on the surface of an asteroid. Unlike the Earth, Moon, and

Mars which have high enough levels of gravity as to enable people to walk on their surfaces,

most asteroids, specifically those near Earth (NEOs), have very low levels of gravity which

would make walking or working on their surfaces almost impossible. Therefore technology

needs to be employed to make this impossibility, a possibility. To that end the author of this

report is proposing a system which secures an astronaut to the surface of an asteroid and provides

them with a perceived level of gravity which enables them to explore the surface. Furthermore in

an effort to accommodate multiple mission architectures and destinations, instead of proposing a

single technology, a family of technologies with alternatives implementations is provided.

Possible Destinations

While astronauts could conceivably visit any asteroid, more practically they will visit an asteroid

which is relatively close to earth, a NEO. Much study has been done to assess which NEOs could

be candidates for possible human missions within the next few decades. It is not relevant to this

report to discuss what makes a NEO a potential mission candidate, other than to say the

requirements are based on mission duration, delta-v, and earth reentry velocity. However it is

important for this report to understand the characteristics of these candidate NEOs.

There are only a small number of known NEOs to date which could be considered possible

candidates for early mission. A high estimate might be 20 different asteroids. These asteroids are

all quite small, on the order of 25 to 100 meters in diameter. Diameters of that magnitude mean

that these objects have extremely low levels of gravity, negligible levels from an EVA mobility

standpoint. Additionally these NEOs most likely have several characteristics that set them apart

from one another, the most important of which is their composition. Some of these asteroids may


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be little more than loose accumulations of rock and dust, while others may be solid bodies of

rock, or something in between. In general this report will refer to two types of aster oids, soft

and solid. Soft asteroids are ones where securing an anchor to the surface would be impossible,

either due to extremely deep layers of regolith, or due to the fact that the asteroid is made up of a

loose pile of rocks. Solid asteroids are therefore ones where the asteroid is for the most part a

single large rock body into which anchors could be secured.

Currently there is not much information available about the physical nature of these candidate

NEOs. Scientists can make assumptions based on information known about other asteroids, and

using correlations to draw conclusions, but in fact the physical properties of these candidate

destinations is still unknown. Thus this report will provide methods to explore several types of

NEOs.

The Overall Technology

The core idea behind this family of technologies is that multiple tethers actively pull down on the

astronaut, providing a net vertical external force downward. Different situations might call for

different amounts of tethers (two, three, or four), and this will be explained later. The tethers are

attached at one end to the surface, and at the other end to a ring which clips around the astronauts

space suit. Depending on the type of asteroid being visited (solid or soft as explained above), the

method by which the tethers attach to the surface is different (as will be explained below).

However in either case one end of the tether is attached to a tension rig. Within this rig is a

spindle which wraps excess tether length around it, and an electric motor connected to the

spindle. The motor acts to generate a tension in the tether, and the tension forces from multiple

tethers acts to keep an astronaut grounded. The resultant force of the multiple tethers should be

zero in all but the vertical direction. Furthermore the tension in each tether is monitored and

actively controlled independently to generate a specific and constant net downward force on the

astronaut. Based on the orientation of the astronaut, their limb joint angles, and any force the

astronaut exudes on the tethers (i.e. in attempting to walk), the tethers will attempt to retract and

extend as required to accommodate the motion of the astronaut while maintaining the desired

constant downward force (downforce).

The Suit Ring

There are two main parts to the suit ring. The first is a ring that is integrated into the suit

(permanently part of the suit), and is situated at the wait of the astronaut on the exterior of the

suit. This feature is referred to herein as the Astronaut Suit Ring (ASR). The second part is the

Suit Connector Ring (SCR), which is essentially two halves of a ring which are hinged at one

end and can clasp at the other end. In use, the SCR attaches around the ASR, and bearings built

into the ASR allow the two rings to rotate freely, one within the other. An encoder system built

into the two rings monitors their relative orientations. Furthermore as part of the whole mobility

system, the SCR attaches to the open end of the tethers, and this is how the astronaut is attached

to the system. This suit ring is exactly the same for all types of missions. This was done to make


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sure that the space suits designed for the one mission would be applicable to all future asteroid

missions, no matter the asteroid type.

Utilization on Solid Asteroids

Figure 1: This is a simple depiction of an astronaut using the mobility system on a solid asteroid. The blue cylinder is

representative of an anchor. The astronaut is exploring the triangular area between three anchors.

The Anchors

For an asteroid of solid monolithic constitution where items can be rigidly attached to its surface,

a system of anchors would be used as the surface attachment points for the tethers. In this setup,

the tension rig would be housed within the anchor, and the free end of each tether would be

connected directly to the SCR. The anchor would have the ability to raise, lower and rotate the

point at which the tether leaves the anchor. This functionality is required by the control system to

provide the correct tension in the tether, as well as by the system to function in an auxiliary mode

that will be explained later (Alternative Parallel Deployment) .The anchor would also house all

of the equipment it would need to attach itself to the surface. Several methods of attachment are

explored in the Anchor Deployment section below. The anchor itself has no other function than

to keep the tension rig (and thereby the astronaut) secured to the surface. However for the

remainder of the section on solid asteroids the use of the term anchor will also refer to the

tension rig as it is housed within the anchor. A concept of this anchor can be seen in Figure 2below. A full description of the anchor design can be found in Appendix One: A.


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Figure 2: A rendering of what the actual anchor might look like. This is an anchor for a solid asteroid. The wide cylinder to the

left is an integrated harpoon. The tether would be wound around the spindle in the center, and would be strung through the

two sets of pulleys on the right. For deployment the tension rig would be covered with a housing, but no housing is shown

here so that the mechanics can be seen. A full description can be found in Appendix One: A - Harpoon Anchor System for

Solid Asteroids a Detailed Description

The Tethers

The tethers in addition to their obvious tethering function, also act as power and data

transmission cables. By enabling the tethers to transmit power, the need to power each anchor

independently is removed. All of the anchors would then be powered by the main spacecraft,

which would connect to one of the anchors via a tether, and that anchor would in turn power all

the other anchors. Additionally the anchors would transmit their sensory data (i.e. tether tension,length, and angle) through the tethers and back to the spacecraft, so that the system could

maintain its desired functionality. This removes the need for the anchors themselves to have

complicated computational capacity or wireless transmission capabilities. However if it was

found that the cabling required to accomplish the data transmission was more massive than

giving each anchor computing and wireless communication capabilities, then that option could

be used as well. This decision would be mission specific though, and depend on the number of


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anchors, the length of the tethers, and the desired distance of the spacecraft from the asteroid.

When the tethers are not attached to the SCR, they are attached directly to a subsequent anchor.

More information on attachment schemes is discussed below. The tether ends would be

data/power plugs, which quick connect to attachment points.

Anchor FormationsThe Anchors could be deployed in many different formations which essentially allow for two

different types of exploration. The most basic formation would be in a line. In this type of

formation only two tethers at a time would be connected to the SCR. This would allow the

astronaut to move in a straight line between the two anchors. Additionally if an astronaut

finished exploring the region between the first two anchors, they could move on to the area

between the second anchor and the third anchor, and so on until the last anchor. The second type

of formation would be one in which three or more tethers are connected to the SCR at any given

time. This would allow the astronaut to explore the region (triangular or otherwise) bound by the

anchors to which they are attached. Similar to the line formation, the astronaut could transfer

between successive sets of anchors to explore multiple areas.

In general, the area formation would make much better use of the available anchors and give the

astronaut much greater freedom in exploration. However in some situations the area formation

might not be advantageous over the line formation. The line formation is useful when it is

desirable for the astronaut to cover a very long distance as opposed to a large area, or when the

asteroid lacks any large area without depressions making it difficult to utilize an area formation.

Additionally, in some instances it might be beneficial to combine the two types of formations.

Such a situation might be when there are two areas to be explored which are separated by a

sizable distance. The two areas would each need at least three anchors to be explored, and the

two areas could be connected by just two anchors, which the astronaut uses to transition between

areas. It all depends on the surface landscape and the mission goals.

1

23

4

5

6

7

KEY

Anchor

Figure 3: Examples of various area formation shapes. In the formation to the right, there are 7 explorable areas.

The astronaut would first explore area-1 and could sequentially move through each of the areas.


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Figure 4: This is a hybrid formation. In it there are two area formations and one line formation. This configuration might be

used if there were very specific targets identified for exploration. However this configuration is less efficient (in terms of

total explorable area per anchor) than using areas which share sides, as in Figure 3.

A broader discussion of the possible anchor formations and an assessment of the efficiency of

each is provided in Appendix One: C. The conclusion of that discussion is that, mission factors

aside, the equilateral triangle formation is the most efficient formation, in terms of system mass

per area explored.

Transitioning Between Anchors

The Following two sections outline possible procedures an astronaut would follow to transition

between sets of anchors. The two protocols below are meant to be illustrative of how such a

system might work, and not an exhaustive study of all of the possible transitions required by the

many types of formations allowed by this family of technologies.

Line Formation

To transition from the region between one set of anchors (i.e. anchors 1 and 2 ) to the next

(anchors 2 and 3), the astronaut would adhere to the following protocol. It is helpful to note here

that the free end of the tether-3 would be attached to anchor-2 before deployment.

1. Walk as close as possible to the second anchor.

2. Clip a safety line from the suit to the anchor.

3. Command tension rigs 1, 2, and 3 to maintain zero tension in their tethers.

4. Clip a second safety line from tether-1 to the anchor.

5. Unhook tether-1 from the SCR and attach it to the anchor.

6. Remove the safety line which connected tether-1 to the anchor.

7. Clip a third safety line from the anchor to the tether-3 (which itself is already connected

to the anchor).

8. Attach tether-3 to the SCR.


10.Remove the safety line which connected suit to the anchor.


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11.Command tension rigs 2 and 3 to reassume tension in their tethers.

12.Begin traversing the line between anchors 2 and 3.

Triangle Area Formation

To transition from the area between one set of anchors (i.e. anchors 1, 2, and 3) to the next

(anchors 2, 3 and 4), the astronaut would adhere to the following protocol. Note in this situationonly one tether is being exchanged for a new one and the second triangular area shares one side

with the first. Also note that tether-4 would be attached to anchor-2 before deployment.

1. Walk as close as possible to the anchor-2.

2. Clip a safety line from the suit to the anchor.

3. Command tension rigs 1, 2, 3, and 4 to maintain zero tension in their tethers.

4. Clip a second safety line from tether-1 to the anchor.

5. Unhook tether-1 from the SCR and attach it to the anchor.


7. Clip a safety line from the anchor to the tether-4 (which itself is already connected to theanchor).

8. Attach tether-4 to the SCR.


10.Remove the safety line which connected suit to the anchor.

11.Command tension rigs 2, 3, and 4 to reassume tension in their tethers.

12.Begin traversing the area between anchors 2, 3, and 4.

Alternative: Parallel Deployment:

For some EVAs it might be beneficial to use an alternative deployment of this system depending

on the activities to be completed during the EVA. The alternative deployment would allow anastronaut to explore the surface of the asteroid while maintaining their body parallel to the

asteroid surface. To do so, a new piece of equipment needs to be introduced, the Parallel

Movement Adapter (PMA). The PMA is basically a rod with a plate on one end, which attaches

to the back of the spacesuit just below the ASR, at the free end of the rod. The disk end has a

copy of the ASR mounted to its diameter, and this fits inside the SCR just as the ASR would.

Once the PMA is connected to both the suit and the SCR, then the astronaut is ready to explore

the surface (see Figure 5: An Astronaut using the Parallel Deployment scheme). This deployment

will keep the astronaut parallel to and a fixed distance from the surface. Locomotion will be

semi-automated with commands given by the EVA astronaut or by a fellow astronaut aboard the

spacecraft. Sensors integrated into the suit and anchors, will orchestrate a cohesive response

from all the tethers connected to the SCR. Part of this sensor package would be distance sensors,

which help automatically maintain the astronaut at a specific height regardless of changes in the

terrain. Their height is determined by the height of the second pulley on the anchor, which is

controllable.


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Figure 5: An Astronaut using the Parallel Deployment scheme

The main advantages of this system deployment are lower energy expenditure by the astronaut,

lower energy consumption by the tension rigs, no dust turned up by foot traffic, and ease of

interaction with the surface. This type of setup would be ideal for situations where an astronaut

needs to use their hands for an extended period of time in close proximity to the surface, or when

an astronaut wants to set up a device on the surface which would be sensitive to dust occlusion.The main drawback of such a deployment is that the astronaut would not have a very large field

of vision. This may or may not hinder the astronaut depending on their duties. However this

shortfall could be rectified though the use of cameras mounted atop the PMA plate. The cameras

could provide views of the landscape in every direction which would be viewed by the astronaut

in a HUD integrated into the spacesuit helmet. Additionally, the astronaut could at any time

easily switch into the standard deployment mode by removing the PMA, and attaching the SCR

around the suit.

Anchor Deployment

Several methods for deploying the anchors are described below, yet they all share two commonfeatures. First, each anchors has three legs which allow it to come to a level position after

landing. Second, all of the anchors will be connected to one another before and after deployment.

The anchors are connected to one another via their tethers, and the first anchor is also connected

to the main spacecraft via a master tether. More precisely, the tether of the first anchor is

attached to nothing during deployment, and each subsequent tether is attached to the previous

anchor.

The reason for choosing one anchor deployment method

over another would come down to TRLs and mass

estimates once the various options were investigatedfurther. Additionally the physical properties of a particular

asteroid might negate the use of one or more of the options

(i.e. an asteroid that is too hard to penetrate with a

harpoon).

KEY

Anchor

Figure 6: Depictions of the way the anchors would connect to one another during deployment


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Harpoon

This method would call for each of the anchors to have a harpoon loaded into the bottom of the

anchor. The main spacecraft would position itself over the location for the first anchor and that

anchor would fire its harpoon towards the surface. Once the harpoon was embedded securely

into the surface, the first anchor would be released from the spacecraft. The spacecraft would

then proceed to position itself over the other anchor locations and allow the remaining harpoons

to embed themselves followed by the release of each associated anchor. Once all of the anchors

were released from the spacecraft, each anchor would simultaneously reel themselves down to

the surface on their harpoon cable1.

Before being fired, the harpoon would be situated in a gas gun on the underside of the anchor.

The harpoon would be propelled away from the anchor by the release of a pressurized cartridge

in the gas gun. Immediately following the exit of the harpoon, the gas from the gun would be

allowed to exit out of the top of the anchor, in an attempt to negate the momentum imparted by

the harpoon. However the main spacecrafts RCS would have to augment this effort. The

harpoon itself would pierce the asteroid burrowing below the surface at which point it would

release barbs from the harpoon shaft which would secure the harpoon to the asteroid2.

Thrusters/harpoon

An alternative harpoon method is one in which the anchors are all free flying vehicles. The

anchors would leave the main spacecraft (tethered together as always), and each anchor would

fly independently3

to a position just above its intended ground location. Close proximity with the

asteroid is required so that when the harpoon is released and the anchor begins moving in the

opposite direction, the harpoon tether is not exhausted before the harpoon is secured in the

surface. Once all of the anchors were in place, they would each fire their harpoon, and then reel

themselves down to the surface. These anchors would be much more complicated than the ones

described previously. The anchors would require fuel, thrusters, reaction wheels, greater thermal

control, and navigational sensors. However their power and data processing needs would be

handled by the main spacecraft as data and power would be transmitted over the tethers.

1The reason to reel all of the anchors down at the same time is to maximize the distance between anchors. For

example, assume two anchors are being deployed and each has a tether length of 20m. The two anchors would be

connected by tether #2, so the farthest apart they could be would be 20m. Assume also that the spacecraft mustmaintain at least 10m between itself and the asteroid. This means that a spacecraft 10m up from the surface could at

most be 17.3 surface meters away from the first anchor (if the first anchor was on the surface, and the second anchor

was still attached to the spacecraft). If the anchors are reeled down simultaneously at the end, they could be their

maximum distance apart (20m).2

The implementation of the harpoon idea was heavily inspired by the harpoon on the Philae lander on the Rosetta

spacecraft [45] [7]. The main difference being the idea presented herein to release the gun exhaust to offset the

momentum transfer.3

The anchors would not fly completely independently. They would be tethered together, and while the tethers would

be limp during flight, they still might generate some coupling between the anchors.


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Tether Length on Solid Asteroids

In the anchor formation analysis, an estimate of 20 meters was used for the tether length, but the

optimal tether length would be asteroid and mission specific. The main reason to optimize tether

length is to reduce mission mass. This reduction in mass comes partially from a reduction in the

tether itself, however the tether material is relatively light per unit length. The majority of the

mass savings from a shorter tether comes from the thickness of tether on the tether spool being

less. This reduction ultimately reduces the necessary mass of the tether spooling motor. The

more layers of excess tether on the spool there are, the lower the tension in the tether will be for

a given torque of the spool by its motor. This is a simple matter of . The more excess

tether there is, the larger the effective spool radius is, and the greater the motor torque required to

produce a given tether tension. Therefore if the tether length is optimized to the mission, then the

motor choice can also be optimized for the mission.

Utilization on Soft Asteroids

Anchors

The implementation of these technologies changes quite a bit for a mission to an asteroid where

securing a traditional anchor would be impossible. In the case of a soft asteroid, the anchor

system would consist mainly of two circumferential ropes. The two ropes would run parallel to

each other, separated by a distance that would determine the explorable area. Furthermore the

circumferential ropes would, as the name implies, each encircle the entire circumference of the

asteroid. Attached to the ropes would be several wide plates which act to keep the ropes from

cutting (or sinking) into the asteroid. These Anti Sinking Plates (ASPs) would be attached to the

ropes at a predetermined intervals prior to deployment. The interval and size of the plates would

be heavily dependent on the physical properties of each individual asteroid. Additionally theplates would have small spikes on the bottom to help them gain traction with the surface.

Tethers and Tension Rigs

In this situation the tension rigs, of which there would be two, connect directly to the SCR. The

tethers emanate from the tension rig just as before, and the free end of the tether connects to a

circumferential rope. The connection between the tethers and the circumferential ropes would be

a free one, such that the tethers could move freely along the ropes as the astronaut walked in the

direction parallel to the ropes. The tethers in this implementation would not need to carry power

or data, as the tension rigs would both be attached to the astronaut. The tension rigs would get

their power in one of two ways. For short EVAs (under 2 hours), power could come from extrabatteries attached to the space suit. For longer duration EVAs, the tension rigs would be powered

by a tether that goes directly to the astronaut.


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Figure 7: Depiction on an astronaut using the soft asteroid mobility system. Two ASPs can be seen just behind the astronaut.

The blue boxes represent the tension rigs.

ExplorationUnlike the system proposed for solid asteroids, this system would allow multiple astronauts to

explore the same area simultaneously4. Additionally using this system, the astronauts would have

the entire area between the two circumferential ropes to explore. If one assumed a distance of 5

meters between the ropes, and assumed the asteroid was 60 meters in diameter, then the

astronauts would have almost 950m2, or more than 10,000ft2 to explore. This would be roughly

the same amount of area as explored by the Apollo 11 crew on the moon, if not a greater area.

Furthermore this area could possibly be increased by moving the circumferential ropes further

apart. However this is subject to the dimensions of the asteroid. The ropes cannot move too far

from the centerline of a spherical asteroid without slipping off of it under tension.

Anchor Deployment

On asteroids that are less cohesive, the anchors as previously described are two circumferential

ropes with anti-sinking plates (ASPs) attached at specific intervals. Each Rope and its ASPs are

initially a single ridged device as depicted in Figure 8 below. Each ASP is connected to the

following ASP by the circumferential rope. Additionally atop each stack of ASPs is a spacecraft,

the Plate Maneuvering Unit (PMU), which plays two roles. First it moves the stack of ASPs

around the asteroid and releases an ASP at every interval. Once that is completed, the PMU

which is attached to the end of the circumferential rope, flies back to the first ASP and docks

with it. Once the PMU is docked to the first ASP, it tightens the circumferential rope until the

whole unit is secured tightly to the surface. See Figure 7 above for more clarification.

4The reason that the circumferential ropes method was not suggested for solid asteroids as well, is because the

system is much more restrictive in terms of the area that can be explored. It may also be more massive. However the

circumferential ropes would work just as well on a solid asteroid.


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Figure 8: A depiction of the PMU on top of a stack of ASPs. The grey coils are the circumferential ropes. The ropes are wound

in such a way that they will uncoil without spinning the ASPs.

Furthermore, the two PMUs (one for each rope) are fully functional spacecraft. They would

possess the ability to operate completely independently of the main spacecraft. The idea of

having the PMUs be tethered to the main spacecraft and possess limited capabilities similar to

the other anchors, was considered. However as the PMUs have to circumnavigate the entire

asteroid, it was determined that having them be tethered to the main spacecraft would be too

cumbersome. However If it is determined that it would be too expensive to develop secondary

spacecraft which such advanced capabilities (i.e. Automatic rendezvous and Docking), then

perhaps it would be possible for the main spacecraft to do the deployment, but this depends on

how close the main spacecraft I allowed to get to the asteroid.

Surface Operation Considerations

For operations in either type of system, there may be times where the astronaut needs to stand

still, or times where the astronaut needs to react a higher level of force to the ground than the

downforce provided by mobility system. In order to allow this functionality, the astronaut can

command the tension rigs to lock. In that way the astronaut can generate much higher forces with

the ground, or the astronaut can bend over and stay grounded.

Astronaut Surface Descent and Ascent Strategy

There are several options for transferring the Astronauts from the spacecraft to the mobilitysystem on the surface of the Asteroid. These options include transferring them on a boom,

allowing them to jet over using an MMU, or having them climb to the surface on a tether. The

boom is a good idea if the main spacecraft is allowed to get close enough for the boom to reach

the surface. However unless the boom was already part of the mission design, it would add a lot

of mass to accomplish one function. The MMU would provide a solution that allows the

spacecraft to maintain a greater distance from the surface of the asteroid. However it too would


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add a not insignificant amount of mass to the mission. Furthermore the MMU introduces

significantly more risk to the sortie. If the unit runs out of fuel or otherwise becomes inoperable

the astronaut would have no way to maneuver back to the spacecraft without the spacecraft

moving dangerously close to the surface. For these considerations and others, this report

concludes that the safest and most efficient method would be for the astronauts to climb along a

tether connecting the spacecraft to the asteroid. Although climb is really overstatement, because

all the astronaut would have to do, is tug once and glide along the tether. In the case of the solid

asteroid, the spacecraft and the anchors would already be linked by a tether (for power and

communications), so this option would add no extra mass to the mission. For the case of the soft

asteroid, the spacecraft could be connected, by a low mass safety tether, to the second ASP

released which remains close to the spacecraft. In either case the astronaut would hand over hand

pull themselves along the tether from the spacecraft over to the surface? This system is by far the

safest. It has the lowest probability for any type of failure, and it requires the smallest physical

footprint which decreases the chance of a debris strike.

There are four possible risks astronauts face as they transfer between the spacecraft and the

asteroid surface, while on EVA. These risks include failure of the device enabling the transfer,

failure of the astronaut to adhere to safety guidelines, incapacitation of the astronaut, and debris

striking the transfer device. A method and technology for enabling this transfer should be chosen

which provides the most safety and reliability to the astronauts who will use it.

Power Requirements

The tension rigs which provide tension to the tethers require a significant amount of energy. The

amount of energy required depends on the configuration being used, the downforce desired on

the astronaut, and the amount of gearing used in the motors. Gearing is an interesting tradeoff

because the more gearing the motors have, the heavier they are, but the less power they require

to generate a particular torque. In Table 1 below, the power requirements for 9 possible

deployments are shown. These values were calculated using the methodologies in Appendix

One: B.

Lunar Gravity Martian Gravity Earth Gravity

Configuration Ave

(kW)

Peak(kW) Ave

(kW)

Peak(kW) Ave (kW) Peak(kW)

Line Formation

(10m apart)1.6 3.5 3.3 7 10 21

Triangular area(10m sides)

1.6 4.2 3.6 8.7 10.8 26.1

Circumferential

Ropes (5m

apart)

0.7 1.7 1.4 3.5 4.1 10.7

Table 1: For the 3 setups shown, the power requirements for 3 different simulated gravity environments are given. These

values are for the whole system, not for individual tension rigs.


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Solid Asteroid Power Requirements

In this case the power for the tension rigs would come from a single tether which connects the

first anchor to the spacecraft. Power would then be distributed from that anchor, through the

tethers, to all of the other anchors on the asteroid surface.

Soft Asteroid Power RequirementsThe power for tension rigs could come in one of two ways. For EVAs of a short duration (2

hours or less on the surface), the power for the tension rigs could be provided by an auxiliary

battery attached to the spacesuit life support system. For longer EVAs, a tether could be attached

to the spacesuit life support system. The battery system could certainly be used for longer EVAs,

however the tension rigs use a significant amount of energy, and the battery pack for surface

stays over 2 hours might become quite cumbersome.

Possible and Proposed alternativesAlthough the study of EVA mobility options has barely been explored in the scientific

community, a few methods have been proposed. These methods rely on the use of either a boom,

an astronaut maneuvering unit, or anchors and tethers (different than those discussed).However,

none of these systems has been explored in any great detail, and only one of the options (as will

be discussed below) has a peer reviewed article written about it. The details of these alternative

systems, and a discussion of their strengths and weaknesses is provided below.

Boom Arm

In 2011 and 2012 NASAs NEEMO project performed simulated asteroid EVA operations at the

Aquarius undersea research laboratory near Key Largo, Florida. In these missions, one of thetechniques the astronauts used to interact with the asteroid was to secure their feet in a boom,

or robotic arm. The boom positioned the astronaut over the surface, so that they could interact

with the asteroid using their hands. NASA has only just begun testing this type of system for use

on an asteroid, and it may be a long time before any data or analysis is released; although one

would assume that this boom system would be something like the one used on the ISS or the

Shuttle. However there are significant differences in the environment of the ISS and an asteroid,

and the boom implementation might change considerably.

There are several options available as to who would be operating the boom. The boom could be

controlled either by the user, by another crewmember, by mission control, or autonomously.Each of these options has its advantages and limitations. Currently the protocol is for another

crew member, who is still in the spacecraft, to operate the robotic arm. This method leaves the

astronaut on EVA free to complete their tasks without having to worry about operating the boom.

However there is some risk involved in having an operator with only limited visibility, as an

onboard operator would have. Additionally, the process of the EVA astronaut describing his

desired movement to the operator could be tedious and slow. This problem does not exist to a


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great extent currently because on station for a given task, an astronaut has very a very specific

place to go on the surface of the station. Conversely at an asteroid, an astronaut on EVA might

not know exactly where they wanted to go. They might have a desired spot to explore, but the

spot might not be what they thought it was, and the astronaut might have to change positions on

the fly. Furthermore due to the significant time delay between the crew and mission control, the

decision might have to be made by the crew. In this case it might be difficult for the person on

EVA to coordinate with the boom operator where to move to. To mitigate this effect, the boom

could be operated primarily by the astronaut on EVA themselves. This would allow the person

on EVA to have full control of their position with respect to the asteroid at all times. This

method seems the most efficient, in that the astronaut on the boom would probably have the best

idea of where they are in relation to the asteroid and where they want to go. The downside to this

method is that the astronaut would have one or both hands occupied while traversing, but this is a

small concern because the astronaut would not have to be interacting with the asteroid while

moving. There is little chance that the boom would be controlled either by a remote operator on

Earth. The remote earth operator would have a large time delay (up to 20 seconds), which wouldmake it impractical for them to control the arm.

One of the problems of the boom is its limited range. If the Asteroid is significantly small in size,

then its curvature would severely limit the reach of the boom without the spacecraft having to

maneuver to a new position. Also in the case of some asteroids, it is believed they could have

large pieces of debris orbiting them, and to avoid such pieces, a spacecraft might have to stay a

significant distance from the surface. Depending on this distance, the boom might become

prohibitively voluminous or massive to be the chose mobility system for the mission.

Another detractor of using a boom system is that the astronaut does not get to walk on the

asteroid. After traveling for 90 days in a spacecraft to a distance from earth never before

experienced to a world never before explored, it is conceivable that an astronaut brought up with

the phrase That's one small step for man; one giant leap for mankind, would want to create

their own first steps for mankind. The public too will not understand why after spending tens of

billions of dollars on a NEO exploration mission NASA is unable to allow their astronauts to

walk on the surface of the Asteroid. The imagery of an Astronaut on the end of a robotic arm just

within reach of the asteroid body, is nowhere near as impressive as the image of that same

astronaut with their feet firmly planted in the surface and the still night of empty space in the

background.

Additionally not only is the imagery and the satisfaction of standing on the surface greater, but

he utility is greater as well. An astronaut attached to a foot restraint on the end of a boom must

be within arms reach of the asteroid, and to do so their head would be very close to the surface.

This means the astronaut would have very low visibility as compared to the visibility they would

have standing on the surface. The astronaut would also have a reduced capacity to maneuver to

more favorable positions and locations at will, which would slow work and frustrate the

astronaut. Also the astronaut might have more trouble adjusting to the their orientation to the


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asteroid as it would appear above them or in front of them, and not under their feet; this could be

disconcerting for a time.

One of the most persuasive advantages of using a boom system is that the astronaut comes in

minimal contact with the surface of the asteroid. In general only the astronauts gloves and tools

would make contact with the surface, and this type of contact would be the same no matter whattype of mobility system is employed. This is advantageous in that significantly less dust will be

churned up, than if the astronaut was walking on the surface, or releasing propellant near the

surface. However the extent to which this is advantageous is certainly dependent on the asteroid.

Some asteroids may already have significant amounts of dust floating around them, even without

human interaction. Other asteroids may have very little dust at all, thus nullifying this advantage.

Although if there is native dust floating above the surface , using a boom system, the astronauts

head would be constantly close to the surface of the asteroid where one would assume the most

dust would be floating about, allowing the astronaut no respite from the obscuring haze.

Astronaut Maneuvering Unit (AMU)Another alternative would be to equip an astronaut going on EVA to the asteroid Surface with a

propulsive personal maneuvering system much like the MMU. This system would consist of a

backpack propulsion device that the astronaut would use to propel themselves from the

spacecraft to the surface, then around the surface, and finally back to the spacecraft. The unit

would be operated by the user most likely and would offer the astronaut a great amount of

freedom in locomotion. However such a system would have many drawbacks. The system unit

would be limited in maneuvering capacity by the amount of fuel it carried. The unit would also

carry a high degree of risk to the safety of the user; if the unit was operated improperly, or

without sufficient caution the operator could collide with the spacecraft, the asteroid, or worse

find themselves out or reach of return. Although the astronaut could be connected to the

spacecraft with a safety tether, but this might limit their mobility. Furthermore the structure of

the unit itself, and its attachment to the spacesuit would severely hinder the astronauts ability to

interact with the asteroid. With some sophisticated controls algorithms, and ergonomic design, it

is possible the unit could allow the astronaut to walk on the surface. However this would require

a significant amount of fuel, and it is likely that the design would not accommodate that type of

functionality. Without the ability for the astronauts to be able to walk on the surface of the

asteroid, the mission would suffer the same losses described in the previous section. The system

would also be prone to churning up dust at high velocities whenever the exhaust comes into

contact with the surface.

Alternative Anchor systems

Other people have vaguely suggested using anchors or circumferential ropes to develop an

astronaut mobility system for asteroids. Gertsch and Gertsch provide six basic methods for attaching anchors

to an asteroid surface [1]:


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1. Single sling around asteroid.

2. Multiple slings around asteroid.

3. Penetration anchor.

4. Expansion anchor in drilled hole.

5. Friction anchor in drilled hole.

6. Glued (grouted) anchor in drilled hole.

They propose tethers can be attached to these anchors, but there is not further elaboration on

how the tethers work, nor on how a person might operate while attached to the tethers.

Furthermore, while anchoring devices have been studied to some extent for other mission types

(i.e. asteroid sample return missions), there has been little research conducted on how a system

of anchors providing safety and functionality to astronauts might be designed. Additionally, there

has been almost no research into the design of a system of tethers for attaching astronauts to said

anchors. Possibly the only academic paper written on the subject proposed a system that uses

circumferential ropes to accomplish this task. Garrick and Carr designed a system which uses

two circumferential ropes as anchors [2]. Astronauts would then don a large rigid harness which

connects at the waist and attaches to the ropes. The harness has four controllable telescoping

arms (two on each side) which rise upward from the waist, and have eyelets on the ends through

which the ropes are threaded. Downforce on the astronaut is provided by the tension in rope

created by the arms pushing upward on the rope. Additionally the harness is gimbaled to provide

astronauts the ability to rotate and bend. Their proposed system would provide a downforce

approximate to lunar gravity.

This system has positive and negative points as well as a few assumptions that may not be

universally true.

Negatives: The astronaut would be constrained it seems to the small path created by their

immediate reach multiplied by the circumference of the astronaut. The astronaut would have

very little ability to move perpendicular to the direction of the ropes. Conceivably the arms of the

harness could extend and retract significantly to make lateral movement a possibility, but it

might be difficult to actuate the linear motion of these arms at the speed an astronaut would

attempt to traverse. Furthermore the more distance you attempt to put between the ropes, the

longer the arms need to be to make this distance traversable, increasing the mass of the system.

The author also mentions that it would be impossible to traverse regions where due to the

asteroid being non-spherical, the ropes were suspended too high above the surface. Additionally,

the arms of the harness and the circumferential ropes would be above waist level, and might

obscure the vision and arm/body movements of the astronaut.

Garrick and Carr provide an option where a pair of astronauts can utilize the system

simultaneously, but it includes the use of a large fulcrum, and couples the astronauts motion to

one another effectively reducing their independent degrees of freedom. In this setup, a long

boom would be attached to an astronaut positioned normally in the system. On one end of the


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boom another astronaut would attach themselves and on the other end of the boom a

countervolume would be attached which rolls along the asteroid surface and keeps the boom

level. This would mean that no matter how the boom was gimbaled, the position of the central

astronaut would define the explorable envelope of the second astronaut. Additionally, any time

this boom was at the end of its range of motion, an attempt by one the astronauts to move further

outside this envelope would directly torque the second astronaut. Furthermore this secondary

setup has greatly increased the mass of the system (because of the boom and the countervolume),

and has increased the amount of dust churned up (because of the countervolume).

The authors also suggest a third mode of operation where the astronaut is parallel to the surface,

and the length of the harness arms would determine the distance between the astronaut and the

surface. However in this setup the eyelets are dragging on the surface, and the astronaut would

have to drag themselves along the surface to change positions. Additionally the harness would

have no way of maintaining a tension in the ropes anymore, which would make it more difficult

for the astronaut to interact with the surface.

Positives: The system does provide the ability for the astronaut to traverse through terrain of

various heights, through the lengthening and shortening of the arms. However the degree to

which the system can compensate for large slopes is again dependent on the range of the arms

and the speed at which they can change length.

Assumptions: The authors assume that the ropes will not cut through the regolith, which may not

always be the case. To derive this assumption, the authors calculate the pressure of the ropes on

the surface to be 42 N/m2 , which the authors assume is very low. However it is conceivable that

there could be an asteroid whose cohesion is not strong enough to withstand a pressure of such

magnitude. Furthermore to obtain this number the authors assumed an asteroid of 3 km indiameter, which is most likely very unreasonable. The greater likelihood, as was described

earlier, is that the first asteroids to be visited by humans will be at most a couple hundred meters

in diameter and more likely they will be under 100 meters. Given an asteroid of radius 60m

(which is much more representative of the field of possible destinations), the pressure exerted by

the ropes on the surface to produce the same downforce would be about 2400 N/m2.

Additionally, this is just an average pressure assuming the tension in the rope is evenly applied

around the circumference of the asteroid which it would not be. This calculation also does not

account for the dynamic loads caused by astronaut movement or tool use. In all likelihood the

maximum pressure the ropes apply to the surface could be 10 times greater or 24 kN/m2 , which

could easily be greater than the cohesive forces of some asteroids.

In addition to the MIT proposal just described, NASA has recently tested a system of

anchors/tethers for asteroid exploration. The tests were conducted three times over the last two

years as part of the NEEMO project performed at the Aquarius undersea research laboratory near

Key Largo, Florida [3]. The system of tethers tested by NASA while not described in full detail,

but it seemed to be very simple. In it there is a central hub which stores six retractable tethers the


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tethers would then be attached to various other anchor points on the surface, and the astronaut

would clip themselves a single tether they wished to traverse along. The Astronaut would then

move themselves by pushing off the surface and/or by pulling on the tether. It is unclear why

these tests are being tested in open water where uncontrollable environmental factors seemed to

be having a significant impact. Motion of the water, as well as vision obscuring particles seemed

to be having a negative effect on the utility of such tests. Hopefully further testing will encourage

the development of more robust methodologies and technologies.

Mission with no EVA

It has also been proposed that the mission could be accomplished without an EVA. In that

scenario, the astronauts would explore the asteroid surface from within either the main spacecraft

or a secondary smaller spacecraft, using robotic arms and tools. This method seems as though it

might provide the greatest risk, and the least reward. The risk comes from the fact that the main

spacecraft would have to spend a great deal of time in very close proximity to the surface. This

would make the spacecraft highly susceptible to accidental contact with the asteroid or to

damage from orbiting debris. Furthermore, it might place a mental strain on the astronauts to

travel for 90 days without being able to get out at their destination. Additionally exploration

tasks would probably be performed very slowly to ensure that the robotic arms interact safely

with the asteroid. This would mean less science could be accomplished. However this system

would have the advantage of not requiring the deployment of any secondary hardware such as

anchors.

Downside to Anchors

If the material type of the asteroid is unknown when the journey begins, then anchor may not be

a good option. There is some chance that if the material properties of the asteroid are too

unknown, then both the schemes for solid and soft asteroids discussed herein might not function

as desired. For solid asteroids, it is possible the anchor harpoons might not penetrate or remain

embedded in the asteroid. For the soft asteroid, it is possible the circumferential ropes and the

ASPs might sink into the asteroid, and render the system ineffective. Therefore if the material

properties of the asteroid are still unknown at the time of the mission, this report recommends

that the boom be utilized for astronaut EVA mobility.

Comparison of alternativesThe most compelling reason though to pick one mobility system over another might be mass, or

a combination of mass and ability to complete mission objectives. For this analysis, a metriccalled mass specific area of exploration is created. This metric assumes that the number of

mission objectives completed is dependent on how much area can be explored. The metric

relates how much mass it takes to explore a given unit of area. To calculate this, it was assumed

that 12 days are spent exploring the surface. A full description of this calculation can be found

in Appendix One: D, and the results are shown in Table 2 below. Based on this calculation using

the harpoon anchors developed in this report are the most economical choice for mobility


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system, when exploring a solid asteroid. For Exploration of a soft asteroid, the circumferential

tether system developed in this report is the best choice based on these calculations. If however

the alternate metric of Mass Specific Unique Area of Exploration is used, then the boom system

becomes the best choice. The choice of which metric to use is dependent on the mission objects.

Furthermore, the results of this analysis may change dramatically based on the specific details of

an actual mission. Although every attempt was made to estimate values that are representative of

a likely mission.

Boom AMUCirc

Ropes

Harpoon

Anchors

Circ

Tethers

Time (hr) Hours/Day of Exploration 6 14 18 18 18

Area (m^2)Explorable/day 940 500 360 2000 1080

Unique Area Over 12 days 11280 6000 620 6200 1860

Power (kW) Ave. Consumption 2 0.1 0 3 0.7

Mass (kg) Total Extra Mission Mass 6905 8066 9884 4452 5704

Mass Specific Area of Exploration 1.63 0.74 0.44 5.39 2.27

Mass Specific Unique Area of Exploration 1.63 0.74 0.06 1.39 0.33Table 2: Mass Specific Area of Exploration is the area explored per day multiplied by the number of days of exploration,

divided by the mission mass due to the implementation of the mobility system. Also shown is the Mass Specific Unique Area

of Exploration, which is calculated using unique area covered instead of area explored each day.

Other Items to discuss

Emergency Astronaut Retrieval

Safety is very important to any human exploration technology, and was a central focus of this

design. This mobility system has been designed so that even if an astronaut becomes

incapacitated, they can be returned to the spacecraft to receive aid. In this system, the astronaut is

always connected to the main spacecraft. In the solid asteroid system, the first anchor is tethered

to the space craft, which is tethered to the other anchors next to it, and ultimately all the anchors

are connected to one another and therefore the spacecraft. An astronaut while on EVA, is always

connected to this system of anchors. If for any reason an astronaut is unable to return to the space

craft, or is having a medical emergency, the space craft can reel in the system of anchors, and

return the astronaut to the vicinity of the spacecraft where they can be brought inside for medical

attention.

To reel in the system of anchors, the anchors have to be first separated from their connection to

the surface. This is done by firing an explosive bolt or other release mechanism that separates the

upper anchor structure from the anchor base. Once all the upper anchor structures have separated

from their bases, they all reel in their tethers, and the spacecraft reels in the tether which

connects it to the first anchor. Thus the astronaut is returned to the vicinity of the spacecraft. The


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SCR can be remotely unclasped to release the astronaut from the chain of anchors. After that, the

astronaut can be retrieved by a robotic arm or by another astronaut.

Likewise in the soft asteroid version of the technology, the astronaut is connected to the main

spacecraft directly by a tether which can be reeled in by the spacecraft. If the astronaut becomes

immobilized and needs to retrieved, the SCR can be remotely unclasped which frees theastronaut from the mobility system and allows them to be reeled into the vicinity of the

spacecraft where then can be brought inside.

Locomotion in the System

Humans evolved their bodies and their locomotive mechanics to function in 1g. When moved

outside of a tradition earth gravitational environment, those standard mechanics of locomotion

may not function as efficiently as they did in 1g. In the lunar gravity environment, it has been

postulated that loping may be the most efficient form of locomotion for humans [4]. In addition

to the gravity environment, the spacesuit worn by astronauts during EVA can also have

significant effect on how they ambulate. Understanding how the downforce level applied to anastronaut by the mobility system effects walking is key to the proper development of this

mobility system. The design driver of this study is to create a system which provides the most

utility to the mission for the lease amount of mass. Previously this study discussed how this goal

could be met in terms of explorable area, and anchor design, but astronaut usability is of equal

importance. Astronaut usability manifests itself through two criterion, comfort and energy

expenditure. Both of these factors intimately affect overall system utility. The more

uncomfortable an astronaut is while using the system, the less effective they will be in

performing their duties. Furthermore the more energy the astronaut uses to ambulate at a given

rate within the system, the less time they will be able use the system before they have to return to

the spacecraft for rest and nourishment5. Therefore optimizing the system to account for these

concerns is quite necessary. The main ways in which these factors can be affected are in

choosing of the gravity replacement load, and in the design of the spacesuit. The gravity

replacement load will have a serious effect on the kinematics of locomotion. It will also

determine how much force is put on the body through the spacesuit, which is a factor in comfort.

The space suit design itself is important in that it determines how the tensions in the tethers will

be transmitted to the body of the astronaut, which in turn affects comfort, muscle fatigue and

locomotion. Additionally the spacesuit will also have a kinetic impact on locomotion as

described by [5]. Thus it would be very important to study how humans walk when utilizing

mobility system such as the one described. This research would help determine which downforcewould be the most beneficial to the mission. In an effort to begin this testing, an experiment was

done to study unsuited locomotion at 5 downforce levels and the results are presented in Part

Two of this report.

5The astronaut might also need to return if one particular muscle becomes too fatigued, before their body as a whole

becomes fatigued.


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Additionally, testing should be conducted over a range or inclines declines, and terrain

topographies. It was found during the Apollo missions that inclination, and terrain roughness had

a significant impact on the metabolic rate of astronauts on EVA [6]. If the terrain of the asteroid

to be visited is well known in advance, then the mobility system can factor this into its

optimization. Keeping this in mind, it might be advantageous to make the loading on the

astronaut dynamic depending on the terrain conditions. For example the SLS could decrease the

load as the astronaut moved up a hill, if this was found to make walking easier.

Further Study Required

The three biggest areas that need to be studied further to implement this design are:

1. How humans wearing a pressurized space suit would walk using the system described

herein for any given downforce. Studying this will decide what the most economical

downforce is.2. The creation of a control scheme which will enable the tension rigs to function as

described.

3. The design and testing of a harpoon system that will withstand the forces likely from this

configuration.


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Appendix One

A - Harpoon Anchor System for Solid Asteroids a Detailed Description

Hardware overview

The harpoon and its launch mechanism designed for this system are conceptually very similar tothat of the Rosetta spacecraft [7]. The harpoon inside of its launch tube is pictured in Figure 9

below. The hardware inside of the launch tube is used to accelerate the harpoon and jettison it

into the surface. At the very rear of the tube is the electrical igniter, and the space where the solid

fuel goes around the igniter. Following that is a combustion chamber which would be covered

with a damning foil. On the other side of the damning foil is a nozzle that accelerates the

combustion gas. A position is glued into the end of the nozzle, and the harpoon is nestled into

the other end of the piston. The harpoon is secured at the front by two shear rods which lock into

the launch tube. Inside the harpoon is a spring and the point where the cable is attached. This

cable attach point can slide up and down the tube. The point of the spring is to allow the cable

reel to create a high tension between the anchor and the surface after landing.

Figure 9: A depiction of the harpoon in its launch tube.

Exhaust valves

Mounting holesIgniter Leads

Shear rod

Penetrating head

BarbsHarpoon Body

SpringCable attach point

Piston Nozzle Combustion chamber

Igniter


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Launch Process

Once the fuel is ignited it begins to turn into a gas. As gas is created, the pressure builds up in the

combustion chamber. Once the pressure is high enough to sustain the reaction, the gas pushes

through the damning foil into the release chamber. Here pressure builds up again until it reaches

its peak. The adhesive which connects the piston to the release chamber is formulated to break at

this peak pressure which allows the gas to begin pushing the piston forward. This motion also

breaks the weak shear rods which had previously helped keep the harpoon centered and in place.

The piston propelled by the gas shoots forward pushing the harpoon in front of it. The end of the

launch tube is tapered, and when the piston reaches the taper it deforms and is permanently

lodged in the front of the tube. The harpoon is now free to enter the surface of the asteroid.

Simultaneous to the release of the harpoon, valves at the rear of the launch tube open which

allow the gas trapped in the tube to exit trough two nozzles in the rear. The release of gas is used

to help counteract the motion of the anchor in reaction to the release of the harpoon 6.

The harpoon is attached to the anchor via a cable. This harpoon cable is spooled around its own

harpoon cable reel. The reel is attached to the bottom of the stationary anchor base and is

positioned at the center of the anchor. As the harpoon travels toward the asteroid, the cable reel

runs in reverse to minimize the drag of the cable coming off the real. Once the harpoon reaches

the asteroid, it loses all of its kinetic energy penetrating into the surface. An accelerometer in the

harpoon senses that the harpoon is no longer moving, and the reel begins to wind the cable back

in7. The reel would continue winding until all of the feet make contact with the surface

8. The legs

of the anchor would have a suspension system built in to absorb the acceleration of the anchor

caused by the reeling.

Cable Reel

The reel has been designed much like a level wind fishing reel. The main feature of which is a

guide on the front of the reel helps wind its cable evenly back onto the spool of the reel. The

guide moves back and forth parallel to the axis of the spool, and its motion is controlled through

gearing by the spin of the spool so that it evenly distributes/unwinds cable along the length of the

spool. This feature ensures that the cable does not snag while being released or while being

rewound. The most important way in which these reels differ from level wind fishing reels is

that they are powered. The reels have the ability to turn themselves in the unwind direction,

which does not actually feed cable off of the spool, but it does ensure that the momentum of the

harpoon is not transferred to the anchor, which would accelerate it toward the surface.

6If the anchor is the variety without propulsion then the rest of the reaction force is counteracted by the main

spacecraft RCS. If the anchor has its own propulsion then the anchor RCS counteracts the remainder of the reaction

force.7

If the anchor is the variety without propulsion then the anchor is released from the main spacecraft before it begins

reeling itself in.8

Due to the spin of the asteroid and to local surface variability, the three feet may not reach the surface

simultaneously.


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Additionally the motor in the reel enables the reel to wind the cable back in, which is its primary

function. The reel also has a break which is used to make sure the spools do not s