A prototype simulation is being developed under contract to the Royal Aerospace Establishment (RAE), Farnborough, England, to assist in the discrimination of artificial space objects/debris. The methodology undertaken has been to link Object Oriented programming, intelligent knowledge based system (IKBS) techniques and advanced computer technology with numeric analysis to provide a graphical, symbolic simulation. The objective is to provide an additional layer of understanding on top of conventional classification methods. Use is being made of object and rule based knowledge representation, multiple reasoning, truth maintenance and uncertainty. Software tools being used include Knowledge Engineering Environment (KEE) and SymTactics for knowledge representation. Hooks are being developed within the SymTactics framework to incorporate mathematical models describing orbital motion and fragmentation. Penetration and structural analysis can also be incorporated. SymTactics is an Object Oriented discrete event simulation tool built as a domain specific extension to the KEE environment. The tool provides facilities for building, debugging and monitoring dynamic (military) simulations

Adamson, J. M.

Marshall, G.

English

NASA Technical Reports Server

OBJECT ORIENTED STUDIES INTO ARTIFICIAL SPACE DEBRIS 
J . M. Adamson, 
Consultant, 1 Brockhurst Cottages, Over Wallop, Stockbridge, 
Hampshire, England 
G. Marshall, 
Consultant, 31 Broomhill Way, Allbrook, Eastleigh, 
Hampshi re , Eng 1 and 
ABSTRACT 
A prototype simulation i s  being developed under contract to 
the Royal Aerospace Establishment (RAE), Farnborough, England, 
to assist in the discrimination of artificial space 
objects/debris. 
The methodology undertaken has been to link Object Oriented 
programming,intelligent knowledge based system (IKBS) techniques 
and advanced computer technology with numeric analysis to 
provide a graphical, symbolic simulation. The objective is to 
provide an additional layer of understanding on top of 
conventional classification methods. 
Use is being made of object and rule based knowledge 
representation, multiple reasoning, truth maintenance and 
uncertainty. Software tools being used include Knowledge 
Engineering Environment (KEE) and SymTactics for knowledge 
representation. Hooks are being developed within the SymTactics 
framework to incorporate mathematical models describing orbital 
motion and fragmentation. Penetration and structural analysis 
can also be incorporated. 
SymTactics [15] is an Object Oriented discrete event 
simulation tool built as a domain specific extension to the KEE 
environment. The tool provides facilities f o r  building, 
debugging and monitoring dynamic (military) simulations. 
I NTRODUCTI ON 
There are currently some 10.000 registered objects orbiting 
the Earth. I t  is estimated that another 40,000 golf-sized 
objects are not tracked along with billions of sttll smaller 
pieces. These objects consist of satellites, extinct rocket 
casings and debris. 
Space debris can be effectively categorised under the 
headings of : particles, fragments and artifacts [lo]. 
Many tiny particles are produced by solid rocket motors 
used in space. Larger particles can be attributed to the 
intense thermal cycling of the space environment (for example 
paint flaking o f f  satellites). Particles are likewise produced 
by explosions, both accidental and deliberate. 
163 
https://ntrs.nasa.gov/search.jsp?R=19890006201 2020-03-20T04:58:40+00:00Z
A principle source of fragments and particles is the 
destruction of spent rocket stages and satellites. Explosions 
of rocket stages can occur many months o r  years after launch. 
Residual propellants may be the cause here. Spacecraft on the 
other hand tend to be destroyed deliberately. This may result 
from testing anti-satellite weapons, o r  spacecraft being 
commanded to self-destruct for various reasons. 
Numerous parts of spacecraft are jettisoned during launch 
and operation; these come under the fragment category. 
Fragments of this type include interstage structures, payload 
shrouds and support structures. Fragments are likewise produced 
during rocket stage separations. 
The term artifacts is applied to derelict items of space 
hardware such as intact payload support structures, spent 
upperstages and spacecraft. Spacecraft can become derelict 
(non-operational) following the malfunction of a launch 
vehicle/upper stage, insertion into an incorrect orbit o r  
following a system malfunction. 
A major concern exists that space debris may cause 
collisions, particularly in orbits ranging from 500km t o  llOOkm 
above the Earth. These collisions can produce many small 
fragments which in turn increase the probability of further 
co 11 is ions. 
Space scientists and mission planners are becoming 
increasingly concerned about the possibility of space objects 
(namely spacecraft, rockets, spaceplanes and space stations) 
being damaged by artificial space debris. Similar problems 
exist, though not to the same extent, when placing 
telecommunication satellites into geostationary orbit. 
Should an operational space object become non-operational, 
i t  is in the interest of space scientists and mission planners 
to determine the cause of this event as soon as possible. I f  
space debris is suspected, the resultant signature data and 
detailed simulations could identify the nature and cause of this 
event. 
SYSTEM CONFIGURATION 
The proposed system configuration for this simulation (see 
Fig. 1) makes use of a library of space objects, intelligent 
sensors and a scenario generator. When a sensor detects an 
object, i t  interrogates the library of space objects to 
determine i f  that object has been identified/categorized. I f  
not, the sensor reasons as to the possible cause and origin of  
the unidentified object. 
The first phase of this prototype simulation is currently 
being modelled on a Lisp machine. The computer used was a 
Symbolics 3640. The simulation is now in the process of being 
164 
Figure 1: Systcrn Configuration 
_ . _ _ - - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ I _ _ _ _ _ _ _ - - -  
, , 
t 
8 
library of space objects in 
objects specified 
volume of Scenario 
generator space I 
L L  I Updates 
T data bank 
I I 
I 
1 -  
I 
Multiple Processor , , - - -__-._-_-_-_.-_-_-------- - - - - - - . - - - - - - - - -  
transferred to a microVAX workstation in order to test the 
concept of the "High Performance Server" [1,2]. The idea of 
this concept is to enhance the host computer with add-on 
extensions. These extensions would be modular, based on a 
combination of any relevant architecture, and would produce a 
fast, high availability processor system for symbolic, numeric, 
graphic and conventional processing. 
CURRENT WORK 
The current phase of work includes orbital dynamics, 
fragmentation, object representation and sensor reasoning. 
ORBITAL DYNAMICS 
Calculations are being undertaken to assess the outcome of 
imparted impulses ( A V ' s )  to objects, fragments following 
explosions, collisions and hypervelocity impacts. 
Modelling discrepancies were minimised by transfering from 
orbital elements (simple Keplerian motion) to Cartesian co- 
ordinates [ 3 , 5 , 6 ]  at the time a simulated event had been 
scheduled t o  take place. Resultant new velocities following 
fragementation were used to calculate individual debris/particle 
orbital elements. 
An assumption was made that an operational space object 
described an unperturbed circular orbit. Following 
fragmentation, i t  was observed that the majority of resultant 
particles described elliptic motion: some achieved escape 
velocity. Simulations showed that the small particles ( < O.lgm) 
can have drastically different orbits from the initial parent 
body orbit. They had either very noticeable differences in 
orbital inclination and longitude of ascending node o r  large 
variations in semi-major axis and eccentricity. Resultant 
debris envelopes should correspond to those described by F u s s  
L4.71. 
FRAGMENTATION 
The number of fragments generated and fragment velocities 
following hypervelocity impacts and explosions were based on 
work by McKnight [ 11 3 ,  Su 6 Kessler [14], and studies undertaken 
in the 1960's. The following assumptions were made in 
simulating fragmentation: 
(i )  Mass distribution of  debris resulting from hypervelocity 
impact is a function of impact velocity and mass of 
projectile [9,11 ,141 
The ejecta mass is represented by 
where Me = ejecta mass in grams 
Me = V' M, 
v = impact velocity, km/sec 
M, = mass of projectile in grams 
( i i )  The ejecta mass distribution takes the form [9,111 
N = 0.447 (m/M,)**(-0.7496) 
m = fragment mass 
Me = total ejecta mass 
where N = number of fragments with mass m o r  greater 
( i i i )  The general equations for debris resulting from the 
1.71E-4 M, exp (-0.02056 m * * 0 . 5 )  : m > 1936gm 
8.69E-4 M, exp (-0.05756 m * * 0 . 5 )  ; m 41936gm 
explosion of a satellite are [11,141 
N = {  
where N = cumulative number of fragments with mass 
greater than m grams 
M, = mass of  target object in grams 
Iiv) The smallest detectable ejecta mass is of order O.lgm 
(VI The ejecta velocity for the smallest detectable fragment 
is 1.3 times impact velocity [9]. Velocities of larger 
ejecta particles were computed on the assumption that 
the kinetic energy was the same for all ejecta 
particles. 
(vi 1 The maximum A V ' s  imparted to fragments following an 
explosion are of order 2km/sec. Larger fragment 
velocities were calculated on the assumption that the 
kinetic energy was the same for all particles. 
166 
Initial results suggest that particle distributions could 
be detected showing the equivalent of a double shock wave 
following a hypervelocity impact and resultant explosion. 
Particle and velocity distributions may likewise indicate the 
orbital plane and velocity of the impacting body. 
OBJECT R EPRESENTAT I ON 
An object oriented approach was used to describe space 
object characteristics and relationships in terms of its 
attributes: namely instance variables and methods. Instance 
variables describe the simple variables or object relations that 
an object may possess; methods are operations that the object 
may perform. 
A class of generic objects, termed operational component 
parts, was used to represent all functional/operational space 
objects such as satellites, rockets, spaceplanes and space 
stations. A sub-class of operational component parts was used 
to describe a typical space object. Terminology such as 
I' f o r wa r d - o f 'I , I' above 'I , 
and "enclosed-by" was then introduced to represent spacially the 
component parts of the space object. In addition, each 
operational component part had assigned attribute fields 
describing mass, shape, material and structural properties. This 
is akin to the approach taken by such products as ICAD [13]. 
be h i nd 'I , bo t t om 'I , 1 e f t - o f 'I  , I' r i g h t - o f 
As an example consider the objects fuel tank, oxidiser 
tank, helium tank and rocket engines. These are a generic sub- 
group of operational component parts which can be used to 
represent the Space Shuttle's orbital maneouvering system [12]. 
These component parts could likewise represent the orbital 
control system for a space station o r  satellite. 
Geometric reasoning was introduced to generate more 
accurate fragmentation models following impacts and explosions. 
The model assumed that debris was equally reflected and 
transmitted when the outer skin of a space object had been 
penetrated by an impacting body. It was further assumed that 
the transmitted debris damaged an operational component part 
causing the part to move into the class of non-operational 
objects. The model used rules to assess whether the damaged 
part was critical o r  non-critical: the outcome of this 
assessment resulted in debris being generated according to 
equations describing explosions or impacts respectively. 
Additional fracture and penetration models could be incorporated 
at this stage for more detailed simulations. 
When a component part became non-operat-ional following an 
impact or explosion, messages were passed to surrounding 
components to evaluate the outcome of this event. The outcome 
could be further explosions, shattering, fragmentation or 
fracture, resulting in adjacent components likewise moving into 
the class of non-operational objects. 
167 
SENSOR REASONING 
This was modelled as an intelligent system having inputs 
from various types of sensors and access to a library of 
currently known space objects. Consider the following scenario: 
A possible cause f o r  the loss of communication with a space 
object could be due to a collision, explosion o r  hypervelocity 
impact: this type of event being termed as a "non-scheduled 
event". The affected object o r  component parts of the affected 
object would transfer.from the class of operational objects to 
non-operational objects. The sensor, having detected non- 
catalogued debris, would use fragmentation-cause-rules to 
determine the nature and origin of the detected debris, using as 
a key the time and known position of the suspected space object. 
Fragmentation-cause-rules were classed under normal 
satellite-and-launch-operations, satellite-deterioration and 
satellite-fragmentation [ 8 ] .  These were further categorized 
(see Fig 2 )  under collisions, loose-hardware, manned-flight, 
rocket-break-up, satellite-out-of-commission, scheduled-events, 
atomic-oxygen, solar-radiation, collision-induced, deliberate- 
action, malfunction and unknown. 
Fig. 2 
1 
. 
When non-catalogued debris was detected, the sensor 
reasoned as to the possible cause and origin. In undertaking 
this task, the sensor had access to an event library of non- 
scheduled and scheduled events. A non-scheduled event, as 
described earlier, could be a sudden loss in communication with 
an operational satellite. A scheduled event could be a 
168 
hard-eject fragment following payload deployment into orbit. In 
this case, the following fragmentation-cause-rule was used to 
test this hypo t hes is : 
HARD-E JECT -FR?iGMEN T 
!(IF (THE EVENT-TALLY OF ?DEBRIS IS ?EVENT) 
(?EV”LNT IS IN CLASS HARD-EJECT) 
(THE ACQUIRED-OBJECTS OF ?OBSERVATORY IS ?SIGHTING) 
(?DEBRIS = (LISP (FIRST ?SIGHTING) ) ) 
(THE CATALOGUE OF ?OBSERVATORY IS ?CATALOGUE) 
(?FRAGMENT-CAUSE = (FOURTH ?CATALOGUE) ) 
(LISP (= ?EVENT ?FRAGMENT-CAUSE)) 
(?RCS = (LISP (SECOND ?SIGHTING) ) ) 
(?RANGE = (LISP (THIRD ?SIGHTING))) 
( L I S P  
(< ?RCS 
(OBJECT-MESSAGE ‘NORMAL-ATMOSPHERE 
’ !ATTENUATE-SIGNATURE 
’ ! RADAR-SIGNATURE 
1.Oe-4 
?RANGE 
’KM) 1 )  
THEN 
(THE DEBRIS-ORIGIN OF ?DEBRIS IS ?EVENT) 
(THE IDENTITY OF ?DEBRIS IS SMALL-FRAGMENT))) 
I f  several fragmentation-cause-rules offered plausible 
solutions, -each one could be tested concurrently. A possible 
confirmation of a rival hypothesis could be obtained by 
calculating orbital parameters at the time of the scheduled 
event and comparing the data with available fragmentation 
models . 
CONCLUSIONS 
The outcome of this study to date has shown that deep 
knowledge based simulations requiring symbolic, numeric and 
visualisation techniques could be linked together and applied to 
the artificial-debris problem. The use of KEE and SymTactics 
enabled rapid prototyping and provided an interactive and rapid 
scenario generation facility. 
I t  is hoped that the outcome of this study will provide a 
better understanding of: 
i )  Object representation, damage assessment 
ii )  Intelligent sensor representation 
i i i )  Object classification, discrimination and hazard/lethality. 
iv) Generation of various space debris scenarios to provide a 
better understanding of the cause and break-up of 
operational space objects. 
-3 169 
ACKNOWLEDGEMENTS 
This work has been carried out with the support of the 
Procurement Executive, Ministry of Defence. 
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Object oriented studies into artificial space debris

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