Probability of depositing viable organisms on Mars during 1964 Mariner missio

Gordon, H. J.

Haynes, N. R.

English

NASA Technical Reports Server

Technicaf Memorandum No. 33-194 
.. 
u” A Study of the Probabifify of Depositing Viable 
Organisms on Mars During the Mariner 1964 Mission 
Norman R. Haynes 
Harofd J. Gordon 
I . .  
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JET P R O P U L S i O N  L A B O R A T O R Y  
C A L l F O R N r A  I N S T l T U T E  O F  TECHNOLOGY 
PAS A DE N A, CALI F o R N I A 
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Technical Memorandum No. 33- 794 
A Study of the Probability of Depositing WaMe Organisms 
on Mars During the Mariner 7964 Mission 
Norman R. Haynes 
Haro/d J. Gordon 
53 L l z L J k L  
T. W. Hamilton, Chief 
Systems Analysis Section 
J E T  P R O P U L S I O N  L A B O R A T O R Y  
C A L I F O R N I A  I N S T I T U T E  O F  TECHNOLOGY 
P A  S A D  E N A ,  C A L I F  0 R N I A 
October 23, 1964 
Copyright 0 1964 
Jet Propulsion Laboratory 
California Institute of Technology 
Prepared Under Contract No. NAS 7-100 
National Aeronautics & Space Administration 
- 
JPL TECHNICAL MEMORANDUM NO. 33-194 
i 
CONTENTS 
II. Probability of Accidental Impact . . . . . . . . . . . . .  2 
111. Probability of Planet Contamination by Expelled 
Particles from Mariner Spacecraft . . . . . . . . . . . . .  4 
Reference . .  . . . . . . . . . . . . . . . . . . . . . .  5 
A p p e n d i x . .  . . . . . . . . . . . . . . . . . . . . . .  6 
FIGURES 
1. Probability of planetary impact model . . . . . . . . . . . .  2 
A-1. Orientation of spacecraft . . . . . . . . . . . . . . . . .  6 
A-2. Geometry where impact is possible . . . . . . . . . . . . .  7 
JPL TECHNICAL MEMORANDUM NO. 33-194 
3 ABSTRACT 
A spacecraft aimed for the close vicinity of the planet Mars cruises 
through space leaving a trail of particles due to outgassing, firing of 
the hot motor used for midcourse maneuvers, and activation of the cold 
gas attitude control system. The ejected particles follow altered trajec- 
tories from their point of separation. It is shown that only the spacecraft 
itself and the attitude control system exhaust contribute significantly 
to the probability of planet contamination. The Mariner 1964 mission 
has been designed so that this probability is less than one part in ten 
thousand. This study considers the actual mission design and assesses 
the modifications which would be required if the desired probability 
were to be reduced to three parts in one hundred thousand. 
pcckl;of 
1. INTRODUCTION 
To protect the planet Mars from possible contamina- 
tion by viable organisms from the Earth, the National 
Aeronautics and Space Administration (NASA) has set 
the policy that any unsterilized spacecraft launched to 
Mars must have less than a lo-* probability of accidental 
impact with the planet. In addition, in recent weeks the 
Committee on Space Research of the International Council 
of Scientific Unions (COSPAR) accepted, as tentatively 
recommended interim objectives, that the probability 
limit for accidental impact by an unsterilized fly-by or 
orbiting spacecraft be reduced to 3 X 
This memorandum outlines in some detail the efforts 
and analyses which have been carried out by the Jet 
Propulsion Laboratory (JPL) to assure that the probabil- 
ity of accidental planet impact by the Mariner Mars 1964 
spacecraft is less than 1 X lo-'. In addition, the effect on 
this mission of adopting the more stringent 3 X le5 
recommendations of COSPAR is assessed. 
An estimate has also been made of the probability of 
viable organisms ejected from the spacecraft entering the 
Martian atmosphere even though the spacecraft does not 
impact the planet. These organisms are ejected from the 
spacecraft by the attitude control gas jets, the midcourse 
motor, and through the mechanism of outgassing by the 
spacecraft itself. 
1 
JPL TECHNICAL MEMORANDUM NO. 33-194 
p2 = 7.7 x 10-10 
IMPACT? 
YES 
II. PROBABILITY OF ACCIDENTAL IMPACT I, 
NO 
Before the probability of the spacecraft’s accidentally 
impacting Mars is investigated a brief description of the 
mission profile is offered. The launch vehicle is aimed to 
send the spacecraft to the nominal aiming point. How- 
ever, expected dispersions in the injection conditions will 
result in a high probability of the necessity for a trajectory 
correction maneuver. The maneuver will attempt to place 
the spacecraft on a trajectory passing through the nom- 
inal aiming point. If the initial maneuver fails to correct 
the trajectory and achieve an acceptable planetary en- 
counter a second maneuver is available. It is obvious that 
following injection and one or both maneuvers, there is 
IS M/C 
REOUIRED? ps = 3.7 x 10-7 
1. The spacecraft is on an impact trajectory at injection 
and cannot be maneuvered because of a malfunc- 
tion. 
IS M/C 
REOUlR E D? 
2. The spacecraft is on an impact trajectory at injec- 
tion. The first maneuver is unsuccessful and a 
second cannot be performed. 
3. The spacecraft is on an impact trajectory at injec- 
tion. The first maneuver is successful but a second 
maneuver, performed to improve the trajectory, 
results in impact. 
YES NO YES 
(0.98) ps = 0.9 x 10-6 ( 1.0) (0.0) 
a finite possibility of having attained an impact trajectory 
by accident. 
4. The spacecraft is on an impact trajectory at injec- 
tion, Each of the two maneuvers result in impact. 
Figure 1 is a probability diagram of seven possible 5. The spacecraft is not on an impact trajectory at 
injection. A maneuver required to improve the ways accidental impact may occur: 
NO 
(0.02) 
p7 = 0.9 x 10-10 I S  M/C 
AVA I L A  E L  E ? 
2 
X IS M/C 
AVAl L A B  L E ?  
Fig. 1. Probability of planetary impact model 
7 
Z Z 6.1 X 
i =  I
YES NO YES NO 
(0.97) (0.03) (0.97) (0.03) 
IMPACT? IMPACT? IMPACT? 
YES NO YES NO YES NO YES 
(0.0000l) (0.99999) (1.0) (0.0) I (0.00001) (0.99999) (0.0) NO (1.0) 
IS 2ND M/C X IS 2NDM/C X IS 2NDM/C I S  2ND M/C 
YES 
(1.0) 
NO YES NO YES NO YES NO 
(0.0) (0.10) (0.90) (1.0) (0.0) (0.10) (0.90) 
IS 2ND M/C 
AVAILABLE? 
IS  2NDM/C I S  2NOM/C I S  2NDM/C 
AVAILABLE ? AVA I L A  E L  E ?  AVA I L A  B L E  ? 
YES 
(0.96) 
NO YES NO YES NO YES NO 
(0.04) (0.96) (0.04) (0.96) (0.04) (0.96) (0.04) 
X L IMPACT? I p2 X X X IMPACT? IMPACT? p5 IMPACT? 
YES 
(0.00001) 
NO YES NO YES NO YES NO 
(0.99999) (0.00001) (0.99999) (0.00001) (0.99999) (0.00001) (0.99999) 
JPL TECHNICAL MEMORANDUM NO. 33-194 
I 
I 
I 
I 
i 
t 
I 
I 
1 
I 
I 
I 
I 
J trajectory results in an impact trajectory and a second 
maneuver cannot be performed. 
6. The spacecraft is not on an impact trajectory. The 
second of two maneuvers required to improve the 
trajectory results in impact. 
7. The spacecraft is not on an impact trajectory. Each 
of the two required maneuvers results in impact. 
The overall probability of impacting the planet is the 
sum of each of the above seven probabilities. In order to 
compute the individual probabilities the following as- 
sumptions are made. 
1. The probability of the spacecraft impacting the 
planet at injection is 2 X le3. This figure is found 
by mapping the injection dispersions given by the 
covariance matrix of injection errors, to the planet 
and integrating over the impact radius of the planet. 
Confidence in the accuracy of this quantity is very 
high. 
2. The probability of requiring a midcourse maneuver 
even though the spacecraft is not on an impact 
trajectory is 0.93. This figure is found by mapping 
the injection dispersions, given by the covariance 
matrix of injection errors, to the planet and inte- 
grating over the area in which an acceptable 
encounter occurs. Once again, confidence in the 
accuracy of this quantity is very high. 
3. The probability that the spacecraft will be able to 
execute a maneuver is assumed to be 0.97. This 
results from assuming the probability of the space- 
craft being operable at injection is 0.98 and the prob- 
ability of a successful motor burn is 0.99. The 
probability of being able to perform a second 
maneuver is assumed to be 0.96. Since the second 
maneuver will normally be performed within two 
weeks of the first maneuver, the probability of the 
spacecraft surviving this time span is assumed to 
be 0.99. Hence, 0.99 X 0.98 X 0.99 = 0.96. The 
rationale and model used to compute these prob- 
abilities is included in detail in Ref. 1. 
4. The probability of requiring a second maneuver is 
0.90. This is computed by h d i n g  statistically the 
size of the first maneuver as a result of the injection 
dispersions. The dispersions in executing this man- 
euver, given by the covariance matrix of execution 
errors, are then mapped to the planet and integrated 
over the region of acceptable planetary encounter. 
5. The probability of impact following a maneuver is 
1 X le5. This probability is computed by mapping 
the dispersions from the execution of the midcourse 
maneuver, given by the covariance matrix of execu- 
tion errors, to the planet and integrating over the 
capture area of the planet. This quantity is highly 
sensitive to the size of the maneuver and the miss- 
distance. For the statistically expected size of the 
maneuver, the expected execution errors, and the 
nominal aiming point, the impact probability is at 
most 1 X le5. 
By utilizing these numbers in the model shown in 
Fig. 1, the seven individual probabilities can be esti- 
mated. Using the above estimates the total probability 
of planet impact is approximately 6.1 X lC5. 
In judging the effect on the mission of reducing the 
impact probability to 3 X le5, the COSPAR recommen- 
dation, it is evident that the overall probability of impact 
can be no smaller than the largest individual probability. 
The dominant term is the probability of impact at injec- 
tion times the probability of being unable to perform a 
corrective maneuver. Using above estimates of probabil- 
ities this is 6 X The only method of reducing this 
probability is to reduce the probability of impact at 
injection by biasing the injection trajectories so that the 
probability of impact at injection is reduced to 1 X lV3. 
However, this implies changing the firing tables for the 
Mariner mission, an alternative clearly impossible at  this 
late date in the mission. Additionally, biasing the trajec- 
tories implies a larger midcourse fuel requirement, which 
in genera1 is an undesirable mission constraint. Hence, it 
is impossible for the Mariner Mars 1964 mission to 
satisfy the 3 X le5 probability of accidental impact 
criteria while easily satisfying the 1 X lV4 required by 
NASA. 
3 
JPL TECHNICAL MEMORANDUM NO. 33-194 
111. PROBABILITY OF PLANET CONTAMINATION BY EXPELLED v 
PARTICLES FROM MARINER SPACECRAFT 
In addition to the possibility of contaminating Mars 
by accidental impact the planet might also be contam- 
inated by viable particles expelled from the spacecraft. 
These particles come from the gas expelled by the 
attitude control jets, the gases expelled by the midcourse 
motor, and the outgassing of the spacecraft. 
It is estimated that the probability of viable organisms 
being expelled from the midcourse rocket motor is 
essentially negligible because of the extremely high tem- 
perature environment the particles must survive. Even 
if a viable organism survived, it would then have to 
travel at least 200 days through space to encounter Mars, 
since the maneuver occurs within the first few days of 
the mission. During this time, the particles would be 
exposed to continuous ultraviolet radiation with a high 
probability of destruction. In addition, as for any out- 
gassed particles, the particle trajectory will be radically 
different than the spacecraft trajectory. Hence, the prob- 
ability is negligible that particles from the midcourse 
motor would contaminate the planet. 
Particles outgassed from the spacecraft have a negli- 
gibIe probability of contaminating the planet for two rea- 
sons. First, since the temperature of the spacecraft will 
be highest during the early phase of flight, it is expected 
that what little outgassing does occur, will take place 
during this time and hence the particles will be subjected 
to months of ultraviolet radiation. Also, the ratio of area 
to mass of the particles outgassed will be from 1,000 to 
1,OOO,ooO times that of the spacecraft and hence the effect 
on the trajectory of the particles clue to solar pressure will 
be proportionally larger than on that of the spacecraft. 
The spacecraft nominally is blown about 20,000 km off 
course by solar pressure. Hence, even if the particles 
survive the ultraviolet environment they will undoubt- 
edly be blown considerably off course by solar pressure. 
The question of the probability of viable organisms in 
the attitude control gas impacting the planet is somewhat 
more complex since the gas jets may operate in the close 
vicinity of the planet. In this event, a small amount of 
the gas might impact the ntmosphere of Mars if there 
were no further pcrturbations. However, four things 
should be pointed out: 
1. The control gases leave the spacecraft with a rela- 
tive velocity of 0.7 km/sec which implies they 
follow a considerably different flight path than the 
spacecraft. 
2. The flight time to the planet will be measured in 
days or hundreds of days except for those particles 
expelled near the planet, and hence it is reasonable 
to expect the interplanetary environment to destroy 
any viable organisms except those emitted very 
near the planet. 
3. With the solar sail control system functioning the 
number of attitude control jet actuations is expected 
to be reduced almost to zero, except during man- 
euvers; and hence, the amount of gas expelled 
should be very small. 
4. The effect of solar pressure on the emitted particles 
will be a thousand to a million times greater than on 
the spacecraft. Particles emitted anywhere but very 
near the planet will be “blown” considerably away 
from the planet. Hence, it is considered highly un- 
likely that any viable organisms emitted prior to a 
few days from encounter will enter the Martian 
atmosphere. 
A further study was made to estimate the probability 
that particles ejected near the planet will impact it. The 
mathematical model for these computations is shown in 
the Appendix in detail. I t  was assumed that the viable 
particles were expelled at constant velocity uniformly 
distributed in an exhaust cone about the jet nozzle axes. 
It was further assumed that only 1 out of every thousand 
organisms in the control system was expelled during 
this period because; (1) the control jets will, in all like- 
lihood, not be operating due to the solar sails, and (2) 
some were already expelled during the flight. Assuming 
a 15,000-km miss-distance the number of particles, n, to 
enter the planet atmosphere was computed to be 
n = 0.26 X N 
where N = assumcd number of viable organisms in the 
system. 
The present filtering system in the attitude control 
system keeps the system extremely clean but not sterile. 
Hence, some viable organisms will exist. To estimate the 
number closely is nearly impossible, but rough estimates 
lie between 1,000 and 1,000,000. If there were on the 
4 
+ JPL TECHNICAL MEMORANDUM NO. 33-194 
order of 1,ooO organisms then the probability of one of 
them impacting the planet is only about le5. If, on the 
other hand, there were roughly l,OOO,OOO then the prob- 
ability of one of them impacting the planet is about le3. 
In summary it can be stated with some assurance that 
outgassing and particle ejection from the midcourse 
I 
motor have a negligible probability of impacting and 
contaminating Mars. However, the probability that viable 
organisms expelled from the attitude control system will 
encounter the planet is estimated to be between l W 3  
and le5 depending upon the number of organisms pres- 
ent in the control system and the number of actuations 
of the gas jets. 
REFERENCE 
1. Billy, John M., Probability of Success Model, Jet Propulsion Laboratory, Pasadena, 
to be published. 
5 
JPL TECHNICAL MEMORANDUM NO. 33-194 
Since the attitude control gas is unsterilized, though 
filtered, some consideration must be given to the possi- 
bility of contaminating the planet Mars with viable or- 
ganisms expelled through the gas jets. Some preliminary 
study shows that the simple model described below is 
fairly accurate and illustrates the dependence on the 
parameters of interest. 
The model is based on the following assumptions: 
1. N viable organisms at launch, expelled continuously 
during flight, such that a fraction, a of the total is ex- 
pelled by encounter time, 0 < a < 1. 
2. Organisms (not gas molecules) are expelled at con- 
stant velocity Vo, uniformly distributed in cone of half 
angle p about nozzle axis. 
3. Capture cross section of Mars may be represented 
by a square that inscribes the actual circular capture 
cross section. 
4. The trajectory may be approximated by a straight 
line (major contribution occurs near planet, but before 
encounter, when on asymptote of hyperbola). Hence, the 
time to go, T ,  is used as the partial derivative of miss with 
respect to velocity, aM/aV. 
5. Gas jets point in direction of roll axis (pitch and yaw 
jets) and in direction of yaw axis (roll jets). The space- 
craft attitude is assumed constant with the actual near 
planet orientation. 
# APPENDIX 
UNIT VECTOR IN H 
DIRECTION OF SUN 
/ 
V 
i U N I T  VECTOR IN DIRECTION OF YAW AXIS  
Figure A- l  shows the orientation of planet and space- 
craft, where H and J the unit vectors along negative roll 
axis (towards Sun) and yaw axis are shown relative to the 
velocity vector V, and miss-distance from the center of 
the planet B. Some thought shows that only the particles 
expelled from the jets pointing in the -H or +J  direc- 
tions can impact the planet. Figure A-2 shows the geom- 
etry at which impact can occur, where $ is the angle 
between jet axis and velocity vector. 
The number dn of particles that impact which are ex- 
pelled in time dt is: 
where T = time from launch to encounter. Therefore, as 
there are 12 jets, of which 4 point in the -H direction 
and 2 point in the J direction: 
where the times may be obtained from the geometry 
shown in Fig. A-2: 
B 
Fig. A-1. Orientation of spacecraft (2) 
I JPL TECHNICAL MEMORANDUM NO. 33-194 
1 IMPACT CAN OCCUR FOR JETS 
POINTED IN -H DIRECTION 
IMPACT CAN OCCUR FOR JETS 
POINTED IN J DIRECTION 
LAST TIME, T FIRST TIME, T LAST TIME, T 
f ; H  0. J 6 J  
Fig. A-2. Geometry where impact is  possible 
With the following typical values: 
To, 
= 20 x 106 S e c  
V,, = 0.7km/sec 
V = 5km/sec 
R, = 5,300km 
$" = 20" 
qlJ = 110" 
/3 = 30" 
Equation (2) becomes 
Na n = 0.039 -B 
Assuming that a provides for the fact that if the solar 
vanes work properly very little gas will be needed and 
only a small portion of the viable organisms will survive 
in the space environment (namely those in large clusters 
shielding the inner organisms from the ultraviolet radia- 
tion), then a = 0.001 would be reasonable. A typical 
value of B would be 15,000 km so that 
n = 0.26 XlO-*N 
7 


A study of the probability of depositing viable organisms on Mars during the Mariner 1964 mission

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