Student Air and Space Corporation

Astronautics - SESA2001

Jupiter Atmosphere and MagnetosphereExploration Satellite - JAMES

Author:Alex GodfreyAhmed MaghrabyNicole MelzackReetam Singh

December 15, 2011

Contents

1 Executive Summary 2

2 Jovian Space Environment 32.1 Magnetosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 The Io Torus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Io Dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.4 Radiation Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Scientific Goals 4

4 Calculations 64.1 Orbit Insertion Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 Instrumentation 85.1 Camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5.1.1 Key Design Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.2 Magnetometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.3 Energetic Particle Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.3.1 Key Design Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.4 Plasma Wave Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.4.1 Key Design Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.5 Dust Detector System DDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.5.1 Key Design Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Subsystems 116.1 Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6.1.1 Key Design Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.2 Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.3 Thermal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.4 Propulsion and Attitude Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7 Budgets 157.1 Mass Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157.2 Power Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157.3 Telemetry Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

8 Conclusion 16

1

1 Executive Summary

As the most massive planet in our solar system, Jupiter sits a mean distance of 778.41 million km[1]. Jupiter and its 62 moons are still relatively unexplored giving rise to huge fascination with thered giant. Due to its scientific richness, many di!erent missions are being proposed and designed atthe moment. These look at huge variety of the currently extremely broad science goals concerningJupiter.

The joint ESA and NASA mission, the Jupiter Icy Moons Explorer, has been designed to fulfillthe first ESA Cosmic Vision L-class mission slot and aims to launch in 2020. [2] [3] Made up oftwo spacecraft, the Jupiter Ganymede Orbiter (JGO) and the Jupiter Europa Orbiter (JEO), it isparticularly investigating the complex couplings with the Jovian system, irregular satellites and ringsas well as a more in depth study of Ganymede, Europa, Callisto and Io and how they fit into theJovian system. [4]

Juno shown in figure 1 , which was launched on the 5th August 2011 and due to arrive at Jupiter in2016, was designed as the second spacecraft under NASAs New Frontiers Program. Specifically Junois will determine the amount of water in Jupiters atmosphere, look deep into Jupiters atmosphere,map Jupiters magnetic and gravity fields, revealing the planets deep structure and also to also toexplore and study Jupiters magnetosphere near its poles. [5]

Figure 1: The Jupiter Near-polar Orbiter

Although there are clearly many di!erent science goals, which will be discussed later, our mainfocuses are;

• To investigate atmospheric conditions on Jupiter’s surface and contribute to producing a globalclimate database

• Investigate the possibility of habitable environments within the Jovian system

• Map and investigate the magnetosphere of Jupiter and study Jupiter’s Auroras

The instruments we will be using to achieve these goals, as well as a brief description is given intable 1.

2

Payload Item Function

Camera To map satellites at roughly 1 km resolution and monitorthe atmospheric circulation on Jupiter’s surface, over aperiod of 20 months while in orbit around the planet

Magnetometer To monitor magnetic field strength, and its changesEnergetic Particle Detector To measure high energy electrons, protons and heavy

ions in and around the Jovian magnetospherePlasma Wave Detector To detect electromagnetic waves, and analyse wave-

particle interactionsDust Detector To measure particle mass, velocity and charge

Table 1: Summary of JAMES’ payload items and their functions

2 Jovian Space Environment

Jupiter’s system contains a huge collection of objects which include, Jupiter itself, four large GalilleanSatellites, Io, Europa, Ganymede and Callisto (1,000 km class objects, four inner satellites Metis,Adrastea, Amalthea and Thebe (10-100 km class objects), more than 60 outer irregular small satel-lites (1-100 km class objects) and lastly, the Jovian ring system. All of these objects experiencegravitational and electrodynamic interactions of variable geometries and strengths with Jupiter’smagnetospheric particles and fields. [1]

2.1 Magnetosphere

All of these interactions make the Jovian system extremely complicated. They drive the mag-netospheric engine which generates Jupiter’s radiation belts and cause a large diversity of binaryinteractions that are shown in figure 2. When considering an orbit for a mission to the Jovian sys-tem the complex interactions must be carefully considered to ensure the correct orbit is chosen. [6]The magnetic field of Jupiter, dwarfing Earth’s at almost 20,000 times as powerful, traps swarms ofcharged particles in it’s magnetosphere. Jupiter’s magnetosphere is the largest structure in the solarsystem and is generated deep within the planet. This ensures that Jupiter’s rings and moons areembedded in an intense radiation belt of electrons and ions, which are trapped in the magnetic field.The Jovian magnetosphere shown in figure 3 balloons to 1-3 million kilometers towards the Sun andtapers into a windsock shaped tail extending more than 1 billion kilometers behind Jupiter as faras Saturn’s orbit. Some instruments can be sensitive to the magnetosphere and so it’s extremelyimportant to both choose the correct instruments and also to protect them.

2.2 The Io Torus

Io, the innermost large moon of Jupiter, is the most volcanically active body in our solar system,which is powered by the 100m tidal flexing due to Io’s orbital resonance. [9] Io’s presence has hada considerable impact on Jupiter’s environment, in particular the Io plasma torus, which radiatesa total EUV power of 1.7 x 1012W with variations of 25% [7]. This is a donut-shaped volume ofionized gas concentrated near Io’s orbit and illustrated in figure 4.

2.3 Io Dust

Io’s volcanoes are the hottest in our solar system and eject plumes of gas and dust as much as 400kmhigh. At the apex of the plumes, some ash and dust is released into space and accelerates away

3

Figure 2: Illustrative summary of the Binary interactions and other mechanism present in the Joviansystem

from Io. These high speed particles can be extremely dangerous to any spacecraft passing near it.The Ulysses spacecraft was hit by a dust ejection 100 million kilometers. Galileo also encountered asimilar dust emission from Io. Great care must be taken when planning missions as Io Dust couldeasily cause a catastrophic failure. [8]

2.4 Radiation Environment

The radiation environment of the Jupiter system is highly complex and unlike the Earth’s radiationbelts, is dominated by electrons. The radiation dose is not uniform throughout the Jovian system.The dose rate near the orbit of Europa is roughly a factor of 30 higher than near the orbit ofGanymede. The reasoning for this is that Europa is predominated by more energetic electrons andso the shielding levels are di!erent. This radiation will pose a technical challenge due to the flightsystem spending a significant time in the Jovian radiation belts. The consideration of this variableradiation is a huge mission design area when considering both the orbit of JAMES and also it’sshielding. [10]

3 Scientific Goals

Jupiter is still relatively unexplored and as a result is extremely scientifically rich. As a result it isnot within the scope of the scope to list all of the current scientific goals regarding Jupiter and sothe most relevant science goals have been chosen and summarised in table 2. [11][12]

4

Scientific Drive Areas of Study Required Instrument

To further investigate the for-mation and evolution of Jupiterwithin our Solar System

Deep probing of the atmosphere Energetic Particle DetectorDust Detector System

Coupling of the Deep Interiorwith the Observable Atmosphere

Deep interior investigation Magnetometer

Time-Variable Atmospheric Phe-nomena over a range of Temporaland Spatial Scales

Quasi-continuous coverage of at-mospheric evolution working withother missions and ground basedobservatories, to create a globalclimate database.

CameraDust Detector System

Is Jupiter’s moon Europa, whichpossesses an interior ocean, capa-ble of providing a suitable habitatfor life in our solar system?

Induct responses fromthe Europa OceanPlasma and radiation envi-ronment

MagnetometerPlasma Wave Detector

Could life be supported on eitherGanyemede or Callisto?

Investigate into whether water isprescence

Magnetometer

Further Investigation intoJupiter’s Auroras

Measure wave emissions associ-ated with auroral phenomena inthe polar magnetosphere

Measure pitch angle and en-ergy Distributions of electronsand ions over both Jovigraphicpolar regions

Plasma Wave Detector

Energetic Particle De-tector

Does the solar magnetic systemsatisfy the conditions in order tosustain life

Understand the interactions be-tween the heliosphere and theplanet’s magnetosphere and at-mosphere

MagnetometerPlasma Wave Detector

How does plasma behave in spaceand what can we learn about it’srelated fundamental processes?

Explore all aspects of Jupitersplasma system

Plasma Wave Detector

Table 2: Summary of the Scientific Drives concerning the Jovian system and the instruments requiredto investigate them

5

Figure 3: Illustration showing Jupiter’s Magnetosphere and its solar reference

Figure 4: Jupiter and it’s moons with Jupiter’s magnetic field shown in blue and the Io torus isshown in red

4 Calculations

4.1 Orbit Insertion Calculations

Insertion Burn "VAssuming the satellite is already at Perijove at 4RJ :

VTP =

!

µ"2

r1! 1

a

#

VTP =

$%%&1.3" 108'

2

4RJ! 1

(4+2702 RJ)

(

VTP =

!

1.3" 108"

3

438400

#

VTP = 29.826km/s

So "V = 30.56! 29.826 Which gives "V = 0.734km/s

6

Propellant Mass Required

"V = Vex ln"M0

Mb

#

"V = g0Isp ln

'M0

M0 !Mf

(

ln(M0 !Mf ) = ln(M0)!"V

g0Isp

M0 !Mf = M0e! !V

g0Isp

Mf = M0

"1! e

! !Vg0Isp

#

Mf = 2500"1! e!

0.7349.81!10"3!310

#

Mf = 536.1kg

Burn Time Calculation

From Newton’s law of motion:F = ma

Hence

a =F

mWhere the force applied is the thrust generated. If the Thrust is constant, and the mass of thespacecraft is a function of time only, acceleration of the craft is a function of time only. i.e.

a(t) =T

m(t)

If we obtain the definite integral of a(t) over the burn time, the obtained value must be the totalchange in velocity over the burn time, i.e. the "V obtained earlier:

) tt

t0a(t) dt = "V

Where t0 is the time at burn initiation and tt is at burn termination. Hence:

) tt

t0

T

m(t)dt = "V

If we assume a steady-state burn with constant mass-flow rate, then the mass of the spacecraft isgiven by:

m = M0 ! !t

where !is the mass-flow rate. Hence:) tt

t0

T

M0 ! !tdt = "V

7

The solution to this integral is: *!T

!ln(M0 ! !t)

+tt

t0= "V

For t0 = 0, this gives:T

!ln

'M0

M0 !Mf

(

= "V

and

! =T

"Vln

'M0

M0 !Mf

(

Since the mass-flow rate is constant:

! =Mf

ttAnd therefore:

tt =Mf"V

Tln

'M0

M0 !Mf

(!1

tt =536.1" 0.734

400" 10!3ln

"2500

2500! 536.1

#!1

= 4075.1s

tt = 1h7m55s

5 Instrumentation

5.1 Camera

The camera on board will provide visual observation of the Jovian moons and the planet itself. Itsprimary task will be to map the moons at roughly 1km resolution and monitor the atmosphericcirculation on Jupiters surface, over a period of approximately 20 months while in orbit aroundthe planet, as specified by the primary mission objectives.The camera will operate in the range ofnear infrared to ultraviolet region of light, to facilitate intuitive visual analysis, as well as observingJupiters ultraviolet auroras and see through the atmosphere for a reasonable depth.

5.1.1 Key Design Issues

Radiation Environment : For this range, a Cassegrain telescope with solid-state detector performswell. This type of detector is vulnerable to damage due to charged particle radiation, thus adequateprotection must be provided while in the hostile Jovian radiation environment, and from the emissionsof the RTGs on board. The heavy apparatus will unfortunately not be steerable, thus to point toa desirable target, the entire spacecraft must be slewed, for which to accurately maintain desirableattitude, high precision attitude control system is required.

Positioning considerations : Although radiation from the RTGs may damage the equipment,however the persistent hostile Jovian radiation environment raises greater concern to maximize thedevices projected life. Hence, the camera is located within the body of the spacecraft under theradiation shield. The device is cylindrical in shape, with 1m length and 0.2m diameter, and weighsapproximately 28kg in total.

8

5.2 Magnetometer

The basic principle of the magnetometer is to map the magnetic field of Jupiter to a very high degreeof accuracy and observe its variability in order to further our understanding regarding it’s formationand evolution. It is designed to make measurements of magnetic field strength and direction and byusing, inboard and outboard magnetic field measurements, which provide the capability to subtractthe contribution from the spacecraft magnetic field. A magnetometer provides a way to see deepinside jupiter, aiming to shed light on how and where Jupiter’s powerful magnetic field is generated.The magnetometer is roughly 7kg and is a small device at the end of a 10m long boom. [12]Thisplacement ensures that the data generated from the magnetometer sensors will not be ”polluted” bymagnetic signals from the spacecraft system.

5.3 Energetic Particle Detector

The Energetic Particles Detector will provide us with information on the quantity of energetic par-ticles (electrons, protons and heavy ions) in the Jovian atmosphere. The EPD will be able to detectparticles of energies over a certain threshold, determine their composition (for example oxygen orsulphur) as well as the velocity and mass of these particles. The EPD will also be able to notechanges in particle density over time and position in its orbit. Ultimately allowing us to determinehow the particles get their energy and how they are transported through the magnetosphere. [13]

5.3.1 Key Design Issues

Detection interference: The RTGs used to generate power also emit high energy particles. If theseare detected by the EPD and counted then it can compromise the data being collected. We do notwant the RTGs emissions to be detected as part of the experiments and so the threshold set willhave to take this into account and be set accordingly. [14]

Radiation environment : The radiation environment is made up of the high energy particles thatthe EPD is expected to detect. The radiation however may still damage the electrical components ofthe detector and so shielding will have to be considered and designed carefully so as not to compromisethe mission. [13]

Positioning considerations : The EPD should be rotating (on a spinning part of JAMES) as wellas being steerable so that it can collect particles from all angles. Similarly to the DDS, the particlesapproaching the EPD could have their velocities disrupted by other instruments, and so placingthe detector away from the boon will help minimise this e!ect. The impact of the RTGs emissionscould cause interference with the data being collected, and could also degrade the electronics of thedetector. The obvious solution here is to keep the EPD away from the RTGs, in particular awayfrom the favoured direction of heat and radiation emissions. [14]

5.4 Plasma Wave Detector

The plasma wave detector will provide measurements on plasma wave and radio wave emission fromthe magnetosphere of Jupiter. This instrument, along with the magnetometer, may reveal the natureof the planets magnetosphere, which then may lead to further discoveries on the composition andthe nature of inner layers of Jupiter; in particular, whether metallic hydrogen exists in the deeperlayers of the planet. The instrument comprises of an electric dipole antenna 6.6m in length.

9

5.4.1 Key Design Issues

To prevent asymmetries in the electric field due to the spacecraft structures, the detector must beperpendicular to the boom. For the detector to be direction-sensitive, the pointing direction of thesensor must be constantly changed. Since our spacecraft is not spin-stabilized, the sensor itself isrotated about the magnetometer boom.

Positioning considerations: The plasma wave detector is placed at the tip of a 10m long boom.Since the instrument is co-operating with the magnetometer, the two are placed adjacently on thesame boom. The two, and the energetic particle detector, are radiation-sensitive. Thus the threeinstruments are placed within the radiation shadows cast by the RTGs.

5.5 Dust Detector System DDS

The dust detector shown in figure 5 will be used to measure mass, charge and velocities of the dustin the Jovian atmosphere. This will allow for further study of Jupiters dust rings and the interactionthe dust has with the strong magnetosphere around the planet. The detector will initiate a measuringcycle if the charge measured on the target in the hemisphere (negative) or ion detector (positive)exceeds a threshold set by telecommand; this will then begin to measure the information of theparticles detected. The DDS will be 4kg, 0.2m long and 0.35m in diameter.

Figure 5: Principle of Dust Detector Operation, Credit: Max-Planck-Institut fur Kernphysik

5.5.1 Key Design Issues

Detection Interference: The hemisphere (where negative charge builds) has a large area which isexposed to the environment. Interplanetary plasma and high-energy radiation will be detected bythe target and this can interfere with the detection of dust particles. To solve this problem thethreshold limit set can be much higher at the main target than at the small ion collector and at

10

times of high activity the initiation of a measurement cycle can be set to begin only if the thresholdon the ion collector is exceeded.[15]

Radiation environment : As mentioned above radiation can interfere with the detection accuracyof the DDS. The high-radiation environment of the inner Jovian magnetosphere can also degrade theelectronics in the detector itself. In the Galileo mission the sensitivity for dust detection dropped bya factor of 7.5 between 1996 and 2003, noise sensitivity also decreased by a factor of 100. [16] TheDDS we are using will be subject to the same radiation, as well as radiation from the RTG systemin the satellite. The damage caused by both these radiation sources can be significant and mayultimately determine the life of the DDS if su#cient protection is not provided. The DDS shouldbe coated in radiation shielding where possible (obviously the detectors do need to be open to theatmosphere to the atmosphere though). [15]

Positioning Considerations: To collect particles to detect the DDS should be moving through theatmosphere, and so it would be beneficial to place it at a rotating point of the craft, either spinningwith a spun section or on its own axis. Other instruments can disrupt the velocity of incomingparticles, and so could invalidate data collected, due to this the DDS should not be positioned nearthe boon of the PWD. Furthermore, the DDS should be placed as far away as possible from theRTGs to limit some of the radiation exposure, although this will be hard to avoid considering theJovian environment.

6 Subsystems

6.1 Power

The Radioisotope Thermoelectric Generators (RTGs) use the radioactive decay of fuels like U-238and Sr-90 as fuel to produce electricity using the Seebeck e!ect. It consists of a sturdy containerhaving fuel(U-238 or Sr-90) along the walls which is connected to a heat sink. The flow of heat thusproduced by the radioactive decay inside the thermocouples to the heat sink generates electricity.The choice of U-238 was done for our missions RTG as it has a low shielding requirement and longhalf-life. Its half life is around 87.7 years which is preferably more than enough for our missiontimeline. A cheaper alternative is present in the terms of Sr-90 which has a shorter half-life of 28.9years.

6.1.1 Key Design Issues

The Total power requirement for our mission at the End of Life(EOL) is 320 W(excluding) thetransmitter. Each of the RTGs has a capability to produce 210W hence we would be requiring threeof those providing us with 630(at EOL including the power for the two transmitters) which leaves uswith su#cient amount of extra power to meet our emergency requirements. It was found that if wehad to use solar arrays with the same specifications; with a solar flux density of 55 W/m2 at Jupiter;we would be requiring solar arrays amounting to the size of 116.80 m2 with a combined weight of735kg. This would compromise with our total mission weight budget of 2500kg. Hence a greaterpreference was done towards using RTGs in our mission. With a combined weight of 120kg and nondependence on the amount of time spent in the eclipse of the Jupiters orbit; RTGs can provide uswith continuous supply of power throughout the mission. It generates 4400W of heat with a fuelusage of 7.8kg of the radioactive material.

The main drawbacks of using an RTG is the environmental consequences and high radiationemission from the generator which has a vide e!ect on the working of other subsystems; hence properchecks like radiation absorbing materials are placed around it so that it wont e!ect the working ofother systems, especially the magnetometer. The other drawback involves the price of the fuel used.

11

Each kilogram of U-238 used costs around 1000000 hence such systems are used only in deep spacemissions were the solar flux intensity is really low.

Regarding the positioning of the RTGs they are generally placed in the lower section of thespace-craft at the farthest distance from the radiation sensitive science experiment at an angle tothe surface of the craft. This is done because we want the radioactive sensitive instruments to fallunder the shadow region of the RTGs to face minimum radiation levels.

To minimize the risk of the radiation, the U-238 is stored in individual modular units with its ownheat shielding. It is surrounded by a layer of iridium metal and encasing of high-strength graphiteblocks. The two materials were corrosion and heat sensitive, surrounding the graphite blocks is anaeroshell. The plutonium fuel was also stored in a ceramic form that is heat-resistant, minimisingthe risk of vaporization and aerosolization which is highly insoluble. The assembly of such an RTGis shown in figure 6.

Figure 6: GPHS-RTG components

6.2 Communications

Our transmission is an antenna size/power input trade o!. With our mass constraint of 2500kg, wecannot a!ord to use extra mass for the antenna or for the power system to generate the necessaryinput. As the diameter increases, the gain of the antenna will also increase proportional to diameter2.This means that for the same power input, the strength of the signal will increase with the diameter.For the signal we need to send, the power required for di!erent diameters are shown in 7

The benefit seen by increasing diameter begins to drop out at around 3.5m, where power requiredhas dropped to 160W. If we were to choose a large antenna diameter (5m for example) then webegin to see new complications for little power di!erence. A larger dish may not fit in the launcherfully deployed, and so the storing and deployment of such a large dish on launch will need to beconsidered in great detail. Galileos large dish antenna (4.8m)[19] needed to be stored on launch.Making it storable out of a thinner sun-sensitive material caused complications when it needed tobe deployed. The antenna was not successfully unfurled and subsequently was unable to be used forcommunications during the mission. Taking this complication into consideration, weighed up against

12

Figure 7: Power requirements for di!erent dish diameters

the small di!erence in input power required, our antenna will not be a deployable type and will fitinto the launch vehicle without much need for complex storage. A dish of 3.5m will be chosen; itrequires low power but will not cause extra storing and mass issues.

6.3 Thermal

Thermal stability is required in a spacecraft because the electric and mechanical equipments functione#ciently only within a narrow temperature ranges and most of the materials have a non-zerocoe#cient of thermal expansion and thus chance in temperature can cause thermal distortion. Therange of functioning of the electronic equipment can be from -15 C to 50 C while the range forrechargeable batteries is is 0 C to 20 C and for the subsystem mechanisms like momentum wheels,gyroscopes; it is between 0 C to 50 C. Hence a system of passive and active thermal controls isneeded to be applied for an e#cient thermal control system. The passive thermal control is achievedthrough the classical thermal insulating multi layer blanket. The blankets are made of aluminizedlayers of metalloids.

The outer surface of such surfaces usually consists of kapton which gives the space-craft its blackappearance. A variety of metals/metalloids/paints can be used for these purposes. Other optionsinclude conduction paths, heat pipes, usage of phase change materials (like hydrocarbon wax) canbe used. The Active thermal control consists of the heaters, Variable Conductance heat pipes anddiodes, liquid loops or the fundamental refrigerators and heat pumps. Brayton and Stirling cycle arethe classical examples of such heat pumps. While the most common active control system which areused are the heaters which can be of the form of a metal-mounted resistors, thermo-coax or usingadhesive sheets of Kapton foils.

A system of heat pipes, radiators and liquid loops run through the space-craft and subsystems tomanage a constant working temperature around. The choice of liquid inside the loops is determinedin order to minimize the power consumption, the specific heat of the fluid shoud be high with a lowermass rate and dynamic viscosity and a higher bouling point. Traditionally Freon-21 and ammoniahave been the most suitable fluids as they have low freezing points.

13

Property Value

Absorbance 0.14Emittance 0.05Absorbance/Emittance 2.80Thermal Conductivity 4.638x10-3 T0.5675W/m K

Table 3: Aluminised Kapton Material Values

6.4 Propulsion and Attitude Control

Although it is known that strong magnetic field exists around Jupiter, its nature is not yet wellunderstood; hence magnetorquers should not be relied on, and hence our satellite is not equippedwith it.

The spacecraft is also equipped by a set of four reaction wheels, for 3-axis stabilization and one forredundancy. The mass of the system may be calculated by the mass of one reaction wheel multipliedby four. The minimum mass of a reaction wheel can be said to be the mass required for the wheelto store enough angular momentum to keep the telescope pointing Jupiter about its sti!est axis,and allow a margin (50%) for the telescope to point to a variety of sources of interest; assumingthe maximum rpm achievable by the reaction wheels is 3000rpm. A large margin has been chosensince it is desirable to minimize the reliance on the thrusters, since the expellant might damage oursensitive science equipments, especially the camera.

Hence, the di!erence between the maximum and the minimum angular velocity of the space-craft required to maintain the line of sight to Jupiter must be calculated. This angular velocity isequivalent to the angular velocity of the spacecraft about the centre of Jupiter, which is given by:

"̇ =h

r2, a(1! e2) =

h2

µ, e =

ra ! rpra + rp

Hence

"̇ =

,µa(1! e2)

r2

Giving

"̇min =

,µa(1! e2)

r2a

"̇max =

,µa(1! e2)

r2p

""̇ =

,µa(1! e2)

r2p!

,µa(1! e2)

r2a

For our orbit,

e =ra ! rpra + rp

=270RJ ! 4RJ

270RJ + 4RJ= 0.971

and

a =ra + rp

2=

270RJ + 4RJ

2= 137RJ

And therefore

""̇ =

,1.3" 108 " 137" 7.2" 104(1! 0.9712)

(4" 7.2" 104)2!

,1.3" 108 " 137" 7.2" 104(1! 0.9712)

(270" 7.2" 104)2

14

Item Mass

Instruments 54PWD 6DDS 4EPD 9Camera 28Magnetometer 7

Communications 200Power 120Propulsion 8.5

400N Thruster 4.310N Thrusters 4.2

Data Handling 50Structure/Thermal 1100

Total Dry Mass 1482.5

Propellent 596.1Orbit Insertion 536.1Station Keeping 60

Margin 421.4

Total 2500

Table 4: JAMES’ Mass budget

Item Power (W)

Instruments and Subsystems 320Communications 170

Total 490

Table 5: JAMES’ Power budget

""̇ = 1.035" 10!4rad/sec

7 Budgets

7.1 Mass Budget

See table 4.

7.2 Power Budget

See table 5.

7.3 Telemetry Budget

See table 6.

15

Item Datarate(bps)

Instruments 54Camera 806.4Magnetometer 0.24Energetic Particle Detector 0.912Dust Detector 0.024Plasma Wave Detector

Margin 161.563

Total 969.379

Table 6: JAMES’ Telemetry budget

8 Conclusion

After taking all payload and sub-system requirements into account a preliminary design of JAMEShas been created and is shown in figure 8. A final summary of each part of the satellite, their positionand the justification for their position is given in table 7.

Figure 8: Design of JAMES

16

Part Justification

Primary AntennaSecondary AntennaMMH Fuel StorageReaction Wheel AssemblyAttitude Control ThrustersMON Oxidizer StorageCameraRTGsInternal Memory ModuleBoom PositionMagnetometer As specified the magnetometer must sit at the end

of the 10m long boom in order to avoid the mag-netic field of the spacecraft polluting it’s results

Plasma Wave DetectorEnergetic Particle DetectorDust Detector

Table 7: Justification of JAMES’ configuration

References

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