technological advances in spacecraft to encounter saturn

8/6/2019 Technological Advances in Spacecraft to Encounter Saturn

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Technological Advances in Spacecraft

To Encounter Saturn

Andrew Hales


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Contents

1. Saturn, Past and Current Missions ........................................................ .............................................. 42. Pioneer 11..................................................................................................................................................... 5

2.1 Introduction ........................................................................................ ................................................. 52.2 Launch and Trajectory .............................................................................. ....................................... 62.2 Spacecraft Layout ............................................................ ............................................................. ...... 72.3 Propulsion and Attitude Control Systems ........... ............................................................. ........ 82.4 Spacecraft Instruments and Degradation ........................................................ ......................... 92.5 Arrival at Saturn .............................................................................................................................. 11

3. Voyager ........................................................ ............................................................. .................................. 123.1 Introduction ...................................................................................................................................... 123.2 Launch and Trajectory ............................................................. ..................................................... 123.3 Spacecraft Propulsion ............................................................... ..................................................... 133.4 Spacecraft Instrumentation .............................................................. ........................................... 143.5 Saturn Encounter .............................................................................................................. .............. 18

4. Cassini ......................................................................................................................................................... 194.1 Introduction ...................................................................................................................................... 194.2 Launch and Trajectory ............................................................. ..................................................... 204.3 Spacecraft Layout ............................................................ ............................................................. ... 214.4 Spacecraft Propulsion ............................................................... ..................................................... 224.5 Spacecraft Instrumentation .............................................................. ........................................... 234.6 Goal and Objectives ................................................................................................ ........................ 264.7 Saturn Orbit Insertion .............................................................. ..................................................... 274.8 Huygens Probe Insertion ......................................................... ..................................................... 274.9 Orbital Tour of the Saturnian System ..................................................... ................................. 29


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4.10 Decommissioning Cassini ............................................................... ........................................... 305. Technological Advances ....................................................... ............................................................. ... 31

5.1 Imaging Instrumentation ....................................................... ...................................................... 315.2 Ultraviolet Imaging Instrumentation ..................................................... ................................. 325.3 Infrared Imaging Equipment ...................................................................................................... 335.4 Magnetosphere Imaging ......................................................... ...................................................... 345.5 Propulsion and Electrical Power ............................................................... ................................ 34

6. Conclusion ............................................................. ............................................................. ....................... 367. Bibliography ............................................................................................................................................. 37


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1. Saturn, Past and Current Missions

The planet Saturn is the second largest in the solar system and has been noted by

virtually every civilisation in ancient history. Saturn has the most extensive system of

rings compared to any other planet in the solar system. The prominent system of rings

mostly consists of icy particles with small amounts of rock debris and dust. The planet is

so massive it has sixty-one moons as well as hundreds of moonlets circling the planet

within the rings. Some of the moons orbiting Saturn can be considered icy satellites

due to their high percentage composition of water. The largest moon orbiting Saturn is

Titan, which is the second largest in the galaxy and is significantly larger than the planet

Mercury. Titan is the only moon in the galaxy to possess a dense veiling atmosphere.

The Pioneer 11 spacecraft was launched in 1973 as part of a flyby mission, where the

probe would encounter Jupiter and then Saturn. The Pioneer spacecraft was the first to

take close-up images of Saturn in 1979. (Chan, 2000)

The Voyager mission would take advantage of a 175 year occurrence, where a rare

geometric arrangement of the outer planets would take place. The layout of Jupiter,

Saturn, Uranus and Neptune would allow the Voyager spacecraft to visit each of the

planets in-turn. Two Voyager spacecraft were launched 15 days apart on slightly

different trajectories in 1977. The two spacecraft returned 2000 images of Saturn and

its moons and information regarding to Titans atmosphere. Voyager One and Two

encountered Saturn in 1980 and 1981 respectively. (Bergman, 2000)

The Cassini Mission was launched in 1997 with the objective of performing close-up

studies of Saturn, its rings, moons and magnetic environment. The moon Titan will be of

special interest because of the atmospheric and surface characteristics it possiblyshared with early planet Earth. The Spacecraft will make in situ and remote

observations under geometric and temporal conditions not available from Earth. The

Cassini spacecraft comprises of an orbiter and probe which arrived at Saturn in 2004.

(Russell, 2005)

Each spacecraft mission was a milestone in terms of space exploration and achievement.

Pioneer 11 was the first spacecraft to visit Saturn and collected vital information about

the environments that further spacecraft would encounter. Information collected from


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Pioneer paved the way for future and more in depth explorations. Voyager spacecraft

further studies Saturns rings and the formation of the moons as well as discovering 21

new moons. The spacecraft also was the first to study Saturns largest moon Titan.

Discoveries made from Titan prompted NASA to study the moon further. The Cassinimission is still in operation to this day and has retrieved in depth information about

Titan and its atmosphere as well as further study of Saturn.

2. Pioneer 11

Figure One. Pioneer 11 Approaching Saturn: 1 September 1979 (Artist Impression)

(Airports Worldwide, 2009)

2.1 Introduction

Pioneer 11 was the first spacecraft to fly past Saturn and return close-up images and

data of the planet and surrounding moons. The spacecraft had a sister probe, Pioneer 10

which was the first to fly past Jupiter. Pioneer 11 was put on a trajectory that would also

fly past Jupiter and use the planet as a gravitational slingshot to redirect the probe on a

course towards Saturn. Pioneer 10 proved that its sister spacecraft could safely

slingshot past Jupiter.


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2.2 Launch and Trajectory

Pioneer 11 was launched on 5 April 1973 from Cape Canaveral, Florida via an

Atlas/Centaur/TE364-4 launch vehicle. (See Figure Two)

Figure Two. The launch of the Atlas-Centaur carrying the Pioneer 11 spacecraft

(Downward, 1973)

After safe passage through the asteroid belt on 10 April 1974, the Pioneer 11 thrusterswere fired and provided a further 64m/s to the spacecrafts velocity. The adjustment

allowed the Jupiter flyby possible by approaching Jupiter at 43000km above the cloud

tops. This close approach accelerated the spacecraft to a velocity of 175000km/h. This

was an acceptable speed to see the aircraft across the Solar System, 2.4 billion

kilometres to Saturn.

By the time Pioneer 11 had reached Saturn in 1979, Voyager spacecraft 1 and 2 had

already passed Jupiter and were on-route to Saturn. It was decided to prioritise the

Voyager mission to Saturn as it had advanced instruments onboard compared to the

Pioneer. The Pioneer 11 spacecraft was acting as a pioneer in a true sense of the word,

making sure the Voyager spacecraft were safe to approach Saturn at the same trajectory.

If Pioneer 11 detected danger, the Voyager spacecraft could be safely re-routed towards

Saturn.


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2.2 Spacecraft Layout

The body of Pioneer 11 was mounted behind a parabolic dish antenna, 2.74m in

diameter and 0.46m deep. The structure of the spacecraft compromised of a 0.36m deep

flat equipment compartment, the top and bottom being hexagonal in shape. The sides of

the spacecraft were 0.71m long, one of the sides attached to a smaller compartment that

carried scientific instruments.

Figure Three. The Pioneer 11 Spacecraft(Lindsay, 2002)

A high-gain antenna was situated at the centre of three protruding struts. A medium-

gain and low-gain antenna were mounted behind the high-gain antenna and extended

0.76m behind the equipment compartment. Communication was maintained via the

low-gain and medium-gain antenna which operated together while connected to a

receiver. The high-gain antenna is connected to a separate receiver, allowing

interchangeable links to command, giving some redundancy.


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2.3 Propulsion and Attitude Control Systems

Power for the Pioneer spacecraft was obtained from four SNAP-19 Radioisotope

Thermonuclear Generators (RTGs). RTGs are nuclear electrical generators that obtain

power from radioactive decay. The heat released by the decay of a suitable radioactive

material is converted into electricity. These were held 3m away from the centre of the

aircraft and were mounted on two trusses 120 apart.

Figure Four. Cutaway of the Pioneer 11 SNAP-19 RTG (Bennett, 2006)

Each RTG unit provided 40.3 watts of electrical energy at maximum output. The mass of

one unit was 13.6 kg. As the spacecraft carries out its mission the overall energy output

of the RTGs will deplete as the radioactive material decays.

Three pairs of rocket thrusters near the rim of the high-gain antenna provided a

threefold function of spin-axis precision, mid-course trajectory correction and spin

control. Each pair of thrusters attained its jet force through the catalytic decomposition

of liquid hydrazine in a small rocket thrust chamber. The hot gas produced expended

through individual thruster nozzles to affect the manoeuvres of the spacecraft. These

thrusters could be pulsed or steadily fired by command on earth. (Anderson, 2004)

Both thrusters and RTGs had an important role in maintaining the spacecrafts

temperature. Temperature control was kept between -23C and 38C.
http://en.wikipedia.org/wiki/Nuclearhttp://en.wikipedia.org/wiki/Nuclear


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2.4 Spacecraft Instruments and Degradation

Pioneer 11 was equipped with eleven main scientific instruments to study the asteroid

belt and the environment around Saturn, solar winds, cosmic rays and eventually the far

reaches of the solar system. Power constraints forced scientists at the Jet Propulsion

Laboratory in California to slowly shut down instruments and communication links.

The scientific instruments used to carry out experiments mentioned above were:

Helium Vector Magnetometer (HVM): Used to measure the fine structure ofthe interplanetary magnetic field around Saturn and provides measurements to

evaluate solar winds.

Quadrispherical Plasma Analyzer (QPA): Peers through a hole in the high-gainantenna to detect particles of the solar wind which originate from the Sun.

Charged Particle Detector Instrument (CPI): Detects cosmic rays in the SolarSystem.

Cosmic Ray Telescope (CRT): Collects and analyses data about the compositionand energy ranges of cosmic ray particles.

Geiger Tube Telescope (GTT): Surveys the intensities, angular distribution andenergy spectra of electrons and protons along the spacecrafts path throughSaturns rings.

Trapped Radiation Detector (TRD): Detects the light emitted in a particulardirection with the use of a Cerenkov Counter, which records electron energy as

particles pass through the counter.

Meteoroid Detector (MD): Panels of pressurized cell detectors are mountedbehind the high-gain dish. Penetrating impacts from small meteoroids are

recorded. Ultraviolet Photometer (UP): Equipment senses ultraviolet light to determine

the quantities of hydrogen and helium in space and around Saturn.

Imaging Photopolarimeter (IPP): This piece of imaging equipment relies uponthe spin of the spacecraft to sweep a small telescope across the planet in narrow

strips. Strips are processed to build a visual image of the planet. The IPP looks at

the planet in red and blue light.

Infrared Radiometer (IR): Detects cloud temperature and heat output fromSaturn. (Grayzeck, 2010)


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Space experiments onboard Pioneer 11 continued to operate for planetary or

interplanetary missions until failure, or until insufficient power was available from the

RTGs to operate all instruments. The meteoroid detector onboard Pioneer 11 failed in

December 1973, only nine months after launch. The Infrared Radiometer (IR) also failedin January 1974 followed by the Helium Vector Magnetometer (HVM) in November

1975. (NASA, 2007)

The onboard RTGs did not have the power capability to operate all instruments for the

full mission duration. The table below shows the degradation of the spacecrafts

scientific equipment over time.

TABLE I: Pioneer 11 Instrument Degradation


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2.5 Arrival at Saturn

Pioneer 11 reached Saturn on 1stSeptember 1979 and was the first spacecraft to take

close-up pictures of the planet during its closest approach of 13000 miles.

Onboard instruments detected two previously undiscovered moons as well as an

additional ring. Pioneer 11 studied Saturn's magnetosphere and magnetic field in great

detail and discovered that its planet sized moon, Titan, would be too cold to support life.

Pioneer 11 sent to Earth, amazing pictures of Saturn and its rings; see figure five.

Figure Five. Pioneer 11 Image of Saturn. (NASA, 1979)

Following the encounter with Saturn, Pioneer 11 explored the outer regions of the Solar

system where it studied solar winds and cosmic rays entering our portion of the Milky

Way.

By September 1995, Pioneer 11 was 6.5 billion km from the Earth. At this distance it

took 6 hours for the radio signal from the spacecraft to reach Earth. However, Pioneer

11 could no longer make any scientific observations at this time. Intermittent contact

continued until November of that year, at which time, the last transmission took place.

There has been no contact with the spacecraft since.


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3. Voyager

Figure Six. Voyager 1 Encounters Saturn: 12th November 1980 (Artist Impression).

(Annon, 2002)

3.1 Introduction

The twin spacecraft Voyager 1 and Voyager 2 were launched by NASA in separate

months during 1977. The spacecraft were designed to conduct close up studies of

Jupiter and Saturn, Saturn's rings and the larger moons of the two planets. Each

spacecraft was built for a five year stint, but successful achievement of all objectivesmeant that flybys of the two outermost giants, Uranus and Neptune proved possible.

Their original five year life was stretched to twelve and is now near thirty-four years.


On 20th August 1977, Voyager 2 was launched from the Kennedy Space Centre at Cape

Canaveral, Florida. Voyager 1 was launched sixteen days later. Both spacecraft were


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launched onboard a Titan III/Centaur Rocket, which stands nearly 50m tall and weighs

635000kg, see Figure Seven. (Sword, 2005)

Figure Seven. Launch of Titan III-Centaur carrying Voyager 1 spacecraft.

(Downward, NASA Galleries, 1977)

Although Voyager 1 was launched after its twin spacecraft, it arrived at Saturn four

months earlier as it was launched on a shorter, faster trajectory.

The trajectory of Voyager 1 was designed to send the spacecraft closely past the large

moon of Titan and behind Saturn's rings; this carried the spacecraft northward out of

the ecliptic plane (the plane in which most of the planets orbit the sun).

Voyager 2 was aimed to fly by Saturn so that it would be automatically sent in the

direction of Uranus. After passing Saturn, Voyager 2 would be able to encounter Uranus

with all its instruments operating properly. (Angrum, 2010) NASA provided additional

funding to continue the operation of the two spacecraft beyond the planned five years.

NASA also authorised the Neptune leg of the mission, the Voyager mission was

subsequently renamed the Voyager Neptune Interstellar Mission.

3.3 Spacecraft Propulsion

Electrical power for the spacecraft is supplied by three Multi-Hundred Watt

Radioisotope Thermoelectric Generators (MHW-RTGs), connected in parallel, capable of

providing 160 Watts of power each. The RTGs are mounted in tandem on a deployable


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boom, each 120 apart. Figure eight shows a cutaway of the MHW-RTG, in which

decaying energy released from the radioisotopic fuel, Plutonium-238 is converted to

heat and is the source of the heat to the thermoelectric converter. (Angrum, Jet

Propulsion Laboratory, 2010)

Figure Eight. Cutaway of MHW-RTG, Propulsion Source of Voyager 1 and 2 (Angrum, Jet


3.4 Spacecraft Instrumentation

The identical Voyager spacecraft are three-axis stabilized systems that used gyroscopic

attitude control to maintain pointing of the high-gain antennas toward Earth. The

mission payload included 10 scientific instruments that are monitored by investigatory

teams, only five of which are still in operation.

Imaging Science Subsystem (ISS) Narrow and Wide Angle Camera

This piece of equipment is a modified version of a slow scan vidicon camera that was

used as part of the Mariner flights.

The ISS consists of two cameras, low resolution (200mm wide angle lens) and high

resolution (1500mm narrow angle lens). Operation of the ISS is controlled by an


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imaging parameter table in one of the onboard computers, the Flight Data Subsystem

(FDS).

Scientific Objectives of the ISS

"Observe and characterize the circulation of the planetary atmosphere, providelimits on atmospheric composition, and determine the wind velocities in the regions

observed.

Map the radial and azimuthal distribution of material in the ring plane; search fornew rings.

Obtain global multi-spectral coverage of all satellites; establish rotation rates andspin axis orientations, study the surface morphology of Triton at spatial resolutionsless than 2 km; search for undiscovered satellites.

Provide support images to assist other onboard investigations in their datareduction." (Angrum, Jet Propulsion Laboratory, 2010)

Plasma Subsystem (PLS)

The PLS is designed to search for low-energy particles in the Plasma. The PLS is also

able to locate particles travelling at a certain speed and has a limited ability to

determine the direction in which they have to come from.

Scientific Objectives of the PLS

"The properties and radial evolution of the solar wind. The interaction of the solar wind with Jupiter, Saturn, Uranus, and Neptune. The sources, properties, and morphology of the magnetospheric plasma from

Jupiter, Saturn, Uranus, and Neptune.

The interactions of magnetospheric plasma with the planetary satellites withparticular emphasis on plasma properties in the vicinity of Io, Titan, and Triton.

Ions of interstellar origin. Detect and characterize the nature of the termination shock, where the solar wind

slows down and becomes denser as it prepares to encounter the heliopause.


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Make the first detection of the heliopause boundary and the first detection of theplasma from outside our solar system, the interstellar medium." (Angrum, Jet


Ultraviolet Spectrometer (UVS)

The UVS is a specialised instrument that is sensitive to ultraviolet (UV) light. It can

determine when certain atoms or ions are present, or when physical processes are

occurring. Elements and compounds emit UV light of a certain frequency or colour, the

UVS is capable of identifying these colours.

Scientific Objectives of the UVS

"To determine the scattering properties of the lower planetary atmospheres. To determine the distribution of constituents with height. To determine the extent and distribution of hydrogen corona of the planets and

satellites.

To investigate night airglow and auroral activity. To determine the UV scattering properties and optical depths of planetary rings. To search for emissions from the rings and from any ring atmosphere" (Angrum,

Jet Propulsion Laboratory, 2010)

Infrared Interferometer Spectrometer (IRIS)

The IRIS acts as three separate instruments. It is capable of determining the distribution

of heat energy a body is emitting, allowing scientists to determine the temperature of

that body or substance.

The IRIS is also to determine when certain types of elements or compounds are present

in an atmosphere or surface.

It is also used as a radiometer by measuring the total amount of light reflected by a body

at ultraviolet, visible and infrared frequencies.


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Scientific Objectives of the IRIS

"Determination of atmospheric vertical thermal structure (which in turn aidsmodeling of atmospheric dynamics).

Measurement of the abundances of hydrogen and helium (as a check on theoriesregarding their ratio in the primitive solar nebula).

Determination of the balance of energy radiated to that absorbed from the sun (tohelp investigate planetary origin, evolution, and internal processes)." (Angrum, Jet


Magnetometer (MAG)

The MAG is capable of detecting the effects of the solar winds on outer planets and

moons; however its main purpose is to measure changes in the Sun's magnetic field

with distance and time. The purpose of this is to determine whether the outer planets

have a magnetic field and how orbiting moons or the planets rings interfere with this

magnetic field.

Scientific Objectives of the MAG

"Measure and analytically represent the planetary magnetic fields of Jupiter,Saturn, Uranus, and Neptune.

Determine the magnetosphere structure of all the giant planets encountered.Investigate the basic physical mechanisms and processes involved both in

interactions between the solar wind and the magnetosphere and in internal

magnetospheric dynamics, in correlative studies with other particles and fields

investigations.

Investigate the interactions of the satellites of these planets with theirmagnetosphere/solar wind environments.

Accurately survey the interplanetary magnetic field beyond 1 AU and continue andextend studies of large-scale characteristics of the interplanetary medium.

Continue and extend studies of the physics of micro scale phenomena in the solarwind.


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Search for the transition region between the interplanetary and interstellar media,and if possible, investigate the magnetic characteristics of the boundary region and

measure the galactic magnetic field and its variations." (Angrum, Jet Propulsion

Laboratory, 2010)

High Gain Antenna (HGA)

The HGA is responsible for transmitting data to Earth via two frequency channels. The

first transmits science and engineering data at a frequency of 8.4 gigahertz, the

downlink data rates are 7.2 kilobits per second. The second channel send only

engineering data on the health and state of the spacecraft at a lower rate of 40 bits per

second. The frequency of the second channel is set at 2.3 gigahertz.

3.5 Saturn Encounter

Voyager One and Two encountered Saturn nine months apart, in November 1980 and

August 1981 respectively.

Both encounters with Saturn increased the knowledge and understanding of the planet.

The extended, close range observations provided high-resolution far different from the

picture assembled during previous interstellar missions.

Scientific findings from both Voyager spacecraft have revolutionized the science of

planetary astronomy, helping to resolve key questions while raising new ones about the

origin and evolution of the planets in the solar system.

Voyager One discovered that only about 7% of the volume of Saturns upper

atmosphere comprises of Helium, the rest of Hydrogen. Saturns Helium abundance

does not fall in line with that of neighbouring planet, Jupiter, which has a higher Helium

composition of 11%. This might imply that heavier Helium atoms are slowly sinking

through the Hydrogen around Saturn.

This theory helps explain the excess heat that Saturn radiates over energy it receives

from the Sun.

Winds blow at speeds of up to 500 metres per second in Saturn, with the strongest

winds located at the equator.


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Voyager 2 penetrated the upper atmosphere with its radio beam to determine

temperature and density. Minimum temperatures of 82 Kelvin were found at the 70-

millibar level (surface pressure on Earth is 1000-miilibar)

Both Voyager spacecraft measured the rotation of Saturn at 10 hours, 39 minutes, 24

seconds.

4. Cassini

Figure Nine. Cassini Approaches Saturn: 1stJuly 2004 (Artist Impression) (Nelson, 2004)

4.1 Introduction

The mission and Orbiter spacecraft is named after the French/Italian astronomer

Giovanni Domenico Cassini. He discovered several of the Saturnian satellites and ring

features between years 1671-1685. The atmospheric Huygens Probe that will explore

Titan is named after Dutch Astronomer Christiaan Huygens, who discovered the moon

in 1655.

The Cassini Program is a cooperative international partnership between the National

Aeronautics and Space Administration (NASA), The European Space Agency (ESA) and

The Italian Space Agency (ASI). The mission is managed by NASAs Jet Propulsion

Laboratory in Pasadena, California. This was also where the Orbiter was developed andmanufactured. Development of the Huygens Titan Probe was carried out by the


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European Space Technology and Research Centre (ESTEC). Huygens batteries and two

of pieces of scientific equipment came from the U.S. ASI is contributing the orbiters

dished high-gain antenna and significant pieces of three science instruments. NASAs

Deep Space Network is used for communications with Cassini during the mission.Stations are located in California, Spain and Australia. Data from the Huygens probe will

be sent to a communications complex in Darmstadt, Germany.


The launch vehicle for Cassini was a Titan IVB with two Solid Rocket Motor Upgrades

(SMRU) attached at the lower stage. On the top of the propulsion stack was a Centaur

rocket, above this on the uppermost stage is the payload, or the Cassini/Huygensspacecraft. The Titan vehicle has two main stages. The SMR|Us are anchored to the first

lower stage. SMRUs are basically strap on rockets to provide extra propulsion required

for launch into orbit. They burn solid fuel, whereas the Titan uses liquid-fuel. The

second stage is the Centaur rocket, which is a versatile, high energy and cryogenic liquid

fuelled. This rocket system had two multiple start engines. The performance of the

combined Titan IVB/SRMU-Centaur system is capable of launching a payload of weight

5760kg into orbit. Above the propulsive system sits the Cassini/Huygens spacecraftwhich is protected by a 20 meter long payload fairing. Protection is required through

the lower atmosphere due to the risk of temperature damage. (Lebraton, 2002)

Lift off took place at night from Cape Canaveral. The launch sequence began with the

ignition of the two SMRUs. The whole stack was lifted off the pad with the thrust

produced. 10 seconds after the ignition of SMRU the stack continued to accelerate and

tilt and rotate. Rotation occurred until the required azimuth (The horizontal angular

distance from a reference direction, usually the northern point of the horizon, to the

point where a vertical circle through a celestial body intersects the horizon) was

reached. Two minutes into the launch, the first stage of Titan was ignited. The altitude

was 192000 feet approximately. A few second after the ignition of Titan, the two SMRUs

were jettisoned because they were spent. The whole system reached an altitude of

360000 feet in a further one and a half minutes and the payload fairing was released.

Five and a half minutes into the flight and 549000 feet was reached and the first stage of

Titan separated and the second stage fired. At launch plus 9 minutes, stage two had fully


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burnt out and was dropped away. The Centaur then fired and boosted the remaining

rocket and spacecraft into a parking orbit and switched off its engines. Sixteen minutes

after the turn off the rockets were re-ignited for a second time. It burnt for a further 8

minutes before the Centaur was separated from the Cassini spacecraft. Cassini/Huygenswas now in an interplanetary trajectory. The whole launch was executed perfectly; the

spacecraft was now heading for swing-bys of Venus, Venus again, Earth and Jupiter,

before reaching and orbiting Saturn. Figure Ten shows the launch sequence events.

(Matson, 2002)

Figure Ten. Launch Sequence to place Cassini/Huygens into Earth Orbit. (Matson, 2002)

4.3 Spacecraft Layout

Before separation the Cassini spacecraft and Huygens probe are as one. Several sections

make up the spacecraft. The bottom of the stack consists of the lower equipment

module. Working up from this section are other parts; these are the propellant tanks

and engines, upper equipment module, twelve-bay electronics compartment and the

high-gain antenna (HGA). These different stacks are arranged vertically upward on top

of one another. Approximately half way up the spacecraft is the Huygens probe. The

probe is a disk-shaped spacecraft, three-meters in diameter. There are eighteen


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specially designed instruments involved in the Cassini/Huygens mission. Twelve of

these instruments are attached to the Cassini orbiter and six on the Huygens probe. The

orbiters equipment is mounted on one of two fixed platforms. These are called the

remote-sensing pallet and the particles-and-field pallet. Each piece of equipment isattached to the corresponding body platform. Above the platforms is a large four-meter

diameter high gain antenna. Centred at the top of the antenna is a smaller low gain

antenna (LGA). Another LGA is attached at the bottom of the bottom of the orbiter. The

completed design for the Cassini spacecraft is shown in Figure 12. (NASA, 2007)

Figure Eleven. Completed design and configuration of the Cassini/Huygens spacecraft.

(Matson, 2002)

4.4 Spacecraft Propulsion

Electrical power to the scientific instruments and to the spacecraft is provided three

General Purpose Heat Source Radioisotope Thermoelectric Generators (GPHS-RTG).

These provide power through the natural radioactive decay of plutonium-238. This

isotope is non weapons grade, so is completely safe. The generation of heat through

decay is changed into electricity by solid-state thermo-electric converters. The benefit

of using RTGs is that they are lightweight, compact and reliable due to no moving parts.


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A drawing of a RTG used on the Cassini orbiter is shown in Figure Twelve. Radioisotope

Thermoelectric Generators were previously used for electrical power in some of the

space program's greatest successes. These include the Apollo lunar landings and the

Viking Landers on mars. RTGs are also currently being used in the Voyager mission,allowing the spacecraft to explore Jupiter, Saturn, Uranus and Neptune, as well as the

Pioneer and Galileo mission. The international Ulysses mission studying the suns polar

region has been made possible thanks to RTG powered spacecraft.

Because of Cassinis scientific objectives, launch systems and travel time to Saturn;

three RTGs were needed on-board the spacecraft for the Cassini spacecraft to

accomplish its mission objectives.

Figure Twelve. Cut-away drawing of a GPHS-RTG. (Matson, 2002)

4.5 Spacecraft Instrumentation

The selection of instrument usage and team coordination was discussed and finalised

between NASA and ESA. The results of the selection and who coordinates which

instrument can be seen in Table II and Table III on the following pages.


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TABLE II

Cassini Orbiter Instrument, Objective and Coordinator.

(Jaffe, 1997)


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TABLE III

Huygens Probe Instrument, Objective and Coordinator.

(Jaffe, 1997)


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Two way communications between Cassini and Earth is carried out through the Deep

Space Network (DSN) via an X-band radio link. These communications use the orbiters

high gain antenna, or one of the low gain antenna. The high gain antenna is also used for

communications between itself and Huygens, as well as carrying out radio and radarexperiments.

The primary data storage device used on the orbiter is called the Solid State Recorder

(SSR). The spacecraft is equipped with two of these, each with a capacity of 1.8 Gigabits.

The SSR will store spacecraft telemetry and attitude articulation and control (AACS),

command and data subsystem (CDS) and instrument memory-loads in separate

partitions.

4.6 Goal and Objectives

The primary goal of Cassini/Huygens is to conduct an in-depth exploration of the

Saturnian System (NASA, 1989).

Although many other spacecraft have explored the Saturnian system before Cassini, a

greater understanding of the system, its moons, its ring and its atmosphere was needed.

Voyager 1 and 2, and Pioneer II each flew through the Saturnian system and gave very

brief but eye opening observations. A clearer more in depth study of Titan in particular

was needed, as it was revealed as a new and unique object.

Cassini/Huygens is designed to determine the present state of Saturn, the rings, Titan,

icy satellites and the magnetosphere. The specialised equipment and mission duration

allows the orbiter and probe to make the same observations at different periods of time,

allowing comparison of results. This allows interactions between different systems to

by observed and understood. These interactions are very complicated; thanks to the

complexity of the instrument sets, the interactions are addressed. Many of the

spacecraft instruments need to operate simultaneously; this has a huge impact on

electrical power usage. This requirement as well as the need for a diverse collection of

equipment able to operate in the Saturnian system is the reason why the

Cassini/Huygens spacecraft is one of the largest to data.

Certain objectives for the different parts of the Saturnian system was established by

NASA and ESA, some of the required observations were prioritised.


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4.7 Saturn Orbit Insertion

Before arriving at Saturn, Cassini began making synoptic observations 2 months before

arrival in order to refine the knowledge of Titan and to characterise the rings and the

planet. This was done as early as possible as soon as a few tenths of pixels came into

view of Cassini's imaging instruments.

The Cassini Orbiter is inserted into Saturn orbit on the 1 July 2004. The Saturn Orbit

Insertion (SOI) burn begins at the spacecrafts closest approach to Saturn. The

manoeuvres just before SOI are planned to insure the correct trajectory for the orbit

insertion manoeuvre. Any manoeuvres after the SOI are only to course correct any

errors during the entry burn. The Cassini/Huygens spacecraft is the forth to pass

through Saturn's rings, and is inserted to pass through a region known to be free of

particles. The spacecraft is designed to withstand small debris collision expected

through the empty region. The SOI manoeuvre is a 97 minute engine burn with a total

increase in velocity (V) of 633m/s. when the required velocity change has been made

an accelerometer will end the burn. The spacecraft is then steered at a constant angular

velocity rate. The engine gimble actuator keeps the main engine pointed near the

velocity vector; this keeps the thrust efficiency at a maximum. After the burn has ended

and sloshing in the fuel tanks has subsided, the spacecraft is rolled 60-70 to allow the

ORS instruments to be switched on to view Saturn's inner rings.

4.8 Huygens Probe Insertion

On the third orbit of Saturn, the Huygens probe is set on course by the Probe Targeting

Manoeuvre to intersect Titan. The Cassini Orbiter turns to aim the probe at Titan. On

Christmas day 2004, the spin eject mechanism releases Huygens and imparts a 5rpm

axial spin. Twenty-two days after release, Huygens reaches Titan. During landing of the

probe, the Orbiter fires an engine and executes the orbiter deflection manoeuvre. This

sets the course 60000 km away from Titan and at the right time to receive transmission

from Huygens. Figure Thirteen shows the relative positions of orbiter and probe away

from Titan, during the landing of Huygens.


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Figure Thirteen. Position of Orbiter and Probe during the Huygens Mission. (Nelson, 2004)

Huygens enters Titans atmosphere 2.1 hrs before the orbiter will reach its closest

approach to Titan. Huygens has a protective thermal shell, to protect it from enormous

flux of heat generated from atmospheric entry. The designated flight path angle for the

probe is 64. Once deceleration to mach 1.5 is achieved, the aft cover is pulled off by a

pilot parachute. The main parachute is then deployed; this has a diameter of 8.3m. This

allows Huygens to initiate a slow and stable descent. The slowing down of the probe

allows the release of the protective shell as well as the main parachute. A smaller 3.0m

drogue chute is instead deployed for the remainder of the descent. The major landing

event for the Huygens probe is shown in Figure Fourteen on the following page.


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Figure Fourteen. Schematic Representation of Huygens Landing Mission. (Nelson, 2004)

4.9 Orbital Tour of the Saturnian System

After Huygens is delivered to the surface of Titan, Cassini is put back into orbits that

take it to the icy satellites. From there Cassini explores the volume of themagnetosphere and to high latitudes to observer the rings. Further flybys of Titan,

provide opportunities to study and observe the moon as well as gravitational assists to

other possible ventures. The tour of Cassini/Huygens consists of 76 Saturn-centred

orbits. They are navigated by using propulsive manoeuvres and 45 Titan gravity assists.

Titan is the only satellite that is large enough to provide sufficient gravity assists. The

complete tour is shown in Figure Fifteen on the following page.


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Figure Fifteen. The Orbits for the Tour of the Saturnian System. On the left the view is from

above Saturns North Pole. On the right the observer is in the planets equatorial plane with

the sun to the right.

4.10 Decommissioning Cassini

The Cassini mission is roughly over halfway through its looping voyage of the Saturnian

system and is still returning a bounty of information. In 2012 the Cassini mission will

need to be decommissioned. This is because of the lack of power available by this time.

There are many options of what to do with the spacecraft when the power is about to

run out. One option is to leave Cassini in orbit around Saturn, continually orbiting the

planet where it is unlikely to impact anything. Or like Galileo did at Jupiter; impact

Cassini into Saturn. This has some inherent risks however. For example Cassini wouldhave to accomplish a risky manoeuvre which could render the spacecraft uncontrollable.

Another option is to crash Cassini into an icy moon of Saturn. This too has risks. The

RTGs on board the spacecraft generate heat, which could potentially melt ice on the

moon. The melted ice could be conducive to the viability of any earth organism that

might have survived on the spacecraft to that point, such as bacterial organisms. NASA

and ESA will make great efforts not to contaminate alien worlds with terrestrial life.

(Anon, 1995)


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5. Technological Advances

The development of spacecraft instrumentation has changed astronomy from a

descriptive science into a measuring science; uncovering new mysteries and

phenomenon has led to further experimentation and exploration of Saturn.

Pioneer 11 transmitted the first low resolution images of Saturn and discovered

additional rings and moons, previously unknown of. This prompted further study of the

Saturnian system, the Voyager mission.

The Voyager mission discovered the enormous complexity of Saturn's rings and the

likeliness of moon Titan, to early conditions on planet Earth.

The Cassini mission was launched in 1997, with the main aim of landing a probe on

Titan. Advances in instrumentation gained a much better understanding of Saturn's

moons, rings, atmosphere and magnetosphere.

5.1 Imaging Instrumentation

The Pioneer 11 spacecraft had an onboard Imaging Photopolarimeter (IPP) that took

spin-scan images of Saturn and Saturn's moons. In order to obtain a full image of Saturn,

the IPP needed to scan the planet in narrow strips, which were time consuming and

required additional instruments to calibrate the IPP and series of images. (Gordon, 1997)

The IPP consisted of a 2.54 cm Maksutov catadioptric telescope which had a focal ratio

of f/3.4. The focal ratio is defined as

"The speed of a telescopes optics, found by dividing the focal length by the aperture. The

smaller the f/number, the lower the magnification, the wider the field, and the brighter the

image with any given eyepiece or camera."(Astronomics / Christophers, Ltd, 2009)

The IPP operates with two colour filters, red and blue. The different filters reflect

different wavelengths of light, allowing an image to be created.

The Voyager spacecraft had an onboard Imaging Science Subsystem (ISS), which is a

modified version of the slow-scan camera used in the Pioneer missions. The ISS consists

of two television-type cameras each with eight filters, allowing greater imaging

accuracy and quality. One of the cameras has a low resolution wide-angle lens with afocal ratio of f/3; the second camera uses a narrow-angle, higher resolution f/8.5 lens.


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This higher focal ratio allows for greater magnification and image quality. In

comparison with the Pioneer IPP, the Voyager ISS is capable of imaging Saturn as a

whole without the need for spin scanning.

The mass of the Voyager ISS is 22.06 kg and the dimensions of the instrument (LxWxH)

0.98m x 0.25m x 0.25m. When is operation the ISS peak operating power was 14W.

(Barros, 2003)

The Cassini spacecraft has a more advanced version of the ISS used on the Voyager

missions. It too has a wide-angle and narrow-angle camera, with f/3.5 and f/10.5

respectively. The wide-angle lens uses 18 filters and the narrow-angle uses 24 filters.

Improved focal ratio and filter usage gives the Cassini ISS higher speed optics, with even

better resolution and magnification capabilities. Improved technological ability of the

ISS has a consequence of larger instrument size (0.95m x 0.4m x 0.33m) and greater

mass (57.83kg). The Cassini ISS peak operating power is also significantly higher at

55.9W. (Porco, 1998) However, the improved capabilities of this instrument surely

outweigh the costs of increased size, mass and power consumption.

5.2 Ultraviolet Imaging Instrumentation

The Ultraviolet Spectrometer (UVS) onboard the Voyager spacecraft has one channel

and can cover wavelength ranges of 53.5 nanometers (nm) to 170.2nm. The instrument

is sensitive to UV light and is capable of determining the presence of certain atoms or

ions. The UVS has a mass of 4.52kg and has dimensions of 0.43m x 0.147m x 0.175m.

(Showalter, 2002)

The Cassini spacecraft has a more advanced piece of equipment called an Ultraviolet

Imaging Spectrograph (UVIS). The UVIS has two separate channels that provide images

and spectra covering data ranges from 56nm to 118nm and 110nm to 190nm. The UVIS

has an increased wavelength range over Voyager's UVS, making it more sensitive to

determining the presence of certain atoms or chemical processes. A summary of the

physical size and capabilities is shown in Figure Sixteen on the following page.


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Figure Sixteen. Cassini UVIS Review(Bloom, 1995)

In comparison with the UVS, the weight of the Cassini UVIS is nearly four times heavier

at 16kg. Although the mass is considerably higher, the scientific benefit of the advanced

piece of equipment outweighs this. (Bloom, 1995)

5.3 Infrared Imaging Equipment

Voyager spacecraft have an onboard Infrared Interferometer Spectrometer (IRIS). The

IRIS camera has a low range spectrograph for the wavelength range 1.0 microns to 2.5

microns. The IRIS is operated at a constant temperature between 80 and 82 Kelvin (K)

and has a mass of 22.3kg. (Hanel, 2003)

The Cassini Composite Infrared Spectrometer (CIRS) was designed in partnership with

NASA and various institutes any companies from France, England and Germany. The

CIRS has an improved wavelength range between 0.7 microns and 10 microns. The

entire instrument is maintained at a temperature of 170K. (Kunde, 1999)

The CIRSs main component is a beryllium telescope which consists of a 50.8cm, f/6

paraboloidal scope, the same size used by the Voyager IRIS. The primary telescope


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mirror for the Cassini instrument has a gold-enhanced surface designed for low-

scattering.

Both Voyager IRIS and Cassini CIRS are comprised of two interferometers, far-infrared

and mid-infrared. Both instruments have a 4.3mrad diameter field of view (FOV) for the

far-infrared detector. The CIRS represents an improvement however in terms of

frequency range and also has greater efficiency over the conventional mirrors in the far-

infrared region.

CIRS sensitivity has improved noticeably over the IRIs in the mid-infrared due to the

use of cooled HgCdTe detectors. The mid-infrared focal plane is thermally isolated from

the rest of the instrument, by tripod supports manufactured from titanium alloys.

Improved technologies of the interferometers has improved the sensitivity and thus

allowed for a much smaller FOV; where each pixel has a 0.273mrad square field of view.

The Voyagers IRIS has a higher FOV of 4.3mrad. Smaller FOV allows for a greater spatial

resolution than the IRIS.

5.4 Magnetosphere Imaging

The Pioneer 11 and Voyager 1 and 2 spacecraft established the existence of a large andcomplicated magnetosphere. The Cassini spacecraft is equipped to fully explore and

investigate the magnetosphere around Saturn.

Charge-exchange neutrals have been detected by Voyager from Saturns magnetosphere.

Cassinis Magnetosphere Imaging Instrument (MIMI) has more capable sensors and in

conjunction with the UVIS, should provide a clearer view of Saturns aurora and

magnetosphere. The excellent capabilities of MIMI will make it feasible to gain new

information on pickup ions and measurements for the first time. (Krimigis, 2002)

5.5 Propulsion and Electrical Power

Pioneer 11, Voyagers 1 and 2 and Cassini all used Radioisotope Thermoelectric

Generators (RTG) as the electrical power and propulsion source. RTG technology was

fairly new at the start of the Pioneer mission in 1973 and has improved greatly and

noticeably in these three spacecraft. Table IV on the following page demonstrates the

adaptation of RTGs over these missions.


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Table IV

RTG Data for Saturn Spacecraft Missions

The need for revolutionary and advanced scientific instruments has led to a clear

increase in spacecraft weight. In order to successfully transport and operate this

equipment, RTG technology has needed to adapt and evolve with the ever improving

scientific technology.

It can be clearly seen from the table above that RTG technology has improved, with

increased electric energy available to power instruments and increase thermal energy

to power the spacecraft.

Degradation of the Pioneer spacecraft meant that after 22 years contact was completely

lost. Degradation of Pioneer started 19 years after launch, when equipment had to be

powered down to reserve energy.

MissionandRTG

RadioactiveMaterial

ElectricalEnergy(W)

(maxoutput)

HeatEnergy(W)

(maxoutput)

Mass(kg)

ElectricalEnergyto

MassRatio

HeatEnergytoMass

Ratio

Pioneer

SNAP-19

RTG

Radioactive

UTP (Uracil

Triphosphate)

40.3 255 13.60 2.96 18.75

Voyager

MHW-RTG

plutonium-

238

160 2400 37.70 4.24 63.66

Cassini

GPHS-RTG

plutonium-238

300 4400 55.90 5.37 78.71


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The Voyager 1 spacecraft is expected to lose contact with Earth in 2025, 48 years after

launch, more than double the lifespan of Pioneer. Power conservation only began in

2007 when the PLS was switched off. Voyager 1 was fully operational for 30 years.

Cassini is presently in full operation around Saturn. Its mission has been extended until

2017. The fate of Cassini after this date is presently undecided. The Cassini spacecraft

produces more electrical and thermal energy than any mission previous and has higher

energy to mass ratios than its two predecessors.

6. Conclusion

Three have been three spacecraft probes to visit the planet Saturn. Pioneer 11, launched

in 1973 was the first to take up-close images of the planet.

Voyager 1 and 2 were the second and third probes to visit Saturn, launched in 1977

with the aim of visiting all four gas giants.

The Cassini-Huygens mission involved landing a probe on the large moon of Titan. It

was launched in 1997 and is to this day orbiting Saturn and returning in depth

recordings and scientific data.

The advancements and development of scientific instruments are clear and present

from each spacecraft mission. Each spacecraft mission has returned new and intriguing

information and data from Saturn, establishing the need for further in-depth

investigation.

Each spacecraft mission was pioneering in its own right, revolutionising space

exploration and discovery. Space exploration will continue to expand our knowledge ofthe Solar System thanks to the spacecraft, Pioneer 11, Voyager and Cassini.


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