the giant planet of the solar system - web viewkey word : uranus, giant ... (important for optical...

The giant planet of the solar system Summer School Alpbach 2012

1 Introduction

The Giant Planets are divided into two groups, where the Ice Giant are Uranus and Neptune because of their different composition. They are called like this because of their composition of ices including water, ammonia and methane. Uranus is especially interesting because of its highly asymmetric configuration of the magnetic field and clearly separates this planets from Jupiter and Neptune. To get a better understanding of the Solar system one must understand all its components. Therefore Uranus is of special interest because so far this is still a cryptic object where its inner structure, atmosphere, magnetic field, ring and satellite system among other aspects are still not completely understood. Also the ESA Cosmic Vision Programme mentioned that the scientific exploration is essential to understand the fundamental processes in the Solar System. The iTOUR mission concept is unique in having an orbiting mission with two satellites so far away from the Sun. The design takes especially the different requirements of a detailed magnetosphere study into account as well as a explicitly high resolution study of the composition and the structure of Uranus and it’s system. The unique possibility to have the time resolution of the magnetosphere and atmosphere at two point measurements will give as a better understanding magnetospheric processes.

2 Scientific objectives2.1 Uranus

Uranus was first discovered by William Herschel in 1781. Uranus is one of the gas giant planets, specifically an icy giant planet, because Uranus is not mainly composed of rock or other solid matter but of water, methane and ammonia [wiki].

Aphelion 20.1 AUPerihelion 18.4 AUOrbital period 84.3 yrNumber of known Satellites

27 (5 larger moons)

Orbital Speed 6.8 km/sAxial tilt 97.8°Magnetic field tilt 59°Equatorial radius 25,559 ± 4 kmPolar radius 24,973 ± 20 kmNumber of Rings 11

Fig 1: The known facts about Uranus [wiki].

After Voyager 2’s fly-by of Uranus in 1986 (for more information see Ref. [A]) some more facts about the system were available as well as the fact that the rotation axis is unlike the other plants of the solar system aligned in the ecliptic plane. The magnetic field is especially interesting because it is unsymmetrical and has a tilt of 59° in respect to the rotation axis. Uranus has a particularly cold planetary atmosphere and has winds in the upper atmosphere as known from Voyager 2.

2.2 Scientific case

Our mission fits with ESA’s cosmic vision goals of exploring how the solar system works, investigating the conditions for planetary formation and matches NASA’s decadal survey’s specific recommendation for a mission to Uranus. Due to the interesting alignment of the rotation axis as well as the magnetic field, Uranus is a very interesting target to investigate. Miranda, one of the main moons of Uranus shows surface traces of a big impact and therefore the surface composition can give us information about the inner structure [?]. Another of the bigger moons, Titania, shows traces of an ocean layer below the surface [E]. For comparison, remote sensing from the Earth could support the measurements of the survey. We will focus our measurement on these four objectives:• Characterise Uranus’ structure• Characterise Uranus’ atmosphere• Characterise & investigate the Uranus’ magnetosphere• Study Uranus’ satellites and ring system

2.2.1 Uranus interior and atmosphere

One of the major goals is tostudy the atmospheric composition, temperature and pressure profiles, atmospheric dynamics and global circulation. These

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ABSTRACT

Jupiter, Saturn, Uranus and Neptune together form the „Giant Planets“. Among these four planets Uranus and Neptune share a special position since they are mainly composed of Ice. Therefore they are also called the “Ice Giants”. Studying these Planets in greater detail could lead to a better understanding of the Solar System and it is evolution. The chemical and physical processes in the Solar System are to date not fully understood and more knowledge will help to understand even other planetary systems. The main goal of the investigative Tour of Uranus (iTOUR) is to fill the described gaps. Mission objectives include the observation of the atmosphere, magnetosphere the ring system and the five main satellites. The exploration of the magnetosphere and atmosphere is of special interest for iTOUR and because of that the mission includes two Orbiters which will make two point measurements possible.

Key Word: Uranus, giant planets, icy giant, solar system, planetary science, atmosphere, magnetosphere, spacecraft.

iTOUR : Investigative Tour Of Uranus

Orange Team : Fabian Duschel, Ingo Gerth, Myrtha Hässig, Kevin Hayes, Kostas Konstantinidis, Piotr Lewkowicz, Jane Mac Arthur, Pedro Machado, Christian Nabert, Jean-Baptiste Ruffio, Joan Stude, Claudia Terhes, Nathalie Themessl, Jorge Vicent Severa, Mingyu Wu.


research lines are also connected with the exoplanetary sciences as the extra solar planets show abundance among the already found exoplanets. Also exobiology is addressed in our research program mainly connected with the study of the presence and abundance of hydrocarbons (methane, ethylene, acetylene…), another important issue is to study the presence of chemical species such as CO, HCN, PH3, NH3.The mission’s scientific approach was based on the current understanding of the Uranus system. Beginning with results obtained with Voyager 2 (Stone E.) and ground based measurements we also studied the proposed models for dynamics and evolution. Some striking aspects of the Uranus’ atmosphere that we will address are: The unexpected high velocity winds in the upper atmosphere and the latitudinal wind profile that presents a prograde wind jet at the equator and retrograde wind jets at mid-latitudes (~ 50°). The main goals of the proposed scientific program focus on the characterization of the atmospheric composition and dynamics. Furthermore the retrieving of pressure, temperature and special species density altitude profiles and structure layers must also be considered. Atmospheric research topics proposed in our scientific mission: - Generate wind velocity maps in order to constrain the

global atmospheric circulation- retrieve wind velocities (zonal and meridianal) by the

cloud tracking method at cloud tops level- obtain altitude pressure, temperature and methane

profiles- determine the 3-D temperature structure in the

mesosphere- constrain the atmospheric weather dynamics with

lightning flashes sounding (night side) and combine with complementary plasma and magnetic field measurements

- obtain maps of material tracers in tropospheric dynamics (1-5 bar)

- Determine the 3-D temperature structure in the mesosphere using selected atmospheric species with a exobiology relevant approach (CH4, H2O, HCN)

- Measure H2O and CO abundances at 250 µm. - Determine abundances of H2, CH4 and other

hydrocarbons as ethane, ethylene and acetylene- First approach map of the stratospheric particles’ and

haze distribution related to photochemistry or auroral energy deposition.

2.2.2 Uranus magnetosphere

The rotation axis near the ecliptic, with a tilted magnetic moment results in Uranus's environment being an extraordinary natural laboratory for magnetospheric investigations. Due to seasonal changes, a configuration with the planetary dipole moment aligned with the solar wind is possible and is unique in the solar system. Starting

from the general magnetospheric structure of Uranus, including the attached magnetopause and bow shock, we want to learn about the solar wind interaction with such unusual magnetic field configuration. The related magnetosphere may lead to unusual dynamics within. For these reasons, the magnetopause and bow shock, as well as the plasma population must be determined. Furthermore, the inner structures containing radiation belt and ionosphere need to be investigated with respect to the solar wind conditions.

The Voyager 2 and ground-based detections found that Uranus has a relatively well developed aurora, which is seen as bright arcs around both magnetic poles [Herbert, 2009, Lamy et al., 2012]. The auroral radio emissions (AREs) which are generated above the ionosphere have also been discovered by Voyager 2 in 1986 [Herbert et al., 1994]. However, because of the lack of in-situ measurements, the heating effect of aurora [Melin et al., 2011], the generation and features of AREs are still not clear.

2.2.3 Uranus planetary system

Uranus has a rich planetary system of both dusty and dense narrow rings and up to 27 regular and irregular satellites.Regarding Uranus’ 5 major satellites, Miranda and Titania showed interesting features based on Voyager-2 measurements during its flyby in 1986. Miranda shows signs of an intensive geologic activity while Titania shows evidence of water ice and a possible sub-surface ocean.Our mission objectives aim to characterize the properties and evolution of these satellites by mapping the surface composition and internal structure, in particular showing the evidence of any sub-surface ocean, together with the analysis of their interaction with Uranus’ magnetosphere.

In addition, the ring's 3-dimensional structure, including the vertical structure of the halo, can be explored by imaging from a variety of viewing geometries. Since complete mosaics of the Uranian system are essential to understand their main characteristics, high resolution images are obtained at different opening angles and phase angles in order to decouple the rings' variations depending on radius, vertical distance from the ring and phase angle. In addition, multi wavelength mapping is used to study the composition of ring particles, but also the photometric behavior of the rings.

Uranus has a rich planetary system of both dusty and dense narrow rings and several regular and irregular satellites. The ring's three-dimensional structure, including the vertical structure of the halo, can be explored by imaging from a variety of viewing geometries. Since complete mosaics of the Uranian system are essential to understand their main characteristics, high resolution images are obtained at different opening angles and phase angles. This is done in order to decouple the rings' variations depending

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on radius, vertical distance from the ring and phase angle. In addition, multi wavelength mapping is used to study the composition of ring particles, but also the photometric behavior of the rings.

2.3 Scientific requirements

2.3.1 Uranus interior and atmosphere

We wish to carry out different altitude measurements for sounding Uranus’: upper atmosphere at µbar pressure levels (UV from Rayleigh scattering + aurora features study). The ultra violet imaging spectrometer (UVIS) is the recommended instrument to perform the requested measurements. At visible wavelengths we will measure the reflected solar radiation at cloud tops (pixelscale< 15km/pixel) using the narrow angle camera (NAC). Thermal infra-red (IR) and spectral measurements will be performed in order to obtain wind velocity measurements by the cloud tracking method (methane clouds, dayside), constrain atmosphere composition and chemical species densities. Visible infra-red high imaging spectrometer (VIRHIS) must include the wavelength range visible (V) and near infra-red (NIR) (~3.7µm) and CH4 absorption bands (V~0.89 µm and NIR~2.3 µm), pixelscale < 15km/pixel and accuracy pointing better than 1 arcmin. At Sub millimeter range we could use the data obtained with the SWI heterodyne based instrument that will obtain the radiation due to collision induced transition absorption of H2 gas and aerosol particles. We will measure de broadening of molecular lines at chosen wavelengths for each chemical studied species and also the associated Doppler shift for dynamic purposes. For the deep atmospheric and ice layers sounding we will benefit from the radio wavelength sounding obtained using the radio and plasma wave instrument (RPWI). Cross measurements with the magnetic field scientific study will also be considered. Performing repeated radio oscillations with the ultra-stable oscillator (USO) from our communication device (latitude and time) will allow us to retrieve pressure profiles as a function of altitude. We will also perform solar and stellar oscillations in the NIR and UV, in order to produce high resolution temperature and methane profiles. The 3-D temperature structure in the mesosphere will be determined using selected species (CH4, H2O, HCN) between 400 mbar – 1 bar. Multispectral imaging on the dayside (0.4– 5.3 µm) and nightside (4.0-5.2 µm) will allow us to observe H2O and CO at 250 µm with submm instrument (SWI) instrument by sounding between 0.5-3 bar (pixelscale<200 km/pixel). Determine the tail of the magnetic field by two observation points close to the planets.

2.3.2 Uranus magnetosphere

Starting from the magnetic field configuration, the best time to explore Uranus is around 2045. The general structure of the magnetosphere with its particles require

magnetic field as well as particle instruments. In most regions, the field strength is comparable to the Earth's field strength and thus, a usual fluxgate magnetometer for the field strength and search coil magnetometer for variations between Hz and kHz can be chosen. The plasma particle instruments have to measure, in particular, protons and electrons in the range of eV up to several keV under low density conditions of the order 0.1/cc to 1/cc (Toth, G.). Two spacecraft will be able to determine the time- and spatial dependent processes of the magnetosphere.

The Ultraviolet Imaging Spectrometer (UVIS) onboard the iTOUR mission should give high resolution images of the aurora which cover the wavelength of the brightest detected features (95~120nm). It is required that the Radio and Plasma Wave Instrument (RPWI) covers at least the frequency range of aurora emission 1~1000kHz. Additionally, the Visble-Infrared image spectrometer, plasma package and magnetometer onboard can also supply useful information for the study of the aurora.

2.3.3 Uranus planetary system

The instruments selected to study the composition and inner structure of Uranus’ moons should provide high spatial resolution (<5m for Miranda-Titania and <100m for the other major moon) surface imaging for geomorphology and cratering rating using a Panchromatic (PAN) and multispectral imager. These measurements should be combined with hyperspectral (VIS to IR region) measurements to exploit the synergies and retrieve information about. In addition, radio measurements can be used for retrieving the s/c orbit and, therefore, the gravitational field and inner structure can be retrieved.

iTOUR shall characterize the physical and chemical properties of Uranus's rings. To determine the phase function and color of the entire ring system a resolution finer than 100 km/pixel is necessary, as well as a sensitivity to reflectivity’s of 10^-8. Therefore, images of at least ten phase angles have to be obtained, as well as several visual and near- infrared images in broad-band filters, which require viewpoints of at least a few degrees out of the ring plane.

2.4 Instruments

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Fig 2

VIRHIS (Visible and InfraRed Hyperspectral Imaging Spectrometer)

UVIS (UltraViolet Imaging Spectrometer) RSI (Radio Science Instrument) SWI (Submm Instrument) NAC (Narrow Angle Camera) RPWI (Radio & Plasma Wave instrument, inc

Search Coil Magnetometer) FGM (Flux Gate Magnetometer) INCA (Imager Neutral Camera) ELS (Electron Spectrometer) HPS (Hot Plasma Spectrometer) DPU (Digital Processing Unit) SCM (Search Coil Magnetometer ELS (Electron Spectrometer) HPS (Hot Plasma Spectrometer)

3 Mission design3.1 Architecture options and tradeoff

Several mission designs were discussed like a multiple cube satellite mission or a balloon but these solutions appeared not to be realistic. The two ideas that remained because of engineering feasibility were an orbiter with a probe and a main orbiter with a slave orbiter.

Design For Against

Orb

iter

&Pr

obe - In situ measurements

of the atmosphere (noble gazes).

- Less magnetic field observations.

Orb

iter

& sl

ave

orbi

ter

- Simultaneous measurements for spatial scale determination of the magnetic phenomena. It becomes possible to connect the magnetospheric response with solar wind variations.- The main spacecraft can be focused on the remote observations while the slave is focused on the magnetic observations.

- No in situ measurements possible of the planetary surface.- Data rate may be low because of the omnidirectional antenna, the 2 Mkm and the power limitation. We can let the slave orbiter store the data and send only the interesting one for solving that issue.

Fig 3

The design with the spinner slave satellite appears to be a good compromised to fill the science cases with the technical requirements. The spinner will embark a magnetometer and particle measurements instruments as well as the main orbiter. But the main orbiter will also provide all the remote sensors for atmospheric studies.

3.2 Orbit justification

A polar insertion orbit can be reached without additional ΔV cost besides the standard OI ΔV. The inclination is constrained by the declination of the incoming trajectory. The orbital constraints lead to 3 characteristic orbits. Intermediate orbits are possible.

Fig 4: The three types of orbits allow by the orbital requirements.

The choice of the orbit has been made with a trade matrix, listing the advantages and disadvantages of each orbit.

Id Advantages Disadvantages

1 - Magneto tail crossing- Close in day side

- Night side all the time- Periapsis on day side

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Instrument (Main)

Mass [kg]

Margin Total mass

Heritage

VIRHIS 17 20% 20,4 JUICEUVIS 6,5 20% 7,8 JUICERSI 4,5 10% 4,95 JUICESWI 9,7 30% 12,61 JUICENAC 8,6 20% 10,32 LORRIFilter wheel for NAC

0,5 20% 0,6 Mars Pathfinder

RPWI 6,8 5% 7,14 CASSINIFGM 3,1 5% 3,255 DOUBLE-

STARINCA 16 5% 16,8 CASSINIPlasma package

JUICE

2 x ELS 1,4 30% 1,82 JUICE2 x HPS 1,6 30% 2,08 JUICEScanner 1,5 30% 1,95 JUICEDPU 2 30% 2,6 JUICED-DPU 1,5 30% 1,95 JUICETotal: 94,3

Instrument (Slace)

Mass [kg]

Margin Total mass

Heritage

FGM 3,1 5% 3,255 DOUBLE-STAR

SCM 2 20% 2,4 THEMISPlasma package

JUICE

ELS - 1 0,7 30% 0,91 JUICEHPS - 1 0,8 30% 1,04 JUICEDPU 2 30% 2,6 JUICETotal: 10,2


2

- Bow shock crossing- Time for remote measurements- Long enough in the magnetosphere

- No Magneto tail- Too long outside the magnetosphere

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- Bow shock crossing.- Time for remote measurements- Day side all the time

- Too long outside the magnetosphere - We can’t study the magneto tail.

Id Magnetosphere study

Remote observation Total

1 10 50 602 90 70 1603 70 80 150Fig 5: a) Tradeoff matrix of the different orbits. b) Rate matrix of the different orbit (100 = great, 0 = No measurement possible) in regard with the principal scientific measurements

The second orbit looks to be the best one but it is possible to make it even better using an intermediate orbit between the second and the third.

Fig 6: a) Magnetosphere and magnetopause description with the interesting regions and the reconnection point. b) Intermediate orbit chosen to fit to the scientific case.

3.3 Interplanetary trajectory

The iTOUR mission will be launched on an Arianne V ECA launch vehicle. The launch is planned to take place on Sep 11, 2026. The combined spacecraft will be put on a direct injenction interplanetary trajectory and will follow a Venus – Earth- Earth – Jupiter (VEEJ) sequence of gravity assists that will reach Uranus after 18.5 years. A deep space maneuver is planned soon after the Jupiter gravity assist to target the desired Uranus Insertion Orbit.Another option for the interplanetary transit would be a Venus – Venus – Earth – Earth flyby sequence. Although this sequence would give less total ∆V from the gravity assists, it would offer greater flexibility for the launch dates, due to the shorter Venus – Earth synodic period.The Jupiter flyby could pose an issue for the spacecraft. In the current trajectory we are spending 42 hours within Europa orbit, the “danger zone” of Jupiter’s

magnetosphere. We can mitigate this by performing the gravity assist at a greater distance from Jupiter, but with impact on the ∆V derived from the gravity assist and on trip duration.Once reaching the vicinity of Uranus the Spacecraft will perform a periapsis burn to enter a baseline polar orbit of 1.5 x 70 RU. Once the spacecraft reach this orbit, the Master and the Slave spacecraft will separate using minor amount of ∆V provided by the Master. The polar orbit was chosen because it allows us a small periapsis distance without intersecting the rings of Uranus, which are assumed to present a hazard within a distance of 2 RU.

Fig 7

Interplanetary Trajectory DataLaunch Date Sep 11, 2026Arrival Date Mar 20, 2045Gravity assists VEEJAR 5 ECA Launch capacity 4300 kg (5160 kg)Mass at launch needed 4100 kgFig 8

Both spacecraft remain on the Baseline orbit and remain in that configuration for the duration of the Uranus scientific phase. They have only a difference in true anomaly. During that phase the Master spacecraft will conduct a survey of the system and will assess possible interesting targets for further investigation during the Moons science phase. In an estimated time of 1 – 2 years, when the requirements of the first Uranus Science Phase are sufficiently satisfied the Master will perform apogee burns to target the moons. The Master will then go into a resonant orbit with that moons that will allow flybys per satellite orbit. The Master will include sufficient ∆V to perform sequential flybys of all 5 of the main moons.

3.4 ΔV budget

Maneuver ∆V (km/s)Interpl. navigation dV 0.125Uranus OI 0.92Miranda orbit 0.08Ariel orbit 0.12Umbriel orbit 0.1Titania orbit 0.18Oberon orbit 0.15

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Moon tour navigation 0.17TOTAL MARGIN 1.93Fig 9

3.5 Observation scheduling

Scheduling and observation modes are programmed for best scientific return, fulfilling downlink, power and time constraints. After the cruise phase, the mission will start its operational scientific phase which can be divided in two main sub-phases:• Uranus phase (~2 years): Baseline orbit for reconnaissance of the whole Uranus system.• Moon phase (~3 years): Exhaustive study of Uranus’ moons by flybying combined with observations of Uranus atmosphere and interior.

Several scientific observation modes have been envisaged, combining measurements from different instruments focusing on particular scientific questions:• Uranus System Survey (USS) mode: Reconnaissance of the Uranus system by imaging the planet, rings combined with measurements of the magnetic field and magnetosphere.• Atmospheres & Interior (A&I) mode: Thorough analysis of Uranus atmosphere composition and dynamics together with gravity field measurements.• Magnetosphere Research (MR) mode: Exhaustive study of Uranus magnetic field and magnetosphere.• Moon Flyby (MF) mode: Detailed observation and analysis of each moon, focusing on surface and inner composition.

4 Spacecraft4.1 Electrical power

Due to the extend duration of the mission and the distance of Uranus from the sun it is unfeasible to use solar panels to provide the power for this mission. For this reason RTG’s were considered. RTG’s are more efficient for a large ΔT and because of the cold temperatures at Uranus are ideal in this case. In order to supply sufficient power to the instruments and subsystems three RTG’s with a power of 160 W each was needed for the Master satellite and one with the same power for the slave. Although Plutonium degrades by 1.2% each year it was found that the remaining power will still be sufficient to support the instruments and subsystem. It’s worth noting that since the efficiency of RTG works on the base of ΔT, during the flyby of Venus the power will be less than that of the Uranus orbit, where solar radiation is small. Table 1 shows an example of the orbits at which each instrument must operate. Table 2 is an example of the power consumption of the iTOUR mission.

Base load Orbiter [W] Slave [W]AOCS 40 16OBDH 8 8Thermal Control 15 5

Communication (receiving) 50 17Power 12 12Total consumption at any time 125 58Total with margin (20%) 150 70Total Watt available at lift-off 480 160Total Watt available after 23 Y 360 121Fig 10: shows the calculated amount for every subsystem and the two mode: payload operation and communication.

Fig 11 shows example of which instruments on which scenario have to work together in Master satellite during the orbit on Uranus. Fig 12 is example of power consumption of iTOURS’s mission.

Instruments 1D 1N 2D 2N 3D 3NVIRHIS[W] 24 - 24 - 24UVIS[W] - - - 28.8 - -RSI [W] - - - - 28.6 28.6SWI[W] - - - 60.8 60.8 -NAC[W] 7.35 - - 7.35 - 7.35Power with Margin [W]

31.35

- 24 96.99

89.4 59.95

Fig 11: The different columns represent the different orbits: 1D, first orbit in day side, 1N, first orbit in night side. The lines show the intruments and when they are used with the power consumption. Only the first orbits or represented.

4.2 Thermal

The transit to Uranus includes a Venus gravity assist. This required the design of a system which can both tolerate the solar radiation experienced during a heliocentric orbit less than 1AU, and maintain an acceptable inner temperature while orbiting Uranus. The method employed for calculating the required radiator area was first carried out for Uranus considering the parameters: heat generated inside of the spacecraft, distance from sun and the α/ε of the materials selected. Once the radiator area was determined the next step was to consider the increased temperature encountered during the Venus flyby. For this stage in the transit the antenna will reflect most of the solar radiation and so the radiator can be considered sufficient to control the thermal system for both satellites. This solution only applies when the radiator is not facing the sun. Two of the cameras in the Master satellite only require passive cooling however the VIRHIS system must be cooled to 70K. Each of these are protected from RTG heating by shades placed near the RTG’s and also it will use Stirling cryo-

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coolers. 23 Kapton layer MLI similar to that used in the Venus Express mission to shield both satellites in the hot environment. Fig G includes the radiator properties.

Sate

llite

Solar radiation

Albedo

α ε Tem. Area

Wm2

- - - K m2

Ura. M 3.4 0.282 0.07 0.74 293 0.84S 0.12 0.9 293 0.30

Ven. M 2657 0.82 0.07 0.74 305 0.84S 0.12 0.9 281 0.30

Fig 13: Material for radiator is white paint (silicate)

4.3 Communication

Communication is the link between Earth and Spacecraft.It is needed not just to get the scientific data from the object of interest, but also to operate and maintain its systems and subsystems. iTOUR Com-System is mostly based on Cassini heritage in order to avoid any pitfalls. Likely advances from 30 years development can be implemented to improve the bandwidth for deep space communications. As this study shows, the downlink to Earth is the crux of the mission. The instruments are able to collect much more data than the scientific requirements. At the time of the arrival (2045), Uranus is almost in his Perihelion of about 18.5 AU. The distance to Earth varies therefor with ± 1AU to 17.5 to 19.5 AU.As later mechanical design constraints showed the diameter had to be reduced to 3.7m for iTOUR. ESA provides 3 ground stations with 35 m antennas for deep space communications: Cebreros (Spain), New Norcia (Australia), Malargüe (Argentina).

The iTOUR Com-System is designed for S (3 GHz),X (8 GHz) and Ka (32 GHz) band transmissions. The budget is calculated with X-band as a present standard in deep space networks. The achievable data rate from Uranus is 3.2 kbps.For an 8h downlink per day and orbit. The communication between the Master and the Slave done via the HGA from the Master and an omnidirectional helical antenna on the Slave. The antenna transmits in the plane of the orbits. To receive data from the Slave some dedicated time per orbit is used to point the HGA to the Slave. This is might be also possible beside earth transmissions. The minimum needed data per orbit is assumed with 80Mbit.With a maximum distance of 2 Mkm a data rate of 3kbps can be achieved and it will take 9 hours (2h@1Mkm) of transmission.

4.4 Propulsion

The propulsion system design is driven by the required ΔV (see Fig 9). In a literature survey of previous mission studies to the outer planets, a chemical propulsion system was identified as preferable over a low-thrust solution for the interplanetary transfer. The selected system for the Master is an MMH/NH4 bi-propellant solution that is available COTS from Astrium Space Propulsion. As thruster, one 500 N/>325 s Isp European Apogee Motor will be used, which is the successor of Astrium’s 400 N thruster with long deep-space heritage. A self-developed sizing tool was used for estimating the system characteristics. One 120 cm diameter/60 kg Ti-6Al-4V tank is used for fuel and propellant respectively, with two 80 cm diameter/38 kg He pressurant tanks. This is advantageous because the tanks are shared with the AOCS thrusters, saving system dry mass. The total system dry mass amounts to 254 kg.

No major Delta V manoeuvres are planned for the slave, so there is no need for a main propulsion unit. For spin-up, momentum dumping, and possible avoidance impulses, 12 CHT-20 monopropellant hydrazine thrusters are used, ensuring redundancy. The tanks are of the same build as for the master, using one 60 cm diameter/8 kg propellant tank, and one 25 cm diameter/3.7 kg tank for the helium pressurant.

4.5 AOCS

Mainly driven by the requirements of 0.8 arc/sec (1 sigma) pointing accuracy with 0.01˚/s pointing stability the following components form the attitude and orbit control system. 2 star trackers and 2 inertial measurement units were used as positioning sensors for both satellites however due to conflicting requirements for the master and slave satellites a different method of stabilisation was needed for each. For the master satellite 3-axis stabilisation (important for optical measurements) was realised through the use of 4 25Nms reaction wheels (1 for redundancy) and 24 5N hydrazine reaction control thrusters. As for the slave satellite spin stabilisation (important for magnetospheric and particle field measurements) was required and was realised through the use of 12 5N hydrazine reaction control thrusters. Because of the need for attitude correction roughly every two orbits of 10m/s and an allowance of 50m/s for safe mode recovery the total delta-v was found to be 1700m/s for the master and 700m/s slave. This resulted in a propellant mass of 1018kg and 85kg respectively. These figures were larger than initially expected however were required considering the duration of the mission.

4.6 Special arrangements

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One of the major requirements of the spacecraft design was the development of a reliable separation mechanism for the slave satellite. After a number of heritage studies it was decided that the separation mechanism for the Huygens probe was the most suited to our needs. Features of this system include: • separation via Pyro-nuts and bolt-cutters• ejection by means of compressed springs which provides an additional delta-v during separation• Spin-up via helical tracks and rollers which imparts an initial spin on the slave satellite• Umbilical connecter’s separation system which separates the electrical systems of the master and slave satellite.• An additional benefit of this system is its low weight (23kg) which is a major driver on this mission considering its duration and distance.

The advantage of a movable antenna is the ability to perform both gravitational field and optical measurements simultaneously. This was a disadvantage of the Cassini configuration as having a fixed antenna led to staggered data collection and a compromise between the respective science groups. For the articulation mechanism it was decided to use a system based on Rosetta heritage. The important features of this system in respect to our requirements were:• High shock and vibration resistance – Ariane 5 launch platform• Low temperature performance: 50K min and 343K max (reflective coating on antenna) • High pointing accuracy: ≈ 0.1°

A significant drawback of this configuration is the increased risk due to possible failure of one of the connections. To mitigate this risk the articulation mechanism will be programmed to return to the optimal static configuration upon failure of one of the connections resulting in a reduced scientific payoff but not catastrophic failure of the mission.

4.7 Configuration and structure

Fig 14

The iTour configuration was determined by conducting a trade-off of previous mission concepts and the final choice

was based on the JEO/ Bepi-Colombo heritage. A stacking sequence, with the slave-satellite on top of the master satellite was considered the best concept as it provided better stress behavior during launch and higher stability when discarding the slave-satellite.

The inner structure of the spacecraft is a hexagonal thrust tube made from Aluminum honeycomb with Aluminum sheets; panels of 3.5[m] in height and 0.917 [m] in width; where the propellant tanks and the pressuring tanks will be accommodated. The hexagonal inner structure will improve the resistance against the loads, which are the highest during launch.

Fig 15: iTOUR structure configuration.

The outer primary structure of iTour spacecraft has an octagonal shape whose dimensions are: height 3.5 [m] and side length of 1[m]. A truss structure was chosen in order to save weight as this is one of the major drivers of our mission. In order to support the HGA, optical instruments, remaining payloads and spacecraft subsystems the top, bottom and two sides were made from reinforced composite.The slave-satellite is a small scale version of the master spacecraft with a similar shape and material properties.

An iterative approach was adopted in order to determine the optimum dimensions of the structure required to support the loads and accommodate the instruments and subsystems.

4.8 Mass

ORBITER SLAVESub-system Mass

without margin [kg]

Total mass [kg]

Mass without margin [kg]

Total mass [kg]

AOCS 60,2 66,2 36,7 40,4Power 110,0 121,0 38,0 41,8Comm 170,0 187,0 30,0 33,0Propulsion 294,8 324,3 14,7 16,2OBDH 65,0 71,5 23,0 25,3Thermal 60,0 66,0 15,0 16,5Structure 200,0 220,0 65,0 71,5Payload 94,3 113,2 11,0 13,2Boom 3,0 3,3 6,0 6,6Sub-system total 1057 1171 240 265

System 20 % 20 %

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marginDry mass Orbiter 1407 317

Propellant 2285 83 91Wet Mass 3692 408Launch Mass 4100

Fig16

4.9 Lifetime and decommissioning

The life time is inferred from the previous Cassini mission. It lasted 20 years and was launched in 1997. Saturn radiation level is worse than Uranus and the temperature is cold. iTOUR will be launched almost 30 years later so we expect technological improvement. The journey to Uranus last 18.5 years and we expect a 5 years mission. A 23 mission duration looks feasible in comparison with Cassini.

5 Key issues 5.1 Planetary protection

The Uranian system falls under Category II of the COSPAR Planetary Protection policy. This means that although the system might be interesting for understanding the chemical evolution and origin of life, chances are still faint that a spacecraft could compromise future investigations through contamination. That is why a brief planetary protection plan suffices for this mission and would be written at a later stage. There is no direct need for end-of-life disposal of the spacecraft.

5.2 Risk

The risk management of iTOUR includes mainly those risks which are of special interest for the mission to the Uranian system. With proven approaches we managed to calculate two major risks with a severe impact on the mission. A failure at orbit insertion and the collision with an unknown object would lead to a fail mission but for bought cases mitigation activities could be set. For the orbit insertion simulations and inhibition of safe mode could minimize the risk but of course a residual risk would remain. To avoid a collision with an unknown object an early investigation of the equatorial disk could definitely lower the likelihood of such an incident. Other known risk such as, the large temperature gradient between the Venus fly-by and Uranus and the risk of RTG’s at launch and earth fly-by would have to be targeted in a more detailed mission study.

5.3 Cost estimation

Cost estimation was done by analysis of expert Denis Moura of the European Defence Agency and is based on comparison to previous missions. The estimate is based on the Launcher used, the number of spacecraft used, mission duration for operational costs and the number of ground

stations and access time. Further, masses of payload, bus and propellant were considered. Gravity assists were considered to add costs because of added operations effort. RTGs were accounted for by adding costs to the launcher and for the RTGs themselves. Based on this information, the total cost estimate is M€1750, rough order of magnitude.

Contributor Cost/M€Ariane 5 with RTG mods. 175Master: Platform 1150Master: Payload 100Slave: Platform 200Slave: Payload 20Total 1750Fig 17

5.4 Descoping options

A decrease in scientific questions leads directly to a reduction of payload and therefor mass. Most obvious the Slave can be eliminated with a saving of about 220M€.If more payload is eliminated it might be possible to go with a smaller launcher. For this study recommended ESA margins has been through fully used which makes it a conservative approach.

5.5 Communications and outreach

6 Conclusion

Voyager 2 was the last probe visiting Uranus by passing by in 1986 and beside pictures from Hubble space telescope no present data is available. The detailed study of one of the ice giants as Uranus will improve our understanding of the dynamics, chemical and physical processes in the Solar System.iTOUR will explore one of these last dark spots in our Solar System.It will be the first dedicated mission to one of the icy giants to study its interior structure & atmosphere, as well as its magnetosphere, rings and moons in great detail. The two orbiter will provide a unique possibility to study the magnetosphere in spatial and time variations.

Despite the great financial & technologic efforts it will help mankind understanding the solar system, even beyond our own!

7 Acknowledgements

The Orange team would like to give special thanks to our two tutors Marcus Hallmann and Anna Milillo for their excellent supervision and support. In addition we would like to thank all the tutors for their help they have provided, their nice feedback and their good ideas. We give a special thanks to Michaela Gitsch and Peter Falkner for organizing

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this summer school. Finally we would like to thank all the speakers, the administration staff, the program committee, and all the students at the Alpbach summer school for an amazing experience.

8 References

(Everett, Wertz, & Puschell, 2011)

(G., D., Keneth, & Grambosi, 2004)

(Larso, Pranke, Connolly, & Giffen)

(Russell, 2003)

(Stark, Swinerd, & Forescue, 2002)

(Study, 2012)

(Udry, Benz, & Steiger, 2003)

(Wertz & Larson, 2003)

(Stone, 1986)

(T. Encremenaz, 2008)

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the giant planet of the solar system - web viewkey word : uranus, giant ... (important for optical...

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