THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

1

1 Introduction

Among the giant planets in the Solar System, Uranus and Neptune are known as the Ice Giants due to their internal composition of ices including water, ammonia and me-thane. Uranus is especially interesting because of its highly asym-metric configuration of the magnetic field and the rotation axis being aligned to the ecliptic plane, different to all the other planets. Its interior, atmosphere, magnetosphere, ring and satellite system are not yet well understood. Based on ESA’s Cosmic Vision statements, the scientific explora-tion of Uranus is essential to understand the fundamental processes leading to a better understanding of formation and evolution of the Solar System. The iTOUR mission concept is unique in having two orbiters around Uranus. The proposed design allows a detailed study of Uranus’ magnetosphere, as well as high-resolution study of the composition and the interior of Uranus and its satellite and ring system. The unique possibility of having two point measurements of the spatial and time variations of the magnetosphere and magnetic field will give a better understanding of the magnetospheres’ processes.

2 Scientific objectives

2.1 Uranus

William Herschel first discovered Uranus in 1781. It was observed to be 14.5 times larger than the Earth, with one orbit taking 84 Earth years to complete.

Aphelion 20.1 AU Perihelion 18.4 AU Orbital period 84.3 yr Number of known satellites 27 (5 larger moons) Orbital speed 6.8 km/s Axial tilt 97.8°

Magnetic field tilt 59° Equatorial radius 25,559 ± 4 km Polar radius 24,973 ± 20 km Number of rings 11

Tab. 1: The known facts about Uranus [1].

After Voyager 2’s fly-by of Uranus in 1986 additional scien-tific information about its system was available, in particu-lar its anomalous rotation axis that, unlike the other planets of our Solar System, is aligned in the ecliptic plane [1]. The magnetic field is especially interesting because it is unsym-metrical with respect to Uranus’ centre of gravity and its magnetic dipole is tilted by 59° with respect to the planet’s rotational axis. Uranus has a particularly cold planetary atmosphere and has winds in the upper atmosphere.

2.2 Scientific case

Based on ESA’s Cosmic Vision 2015-2025, iTOUR aims at exploring how the Solar System works and investigate the conditions for planetary formation. In addition, iTOUR also 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 has many mysteries to be solved. Miranda, one of Uranus’ major moons, shows surface traces of a big impact and therefore its surface composition can reveal information about the inner structure [2]. Another of the bigger moons, Titania, could have an ocean layer below its surface [3]. Remote sensing and in-situ measurements would allow surveying Uranus’ system focusing on the following objec-tives: • Characterise Uranus’ interior and atmosphere • Characterise and investigate Uranus’ magnetosphere • Study Uranus’ satellites and ring system

ABSTRACT The iTOUR “Investigative Tour Of URanus” mission will study the Uranus system to give crucial answers about its current state and evolution, paying particular regard to the unusual inclination and characteristics of the magnetosphere by flying a slave satellite in addition to a main orbiter. Little is known about the “Ice Giants”, Uranus and Neptune, as the only observations have been from Voyager 2’s fly-bys and Earth (and space) based telescopes. Studying the chemical and physical processes occurring on these planets in greater detail would lead to a better understanding of our Solar System, its evolution, and provide a reference for exo-planetary systems. Uranus is unique in the Solar System in having an axial tilt of 97.77

o and its magnetic field is inclined 59

o from the axis of

rotation and does not originate from the geometric centre. The main mission objectives of iTOUR are to characterise Uranus’ interior and atmosphere, magnetosphere, the ring system and the five major satellites. The proposed mission architecture includes two orbiters, allowing two-point measurements to show spatial and time variation, which is crucial for the detailed mapping of Uranus’ magnetic field and magnetosphere. Key words: Uranus, giant planets, icy giant, Solar System, planetary science, atmosphere, magnetosphere, spacecraft.

iTOUR : investigative Tour Of URanus

Team Orange: Fabian Duschel, Ingo Gerth, Myrtha Hässig, Kevin Hayes, Kostas Konstantinidis, Piotr Lewkowicz, Jane MacArthur, Pedro

Machado, Christian Nabert, Jean-Baptiste Ruffio, Joan Stude, Claudia Terhes, Nathalie Themessl, Jorge Vicent, Mingyu Wu.

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

2

2.2.1 Uranus’ interior and atmosphere

One of iTOUR’s major goals is to study Uranus’ atmospheric composition, temperature and pressure profiles, atmos-pheric dynamics, and global circulation. These research lines are strongly connected with exoplanetary sciences as a large number of known exoplanets are thought to be similar to these Ice Giants. Also exobiology is addressed in the iTOUR research program, mainly connected with the study of the presence and abundance of hydrocarbons (methane, ethylene, acetylene), together with the im-portant issue of studying the presence of chemical species such as CO, HCN, PH3, NH3. The mission’s scientific approach is based on the current understanding of the Uranus system. Beginning with results obtained with Voyager 2 and ground based measurements, iTOUR will also study the proposed models for dynamics and evolution. In addition, Uranus’ atmosphere shows some striking aspects that are addressed in the mission objectives: 1) the unexpected high velocity winds in the upper atmosphere and 2) the latitudinal wind profile, presenting a prograde wind jet at the equator and retro-grade wind jets at mid-latitudes (~ 50°). The main goals of this proposed scientific program focus on the characterization of the atmospheric composition and dynamics. Furthermore the retrieving of pressure, tempera-ture and special species density altitude profiles and struc-ture layers must also be considered. Atmospheric research topics are summarized in the following items:

Generate wind velocity maps in order to constrain the global atmospheric circulation

Retrieve wind velocities (zonal and meridian) by the cloud tracking method at cloud tops level

Obtain altitude pressure, temperature and methane profiles

Constrain the atmospheric weather dynamics with lightning flash sounding (night side) and combine with complementary plasma and magnetic field measure-ments

Obtain maps of material tracers in troposphere dy-namics (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 hydrocar-bons as ethane, ethylene and acetylene

Map the stratospheric particles and haze distribution related to photochemistry or aurora energy deposition

2.2.2 Uranus’ magnetosphere

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

spheric structure of Uranus, including the attached magne-topause and bow shock, iTOUR aims to provide information about the solar wind interaction with such unusual magnet-ic field configuration. The related magnetosphere may lead to unusual dynamics within. For these reasons, the magne-topause and bow shock, as well as the plasma population, must be determined. Furthermore, the inner structures containing radiation belt and ionosphere need to be inves-tigated 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. The Auro-ral Radio Emissions (AREs), which are generated above the ionosphere, were also discovered by Voyager 2 in 1986. However, because of the lack of in-situ measurements, the heating effect of aurora, as well as the generation and features of AREs, are still not clear.

2.2.3 Uranus’ satellites and rings

Uranus has a rich planetary system with both dusty and dense narrow rings and up to 27 regular and irregular satellites. Regarding Uranus’ five major satellites, Miranda and Titania showed interesting features based on Voyager 2 measure-ments during its flyby in 1986. Miranda shows signs of intense geologic activity while Titania shows evidence of water ice and a possible sub-surface ocean. iTOUR’s objectives aim to characterise the properties and evolution of these satellites by mapping the surface geology and internal structure, in particular showing the evidence of any sub-surface ocean, together with the analysis of their interaction with Uranus’ magnetosphere. The ring's 3-D structure, including the vertical structure of the halo, can be explored by imaging from a variety of viewing geometries. Since complete mosaics of the Uranus’ system are essential to understand their main characteris-tics, 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, hyperspectral imaging could be used to study the composition of ring particles and also the photometric behaviour of the rings.

2.3 Scientific requirements

2.3.1 Uranus’ interior and atmosphere

iTOUR will carry different optical instruments to perform different altitude measurements for sounding Uranus’ upper atmosphere at µbar pressure levels (UV from Ray-leigh scattering and aurora features study). An Ultra Violet Imaging Spectrometer (e.g. UVIS/JUICE) is the recommend-ed instrument to perform the requested measurements. At visible wavelengths, a narrow angle camera (e.g. NAC/LORRI) will measure the reflected solar radiation at cloud tops (pixel scale <15km/pixel). Thermal InfraRed (TIR) spectral measurements will provide information by retriev-

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

3

ing wind velocity measurements using the cloud tracking method (methane clouds, dayside), constrain atmosphere composition and chemical species densities. The Visible and Infra-Red Hyperspectral Imaging Spectrometer (VIR-HIS/JUICE), with a spectral range covering visible (VIS), Near InfraRed (NIR) (~3.7µm) and CH4 absorption bands (V~0.89 µm and NIR~2.3 µm, pixel resolution <15km/pixel and accuracy pointing better than 1 arcmin), is proposed as a basic payload. At sub-millimeter range, the data obtained with the SWI heterodyne-based instrument can be used obtaining the radiation due to collision induced transition absorption of H2 gas and aerosol particles. In addition, it will measure the broadening of molecular lines at chosen wavelengths for each chemical species studied and the associated Doppler shift for dynamic purposes. Radio wavelength sounding by using the Radio and Plasma Wave Instrument (RPWI) could be used for deep atmospheric and ice layer sounding. Cross measurements with the magnetic field scientific study will also be considered. Performing repeated radio occultations with the Ultra-Stable Oscillator (USO) from the spacecraft communication device (latitude and time) would allow retrieving pressure profiles as a function of altitude. By performing solar and stellar occulta-tions in the NIR and UV ranges, high-resolution tempera-ture and methane profiles could be determined. The 3-D temperature structure in the mesosphere will be deter-mined using selected species (CH4, H2O, HCN) between 400 mbar–1 bar. Multispectral imaging on the day-side (0.4– 5.3 µm) and night-side (4.0-5.2 µm) will allow observing H2O and CO at 250 µm. A Submm Wave Instrument (SWI) would achieve these measures by sounding between 0.5-3 bar (pixel resolution <200 km/pixel).

2.3.2 Uranus’ magnetosphere

Starting from the magnetic field configuration, the best time to explore Uranus is around 2045. The general struc-ture of the magnetosphere and its particles requires mag-netic 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 should 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. Two space-craft would be able to determine the time- and spatial dependent processes of the magnetosphere. The UVIS instrument onboard the JUICE 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 RPWI covers at least the frequency range of aurora emission 1-1000 kHz. Additionally, VIRHIS, the plasma package, and magnetometer onboard can also supply useful information for the study of the aurora.

2.3.3 Uranus’ satellites and rings

The instruments selected to study the composition and inner structure of Uranus’ moons shall provide high spatial resolution (<5m for Miranda-Titania and <100m for the other major moons) surface imaging for geomorphology and cratering rate using a Panchromatic (PAN) and multi-spectral imager. These measurements shall be combined with hyper spectral (VIS to IR region) measurements to exploit the synergies and retrieve information about. In addition, radio measurements can be used for retrieving the spacecraft’s orbit and therefore the gravitational field and inner structure can be retrieved. iTOUR shall characterise the physical and chemical proper-ties of Uranus's rings. To determine the phase function and colour of the entire ring system a resolution finer than 100 km/pixel is necessary, as well as a sensitivity to reflectivities 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 broadband filters, which require view-points of at least a few degrees out of the ring plane.

2.4 Instruments Based on the scientific measurements required to answer the objectives, suitable instruments with flight heritage were chosen which would take measurements in the required ranges. Trade offs were carried out between payload weight, data download restrictions, mission length and orbit options which resulted in the choice of the following instruments:

VIRHIS (Visible and InfraRedHyperspectral Imaging Spectrometer)

UVIS (UltraViolet Imaging Spectrometer)

RSI (Radio Science Instrument)

SWI (Submm Wave Instrument)

NAC (Narrow Angle Camera)

RPWI (Radio & Plasma Wave Instrument, inc Search Coil Magnetometer)

FGM (Flux Gate Magnetometer)

MENA (Medium Energy Neutral Atom Imager)

ELS (Electron Spectrometer)

HPS (Hot Plasma Spectrometer)

DPU (Digital Processing Unit)

SCM (Search Coil Magnetometer)

ELS (Electron Spectrometer)

HPS (Hot Plasma Spectrometer) A wide angle camera was rejected as it would be difficult to transmit such volumes of data back to Earth. A cosmic dust analyser was rejected on weight grounds as more science data could be obtained with other lighter instruments.

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

4

Tab. 2: Instrument configuration for the main and slave [4]

3 Mission design

3.1 Architecture options and trade-off

Several mission designs were pondered, like a multiple cube satellite mission or a balloon, but these solutions were found to be unsuitable. The two ideas that remained after considering engineering feasibility were an orbiter with a probe and a main orbiter with a slave orbiter.

The design with a slave satellite is an excellent compromise to fulfil the science desires within the technical boundaries. The slave satellite will be in a spin-stabilised configuration, and contain a magnetometer and particle measurement instruments like the main orbiter, but the main spacecraft will also provide all the remote sensing instruments for atmospheric studies.

Design For Against

Orb

ite

r

&P

rob

e - In-situ measurements

of the atmosphere (noble gases).

- Less magnetic field observations.

Orb

ite

r

& S

lave

Orb

iter

-Simultaneous meas-urements for spatial scale determination of the magnetic phenom-ena. It becomes possi-ble to connect the magnetosphere re-sponse with solar wind variations. - The main spacecraft can be focused on the remote observations while the slave focuses on magnetic observa-tions.

- No in situ measure-ments possible of the planetary composi-tion. - Data rate may be low because of limited inter-satellite com-munications and power limitations. The slave orbiter can store the data and send only interesting sets for solving that issue.

Tab. 3: Advantages and disadvantages for different archi-tectural designs.

3.2 Orbit design

The inclination of the insertion orbit is constrained by the declination of the incoming trajectory. Therefore, in this case a polar insertion orbit can be reached without addi-tional ΔV cost besides the standard orbital insertion ΔV.

Taking into account the scientific requirements along with the orbital engineering constraints, three general classes of orbits were considered, roughly depicted in Fig. 1.

Fig. 1: The three types of orbits allowed by the orbital

requirements.

Instrument (Main)

Mass [kg]

Margin Total mass

Heritage

VIRHIS 17 20% 20.4 JUICE

UVIS 6.5 20% 7.8 JUICE

RSI 4.5 10% 5.0 JUICE

SWI 9.7 30% 12.6 JUICE

NAC 8.6 20% 10.3 LORRI

Filter wheel for NAC

0.5 20% 0.6 Mars Pathfinder

RPWI 6.8 5% 7.1 CASSINI

FGM 3.1 5% 3.3 DOUBLE-STAR

MENA 16 5% 16.8 CASSINI

Plasma package

JUICE

2 x ELS 1.4 30% 1.8 JUICE

2 x HPS 1.6 30% 2.1 JUICE

Scanner 1.5 30% 2.0 JUICE

DPU 2 30% 2.6 JUICE

D-DPU 1.5 30% 2.0 JUICE

Total: 94.4

Instrument (Slave)

Mass [kg]

Margin Total mass

Heritage

FGM 3.1 5% 3.3 DOUBLE-STAR

SCM 2 20% 2.4 THEMIS

Plasma package

JUICE

ELS - 1 0.7 30% 0.9 JUICE

HPS - 1 0.8 30% 1.0 JUICE

DPU 2 30% 2.6 JUICE

Total: 10.2

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

5

The final choice of orbit was done using a trade matrix, listing the advantages and disadvantages of each orbit.

Id Advantages Disadvantages

1 - Magnetotail crossing - Close passes at day- side

- Night side all the time - Periapsis on day side

2 - Bow shock crossing - Enough time for remote measurements - Enough time in the magnetosphere

- No time in magnetotail - Too much time outside the magnetosphere

3 - Bow shock crossing - Enough time for remote measurements - Continuous dayside illumination

- Too much time outside the magnetosphere - No time in magnetotail

Id Magnetosphere study

Remote observation

Total

1 10 50 60 2 90 70 160 3 70 80 150

Tab. 4: a) Trade-off matrix of the different orbits. b) Rating matrix for the different orbits (100 = perfect, 0 = no meas-urement possible) in regard to the principle scientific measurements

The second orbit option is the best trade-off choice. To further maximize its advantages, an intermediate orbit that combines the advantages of the second and the third options is chosen, namely it crosses the reconnection regions, provides good illumination conditions for remote sensing measurements, and it spends enough time inside Uranus’ magnetosphere (Fig. 3a).

Fig. 3: a) Illustration of the magnetosphere of Uranus, including regions of particular interest, such as the magne-topause, the bow shock and the reconnection region. b) Intermediate orbit chosen to fit to the scientific case.

3.3 Interplanetary trajectory

The iTOUR mission will be launched on an Ariane 5 ECA launch vehicle. Launch is planned to take place September 11, 2026. The stacked Master-Slave spacecraft will be put on a direct injection interplanetary trajectory and will follow a Venus- Earth-Earth-Jupiter (VEEJ) sequence of gravity assists that will reach Uranus after 18.5 years. Insertion is done to escape velocity directly by the upper stage. A deep space manoeuvre 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 (VVEE) 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 and would also avoid Jupiter with its hazardous radiation belts. 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 (radii of Uranus). Once the spacecraft reaches this orbit, the Master and the Slave spacecraft will separate using minor amount of ∆V provided by the Master. The polar orbit allows for a minimum radius of periapsis without intersecting the rings of Uranus, which are assumed to present a hazard within a distance of 2 RU.

Fig. 2: Mission sequence

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

6

Launch Date Sep 11, 2026 Arrival Date Mar 20, 2045 Gravityassists VEEJ AR 5 ECA Launch capacity 4300 kg (5160 kg) Mass at launch needed 4100 kg Tab. 5: Interplanetary Trajectory data.

Both spacecraft remain on the baseline orbit and in the initial configuration for the duration of the Uranus scientific phase, having 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. When the scientific goals of the first Uranus science phase are sufficiently satisfied (1-2 years), the Moons science phase will be initiated. The Master will perform an apoapsis burn, raising the orbit’s periapsis and going into a resonant orbit with one of the moons, allowing for two flybys per satellite orbit for that moon. Once observations of that moon are deemed satisfactory, the Master will perform another apoapsis burn to target a resonant orbit with the next moon. This process is repeated, going from the inner-most to the outermost moons. The ∆V, see Tab. 6, budget is sufficient to successively visit the orbits of all five major moons. Maneuver ∆V (km/s)

Interpl. navigation dV 0.13 Uranus OI 0.92 Miranda orbit 0.08 Ariel orbit 0.12 Umbriel orbit 0.10 Titania orbit 0.18 Oberon orbit 0.15 Moon tour navigation 0.17

TOTAL+ MARGIN 1.93

Tab. 6: ∆V budget, different orbits

3.4 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 that can be divided in two main phases:

1) Uranus phase (2 years): Baseline orbit for recon-naissance of the whole Uranus system.

2) Moon phase (3 years): Exhaustive study of Uranus’ moons by flyby 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 Spacecraft

4.1 Electrical power

Due to the extended mission duration 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 Radioi-sotope Thermoelectric Generator (RTGs) were considered. RTGs are more efficient for high temperature differences and so ideal for the low temperatures encountered at Uranus. The power budget for the payload and subsystems requires the use of three RTGs with a power of 160 W each for the Master orbiter, and one RTG with the same charac-teristics for the Slave satellite. Although Plutonium de-grades by 1.2% each year, calculations showed that the remaining power would still be sufficient to support the instruments and subsystem. It is worth noting that since the efficiency of RTGs works on the base of ΔT, the power produced during the Venus fly-by will be less than that of the Uranus orbit, where solar radiation is lower. Table 7 summarises the power budget for iTOUR in two different operation modes. Table 8 shows an example of the power consumption of each instrument for different orbit.

Base load Orbiter [W] Slave [W]

AOCS 40 16

OBDH 8 8

Thermal Control 15 5

Communication (receiving) 50 17

Power 12 12

Total consumption at any time 125 58 Total with margin (20%) 150 70

Total Watt available at lift-off 480 160

Total Watt available after 23 Y 360 121

Tab. 7: Calculated amount for every subsystem and the two modes: payload operation and communication.

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

7

Instrument 1D 1N 2D 2N 3D 3N

VIRHIS [W] 24 - 24 - 24 UVIS [W] - - - 28.8 - - RSI [W] - - - - 28.6 28.6 SWI [W] - - - 60.8 60.8 - NAC [W] 7.35 - - 7.35 - 7.35

Power with Margin [W]

31.35

- 24 96.99

89.4 59.95

Tab. 8: The different columns represent the different orbits: 1D, first orbit in dayside, 1N, first orbit in night side. The lines show the instruments and when they are used with the power consumption. Only the first orbits or represented.

4.2 Thermal

The transfer to Uranus includes a Venus gravity assist. This requires the design of a system which can both tolerate the Solar radiation experienced during a heliocentric orbit at distances less than 1 AU 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 deter-mined the next step was to consider the increased temper-ature encountered during the Venus flyby. For this stage in the transfer the antenna will be used as a heat-shield and reflects 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 is protected from RTG heating by shades placed near the RTGs and will also use Stirling cryo-coolers. 23 Kapton layers for multi-layer insulation are used similar to those in Venus Express, to shield both satellites in the hot environment. Table 9 includes the radiator properties.

Pla

ne

t

Sate

llite

Sola

r ra

dia

-

tio

n

Alb

edo

Ab

sorp

tan

ce

Emit

tan

ce

Tem

per

atu

re

Are

a

- - - K

Uranus M 3.4 0.282 0.07 0.74 293 0.84

S 0.12 0.9 293 0.30

Venus M 2657

0.82 0.07 0.74 305 0.84

S 0.12 0.9 281 0.30

Tab. 9: Sizing of the thermal control system (M for Master

and S for Slave)

4.3 Communication

Communication is the link between Earth and the space-craft. It is needed not just to get the scientific data from the object of interest, but also to operate and maintain its systems. The iTOUR communication system is mostly based on Cassini heritage in order to avoid any pitfalls. As this study shows, the downlink to Earth is one of the cruxes of the mission. The instruments are able to collect much more data than the maximum that can be down-linked to Earth. At the time of the arrival (2045), Uranus is almost in its perihelion of about 18.5 AU which gives a maximum distance to Earth of 19.5 AU. The iTOUR communication system is designed for S- (3 GHz), X- (8 GHz) and Ka- (32 GHz) band transmissions. The High Gain Antenna (HGA) has a diameter of 3.7 m to fit into the launcher fairing. The budget is calculated with X-band as a present standard in deep space networks. With the given constraints above the achievable data rate from Uranus is 3.2 kbps. For an 8h downlink per day a volume of 2 Gbit can be transmitted per orbit. The communication between the Master and the Slave is 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. The minimum needed data per orbit is assumed to be 80Mbit. With a maximum distance of 2 Mkm a data rate of 3kbps can be achieved so it will take 9 hours (2hours at 1Mkm) to transmit 80 Mbit. On Earth the link to the spacecraft is provided by one of the three ESA ground stations with 35m antennas for deep space communications: Cebreros (Spain), New Norcia (Australia), Malargüe (Argentina). One station can hold contact for an average of 8h per day. It can be possible to use a second or even a third station to increase the data volume for a short period of time.

4.4 Propulsion

The propulsion system design is driven by the required ΔV (see Tab. 6). 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 propel-lant respectively, with two 80 cm diameter/38 kg He pres-suring 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.

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

8

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 pressuring.

4.5 AOCS

Mainly driven by the requirements of 0.8 arcsec (1 sigma) pointing accuracy with 0.01˚/s pointing stability the follow-ing components form the attitude and orbit control system. Three star trackers for the Master and two for the slave, and two inertial measurement units are used as attitude sensors for both satellites. However, due to conflicting requirements for the Master and Slave, a different method of stabilisation is needed for each. For the master satellite 3-axis stabilisation (important for optical measurements) is realised through the use of 4 25 Nms reaction wheels (1 for redundancy) and 24 5 N hydrazine monopropellant reaction control thrusters. As for the slave satellite spin stabilisation (important for magnetospheric and particle field measure-ments) is required and is 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 AOCS ∆V is found to be 1700m/s for the master and 700m/s slave. This results 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

One of the major requirements of the spacecraft design is 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 is the most suited to iTOUR. Features of this system include: • Separation via pyro-nuts and bolt-cutters • Ejection by means of compressed springs which provides an additional ∆V during separation • Spin-up via helical tracks and rollers which imparts an initial spin on the slave satellite • Umbilical connector’s separation system which separates the electrical systems of the master and slave satellite • Low system mass (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, especially during moon flybys. 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 Roset-ta heritage. The important features of this system in re-spect to the requirements are: • High shock and vibration resistance – Ariane 5 launch platform • Low temperature performance: 50 K min and 343 K 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 pro-grammed 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. 4: Configuration of the spacecraft

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 provides better stress behaviour during launch and higher stability when releasing the slave-satellite.

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

Fig. 5: iTOUR structure configuration

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

9

The outer primary structure of iTOUR 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 mass, as this is one of the major drivers of this mission. In order to support the HGA, optical instruments, remaining payloads, and spacecraft subsystems the top, bottom, and two sides are 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 budget

ORBITER SLAVE

Sub-system

Mass without margin [kg]

Total mass [kg]

Mass without margin [kg]

Total mass [kg]

AOCS 60.2 66.2 36.7 40.4

Power 110.0 121.0 38.0 41.8

Comm 170.0 187.0 30.0 33.0

Propulsion 294.8 324.3 14.7 16.2

OBDH 65.0 71.5 23.0 25.3

Thermal 60.0 66.0 15.0 16.5

Structure 200.0 220.0 65.0 71.5

Payload 94.3 113.2 11.0 13.2

Boom 3.0 3.3 6.0 6.6

Sub-system total

1057 1171 240 265

System margin

20 % 20 %

Dry mass master

1407 317

Propellant 2285 83 91

Wet Mass 3692 408

Launch Mass

4100

Tab. 10: Mass estimations for orbiter and slave

4.9 Lifetime and decommissioning

The lifetime is inferred from the previous Cassini mission, which should last 20 years after its launch in 1997. Saturn’s radiation level is higher than Uranus’ and the temperature is cold. iTOUR will be launched almost 30 years later so technological improvement is expected. The journey to Uranus lasts 18.5 years and science operations will be 5 years at least. A 23-year mission duration looks feasible in comparison with Cassini.

5 Key issues

5.1 Planetary protection

The Uranus 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 Uranus’ system. With proven approaches two major risks with a severe impact on the mission could be calculated. First, a failure at orbit insertion and second, the collision with an unknown object. Both would lead to mission failure but for both cases mitigation activities can be set. For the orbit insertion, simulations and inhibition of safe mode could minimize the risk but of course a residual risk would re-main. 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 risks 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. Also masses of payload, bus, and propellant were considered. Gravity assists were consid-ered to add costs because of added operations effort. RTGs were accounted for by adding costs to the launcher for necessary adaptations and for the RTGs themselves. Based on this information, the total cost estimate is €1,750M, rough order of magnitude.

THE GIANT PLANETS OF THE SOLAR SYSTEM Summer School Alpbach 2012

10

Contributor Cost/M€

Ariane 5 with RTG mods. 175 Master: Platform 1150 Master: Payload 100 Slave: Platform 200 Slave: Payload 20 Operations 100 Total 1750 Tab. 11: Total lifecycle cost estimate

5.4 Descoping options

By reducing the number of questions that the mission hopes to solve and decreasing the amount of scientific data requested, some instruments could be removed from the payload to achieve a reduction of mass. Most obviously the Slave could be eliminated with a saving of about €220M. If more payload is eliminated it might be possible to use a smaller launcher. For this study, recommended ESA margins have been used which makes it a conservative approach.

5.5 Communications and outreach

iTOUR would be a great opportunity for ESA to communi-cate about space science and exploration. It would be:

The first orbiter of an ice giant.

The first detailed study of the Uranus system.

The first detailed investigation of Uranus’ atmosphere.

The first detailed study of Uranus’ magnetic field.

The first outer planet mission with two orbiters.

Cassini-like public outreach is expected. The icy giants have been visited only once by Voyager 2’s fly-by. Therefore, Uranus stays a mysterious and distant planet. iTOUR could provide high quality pictures that would capture press headlines and public attention. Innovative space explora-tion missions like iTOUR always attract children and stu-dents to physics and astronomy to continue the work of generations of scientists and engineers.

6 Conclusion

Voyager 2 was the last probe that visited Uranus, passing by in 1986, and besides pictures from the Hubble space telescope and Earth based observations, no other data is available. The detailed study of one of the ice giants like Uranus will improve our understanding of the dynamics, chemical and physical processes in the Solar System. iTOUR will be the first dedicated mission to one of the icy giants to study its interior structure and atmosphere, as well as its magnetosphere, rings, and moons in great detail. The master and slave will provide a unique possibility to study the magnetosphere in spatial and time variations. Despite the great financial and technological difficulties, iTOUR will help mankind better understand the Solar System and perhaps exo-planetary systems beyond.

7 Acknowledgements

Team Orange would like to give special thanks to our tutors Marcus Hallmann and Anna Milillo for their excellent supervision and support. In addition we would like to thank Christian Erd, Chris Arridge, Tilmann Denk, and all the tutors for the help they have provided, their feedback, and good ideas. We give a special thanks to Michaela Gitsch and Peter Falkner for organizing this summer school. Finally we would like to thank all speakers, the administration staff, the programme committee, and all the students at the Alpbach summer school for an amazing experience.

8 References

[1] E. C. Stone and E. D. Miner; The Voyager 2 Encoun-

ter with the Uranian System; Science, New Series,

Vol. 233, No. 4759 (Jul. 4, 1986), pp. 39-43

[2] I. de Pater and J.Lissauer, Planetary science,

Cambridge University Press, 2010

[3] Hussmann, H.; Sohl, Frank; Spohn, Tilman (Novem-

ber 2006). "Subsurface oceans and deep interiors

of medium-sized outer planet satellites and large

trans-neptunian objects". Icarus 185 (1): 258–273.

Bibcode2006Icar..185..258H.

[4] JUICE assessment study report, ESA, SRE(2011)18

[5] C. S. Arridge et al; Uranus Pathfinder : Exploring

the Origins and Evolution of Ice Giant Planets, Exp

Astron, doi :10.1007/s10686-011-9251-4