THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
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.
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.
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 objectives
2.1 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 AU
Perihelion
18.4 AU
Orbital period
84.3 yr
Number
of
known 27 (5 larger moons)
Satellites
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
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].
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THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
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
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 midlatitudes (~ 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,
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THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
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
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
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THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
require viewpoints of at least a few degrees out of the ring
plane.
2.4 Instruments
20%
20%
10%
30%
20%
20%
Total
mass
20,4
7,8
4,95
12,61
10,32
0,6
6,8
3,1
5%
5%
7,14
3,255
INCA
Plasma
package
2 x ELS
2 x HPS
Scanner
DPU
D-DPU
Total:
16
5%
16,8
1,4
1,6
1,5
2
1,5
30%
30%
30%
30%
30%
1,82
2,08
1,95
2,6
1,95
94,3
JUICE
JUICE
JUICE
JUICE
JUICE
Heritage
Instrument
(Slace)
FGM
Mass
[kg]
3,1
Margin
5%
Total
mass
3,255
SCM
Plasma
package
ELS - 1
HPS - 1
DPU
Total:
Fig 2
2
20%
2,4
0,7
0,8
2
30%
30%
30%
0,91
1,04
2,6
10,2






Heritage
JUICE
JUICE
JUICE
JUICE
LORRI
Mars
Pathfinder
CASSINI
DOUBLESTAR
CASSINI
JUICE
DOUBLESTAR
THEMIS
JUICE
JUICE
JUICE
JUICE
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
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
Orbiter
&Probe
Margin
Orbiter
& slave orbiter
Mass
[kg]
17
6,5
4,5
9,7
8,6
0,5





ELS (Electron Spectrometer)
HPS (Hot Plasma Spectrometer)
3 Mission design
3.1 Architecture options and tradeoff
Instrument
(Main)
VIRHIS
UVIS
RSI
SWI
NAC
Filter wheel
for NAC
RPWI
FGM



For
Against
- In situ measurements
of the atmosphere
(noble gazes).
- Less magnetic field
observations.
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.
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THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
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.
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
1
2
3
Advantages
- Magneto tail crossing
- Close in day side
- Bow shock crossing
- Time for remote
measurements
- Long enough in the
magnetosphere
- Bow shock crossing.
- Time for remote
measurements
- Day side all the time
Disadvantages
- Night side all the time
- Periapsis on day side
- No Magneto tail
- Too long outside the
magnetosphere
- Too long outside the
magnetosphere
- We can’t study the
magneto tail.
Id
Magnetosphere
Remote observation
Total
study
1 10
50
60
2 90
70
160
3 70
80
150
Fig 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.
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 – EarthEarth – 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
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THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
Interplanetary Trajectory Data
Launch Date
Arrival Date
Gravity assists
AR 5 ECA Launch capacity
Mass at launch needed
Sep 11, 2026
Mar 20, 2045
VEEJ
4300 kg (5160 kg)
4100 kg
Fig 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
Interpl. navigation dV
Uranus OI
Miranda orbit
Ariel orbit
Umbriel orbit
Titania orbit
Oberon orbit
Moon tour navigation
TOTAL MARGIN
Fig 9
∆V (km/s)
0.125
0.92
0.08
0.12
0.1
0.18
0.15
0.17
1.93
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.
• 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 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
AOCS
Orbiter [W]
40
Slave [W]
16
OBDH
8
8
Thermal Control
15
5
Communication (receiving)
Power
50
12
17
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
Fig 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.
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.
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THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
Instruments
VIRHIS[W]
UVIS[W]
RSI [W]
SWI[W]
NAC[W]
Power with
Margin [W]
1D
24
7.35
31.3
5
1N
-
2D
24
24
2N
28.8
60.8
7.35
96.9
9
3D
28.6
60.8
89.4
3N
24
28.6
7.35
59.9
5
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
Satellite
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-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.
Ura.
M
Solar
radiatio
n
𝑊
𝑚2
3.4
Albed
o
α
ε
Tem.
-
-
-
K
0.282
Area
𝑚2
0.07 0.74 293 0.84
0.12 0.9
293 0.30
Ven.
2657
0.82
0.07 0.74 305 0.84
S
0.12 0.9
281 0.30
Fig 13: Material for radiator is white paint (silicate)
S
M
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 deepspace 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,
7
THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
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 deltav 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
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,
Fig 15: iTOUR structure which are the highest during
launch.
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
8
THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
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
Sub-system
SLAVE
Total
mass
[kg]
AOCS
Mass
without
margin
[kg]
60,2
Total
mass
[kg]
66,2
Mass
without
margin
[kg]
36,7
Power
Comm
110,0
121,0
38,0
41,8
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
1057
1171
240
265
Sub-system
total
System
margin
Dry
mass
Orbiter
Propellant
Wet Mass
Launch
Mass
Fig16
40,4
20 %
20 %
1407
317
2285
83
3692
91
408
4100
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.
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
Ariane 5 with RTG mods.
Master: Platform
Master: Payload
Slave: Platform
Slave: Payload
Total
Cost/M€
175
1150
100
200
20
1750
Fig 17
5.4 Descoping options
5 Key issues
5.1 Planetary protection
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THE GIANT PLANET OF THE SOLAR SYSTEM Summer School Alpbach 2012
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.
(Udry, Benz, & Steiger, 2003)
(Wertz & Larson, 2003)
(Stone, 1986)
(T. Encremenaz, 2008)
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
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)
10