3

Paper

AAS

06-185

Jupiter Icy Moons Orbiter

Mission Design Overview

Jon A. Sims

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

AAS/AIAA Space Flight

Mechanics Conference

Tampa, Florida

January 22-26, 2006

AAS Publications Office, P.O. Box 28130, San Diego, CA 92198

AAS

06-185

JUPITER ICY MOONS ORBITER MISSION DESIGN OVERVIEW

Jon A. Sims

An overview of the design of a possible mission to three large moons of Jupiter
(Callisto, Ganymede, and Europa) is presented. The potential Jupiter Icy Moons
Orbiter (JIMO) mission uses ion thrusters powered by a nuclear reactor to
transfer from Earth to Jupiter and enter a low-altitude science orbit around each
of the moons. The combination of very limited control authority and significant
multibody dynamics resulted in some aspects of the trajectory design being
different than for any previous mission. The results of several key trades,
innovative trajectory types and design processes, and remaining issues are
presented.

INTRODUCTION

Europa is one of the highest priority targets for planetary science. The potential

exists for a liquid water ocean under a crust of ice, and with liquid water comes the
enticing prospect of life. Unfortunately, Europa is deep in the gravity well of Jupiter and
within an intense radiation environment, making an extended mission at Europa
extremely challenging. The Galileo mission flew by Europa several times and acquired
evidence for the existence of an ocean. The Europa Orbiter project began in the late
1990s and neared completion of the formulation phase before being cancelled in 2001.

In 2002, NASA began seriously considering the use of nuclear reactors for

planetary missions. The immense power available from the reactors would open up a
new era in planetary exploration, enabling the use of high-power science instruments,
high data rate communications, and advanced electric propulsion. Concept studies for a
mission to three large moons of Jupiter, including Europa, were completed in 2002, and
the Jupiter Icy Moons Orbiter (JIMO) project began Phase A in 2003. The objectives of
JIMO were both technological (develop a safe nuclear reactor powered spacecraft) and
scientific (explore the three icy moons of Jupiter). Several requirements were placed on
the project, including the use of nuclear electric propulsion, accommodation of a large
scientific payload, reaching the Jovian system by 2021, and achievement of low-altitude
science orbits around Callisto, Ganymede, and Europa.

A mission transferring from Earth to Jupiter and into low-altitude orbits at three

massive moons at Jupiter requires a large amount of

∆

V. In order to make this mission

feasible, an efficient propulsion system is required. With the large amount of power
available from the nuclear reactor, nuclear electric propulsion was a logical choice.

Senior Member of Engineering Staff, Guidance, Navigation, and Control Section; Jet Propulsion Laboratory, California Institute of

Technology, Mail Stop 301-140L, 4800 Oak Grove Drive, Pasadena, California 91109-8099

However, even though the power available for propulsion is substantial, the large mass
inherent in a system using a nuclear reactor results in a low thrust-to-mass ratio for the
spacecraft.

The dynamical environment at Jupiter is complex. The trajectories at Jupiter are

governed by multiple gravitational fields and spend considerable time in regions of space
in which more than one body is exhibiting significant influence on the spacecraft. With
appropriate design techniques, we can find very efficient pathways by taking advantage
of these intricate dynamics. An additional complexity results from the very low
acceleration capability of the spacecraft. We are virtually being churned around by the
ocean while using an oar for control. We must choose our strokes carefully and
deliberately. The combination of very limited control authority and significant multibody
dynamics results in some aspects of the trajectory design being different than for any
previous mission.

A challenging aspect for low-thrust mission design in general is that the trajectory

design is closely coupled with other project elements, even at an early stage. The
trajectory depends on the launch vehicle capability, the mass of the spacecraft,
characteristics of the power and propulsion subsystems, and capabilities of the attitude
control. With JIMO being the first mission powered by a nuclear reactor, this coupling
proved even more challenging since the system parameters had large uncertainties
initially and significant external constraints as the design progressed.

This paper presents an overview of the mission design for the JIMO mission

through the cancellation of the project in 2005. As has been described, there were many
new and challenging aspects of the mission, requiring new tools and innovative
techniques to be developed as we proceeded. We performed a wide variety of trades and
developed fully integrated, high fidelity trajectories from interplanetary injection through
the end of the mission. (The reference trajectory referred to later in this paper is the latest
one that was completed.) Much of the work has been documented in papers. (See
References 1-41.) This paper summarizes some of the key trades and results. More
details are provided in the references.

INTERPLANETARY TRANSFER

An extensive database of direct interplanetary trajectories was created in order to

be able to quickly perform broad trades in system parameters such as power, specific
impulse, and mass. When coupled with subsystem mass models and potential launch
vehicle capabilities, the database was used to explore a large trade space constrained by
technological and other practical considerations. We could then focus on regions with
reasonable system parameters and good mission characteristics.

Several options were considered for departure from Earth. The most appropriate

option depends on the capabilities of the launch vehicles being considered. One option is

to launch into an orbit about the Earth and use electric propulsion to gain energy and
escape the Earth. Another option is to use the launch vehicle or a chemical transfer stage
to escape the Earth. Spiraling out with electric propulsion provides more delivered mass
or requires less launch vehicle capability but typically has a longer flight time. The
project decided early on to escape the Earth using a chemical propulsion system. The
requirement on arrival date forced a flight time that would have been difficult to meet
with a spiral out option.

The project also had a guideline that JIMO would not require a significant new

launch vehicle development. We baselined a launch vehicle that was a reasonable
evolution from current launch vehicles, although we had to consider many different
options. Using a chemical system to escape the Earth and relying on launch vehicle
capability not much beyond the current level led to a scenario with three launches: one
launch for the fully fueled JIMO spacecraft and two launches for two chemical
propulsion transfer stages. The three vehicles would rendezvous and mate in low Earth
orbit.

The reference interplanetary trajectory is a direct trajectory with no planetary

gravity assists (Figure 1).

We performed an extensive analysis of a variety of gravity

assist options with Earth, Venus, and Mars.

11,23,24

Results of one of these analyses are

shown in Figure 2. The solid black line in Figure 2 is the direct case with optimized
launch energy. All the other cases include at least one planetary gravity assist. There are
many gravity assist options that both increase the delivered mass and decrease the flight
time. Some of the Earth gravity assist options are particularly interesting because they
are among the best performers and provide consistent performance at regular intervals of
launch opportunities.

Figure 1 Reference Interplanetary Trajectory and Jupiter Arrival

Figure 2 Gravity Assist Trajectories

The injection period for the reference trajectory can be quite long without

sacrificing significant performance. For example, the injection period is potentially as
long as 84 days at the cost of 0.3% of delivered mass to Jupiter. If we were to allow
slightly longer transfer times for backup injection opportunities, the injection period
could be extended indefinitely at a reasonable cost in performance. The mission is also
extremely robust to injection vehicle delivery dispersions.

TRAJECTORY NEAR JUPITER

The reference trajectory flies by Callisto on the initial approach to Jupiter and

uses additional Callisto gravity assists prior to capture at Callisto (Figure 1). These
gravity assists reduce the required propellant for this phase of the mission by about 80%
and also decrease the flight time. We analyzed using Ganymede for gravity assists prior
to capture at Callisto, but the results showed that Ganymede did not help when Callisto
was to be the first moon orbited.

The reference trajectory orbits Callisto, Ganymede, and Europa, in that order. We

also analyzed orbiting Europa first, then Ganymede, then Callisto. The delivered mass
performance was very similar between the two cases. The Callisto first case has a
slightly shorter flight time and lower radiation – potentially much lower radiation
depending on the end-of-mission orbit.

Capturing at a body using low-thrust propulsion is different than for high-thrust

missions. The reduction in orbital energy is necessarily slower; hence, a substantial
amount of time is spent in a transition region between escaped from the moon and
captured at the moon. During this transition, the multibody effects are significant, and in
many cases an uncontrolled spacecraft would impact in a matter of days (Figures 3 and
4). This was particularly true when we tried to capture directly into near-polar inclination
orbits. We did find very stable near-equatorial, retrograde orbits that we could capture
into, but to avoid the unstable regions, we had to change the inclination at relatively low
altitudes which is very costly in terms of propellant and flight time. At Callisto and
Ganymede, we could follow paths to the science orbit that would not impact for at least a
couple weeks; however, this is extremely costly at Ganymede since the relatively safe
region is at a much lower altitude with Ganymede being closer to Jupiter. At Europa, the
relatively safe region essentially disappears within about 45 deg of the poles. With the
extremely high radiation environment at Europa, the decision was made to get to the
science orbit as fast as reasonably possible, allowing the uncontrolled lifetime to be very
short.

Figure 3 Orbit Lifetime Maps for Ganymede and Callisto

Higher vulnerability to
unplanned missed thrusting,
lower

∆

V, shorter flight time

Lower vulnerability to
unplanned missed thrusting,
higher

∆

V, longer flight time

Figure 4 Orbit Lifetime Map for Europa

We recently began exploring and discovering other types of captures that are very

promising. These “manifold captures,” as we referred to them, approach the moons along
stable manifolds of unstable periodic orbits (of which there are many) near the moons.
The manifold captures performed well in terms of propellant mass, flight time, and
controllability with reasonable lifetimes.

34,35

These types of captures would have been

explored more fully given more time. They may also be very useful for high-thrust
missions.

Overall,

significant

trades are available between propellant mass, flight time, and

stability for a variety of capture types. The requirements on trajectory lifetimes and
acceleration levels (translational and rotational) will drive the design of the captures and,
hence, many other aspects of the mission.

Figure 5 illustrates the capture at Callisto and transfer down to the science orbit.

Figure 5 Capture at Callisto and Transfer to the Science Orbit

The transfers between the moons take advantage of multibody effects and gravity

assists to reduce the required propellant for these phases of the mission by about 80%.
We explored many different types of transfers, including various combinations of
resonances with the moons. The best transfers depend on the type of escapes and
captures used at the moons and the available level of acceleration. The transfer from
Callisto to Ganymede for the reference trajectory is shown in Figure 6.

Figure 6 Transfer from Callisto to Ganymede

SCIENCE ORBITS

We knew from previous studies that low-altitude orbits around the moons with

inclinations within about 45 deg of the poles are unstable due to the gravitational
influence of Jupiter, that is, if left uncontrolled, they impact the moon in a relatively short
time.

Since Europa is the closest to Jupiter of the icy moons and also the smallest, the

time scale for this effect is the shortest at Europa with impact occurring on the order of
10s of days. The previous studies considered only a very simple gravity field for the

moons, including only the effect of J2. When we started considering more detailed
gravity fields, we discovered that higher order terms can have a significant effect on the
stability of the orbits. For example, a significant value for J3 makes orbits at essentially
all inclinations unstable. We did discover very special cases of near polar “frozen orbits”
that have relatively long lifetimes,

but the exact orbital conditions for these orbits

depend on the details of the gravity field which we won’t know until we have been at the
moon for awhile.

Stability of the orbits also has a direct effect on the science orbit maintenance and,

hence, the orbit determination. A trade exists between the frequency and total delta-V
required for the maintenance maneuvers, with smaller, more frequent maneuvers
potentially resulting in less

∆

V overall. Lower total

∆

V results in less total time

interruption to the science, but the more frequent maneuvers may significantly degrade
the orbit determination. So the selection of the precise elements for the science orbits and
the orbit maintenance strategy are still unclear.

The mission ends with the spacecraft in the science orbit at Europa. We explored

options for transferring to orbits that do not impact Europa for an extended duration (>
1000 years), but the transfers require more propellant and more time in the high radiation
environment at Jupiter.

CONCLUSION

Designing the ambitious JIMO mission presented many interesting and new

challenges. The spacecraft configuration and parameters placed severe constraints on the
mission design, including a low thrust-to-mass ratio. The dynamical environment at
Jupiter is complex, and the radiation environment is harsh. Even with all of these
constraints and challenges, we were able to meet all of the high-level mission design
requirements placed on JIMO.

A tremendous amount of innovative work was completed over the past three

years. Many new tools and trajectory design techniques were developed and could play
an important role when we return to seriously considering reactor powered missions of
exploration. In addition, much of this work also applies to other types of missions,
including low-thrust missions in a multi-body environment (e.g., the Earth-Moon system)
or even high-thrust missions to any massive moon (e.g., Europa).

ACKNOWLEDGMENT

The author would like to thank the entire JIMO Mission Design Team for their

tremendous efforts and innovation. This work was performed at the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with the National
Aeronautics and Space Administration.

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