Planning A Journey To The Beginning of the Solar System
Carol A. Raymond
Dawn Deputy Principal Investigator, Jet Propulsion Laboratory

The Dawn mission officially started in September, 2002. During January,
the mission team at JPL and Orbital Sciences Corp. reached full staffing
levels and contracts for science team support were signed. The European
team members at DLR (Berlin) and IFSI (Rome) have begun work on the
framing cameras and mapping spectrometer, respectively. We are now sailing
smoothly towards our Preliminary Mission and Systems Review in April,
followed by the Preliminary Design Review (PDR) in August, 2003. The PDR
is also the official mission confirmation review.

Any successful journey requires careful route planning and efficient
packing, and Dawn is no exception. Our journey will take us on a trip of
5.5 billion kilometers over eight years, with major stopovers at Vesta and
Ceres. Thus careful planning of the spacecraft trajectory is critical to
mission success. The Dawn mission design and navigation team has been hard
at work doing just that, and a report of their progress by Marc Rayman is
featured in this newsletter. The mission team is now reviewing the
availability, cost and performance of the payload and spacecraft systems
and making sure everything fits within the mission’s technical and cost
resources. Thus far no technical obstacles have been identified, but we
did have to abandon our plan to use a lightweight composite tank for the
xenon propellant, and instead will carry a heavier but more reliable
titanium tank with composite overwrap.

The Dawn Science Team will be meeting in Houston, Texas on March 16
(in conjunction with the Lunar and Planetary Science Conference) and in
Nice, France on April 5 (in conjunction with the joint European Geophysical
Society / American Geophysical Union meeting), to verify the mission plans
and requirements, and begin planning for the mission operations and data
analysis. A paper describing the mission will appear in Planetary and
Space Science later this year.

How Do We Get There?
Marc D. Rayman
Dawn Project Engineering Team, Jet Propulsion Laboratory

The design of Dawn’s trajectory is difficult, unusual, and interesting
because of the use of solar electric propulsion, implemented on Dawn as an
ion propulsion system (IPS). While providing performance far in excess of
what conventional chemical propulsion would deliver, the IPS necessitates
the use of design tools and methods quite different from what has been used
for the development of trajectories since the dawn of the solar system (or,
at least, since the dawn of space exploration). Rather than finding a few
points at which impulsive maneuvers are required, this problem involves the
determination of IPS thrust vectors over years of continuous thrusting.
Unlike trajectories for ballistic missions, Dawn’s depends sensitively on
the spacecraft’s power system (because power translates directly into IPS
thrust). The tools that generate the trajectories require much more coaxing
and cajoling (and sometimes pleading) than the tools that have been used for
conventional missions.

In addition to the different underlying mathematical problem, the use of the
IPS necessitates unfamiliar constraints on the mission. For example, because
IPS thrusting is needed for years at a time, the mission could be vulnerable
to an unexpected loss of thrust. Therefore, a substantial effort is devoted
to designing a trajectory with enough “mission margin” that most spacecraft
problems that interfere with IPS thrusting do not jeopardize reaching both
Vesta and Ceres. (Missions relying on chemical propulsion tend to have
greater vulnerability for shorter times.)

The initial work is focused on obtaining an understanding of the sensitivity
of the trajectory to parameters that we can control. Ultimately we will
develop a baseline trajectory that accounts for constraints such as the
finite launch period, launch window, Vesta arrival window (to ensure good
lighting for framing camera and mapping spectrometer observations of the
south pole), Ceres arrival window (for lighting at one of the poles),
mission margin, periods in which spacecraft activities preclude thrusting in
the optimal direction, spacecraft power characteristics, flybys of other
asteroids during the interplanetary cruise, and others. We separately
analyze the orbit insertion, departure, and orbit transfers at each primary
science target, where the complexity of spiraling around the bodies requires
different analytical techniques.

Steve Williams and Dr. Greg Whiffen of JPL are the principal trajectory
analysts on Dawn. Steve designed the trajectory for Deep Space 1 (DS1), the
mission that tested the IPS design Dawn uses. Many issues that an
operational IPS flight would face were revealed during that work; prior
analyses had rarely, if ever, exceeded the depth necessary for conceptual
studies. Greg has written a powerful new trajectory design tool that
complements the one used for DS1. With his new software, Greg has generated
our first looks at the Vesta orbit transfers. The first baseline trajectory
will be completed by early April. Although preliminary, it will be
significantly more accurate than previous calculations.

Dawn’s Attractive Science
Christopher T. Russell
Dawn Principal Investigator, UCLA

The solar system contains a spectrum of magnetic dynamos in large bodies
like the Sun and Jupiter, in more modest-sized bodies like the Earth and in
smaller bodies like Mercury and Ganymede.

In ancient times even more solar system objects had operating magnetic
dynamos. There have been so many dynamos that comparative planetology in
this area shows much promise of providing insight both into the dynamo
mechanism and the properties of the dynamo regions. In fact geochemical and
paleomagnetic evidence from the HED meteorites suggests that Vesta formed an
iron core and once had an internally generated magnetic field. This if
confirmed would put Vesta as the smallest body on a sequence with the Moon,
Mercury, Mars and the Earth as rocky planets at least once having a magnetic
dynamo. Dawn allows us to survey Vesta and Ceres at orbital altitudes well
below one body radius, and determine if they possess natural remanent
magnetization and, through geologic correlations, when it was produced. The
same instrument measures the transient response of Vesta and Ceres as
discontinuous changes in the external magnetic field occur, placing
constraints on the electrical conductivity of the interior. The response
time of the Moon to step transients in the solar wind magnetic field is
80 s, and at Vesta should be in the range 2 to 8 s, easily resolvable by the
10 Hz bandwidth and 0.1 nT resolution of the magnetometer. Detection of
remanent magnetization or an electrically conducting interior at Ceres would
lead to a major revision of our understanding of this body.

The ranges and sensitivity of the magnetometer are tailored to the expected
environments to be found at Vesta and Ceres as well as the magnetic
environment of the spacecraft. We have conservatively chosen a ± 1000 nT
range for the magnetometer, sampled at 20 Hz. The data are digitized to
16 bits providing ± 0.015 nT digitization. The UCLA magnetometer derives
from a long line of missions including OGO5 (1968); ISEE 1 and 2 (1977),
Pioneer Venus (1978); Galileo (1989); Polar (1996) and ST5 (in fabrication).
The main electronics unit, a sensor triad, and a block diagram are shown in
Figure 1. The main electronics and the sensors are completely redundant, use
a single range and data rate and operate continuously. A third new
technology magnetometer, with a design based on the sigma delta modulator,
adds further redundancy. This overall instrument design was chosen because
of its low-noise level, simplicity, low cost and its high inheritance from
recent missions.