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3\15 Dawn's Early Light - March 2003
Part 1 of 2

The Dawn mission has been selected as NASA's ninth Discovery mission 
to be launched in May 2006 to orbit both Vesta and Ceres.

Dawn's Early Light
Volume 2, Issue 1
March 2003

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. 

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How Do We Get There?
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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.

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Dawn's Attractive Science
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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.

(continued)

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