From: University of California Los Angeles
Posted: Monday, March 17, 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.
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.
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