Exploring the asteroid belt with ion propulsion: Dawn mission history, status and plans

Abstract

In this report, we describe the journey Dawn has taken in the recent past, its present status, and its future mission. The overall objective of Dawn is to explore backward in time via its observations of the primitive bodies, Vesta and Ceres. Thus Dawn embarks on three journeys. The first is its tumultuous temporal terrestrial trek during development. The second is its soon-to-be voyage in space to 4 Vesta, the second most massive asteroid in the main belt, and to 1 Ceres, the most massive. The third is its journey backward in time to infer the conditions as the solar system was formed. Finally, we discuss how it is possible to go back even further in time, beyond the horizon of the Dawn mission to obtain “pre Dawn” observations at 10 Hygiea, the fourth most massive asteroid, and one more primitive than Vesta and Ceres.

Introduction

Planetary systems begin in cold clouds of gas and dust, in which minerals precipitate as the gas cools. Then follows the accretion of ever larger bodies: planetary embryos, protoplanets and the planets themselves, in a progression that takes tens of millions of years (see Fig. 1). In the neighborhood of the present asteroid belt the process was cut short by the gravitational stirring of the gas giant, Jupiter. Following the condensation of Jupiter instead of growing, the existing bodies began to collide at velocities that led to erosion. Today only a few of the largest of these bodies exist intact, but those that do allow us to journey backward in time to obtain information on the state of the solar system before the planets reached their present masses. We cannot learn much about this epoch from terrestrial rocks, for example, because the last major impact on Earth, the one thought responsible for forming the Moon, destroyed most of the terrestrial evidence about earlier events.

There are many mysteries about the earliest epoch that we can only unlock by visiting these primitive bodies in the asteroid belt. The asteroid belt is clearly a region of transition from the rocky planets of the inner solar system to the icy, water-rich bodies of the outer solar system. As illustrated in Fig. 2 the bodies in the asteroid belt, as judged from their surfaces, exhibit a strong compositional gradient from the inner to the outer edge, changing from very evolved objects nearest the Sun to very primitive bodies far from the Sun (Cellino, 2000 and references therein). In order to evolve the minerals we see on 4 Vesta, as exhibited by the Howardite–Eucrite–Diogenite (HED) meteorites, Vesta needed to melt to form a magna ocean around a solid mantle or perhaps to melt completely. For a body the size of Vesta the release of gravitational energy during accretion is not sufficient to melt the entire body. Since it does appear Vesta melted completely, we look to short-lived radionuclides for assistance. This imposes a very strict time history starting with the supposed supernova explosion as the radionuclides produced in this way provide little heating several half-lives after they were created.

Ceres sits a little further out in the asteroid belt (2.77 vs. 2.34 AU) and if there were little migration during the initial stages of formation, this body should be cooler and more primitive, all else being equal. However, all else cannot be equal because Ceres is clearly a wet planet, containing hydrated minerals, contrasting sharply with the dry basaltic surface of Vesta. Keeping water on Ceres is not easy. While Ceres is bigger, it releases more gravitational energy during accretion. If it formed contemporaneously with Vesta it too should have gathered radionuclides, and would become even hotter than Vesta, but Ceres did not proceed in this way. Did it form too late for the radionuclides to be helpful in the heating the body? We do not now know, but we do hope to find out with the Dawn mission.

Dawn is enabled by a solar-powered set of three ion thrusters, used one at a time, that ionize and accelerate their xenon fuel to 35 km/s, a far higher and therefore more efficient use of fuel than in chemical engines. Sufficient fuel can be carried on the spacecraft to allow Dawn to first rendezvous with Vesta, spiral into its gravitational potential, orbiting it for the order of 8 months down to altitudes of approximately 200 km. Then Dawn spirals outward from Vesta and heads for Ceres arriving there in about 3 years at which point it repeats its series observations at different altitudes. Dawn’s ability to do this double planetary exploration in a reasonable and therefore affordable length of time is made possible by a fortunate planetary alignment. Every 17 years Vesta swings by Ceres as illustrated by a series of snapshots of their positions during the period Dawn is making its transit between them. They are especially close this conjunction. For Dawn, there is only a 2-year launch opportunity in which the mission can reach both targets; it will be 15 years before favorable alignment of the bodies allows the mission to take place again. Dawn must launch by the end of October 2007, when the current opportunity closes.

The purpose of this report is to provide an update on Dawn’s status, to describe the mission strategy and how this strategy can be extended by future exploration of complementary objects. Herein, we also give a very brief description of the spacecraft, science goals, mission operations and project history. Several more detailed papers exist providing further information. The first of these papers was written in the Proceedings of the Asteroids, Comets, and Meteors meeting in August 2002 and describes the mission prior to any descopes (Russell et al., 2002). Later in that year a second paper was written, but after the descoping of the laser altimeter, when because of a change in accounting rules, the instrument could not be built for the funds available. After this second paper was in the publication process the magnetometer was deselected by NASA for reasons that remain unclear to this day. This second paper contains a detailed description of the mission objectives, the mission plan and the instruments as they were proposed (Russell et al., 2004).

More recently a detailed description of the flight system and mission trajectory has appeared (Rayman et al., 2006), as has a brief report on the symbiosis of the remote sensing and theoretical communities with exploration of Vesta and Ceres (Russell, 2003). The Asteroid, Comets and Meteors conferences occur on a triennial basis and in 2005 we updated our scientific understanding of Ceres and Vesta and described the Dawn operations in greater detail (Russell et al., 2006). Later that year two meetings were held back-to-back in Japan in which three focused papers were presented. At the International Conference on Low Cost Planetary Missions we presented a status report roughly as the program entered phase D (Russell et al., 2005) and a more management-oriented lessons-learned report (Fraschetti et al., 2005). At the IAF/IAA conference the following week. Rayman et al. (2005) described the interrelationship of technical resources on an ion propulsion mission and how the margins can be managed and used to increase the scientific potential of the mission. Simultaneous with these conferences a decision was made by NASA to stand-down the project and assess its readiness to be completed and launched successfully. The stand-down added 6 months to the project. After the successful reconfirmation of the project at the end of March 2006, the consequent start up transient added further delay, as did the addition of generous schedule reserves. Launch has now slipped from June 2006 to July 2007. Since these events have much to do with Dawn’s present status, we will spend some time reviewing the history of the project.