The Dawn of Exploration
July 7 update: The launch has been rescheduled for September, due to the limited launch timetable for Dawn and potential conflicts with the upcoming launch of the Mars Phoenix Lander.
Christopher T. Russell, UCLA professor of geophysics and space physics, has spent 15 years working on NASA’s Dawn mission to the doughnut-shaped asteroid belt between Mars and Jupiter. As the scheduled July 7 launch from Cape Canaveral nears, Russell is ready, and so is Dawn.
"The spacecraft will spend much less time in space than we put in preparing for the mission," said Russell, the mission’s principal investigator. "I want to get this spacecraft up in space, where it belongs. I’m really confident about the spacecraft. We’ve been testing and retesting."
Dawn will conduct a detailed study of the structure and composition of two of the first bodies formed in our solar system: the "dwarf planet" Ceres and the massive asteroid Vesta. The mission’s goals include determining the shape, size, composition, internal structure, and the tectonic and thermal evolution of Vesta and Ceres. Dawn, which will be the first spacecraft to orbit two planetary bodies on the same mission, is expected to reveal the conditions under which these objects formed. Comparing their different evolutionary paths will provide evidence about the role of size and water in planetary evolution. In addition, studying these celestial bodies may yield clues about how asteroids could have participated in the origin of life on Earth by delivering important chemical precursors during our planet’s early years.
Dawn is scheduled to fly past Mars by April 2009, and after more than four years of travel, the spacecraft will rendezvous with Vesta in 2011. The spacecraft will orbit Vesta for approximately nine months, studying its structure and composition. In 2012, Dawn will leave for a three-year cruise to Ceres. Dawn will rendezvous with Ceres and begin orbit in 2015, conducting studies and observations for at least five months.
"I think of Dawn as two journeys," said Russell, who proposed the mission to NASA. "One is a journey into space. This is analogous to what ancient explorers did, who knew there was unexplored territory and wanted to discover what was there. We’re going to explore a region for the first time to find out what the conditions are today.
"Dawn is also a journey back in time. Ceres and Vesta have been altered much less than other bodies. The Earth is changing all the time; the Earth hides its history, but we believe that Ceres and Vesta, formed more than 4.6 billion years ago, have preserved their early record. They’re revealing information that was frozen into their ancient surfaces. By looking at the surface and how it was modified by the bombardment of meteoroids, we will get an idea of what the early conditions of Ceres and Vesta were and how they changed. So Dawn is a history trip too. We’re going back in time to the early solar system."
Could Ceres harbor life?
Another important question concerns the potential for water ice on Ceres. "Evidence indicates it has substantial water or ice beneath its rocky crust," Russell said. "Our instruments on board will be able to determine whether there is water." Dawn’s instruments include a gamma ray and neutron spectrometer that can detect the hydrogen from water.
Evidence of whether water still exists on Ceres could come from frost or vapor on the surface, and possibly liquid water under the surface. The water kept Ceres cool throughout its evolution. In contrast, Vesta was hot, melted internally and became volcanic early in its development. Ceres remains closer to its primordial state, while Vesta evolved further over the first few millions of years of its existence, Russell said. Water is essential for life as we know it, and understanding the history and presence of water in the Solar System is the first step in the search for life beyond our planet.
Ceres, named for the Roman goddess of agriculture, revolves around the sun every 4.6 terrestrial years and has an average diameter of approximately 600 miles. A roughly round object, Ceres orbits the sun in the asteroid belt between Mars and Jupiter, approximately 258 million miles from Earth. Ceres is much larger than Vesta – more than two times further across, with a volume eight or nine times greater – but is less dense, as the material in it is lighter.
Vesta, the brightest asteroid, is named for the ancient Roman goddess of the hearth. Approximately 220 million miles from Earth, it appears to be essentially solid rock, with a density similar to Mars. It orbits the sun every 3.6 terrestrial years, has an oval, pumpkin-like shape, and has an average diameter of approximately 320 miles. Vesta’s basaltic dust layer reflects its crustal composition, and its dry surface includes a huge crater near its southern pole.
Studies of meteorites believed to be from Vesta that were found on Earth suggest that this body formed from galactic dust during the solar system’s first 3 to 10 million years. Although no meteorites from Ceres have been found, it is believed this body also formed during the first 10 million years of the solar system’s existence, Russell said.
Dawn is expected to bring high-resolution images of previously unseen worlds to the public, including, perhaps, mountains, canyons, craters and ancient lava flows. In addition to the images, Dawn will generate data that will help scientists identify geologic minerals and will take measurements of gamma rays and neutrons. The gamma-ray measurements will reveal which elements are in the minerals. The data are expected to arrive a scant 30 minutes after the spacecraft obtains them.
Dawn, which will orbit as close as 125 miles from Ceres and Vesta, is the first purely scientific mission designed to be powered by an advanced NASA technology known as ion propulsion. Unlike chemical rocket engines, ion engines accelerate their fuel nearly continuously, giving each ion a tremendous burst of speed. The fuel used by an ion engine is xenon, a gas also used in photo-flash units, which is more than four times heavier than air. This results in a xenon atom with a positive charge – a xenon ion. The xenon ions shoot out the back of the engine at a speed of 78,000 miles per hour.
At full throttle, the ion engine consumes 2,300 watts of electrical power and produces 1/50th of a pound of thrust – about the same pressure as a sheet of paper resting on the palm of a hand, and far less thrust than is produced by even small chemical rockets. This engine, for a given amount of fuel, can gradually increase a spacecraft’s velocity 10 times more than can a conventional rocket powered by liquid or solid fuel.
UCLA is in charge of Dawn’s science and public outreach. Russell leads the science team, brings together the mission’s partners, manages the budget and participates in all major decisions. Russell and his colleagues will make science decisions and develop the operations plans through the science center at UCLA’s Institute of Geophysics and Planetary Physics. His science team has the lead role for analyzing and interpreting the data from Dawn. UCLA graduate students and postdoctoral scholars will work on the mission, including helping to analyze the data from Dawn.
Dawn is part of NASA’s Discovery Program, in which scientists find innovative ways to unlock the mysteries of our solar system by answering some of humanity’s oldest questions. The Discovery Program is managed by NASA’s Marshall Space Flight Center for the Science Mission Directorate. Team members include scientists from the Jet Propulsion Laboratory, the NASA Goddard Space Flight Center, the U.S. Department of Energy’s Los Alamos National Laboratory, the Massachusetts Institute of Technology and other institutions. Orbital Sciences Corp. is building the spacecraft, and JPL is providing the ion engines.
"Why do we explore the solar system?" asked Russell. "Why did Lewis and Clark go across the U.S. at the start of the 19th century? We’re not going to expand the human race off this planet for a long time, but discovering our origins and how the solar system evolved is valuable in itself. Mankind has always expanded horizons. Exploration is a human imperative."