Aspacecraft set to launch early next month will explore the two largestresidents of the asteroid belt to look for clues to our solar system?sbeginnings.
NASA?s Dawn mission willhelp scientists understand the first stages of planet formation by studyingVesta and Ceres, two rocky bodies whose growth stalled 4.5 billion years agowhen they failed to gather enough raw materials to become planets themselves.
?We believethey?re the building blocks of planets,? said Dawn mission co-investigator LucyMcFadden from the University of Maryland, ?which is one reason why we want togo there and learn what we can about them. It will tell us about the early partof the solar system, sort of like looking back in time.?
Bytargeting both worlds, Dawn will be the first spacecraft to orbit and study thetwo objects after leaving Earth.
A studyin contrasts
Vesta is abone-dry asteroid, while Ceres is a dwarfplanet suspected of harboring a layer of ice up to 75 miles (120 kilometers)deep or even liquid water beneath its dusty surface. Both are located in thesolar system?s asteroid belt, a 130-million-mile wide rock-strewn corridorlocated between the orbits of Mars and Jupiter. Ceresis spherical and about as wide as the state of Texas. Vestais more elongated, and has a diameter about the width of Arizona.
Vesta?sphysical characteristics are like those of the inner terrestrial planets,whereas Ceres resembles the icy moons of the outer planets.
?They?reworlds in their own rights,? McFadden told SPACE.com. ?These are smallbodies that did not grow into big giant planets, so we believe they holdinformation about what the planet forming processes were 4.5 billion yearsago.?
Dawn willcarry an infrared and visible camera to study Vesta and Ceres? surfaces, agamma ray and neutron spectrometer to determine their composition, andinstruments to measure their gravity fields and reveal details about theirinteriors.
Dawn isscheduled to enter orbit around Vesta in October 2011. Past observations offeatures such as lava flows and magma oceans on the asteroid?s surface suggest itwas partially molten early in its history. Studies of meteorites believed tohave come from Vesta suggest the asteroid?s surface was once soft enough forheavy elements like iron to sink and form a dense core.
?That?sinteresting, and a bit puzzling,? said Dawn principal investigator ChrisRussell of the University of California, Los Angeles.
Meltingrequires a source of heat. One way to produce that heat is through thegravitational energy released when rocky particles collide to form an asteroid.But Vesta is so small that ?there would not be enough gravitational energy tomelt the asteroid when it formed,? Russell said.
Onepossible explanation is that two supernovasbelieved to have exploded around the time of our solar system?s birth spatteredVesta with radioactive forms of elements like aluminum-26 and iron-60. The hotatoms could have provided the extra heat needed to melt Vesta, scientistsspeculate. Once the radioactive atoms decayed, the asteroid would have cooledto its present form.
What Dawnfinds on Vesta could change how scientists think about planet formation.
?When Iwent to school, the thought was that the Earth got together, heated up, and theiron went to the center and the silicate floated on top, producing acore-forming event,? Russell said. This scenario assumes that rocky planetswere formed by the collision and merger of smaller ?planetoids? that hadn?t yetformed their own iron cores. But if objects the size of Vesta could melt andform dense cores, ?it would affect the way the planets and their cores grew andevolved,? Russell said.
Afterorbiting Vesta for seven months, Dawn will make its way towards Ceres. If Dawnused conventional rocket fuel, such a maneuver would be impossible because ofthe amount of fuel required.
?We wouldneed one of the largest rockets the U.S. has to carry all the propellant,? saidMarc Rayman, project system engineer for Dawn at NASA?s Jet PropulsionLaboratory.
Instead,Dawn uses an innovative ion propulsion engine that relies on acceleratedcharged particles to create the thrust required to accelerate the spacecraft.Rather than a burst of energy, the thrust is generated gradually, causing smallchanges in trajectory and speed over long periods of time.
Dawn isscheduled to arrive at Ceres by February 2015, a few months before NASA?s NewHorizon probe reaches the dwarf planet Pluto. Scientists think Ceres couldhelp answer some longstanding questions about the role water played inplanetary evolution.
Forexample, why can some rocky worlds like Ceres and Earth hold on to largeamounts of water, while others, like Vesta, are completely desiccated?
?Vesta willtell us about the earliest epoch, and Ceres will tell us about what happenedlater,? Russell said.
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