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The Dawn mission is a NASA undertaking to study Ceres and Vesta, the two largest objects in the asteroid belt between Mars and Jupiter. Scheduled for a June 2006 launch, Dawn is the tenth mission if NASA's Discovery Program. Orbital Sciences Corporation will design and build the spacecraft, while NASA's Jet Propulsion Laboratory will provide the ion propulsion engine.

Intrigue of Vesta and Ceres

Representing the oldest known worlds in the solar system, Vesta and Ceres are essentially time capsules from the formation of the planets. Although they are comprised of similar matter as the other inner planets, their growth was stunted by the presence of Jupiter, which gravitationally discouraged the further accretion of nearby asteroids. Meteorites found on Earth that appear to have originated from Vesta suggest a howardite, eucrite, and diogenite (HED) composition. Based on these meteorites, scientists believe Vesta formed only 5-15 million years after the beginning of the solar system. Contrasted with Earth's 50 million year accretion period and Mars' 30 million years, Vesta holds clues as to the composition of the inner planets early in their development. Using cosmic ray exposure dating of Vestan meteorites, scientists believe at least five major impacts have altered Vesta's landscape in the last 50 million years. These craters will allow Dawn to peek inside the massive asteroid, revealing the composition of its interior. While no meteorite specimens from Ceres have been found on Earth, scientists believe it must have formed around the same time as Vesta.

Spectrometry suggests Ceres contains water-bearing minerals and perhaps a thin atmosphere. Discovered in 1801 by Giuseppe Piazzi, it was the first asteroid to be identified, giving it the title of "1 Ceres". It is named for the Roman goddess of agriculture. With a diameter of 932 kilometers, it is the largest asteroid in the solar system, yet is less spherically developed of the two protoplanet targets of the mission. While Ceres might have seasonal polar ice caps, Vesta, on the other hand, is a warmer and dryer world, with evidence of lava flow on the surface and a more spherical shape with a diameter of 525 km. Heinrich Olbers discovered Vesta in 1804, making it the fourth asteroid discovered, and so it is traditionally cited as "4 Vesta". It is the brightest asteroid, and the only one visible with the naked eye. Vesta is named for the Roman goddess of the hearth.

Onboard Instrumentation and their Scientific Objectives

The Dawn mission seeks to measure the composition, mass, volume, and spin rate of Ceres and Vesta using three basic onboard instruments. A Framing Camera will offer photographs of the protoplanets to study their surface morphology and craters.  To obtain data regarding the elemental composition of the worlds, Dawn will employ a Visible and IR Mapping Spectrometer. This spectrometer will also give insights into the thermal history of the bodies. A Gamma Ray and Neutron Detector will assist in the search for water-bearing minerals.

Advantages and Disadvantages of Ion Propulsion

Ion propulsion in spacecraft was first successfully by the Deep Space 1 mission in 1998 and 1999. This advanced technique allows spacecraft to achieve great speeds with very little onboard fuel. Electricity generated from large solar panels is used to charge tiny amounts xenon atoms which are then thrust from the back of the spacecraft. Since there is no friction in space, these tiny spurts cumulate into a great deal of acceleration for the probe. Dawn will only use 275 kg of xenon fuel to arrive at Vesta, and another 110 kg to reach Ceres. Still, not enough speed will be generated after launch to take advantage of a direct route to Ceres and Vesta. Instead, Dawn will take a roundabout trip, utilizing a flyby of Mars and Earth to gain acceleration en route to its targets.

Critical Mission Dates

  • June 2006 - Launch
  • February 2009 - Mars flyby
  • October 2011 - Vesta arrival
  • May 2012 - Vesta departure
  • August 2015 - Ceres arrival
  • January 2016 - End of operations

www-ssc.igpp.ucla.edu/dawn/newsletter - The Dawn's Early Light (Newsletter following the Dawn Mission)

Other NASA Discovery Program Missions
·Mars Pathfinder·
·Lunar Prospector·
·Deep Impact·