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.solarviews.com/eng/asteroid.htm
discovery.nasa.gov/dawn.html
www-ssc.igpp.ucla.edu/dawn
www-ssc.igpp.ucla.edu/dawn/newsletter - The Dawn's Early Light
(Newsletter following the Dawn Mission)
Other NASA Discovery Program Missions
·NEAR·
·Mars Pathfinder·
·Lunar Prospector·
·Stardust·
·Genesis·
·CONTOUR·
·MESSENGER·
·Deep Impact·
·Kepler·
·ASPERA-3·