NASA Dawn Spacecraft Reaches Milestone Approaching Asteroid

WASHINGTON -- NASA's Dawn spacecraft has reached its official approach
phase to the asteroid Vesta and will begin using cameras for the
first time to aid navigation for an expected July 16 orbital
encounter. The large asteroid is known as a protoplanet - a celestial
body that almost formed into a planet.

At the start of this three-month final approach to this massive body
in the asteroid belt, Dawn is 752,000 miles (1.21 million kilometers)
from Vesta, or about three times the distance between the Earth and
the moon. During the approach phase, the spacecraft's main activity
will be thrusting with a special, hyper-efficient ion engine that
uses electricity to ionize and accelerate xenon to generate thrust.
The 12-inch-wide ion thrusters provide less thrust than conventional
engines, but will provide propulsion for years during the mission and
provide far greater capability to change velocity.

"We feel a little like Columbus approaching the shores of the New
World," said Christopher Russell, Dawn principal investigator, based
at the University of California in Los Angeles (UCLA). "The Dawn team
can't wait to start mapping this Terra Incognita."

Dawn previously navigated by measuring the radio signal between the
spacecraft and Earth, and used other methods that did not involve
Vesta. But as the spacecraft closes in on its target, navigation
requires more precise measurements. By analyzing where Vesta appears
relative to stars, navigators will pin down its location and enable
engineers to refine the spacecraft's trajectory. Using its ion engine
to match Vesta's orbit around the sun, the spacecraft will spiral
gently into orbit around the asteroid. When Dawn gets approximately
9,900 miles (16,000 kilometers) from Vesta, the asteroid's gravity
will capture the spacecraft in orbit.

"After more than three and a half years of interplanetary travel, we
are finally closing in on our first destination," said Marc Rayman,
Dawn's chief engineer, at NASA's Jet Propulsion Laboratory (JPL) in
Pasadena, Calif. "We're not there yet, but Dawn will soon bring into
focus an entire world that has been, for most of the two centuries
scientists have been studying it, little more than a pinpoint of light."

Scientists will search the framing camera images for possible moons
around Vesta. None of the images from ground-based and Earth-orbiting
telescopes have seen any moons, but Dawn will give scientists much
more detailed images to determine whether small objects have gone undiscovered.

The gamma ray and neutron detector instrument also will gather
information on cosmic rays during the approach phase, providing a
baseline for comparison when Dawn is much closer to Vesta.
Simultaneously, Dawn's visible and infrared mapping spectrometer will
take early measurements to ensure it is calibrated and ready when the
spacecraft enters orbit around Vesta.

Dawn's odyssey, which will take it on a 3-billion-mile journey, began
on Sept. 27, 2007, with its launch from Cape Canaveral Air Force
Station in Florida. It will stay in orbit around Vesta for one year.
After another long cruise phase, Dawn will arrive at its second
destination, an even more massive body in the asteroid belt called
Ceres, in 2015.

These two icons of the asteroid belt will help scientists unlock the
secrets of our solar system's early history. The mission will compare
and contrast the two giant asteroids, which were shaped by different
forces. Dawn's science instrument suite will measure surface
composition, topography and texture. In addition, the Dawn spacecraft
will measure the tug of gravity from Vesta and Ceres to learn more
about their internal structures.

The Dawn mission to Vesta and Ceres is managed by JPL for NASA's
Science Mission Directorate (SMD) in Washington. Dawn is a project of
SMD's Discovery Program, which is managed by NASA's Marshall Space
Flight Center in Huntsville, Ala. UCLA is responsible for overall
Dawn mission science. Orbital Sciences Corp. of Dulles, Va., designed
and built the Dawn spacecraft. The framing cameras have been
developed and built under the leadership of the Max Planck Institute
for Solar System Research in Katlenburg-Lindau in Germany, with
significant contributions by the German Aerospace Center (DLR)
Institute of Planetary Research in Berlin, and in coordination with
the Institute of Computer and Communication Network Engineering in
Braunschweig. The framing camera project is funded by NASA, the Max
Planck Society and DLR.

For more information about Dawn, visit:



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