Dawn Reaches Milestone Approaching Asteroid Vesta

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE 818-354-5011
http://www.jpl.nasa.gov

Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov

Dwayne C. Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown@nasa.gov

News release: 2011-130 May 3, 2011

Dawn Reaches Milestone Approaching Asteroid Vesta

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-130&cid=release_2011-130

PASADENA, Calif. – 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 1.21
million kilometers (752,000 miles) from Vesta, or about three times the distance between 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. 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 16,000 kilometers (9,900 miles) 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 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 journey of 4.8-billion kilometers (3-billion miles), 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 bodies, 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 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.

JPL is a division of the California Institute of Technology, Pasadena.

For more information about Dawn, visit: http://www.nasa.gov/dawn and http://dawn.jpl.nasa.gov

To learn more about Dawn's approach phase, read the latest Dawn Journal at
http://blogs.jpl.nasa.gov/2011/05/dawn-begins-its-vesta-phase/

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