[meteorite-list] NASA's Dawn Mission to Asteroids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 21 Jun 2007 09:12:58 -0700 (PDT)
Message-ID: <200706211612.JAA13498_at_zagami.jpl.nasa.gov>

http://www.itwire.com.au/content/view/13090/1066/

NASA's Dawn mission to asteroids powered with high-tech ion drive
By William Atkins
iTWire
21 June 2007

Preparations are being made on Launch Pad 17-B at the Cape Canaveral Air
Force Station, Florida to launch the robotic Dawn interplanetary
spacecraft on its eight-year, 3.2-billion-mile mission to the main
asteroid belt.

The Dawn space probe is scheduled to lift-off on July 7, 2007, with a
launch window of opportunity between 4:09:31 and 4:36:22 p.m. EDT. The
Dawn mission is the first NASA scientific mission to use the technically
advanced ion propulsion, and the first mission to orbit two planetary
bodies during a single mission.

The Dawn mission will involve the Dawn space probe to rendezvous, orbit,
and examine two large members of the asteroid belt: the dwarf planet
Ceres and the asteroid Vesta. The asteroid belt is located between the
orbits of the planets Mars and Jupiter.

The members of the asteroid belt hold important information about the
origins and early evolution of our solar system. The asteroid belt
members consist of gases and dust that have slowly come together to form
larger and larger pieces. For the first time ever, one spacecraft will
enter the orbit of two different planetary bodies (other than the Moon
and the Earth).

Ceres and Vesta are two of the largest protoplanets left intact after
the original formation of our solar system. (A protoplanet is any
celestial body that had the potential at one time to turn into a
planet.) Both protoplanets are believed to be over 4.5 billion years old.

As protoplanets, the study of Ceres and Vesta by the Dawn probe will
allow astronomers to better understand the conditions and processes
going on during the solar system's earliest period of development. Since
Ceres and Vesta are located in two different regions of the solar
system, scientists contend that they may have developed differently.

In fact, Ceres is thought to have experienced a cool and wet (icy)
beginning that produced subsurface water. On the other hand, Vesta is
believed to have experienced a hot and dry (rocky) start that gave it
volcanism both on its surface and beneath it.

In its mission schedule, the probe will first fly past the planet Mars
in March 2009 for a gravity assist maneuver to gain speed for its
eventual arrival at Vesta (officially called 4 Vesta). It will arrive at
Vesta in October 2011 to perform remote-sensing observations of the body.

Vesta, the second most massive object in the asteroid belt, is a
water-poor achondrite (a stony meteorite that does not contain
chondrules, which are material composed primarily of silicate minerals).
It experienced extreme heating and differentiation (development) during
its lifetime. Vesta has a diameter of about 530 kilometers (330 miles)
and an estimated mass of 9% of all the asteroids in the asteroid belt.
It is believed to contain a metallic core and basaltic (volcanic) flows.

Earlier in its history, Vesta may have been partially molten, which
allowed heavily elements such as iron to sink into a dense core.
However, scientists are puzzled with Vesta's history because of its
relatively small size. It does not have enough size to produce the
gravitational forces needed to melt the asteroid. Some cosmologists
think that one or more supernovas exploded during the birth of the solar
system, giving Vesta enough energy to melt.

Much can be learned about Vesta and the early times of our solar system.
Dawn's instruments such as spectrometers will investigate the minerals
and elements that make up its surface. Vesta's gravitational field will
also be tracked by the spacecraft.

The Dawn probe will then leave Vesta in April 2012, performing the
first-time ever maneuver of leaving the orbit of one distant body and
flying to another body to orbit about it (other than the Earth and Moon).

In February 2015, Dawn will arrive at Ceres, which is classified as a
dwarf planet. Pluto is also now considered a dwarf planet, after a new
international definition instituted in 2006 demoted it from a
full-fledged planet. Ceres is about 25% as massive as all of the
asteroids within the asteroid belt. It is 950 kilometers (590 miles) in
diameter.

The mission will study the composition of Ceres, which now is believed
to be similar to a water-rich carbonaceous (carbon-like) chondrite (a
stony meteorite that contains chondrules). It is believed to contain a
layer of ice that is 60 to 120 kilometers (37 to 74 miles) in thickness.
Scientists hope to learn how water assisted in the development of Ceres
and the early formation planetary bodies.

The end of Dawn's operational life is expected to be July 2015, although
visits to other asteroids are being left as possibilities if its
operational lifetime is greater than expected.

A Delta II-Heavy (Delta 7925H) rocket, manufactured by United Launch
Alliance (a joint venture of Boeing and Lockheed Martin), will lift the
Dawn spacecraft off its launch pad. The rocket has three stages and nine
solid rocket boosters. It was prepared for launch at the Astrotech Space
Operations subsidiary of SPACEHAB, Inc. (Titusville, Florida).

The Dawn probe is propelled with solar electric ion thrusters, sometimes
called collectively as an ion drive. The thrusters were first used in
the NASA Deep Space 1 mission. The propulsion system for Dawn uses beams
of ions (electrically charged atoms or molecules) to move it about. Ion
propellant is ten times more efficient than conventional propellant.

The spacecraft is about 19.7 meters (65 feet) across and has a launch
mass of about 1,210 kilograms (2,668 pounds). It carries a framing
camera, gamma ray and neutron detector, visible and infrared mapping
spectrometer, and other scientific instruments to make observations
during its trip.

The major contractors for the mission are Orbital Sciences Corporation,
NASA's Jet Propulsion Laboratory (JPL), and the University of California
at Los Angeles (UCLA).

Three websites of the NASA Dawn mission are http://dawn.jpl.nasa.gov/
and http://www.nasa.gov/mission_pages/dawn/main/ and
http://tinymce.moxiecode.cp/mce_temp_url.
Received on Thu 21 Jun 2007 12:12:58 PM PDT