[meteorite-list] Ion Engine Goes On Dawn Patrol

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:10:09 2004
Message-ID: <200304181822.LAA11886_at_zagami.jpl.nasa.gov>

http://iol.co.za/index.php?click_id=31&art_id=iol105059084577N350&set_id=1

Ion engine goes on Dawn patrol

These articles appear in the April issue of the South African edition of
Popular Mechanics

April 17, 2003

It sounds more like a joke than an example of cutting-edge propulsion
technology. Here's a complex and expensive spacecraft engine designed by
some of the world's finest minds, and it produces just 10 grams of thrust at
full throttle - about the same pressure as a sheet of paper resting on the
palm of your hand!

In reality, of course, the ion engine is a very serious piece of machinery.
And it works rather well. So well that Nasa and UCLA are fitting it to their
Dawn spacecraft, due for launch from Cape Canaveral, Florida, on May 27,
2006. The goal of the Dawn mission is to help us understand the conditions
and processes during the earliest history of our solar system.

To accomplish this, Dawn will explore the structure and composition of Ceres
and Vesta, two asteroids (or minor planets) that have many contrasting
characteristics and have remained intact since their formation more than
4.6-billion years ago. After more than four years of travel, the spacecraft
will rendezvous with Vesta on July 30, 2010. It's due to orbit Vesta for
almost a year, studying its basic structure and composition.

On July 3, 2011, Dawn will leave Vesta for a three-year cruise to Ceres,
where it begins its new orbit on August 20, 2014. After a year-long
exploration of Ceres, and more than nine years of space travel, Dawn may
continue with additional exploration of the asteroid belt. Dawn will capture
images of Ceres' and Vesta's surfaces to determine their bombardment and
tectonic history, using gravity, spin state and magnetic data to limit the
size of any metallic core, and infrared and gamma ray spectrometry to search
for water-bearing minerals.

The Dawn Mission - part of Nasa's Discovery Programme, an initiative for
lower-cost, highly focused, rapid-development scientific spacecraft -is led
by UCLA space scientist Christopher Russell. Team members include scientists
from the German Aerospace Centre, the Institute for Space Astrophysics in
Rome, Nasa's Jet Propulsion Laboratory (JPL), Los Alamos National
Laboratory, the Massachusetts Institute of Technology, and other US
universities. Orbital Sciences Corporation will construct the spacecraft,
and JPL will provide the ion engines and management of the overall flight
system development.

Because Ceres and Vesta lie near the plane of Earth's orbit they can both be
studied with a single mission. The Dawn mission will help us understand the
evolution of the interior structure and thermal history of Ceres and Vesta -
information that provides keys to the secrets of the creation of our solar
system.

Ceres, the largest asteroid in our solar system, is a roughly round object
about 960km in diameter. It orbits the sun in the asteroid belt between Mars
and Jupiter about 412-million km from Earth, completing an orbit in
4.6-terrestrial years. The year 2001 marked the 200th anniversary of the
discovery of Ceres by Giuseppe Piazzi, using a small telescope atop the
royal palace in Palermo. At first Piazzi believed he had found the missing
planet expected to be in the region we now call the asteroid belt.
However, this minor planet turned out to be very small indeed - only one
quarter of the diameter of Earth's moon.

Further observations by Piazzi were cut short due to illness. Carl Friedrich
Gauss, at the age of 24, was able to solve a system of 17 linear equations
to allow Ceres (named after the Roman goddess of agriculture) to be
rediscovered, a remarkable feat for this time. Within one year of its
initial discovery, both Heinrich Olbers and Franz von Zach were also able to
re-identify Ceres.

Vesta is the brightest asteroid in our solar system and is the only one
visible with the unaided eye; its oval, pumpkin-like shape has an average
diameter of about 515km. Discovered by Olbers on March 29, 1807, it was the
fourth minor planet to be found. Named for the ancient Roman goddess of the
hearth, Vesta is approximately 354 million km from Earth. It circles the sun
in 3,6-terrestrial years.

Studies of meteorites found on Earth (and thought to originate from Vesta)
suggest that the body formed from dust in the solar nebula within 5 to
15-million years of the solar system's birth about 4.6-billion years ago.
Although no meteorites from Ceres have yet been found, it too was formed
very early, perhaps during the first 10-million years of the solar system's
existence. Ceres and Vesta are among the few large protoplanets that have
not been heavily damaged by collisions with other bodies.

Ceres and Vesta are strikingly different in composition. Scientists believe
many of these differences stem from the conditions under which they formed,
with Ceres forming wet and Vesta dry. Evidence of water - frost or vapour on
the surface, and possibly liquid water under the surface - still exist on
Ceres; this water kept Ceres cool throughout its evolution. However, Vesta
was hot, melted internally and became volcanic early in its development.

As a result of these two different evolutionary paths, Ceres remains in its
primordial state, while Vesta has evolved and changed over millions of
years. Microwave studies suggest that Ceres is covered with a dry clay, in
contrast with Vesta's basaltic dust layer, which reflects its crustal
composition.

*Sources: Nasa/UCLA

Ion propulsion

Unless something dramatic happens in the interim, the Dawn spacecraft will
be the first purely scientific mission to be powered by ion propulsion, an
advanced technology used by Nasa's earlier Deep Space 1 mission.

The principle behind the ion engines is much the same as the phenomenon you
experience when you pull hot socks out of the tumble dryer on a cold winter
day. The socks push away from each other because they are electrostatically
charged, and like charges repel. The challenge in electric space propulsion
is to charge a fluid so its atoms can be expelled in one direction, and thus
propel the spacecraft in the other direction.

Unlike chemical rocket engines, ion engines accelerate nearly continuously,
giving each ion a tremendous burst of speed. The fuel used by an ion engine
is xenon, a gas also used in high-intensity light systems such as
photographic flash units and some car headlights. It's more than four times
heavier than air. When the ion engine is running, electrons are emitted from
a hollow tube (the cathode). These electrons enter a magnet-ringed chamber,
where they strike the xenon atoms. The impact of an electron on a xenon atom
knocks away one of xenon's 54 electrons. This results in a xenon atom with a
positive charge - a xenon ion.

At the rear of the chamber, a pair of metal grids is charged positively and
negatively. The force of this electric charge exerts a strong electrostatic
pull on the xenon ions. The xenon ions shoot out the back of the engine at a
speed of over 100 000km/h. At full throttle, the ion engine consumes 2 500
watts of electrical power and produces a tiny thrust - about the same
pressure as a sheet of paper resting on the palm of a hand. That's far less
thrust than is produced by even small chemical rockets.

But here's the key: as proved by Nasa's Deep Space 1 mission, an ion engine
can run for months and even years, and despite the almost imperceptible
thrust, this engine, for a given amount of fuel, can gradually increase a
spacecraft's velocity 10 times more than can a conventional rocket powered
by liquid or solid fuel.

In fact, Deep Space 1 validated 12 high-risk advanced technologies that had
never flown before. Launched in 1998 as an 11-month mission, it performed so
well that Nasa kept it going for a few more years to continue testing its
ion engine while doing a little science. The spacecraft completed its
primary mission to test these technologies, going on to exceed expectations
by cashing in on its frequent flyer miles and flying by Comet Borrelly. It
captured the best close-up pictures of a comet and returned the best science
data from a comet ever. The spacecraft was retired in December 2001, but
back home its spare engine kept on running.

In August last year the thruster passed a major milestone by processing
200kg of xenon propellant. This amount of propellant, in addition to being a
nice round number, is also the amount required for the ion propulsion system
on the Dawn spacecraft.
Received on Fri 18 Apr 2003 02:22:08 PM PDT