[meteorite-list] Dawn Journal - December 28, 2006

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 4 Jan 2007 09:10:48 -0800 (PST)
Message-ID: <200701041710.JAA12729_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_12_06.asp

Dawn Journal
Dr. Marc D. Rayman
December 28, 2006

Dear Dawnnewyears,

The Dawn spacecraft has made its new year's resolution: to leave Earth
behind in 2007 and embark upon its celestial voyage of adventure and
discovery. (Actually, it was either that or spend more quality time with
friends and family. As much as we all like Dawn, I think we can be
grateful it made the choice it did.) The spacecraft is well on its way
to achieving its goal.

Over the past few months, Dawn has completed all of the demanding
environmental tests planned for it at Orbital Sciences Corporation. In
the last log <http://dawn.jpl.nasa.gov/mission/journal_10_06.asp#testing>,
we saw why such tests are so important. Since then, Dawn has been spun,
vibrated, and blasted by noise, and careful testing afterwards has
verified that it can withstand these insults and still operate as planned.

One of the tests included attaching Dawn to the structure that will
connect it to the upper stage of the Delta II 7925H-9.5 rocket so
essential to keeping its new year's resolution. Part of the objective of
this test was to verify that the spacecraft and the rocket, although
manufactured separately, really will fit together when they meet at Cape
Canaveral in June. In addition, this test was used to subject Dawn to
another special condition it will experience in its mission. Following
the burn of the Delta's third (and last) stage, the rocket will
relinquish its firm grasp on the spacecraft. The firing of the release
mechanism will cause a shock (certainly physical, possibly emotional) to
go through the spacecraft as it is freed to operate in space on its own.
Feeling this shock is part of the battery of tests the spacecraft has
now completed. Continuing with its perfect record, Dawn passed
beautifully, demonstrating that it can tolerate the shock and separate
cleanly, with no structures impeding its departure from the rocket.

Following all these tests, the two large solar array wings
<http://dawn.jpl.nasa.gov/mission/journal_09_06.asp#solar_wings> were
extended, allowing engineers another test of the deployment system and
the opportunity to verify that the delicate cells were still healthy.
Each wing extends 8.3 meters (more than 27 feet) and weighs almost
63 kg (139 pounds). The system is not designed to be strong enough to
support them under the strong pull of Earth's gravity; of course, when
Dawn is in its natural environment of spaceflight, no such force will
be exerted upon the arrays. For working in the exotic conditions here
on the surface of our planet, a special structure is erected to bear the
weight of the wings yet allow them to unfold smoothly. After the tests,
the solar arrays were removed, and they will not be reattached until
the spacecraft is in Florida.

Now Dawn is being prepared for its departure from Orbital Sciences in
Dulles, VA. Next month it will be transported to the Naval Research
Laboratory (NRL) in Washington, D.C. for the final phase of
environmental tests, all of which will be conducted with the spacecraft
in a vacuum. Orbital has the vacuum facilities to accommodate the
spacecraft (see the description in the July 29, 2006 log
<http://dawn.jpl.nasa.gov/mission/journal_07_29_06.asp#vacuum>), but
this upcoming series of tests will include a brief firing of the ion
thrusters, and that requires a different vacuum system. Because NRL
has the needed capability and is near Orbital, it was a natural
location for this work. Dawn will spend about 3 months there, and the
next log will report on the activities, including the operation of the
ion propulsion system.

Devoted readers have asked for more information on ion propulsion. This
is only one of the important subsystems onboard (see the overview and
relative importance of all the subsystems and systems on September 17,
2006 <http://dawn.jpl.nasa.gov/mission/journal_09_06.asp#ion_prop> and
October 29, 2006 <http://dawn.jpl.nasa.gov/mission/journal_10_06.asp#ion_prop2>),
and Dawn will rely upon all of them in order to explore the remote,
alien worlds Ceres and Vesta. Over the many years of the mission, we
shall have occasion to learn a great deal more
about many facets of the engineering and science of this exceptional
adventure, but starting in this log, and continuing in the next, we will
take a more detailed look at the ion propulsion system.

While most of our audience is, of course, quite familiar with this
topic, we should recall that our readership extends to planetary systems
that have had little experience with this technology, and it is to them
that this material is directed. Although it may be surprising,
apparently there are even some readers who did not follow NASA's Deep
Space 1 (DS1) mission, which tested ion propulsion and other high-risk
technologies to protect subsequent missions from the risk and cost of
being the first users of such advanced systems. Dawn is one of DS1's
beneficiaries, and being the first spacecraft ever built to orbit 2
target bodies after leaving Earth, it would be effectively impossible
without ion propulsion.

Ion propulsion had its origins in solid science, but despite some
scientific and engineering work, it resided principally in the fictional
universes of Star Trek, Star Wars, and other fanciful stories. DS1
helped bring ion propulsion from the domain of science fiction to
science fact.

First let's recall how a propulsion system works. Most conventional
systems use high pressure or temperature to push a gas through a rocket
nozzle. The action of the gas leaving the nozzle causes a reaction that
pushes the craft in the opposite direction. This is what causes a
balloon to fly around when the end is opened and the stretched rubber
squeezes the air out. Ion propulsion works on the same principle, but
the method of pushing the gas out is unique.

The inert gas xenon, which is similar to helium and neon but heavier, is
used as propellant. The composition of xenon is simple: each atom
consists of a tiny and dense nucleus surrounded by a cloud of electrons.
The nucleus is 54 positively charged protons plus about 76 neutral
neutrons. (Xenon gas is a mixture of 9 isotopes, meaning there are 9
different values for the number of neutrons. From a low of 70 to a high
of 82, the number of neutrons makes only very modest differences in the
behaviors of the atoms.) The 54 positive charges in the nucleus are
precisely balanced by 54 negatively charged electrons, rendering the
atom electrically neutral -- until the ion propulsion system gets in the
act.

Inside the ion thruster, an electron beam, somewhat like the beam that
illuminates the screen in a television, bombards the xenon atoms. When
this beam knocks an electron out of an atom, the result is an
electrically unbalanced atom: 54 positive charges and 53 negative
charges. Now with a net electrical charge of 1 unit, such an atom is
known as an "ion." Because it is electrically charged, the xenon ion can
feel the effect of an electrical field, which is simply a voltage. So
the thruster applies more than 1000 volts to accelerate the xenon ions,
expelling them at speeds as high as 35 kilometers/second (more than
78,000 miles/hour). Each ion, tiny though it is, pushes back on the
thruster as it leaves, and this reaction force is what propels the
spacecraft. The ions are shot from the thruster at roughly 10 times the
speed of the propellants expelled by rockets on typical spacecraft, and
this is the source of ion propulsion's extraordinary efficacy.

All else being equal, for the same amount of propellant, a spacecraft
equipped with ion propulsion can achieve 10 times the speed of a craft
outfitted with normal propulsion, or a spacecraft with ion propulsion
can carry far less propellant to accomplish the same job as a spacecraft
using more standard propulsion. This translates into a capability for
NASA to undertake extremely ambitious missions such as Dawn.

The rate at which xenon is flowed through the thruster is very low. At
the highest throttle level, the system uses only about 3.25
milligrams/second, so 24 hours of continuous thrusting would expend only
10 ounces of xenon. Because the xenon is used so frugally, the
corresponding thrust is very gentle. The main engine on some
interplanetary spacecraft may provide about 10,000 times greater thrust
but, of course, such systems are so fuel-hungry that their ultimate
speed is more limited.

The force of the ion thruster on the spacecraft is comparable to the
weight of a single sheet of paper. So here is an ion propulsion
experiment you may conduct safely at home: hold a piece of paper in your
hand, and you will feel the same force that the ion thruster exerts.
Because the fuel efficiency is so great, the thruster can provide its
push not for a few minutes, like most engines, but rather for months or
even years. In the weightless and frictionless conditions of
spaceflight, the effect of this thrust can gradually build up to allow
the spacecraft to achieve very very high speed. Ion propulsion delivers
acceleration with patience.

Throughout its mission, Dawn will be farther from the Sun than Earth,
but as long as it is less than about twice Earth's distance from the
Sun, those huge solar arrays will generate enough power to operate the
ion propulsion system at its maximum throttle level. At that setting,
the acceleration will be equivalent to about 7 meters/second/day, or
slightly more than 15 miles/hour/day: one full day of thrusting would
change the spacecraft's speed by 15 miles/hour. That means it would take
Dawn 4 days to accelerate from 0 to 60 miles/hour. Perhaps this does not
evoke the image of a hot rod, but its parsimonious consumption of xenon
lets it thrust for much longer than 4 days.

To put this in perspective, consider a greatly simplified example based
upon the remarkable probes NASA has in orbit around Mars now. When they
arrived at the planet, these spacecraft had to burn their engines to
drop into orbit. While each mission is different, such a maneuver might
be about 1000 meters/second (2200 miles/hour) and could consume about
300 kilograms (660 pounds) of propellants. With its ion propulsion
system, Dawn could accomplish the same change in speed with less than 30
kilograms of xenon. A typical Mars mission might complete its maneuver
in less than 25 minutes, while Dawn might require more than 3 months. If
one has the patience, the ion propulsion can be very effective. Now for
many missions, the greater complexity and cost of ion propulsion is
unnecessary, and it is quite clear that we can get into orbit around
Mars without it. But as humankind engages in ever more ambitious
missions in deep space, opening our frontiers, revealing otherwise
inaccessible vistas, and seeking answers to new and more exciting
questions about the cosmos, the tremendous capability of ion propulsion
will be an essential ingredient.

By the end of its mission, having operated from its maximum throttle
level down to lower levels when Dawn was much farther from the Sun, the
spacecraft will have accumulated over 5 years of total thrust time,
giving it an effective change in speed of 11 kilometers/second, or well
over 24,000 miles/hour. That is about the same as the entire Delta
rocket with its 9 solid motor strap-ons, first stage, second stage, and
third stage, and it is far in excess of what any single-stage craft has
accomplished.

In the next log, we will see how the Dawn mission takes advantage of ion
propulsion and how its use makes the profile of the mission different
from most interplanetary flights. In the meantime, the spacecraft will
use conventional transportation technology to travel to NRL for more
rigorous tests in preparation for the challenging mission it has
resolved to begin in 2007.
Received on Thu 04 Jan 2007 12:10:48 PM PST