[meteorite-list] Dawn Journal - February 19, 2007

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 4 Mar 2007 20:54:51 -0800 (PST)
Message-ID: <200703050454.UAA28105_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_2_07.asp

Dawn Journal
Dr. Marc D. Rayman
February 19, 2007

Dear DawNRLs,

The Dawn spacecraft has just completed the final and most challenging of
the environmental tests needed to prepare for its launch and travels
through space. During the past month, it has endured the extreme heat
and cold of spaceflight in a large vacuum chamber at the Naval Research
Laboratory (NRL) in Washington, DC.

In the last few months of 2006, the spacecraft underwent a broad range
of tests at Orbital Sciences Corporation in Dulles, Virginia. It passed
all of them, and for graduation the spacecraft, along with its retinue
of mechanical and electrical test and support equipment, was sent on a
pleasingly uneventful drive to NRL during the first weekend in January.
Once it arrived, preparations began immediately for the next set of
tests in NRL's thermal vacuum chamber.

After the spacecraft and special monitoring equipment were installed in
the chamber, plates whose temperature could be individually controlled
were positioned around the spacecraft to ensure the desired thermal test
conditions could be achieved. Pumps began removing air early in the
evening of January 23, and by the next morning, the interior of the
chamber was more than 100 million times below normal atmospheric
pressure. Dawn experienced the same vacuum condition last summer, but
that was to drive off contaminants. This time, the goal was to operate
the spacecraft in ways similar to what it will face during its mission.
By subjecting it to heat and cold, and testing the performance of its
subsystems under both extremes, the engineering team could verify that
the critically important thermal control subsystem
will be able to keep temperatures within desired limits throughout the
full range of conditions Dawn will encounter in space.

The enormous solar arrays had been removed at Orbital and will not be
reconnected to the spacecraft until April. The vacuum chamber at NRL is
not large enough to accommodate the arrays when they are open, extending
19.7 meters (almost 65 feet) tip-to-tip. If they had been in their
stowed, or folded, position for these tests, as they will be when on the
rocket, they would have covered parts of the structure that are supposed
to be exposed in space. That would have retained heat inside the probe
and prevented these tests from accurately duplicating the temperatures
it will experience in space.

After the chamber pressure was reduced, the temperature was raised
gradually to 45?C (113?F) and held there for almost a week while
engineering and science subsystems were put through their paces.
 Following that, quite unaware of the hibernal, but comparatively balmy,
conditions outside NRL, the spacecraft was brought to -25?C (-13?F) for
several more days of tests. Although it is designed to operate under
these conditions, few of the Dawn team members are well suited either to
working in the absence of air or at such temperatures. Therefore, on
February 8, when the time came to make planned changes in the test
configuration, the chamber was brought back to normal atmospheric
pressure and temperature so people could enter.

Tests resumed the next day under vacuum. Among other activities in this
second phase, some mission scenario tests were conducted. Unlike the many
tests focused on individual subsystems, these were designed to make the
subsystems work together as they must when Dawn is operating in orbit
around distant Vesta, the first of its mysterious and enticing
destinations. In fact, the mission scenario tests provide an opportunity
to assess even more than the collective performance of the subsystems;
the spacecraft and some of the systems in mission control operate
together in much the same way they will during the mission.

On February 14, Dawn's ion propulsion system was powered on for its
long-awaited "hot fire test." (Your correspondent -- ever the romantic
-- conducted what proved to be an unsuccessful search for a Valentine's
Day card that appropriately expressed the sentiments associated with
such an experience.) The ion propulsion system cannot operate in normal
atmospheric pressure, so although its individual components had been
tested quite extensively, this was the only opportunity to test them all
together. The digital control interface units, power processor units,
xenon feed system, and thrusters all performed beautifully. The
extremely gentle thrust, as described in the last log, caused no more
movement of the spacecraft than if a piece of paper had been lain on it,
but the team certainly felt a powerful boost to see the bluish glow of
the thruster and gain one more indication that Dawn is getting close to
flight.

The spacecraft has 3 thrusters, with only 1 to be operated at a time in
the mission. In this test, one of them could not be fired because it was
blocked by hardware supporting the spacecraft. That thruster still had a
nearly full test. It ionized xenon but did not apply the voltage needed
to accelerate the ions. Between the other 2 thrusters, the ion propulsion
system operated at 5 different throttle levels for a total of 34 minutes
of thrusting.

To generate propulsion, the thrusters emit high velocity xenon ions.
The impingement of those ions on nearly
any object, including the chamber wall, could erode it, blasting off
contaminants that could settle on the spacecraft. Therefore, specially
designed targets were positioned about 2 meters (almost 7 feet) from the
thrusters. Myriad specially oriented fibers of carbon on the targets
captured most of the materials that the ions would otherwise have
liberated. Sensors located in the vicinity proved that this system did
indeed prevent adverse levels of contamination from accumulating. In
fact, it worked so well that the hot fire test could have continued
longer than planned, but there was no need for an extension.

The thermal vacuum testing concluded on February 17, and the spacecraft
was removed from the chamber two days later. As the operations in the
vacuum chamber were so intensive, with the team working around the
clock, a tremendous amount of data was collected, and it will require
several weeks for engineers to analyze all of them. Serving its purpose
well, however, the test has already pointed to several changes that will
bring the maturing space probe closer to its final readiness for flight,
such as adjusting the size of some heaters, the amount of insulation
over certain components, and the values of parameters in software that
control temperatures. While this and other work is being conducted on
the spacecraft, the unit that governs the delivery of electrical power
from the solar arrays to the onboard subsystems will be removed and
returned to JPL where engineers will make some modifications.

With the thermal vacuum work having been completed, representing the end
of Dawn's many months of environmental testing, this would conclude our
update on Dawn's progress, were it not for an item we report on with
some pride. The last log was chosen by readers on a majority of planets
surveyed as being among the 1000 "Most Interesting Dawn Articles Written
on December 28, 2006." Lest this be misinterpreted as being even more
prestigious than it is, it should be revealed that this recognition
applies only in the category of articles of 1900 - 2000 words (in the
original language). Nevertheless, it is this kind of appreciation that
makes the many seconds of writing seem worthwhile. This surge in
interest may be attributed to the inclusion of an explanation of the
principles underlying the ion propulsion system (IPS),
the remarkable technology that enables Dawn to
undertake its unique and exciting mission. Thus, to build upon the
success of the previous log (and, by the way, to fulfill the promise in
that log's final paragraph), it may be interesting to explore how the
IPS is used and why it makes operating Dawn different from deep space
missions with conventional propulsion. We'll see much much more about
this as we join Dawn on its long flight through space, but for now,
let's take a very brief look at how spacecraft reach their
extraterrestrial destinations and see some of the differences when ion
propulsion is used.

The physics that explains the complex beauty of orbital ballet tells us
that the velocity of any object in orbit around the Sun, be it one of
humankind's interplanetary robotic explorers or the solar system's
natural residents of planets, asteroids, and comets, depends upon the
exact shape and size of its orbit and where it is in the orbit. All
orbits are ellipses (like squashed circles, or ovals in which the ends
are the same size), but the degree of flattening and the overall size
allow for an infinite range of theoretically possible orbits (including
perfect circles). Because of our understanding of the mathematical
principles, if we know the orbit, we can calculate the velocity at any
position in the orbit; if we know the velocity at any one position, we
can calculate the entire orbit. We also know that if we change the
velocity at any position, the overall shape and size of the entire orbit
will change.

Celestial navigators have developed remarkably sophisticated methods of
using these seemingly simple principles to achieve astronomical accuracy
in flying throughout the solar system. To deliver a spacecraft to its
remote destination, engineers use the nature of orbits to choreograph
the perfect cosmic dance, ensuring that the individual performers (such
as the spacecraft and the planet it will explore) arrive at the same
spot at the same time.

When the paths of the spacecraft and the target cross, the laws of
celestial motion dictate that the objects following the orbits travel at
very different velocities, generally many kilometers per second (many
thousands of miles per hour) for interplanetary missions. If the goal is
to fly by the target, the spacecraft conducts its observations during
the brief time they are near each other; no additional orbit changes are
needed. Suppose instead the objective is to go into orbit around the
destination. That means the spacecraft will join the target as the
latter follows its own orbit around the Sun, just as the moon and
satellites in Earth orbit accompany our planet on its annual
heliocentric loop. To accomplish this, the spacecraft must swerve from
its original solar orbit in order to match the speed and direction of
its new solar orbit, which is precisely the same as the target's orbit
around the Sun. (For our purposes here, we will not attend to the
details of the spacecraft's orbit around its destination. We shall
return to that in a future log however, as it is an important part of
Dawn's story. You are encouraged simply to accept that it is adequate to
consider only orbits around the Sun for now.)

Perhaps imagining this, as one gazes thoughtfully into the cosmic void,
it becomes clear why spacecraft usually have to execute a large burn of
their propulsion systems to get into orbit around another solar system
body. That maneuver accomplishes the swerve to change the craft's path
around the Sun. (If the objective of the mission is to slam into the
target or its atmosphere, the energy of the collision changes the orbit
by just the amount that otherwise would be effected by the spacecraft's
thrusters.)

Interplanetary missions with conventional chemical propulsion rely on a
powerful rocket to be thrown from Earth into orbit around the Sun, after
which they spend months or years following that orbit, coasting to their
targets. On occasion, the flight may be interrupted by a brief firing of
the spacecraft's engine to adjust its course or a more dramatic passage
by a planet whose gravity alters the orbit, thereby boosting the probe
on its way, but for the most part, the journey is a very passive one,
with the craft doing little to help itself along. Upon reaching its
target, it fires up its engine again to veer into its new orbit. In
effect, most missions floor it very briefly and coast most of the time.

The use of the IPS creates a very different situation. The rocket does
not place Dawn into an orbit that will intersect the orbit of its
target. Dawn is so much more capable of its own maneuvering, that it
relies on the rocket only to propel it away from Earth. Once its journey
has begun, it steers its own course. By thrusting gently but
persistently for years, Dawn constantly reshapes its orbit around the
Sun. The flight profile -- the direction and timing of the thrusting --
is calculated to smoothly sculpt Dawn's orbit, gradually changing the
trajectory so that it is identical to that of its quarry. With its
amazingly low rate of fuel consumption, Dawn will spend most of its
mission with a light touch on the accelerator.

Contrary to common intuition, unlike missions with chemical propulsion,
Dawn will not have to execute a special, dedicated ion thrusting
maneuver to get into orbit around Vesta or Ceres. Indeed, the thrusting
to arrive in orbit will be no different from the years of thrusting that
precede it. With the utmost elegance, Dawn will approach each target
very slowly because, under the influence of its IPS, its orbit around
the Sun will slowly take the required shape. Instead of veering and
swerving, Dawn's maneuvering will be characterized more by grace and
delicacy. As it creeps up on an asteroid, it will slip into orbit so
gently that a casual observer would not even notice the transition.

One of the many consequences of the whisper-like force of the IPS is
that engineers must ensure that the flight profile allows enough time to
accomplish the needed thrusting. Because of the rigors of space travel
and the complexity of spacecraft engineering, all probes experience the
occasional unexpected event that interferes with planned activities, and
much sophisticated work is devoted to developing systems to safeguarding
the lonely craft when such anomalies arise. So the Dawn mission must be
designed to account for the inevitable glitches that will interrupt
thrusting, whether they be from a burst of cosmic radiation, a software
bug, or a balky component.

For a conventional planetary orbit insertion, if the maneuver of a few
tens of minutes were missed, the objectives of the entire mission would
be lost -- there can be no second chance. Propulsion then is truly
vital, so most missions have very short periods of extremely high
vulnerability and long periods of no vulnerability at all. A great deal
of effort is devoted to protecting the critical minutes. With ion
propulsion, missions generally have very long intervals of low
vulnerability. Part of the arcane science of formulating Dawn's flight
profile is ensuring that the mission can tolerate weeks of missed
thrusting at any time and still make its way to the distant worlds Vesta
and Ceres to help unlock the secrets they hold.

Dawn's next destination is Cape Canaveral. We will check in again in
April once the spacecraft has arrived at that familiar part of the solar
system to begin final preparations for launch.
Received on Sun 04 Mar 2007 11:54:51 PM PST