[meteorite-list] Dawn Journal - October 7, 2007

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 10 Oct 2007 08:23:31 -0700 (PDT)
Message-ID: <200710101523.IAA02902_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_10_07_07.asp

Dawn Journal
Dr. Marc Rayman
October 7, 2007

Dear Xedawnions,

Joining an elite club among spacecraft, Dawn successfully fired up its
xenon ion propulsion system on October 6. This important milestone in
Dawn's 80-day checkout phase followed ongoing work by the mission
operations team members to become accustomed to flying this new
spacecraft, as they continue monitoring telemetry, adjusting onboard
parameters, and conducting special activities to keep the spacecraft
performing smoothly.

The ion propulsion system (in the interest of environmental
responsibility, we will conserve keystrokes in this log by using the
acronym IPS) will be used to climb away from the Sun, pass Mars,
rendezvous with Vesta, maneuver into different orbits around it to
provide the science instruments with varied views of the alien world,
leave orbit, push still deeper into space to dwarf planet Ceres, and
orbit it for the same scientific scrutiny. All systems on the spacecraft
are complex and important (the relative importance was considered on
September 17, 2006 and October 29, 2006, but the IPS has been the
focus of the Dawn team in recent days.

While the 3 ion thrusters are the most familiar part of the IPS, they
are not its only elements. The system includes 2 computer controllers
(only 1 is used at a time). When mission control selects 1 of the 112
throttle levels (each corresponding to a certain power consumption and
thrust), the operating controller translates the command into the
currents and voltages that must be applied to parts of the thruster and
the flow rate of xenon propellant to the thruster. The controller also
provides the principal communications between the main spacecraft
computer and the rest of the IPS, accepting commands and reporting on
the IPS performance. While the controller is the brain of the system,
the 2 power units (again, only 1 is used at a time) are the brawn.
Following instructions from the controller, a power unit receives power
from the solar arrays at about 100 volts and converts it to more than
1000 volts for use by the thruster.

On October 2, a controller and a power unit were activated and verified
to be operating correctly. When that was complete, those devices were
turned off and the other ones were powered on and checked out.

The 3 ion thrusters are mounted on different parts of the spacecraft.
The one located along the central axis
of the spacecraft (at the "bottom" in many depictions) is known
affectionately as thruster #3, and that was the focus of this past
week's tests. The thruster near the visible and infrared mapping
spectrometer (on what might be considered the "back" of the spacecraft)
is thruster #1. Given that sophisticated nomenclature, the astute reader
might presume that the thruster near the main antenna (on the "front" of
the spacecraft) is thruster #2. As our readers are well known all to be
astute, it will be no surprise that indeed that is its designation.

Each thruster is mounted to a gimbal system that allows the direction it
is pointed to be changed by a few degrees. The angle is not large; the
total range corresponds to less than the movement of the minute hand of
a clock in 3 minutes. While the purpose of IPS is thrusting is to propel
the spacecraft in its orbit (around the Sun now, and later around Vesta
or Ceres), the attitude control system uses the
thrust as one of its means to control the spacecraft's orientation by
slowly swiveling the thruster. The gimbal for thruster #3 was commanded
to execute a preprogrammed set of movements on October 2, and its health
was verified.

Some of the components in the thrusters are very sensitive to
contamination, particularly water. While every effort was made to
prevent air, and its normal inventory of water vapor, from becoming
trapped in the system while on Earth, it is inevitable that some stray
molecules of water would be in the stainless steel lines that deliver
xenon to the thrusters and in the thrusters themselves. To reduce the
presence of contaminants, several days of activities were devoted to
purging the system by baking it out. Around noon PDT on October 2,
mission controllers transmitted commands to raise some parts of the
spacecraft to about 50?C (approximately 120?F). The temperatures were
restored to normal values 30 hours later.

As most space enthusiasts know, October 4 was the 50th anniversary of
the launch of Sputnik 1 and, quite remarkably given the different
calendric system, the 5,000,050th anniversary of the first mission to
the eventual site of the Tribute to Coincidence. The Dawn project
recognizes with great admiration those missions and all others that
have ventured into space in the pursuit of knowledge and the spirit
of exploration. As the Dawn team prepared for the day's activity, 7
days after launch, the spacecraft was almost 2300 times farther from
Earth than Sputnik 1 was at its maximum range. Yet Dawn's journey is
still just beginning, and its travels should take it more than 250
times still farther from home.

To ionize its propellant, the thruster bombards the xenon atoms with
electrons, as explained on December 28, 2006. The device that emits
electrons was heated for several hours on October 4 as another step
in preparing the thruster for operation.

The last operation before thrusting, undertaken on October 5, was to
ionize xenon inside the thruster but not accelerate the propellant,
obliging the IPS to do almost everything required for normal thrusting.

Because the thrust is so gentle, there are no sensors on board that
directly measure it. To verify that the thruster performs as expected,
the remarkable accuracy of the techniques of deep-space navigation
are employed. With measurements of the change in the frequency (or
pitch) of the radio signal, engineers can calculate the change in
the spacecraft's speed. This capability relies on the Doppler effect,
which is familiar to most terrestrial readers as they
hear the pitch of a siren rise as it approaches and fall as it recedes.
Other readers who more commonly travel at speeds closer to that of light
recognize that the well-known blueshift and redshift are manifestations
of the same principle, applied to light waves rather than sound waves.
Although like all spacecraft built by humans, Dawn's speed is only a
tiny fraction of the speed of light, with the astonishing sensitivity of
the Doppler measurements, the gradual effect of the thrusting can be
sensed. With the spacecraft coasting away from Earth at more than 3.3
kilometers per second (7400 miles per hour), the radio measurements can
detect changes smaller than 0.5 millimeters per second (less than 6 feet
per hour). Snails take note.

Others take note that the speed relative to Earth is most assuredly not
the speed relative to the Sun. The spacecraft is in its own orbit around
the Sun, and at this point in the mission, it is traveling at about 33.1
kilometers per second (74,000 miles per hour) in that orbit. This writer
and others on his homeworld are orbiting the Sun at 29.8 kilometers per
second (66,700 miles per hour). The difference is the speed at which
Dawn is leaving Earth.

The Doppler effect applies only for motion toward or away from the
observer; movement across the line of sight does not change the pitch
of the signal. Therefore, to maximize the effect in the test of the IPS,
the thruster should propel the spacecraft toward or away from Earth.
With the present relative positions of Earth, our favorite
interplanetary probe, and the Sun, when thruster #3 is pointed toward
Earth, the incident sunlight, in combination with the heat generated by
the operation of the thruster itself, would cause the unit to overheat.
With the thruster pointed directly away from Earth, the temperature is
fine. That has the curious consequence of engineers choosing to propel
the spacecraft toward Earth during the first thrust test.

The objective of all the thrusting during the first 80 days of the
mission is not to change the spacecraft's trajectory but rather to
evaluate the performance of all systems and prepare for the thrusting
after this checkout phase. As the effect of the ion propulsion only
becomes significant after long intervals, the short thrust periods for
testing do not cause important changes in the trajectory.

On October 6, the mission control team instructed Dawn to turn to point
thruster #3 away from Earth. Following that, and after one final
verification that all onboard systems were healthy and ready for the
next step, the command to initiate thrusting was sent. The drama was
captured in the stirring name of the file that was transmitted to the
spacecraft: dz002e.scmf. (Our readers who are not versed in neutron star
orbital opera may not fully appreciate the drama of that name and are
requested to accept that others may find great passion in the command
file name.)

In the silent depths of space, far from its designers and controllers,
connected to Earth only by the faintest whisper of a radio signal, Dawn
dutifully executed the programmed steps. The craft had no appreciation
of the hopefulness of its terrestrial handlers as it began emitting a
bluish beam of xenon ions at 6:07 pm PDT.

When data revealing the thruster's electrical currents and voltages
showed up in mission control, the excitement remained controlled but was
clearly rising much much faster than the gradual acceleration of the
spacecraft. Experienced team members, huddled around the monitors in
mission control, kept in mind that while starting thrusting was
wonderful progress, success required sustaining it. Within 5 minutes
though (in fact, shortly after 4.5 minutes for team members who
also worked on Deep Space 1), the enthusiasm could no longer be
contained, as all indications were that Dawn was quite content to keep
thrusting.

Much of the joy was in sharing the success with colleagues who have
worked very very hard together for years, each perhaps with his or her
own personal motivations and rewards, but each contributing to a common
goal of pushing the frontiers of space exploration. Still more of the
happiness is in sharing the accomplishment with supportive family
members and friends -- and loyal readers! While, like launch, this is
but one more step in Dawn's very long journey to unlock the secrets of
Vesta and Ceres, it is an important one, and the many feelings of having
a probe in powered flight in deep space are all? well, perhaps "out of
this world" is the best descriptor.

The test sequence operated the IPS at throttle level 28 (in the range
from 1 to 112) for nearly 12 hours. Next, it throttled up to level 49,
and then it pushed still higher every 4 hours after that, operating at
levels 70, 91, and finally 112. Thrust was commanded off on October 7 at
7:12 pm as the test completed successfully. In about 25 hours of
thrusting, the acceleration amounted to 3.6 meters per second (8 miles
per hour), truly negligible compared to the spacecraft's speed relative
to Earth or the Sun.

After the checkout phase, except in special circumstances, Dawn always
will use the highest throttle level it can. As it travels farther from
the Sun, eventually its enormous solar arrays, the most powerful ever
used on an interplanetary spacecraft, will not produce enough power to
permit operation at level 112, so the IPS will be throttled down. That
is why it is necessary to certify operation over a range of throttle levels.

In addition to the Doppler measurements to reveal the thrust,
engineering data were collected on the performance of the IPS, attitude
control system, electrical power system, thermal control system, and all
other onboard participants in the thrusting. More tests are ahead for
thruster #3 as well as the other thrusters and other spacecraft systems.

Dawn is 3.21 million kilometers (2.00 million miles) from Earth or more
than 8 times farther than the moon. Radio signals, traveling at the
universal limit of the speed of light, take more than 21 seconds to make
the round trip.
Received on Wed 10 Oct 2007 11:23:31 AM PDT