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Posted by baalke on April 18, 2007, 11:42 am
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http://dawn.jpl.nasa.gov/mission/journal_4_07.asp
Dawn Journal
Dr. Marc D. Rayman
April 15, 2007
Dear Dawnthecoasts,
The Dawn spacecraft has completed its longest terrestrial journey on
its
path to asteroid Vesta and dwarf planet Ceres. While it will be
propelled by exotic ion propulsion during most of its mission, this
segment of its travels was accomplished using decidedly more
conventional chemical propulsion. After being packed with great care
at
the Naval Research Laboratory (NRL) in Washington, DC, the spacecraft
and a great deal of additional equipment left on a truck a few hours
before dusk on April 9. Less than 18 hours later, a few hours after
dawn, it arrived at its home for the next two months, Astrotech Space
Operations in Titusville, Florida, near Cape Canaveral.
When last we checked in with the spacecraft,
it had completed an extensive series of tests in a
thermal vacuum chamber at NRL. The pace of activities has not let up
since then, with engineers and technicians rarely letting the
spacecraft
have a rest. Myriad tasks are being completed and checked off the long
and carefully planned list of steps necessary before the probe may
begin
its ambitious mission in harsh and remote parts of the solar system.
For example, thorough checks for any possible leaks in the ion
propulsion system and the reaction control system (the system that
uses
small conventional thrusters to aid in orienting the spacecraft in
the
zero-gravity of spaceflight) verified their integrity, certifying
them
for many years of operation in space. More tests have been conducted
to
confirm the flow of information between the many elements of the
mission control systems and all of the computers onboard the
spacecraft.
As expected, some of the thermal vacuum tests had revealed the need to
make some minor changes in a few of the 9000 wires connecting
different
elements of the spacecraft. As these updates were in progress, the
device that controls the high voltage, high power electricity from
Dawn's large solar arrays was removed from the spacecraft and shipped
to
JPL. There is always a risk of accidentally damaging hardware or
introducing an error, even in ways that may not be noticed
immediately.
Therefore, after this unit was modified, it was subjected to
additional
vibration testing as well as operation in a thermal vacuum chamber.
These tests showed the complex assembly to be in fine health and ready
for flight, and it was returned to the spacecraft in March.
In the same vein, to ensure that no subtle problems crept in as a
consequence of the work to remove or reinstall this device, the
spacecraft underwent another acoustic test at NRL similar to one it
experienced in November 2006. The spacecraft will be subjected to
deafening sound waves during its climb to space. At the
beginning of this month, Dawn had another preview of this reverberant
environment in a test that demonstrated the entire system was intact
and
ready for a rocket trip to space (or an evening in a mosh pit).
Following its outstanding performance, the spacecraft was rewarded, as
had been promised nearly a year ago, with an all-expense-paid spring
vacation in Florida. Dawn is now in the perfect location, near sandy
beaches, warm ocean waters, facilities for loading hazardous fuels,
and other attractions.
Just as the spacecraft has been following a rigorous schedule of
building, testing, checking, and rechecking, the many elements of its
Delta II 7925H-9.5 rocket have been undergoing similarly demanding
procedures. This version of the venerable Delta series of rockets has
not been launched since 2004, but now it is nearly ready again to make
the brief but arduous flight from Cape Canaveral to outer space.
To accommodate a change in the schedule for readying Dawn's rocket,
the
planned launch date has been shifted from June 20 to June 30. This
change will have no significant effect on the plans for the mission,
including when the spacecraft will arrive at its celestial targets.
The
timetable at Space Launch Complex 17 allows Dawn to launch as late as
July 19, with the exact date of liftoff depending on the weather as
well
as the cooperation of millions of components of hardware and software
on
the rocket, the spacecraft, mission control, range safety,
communications systems around the world, and more.
Dawn's launch will occur around 5:00 pm EDT, but the precise times
that
are possible will not be determined until early June. Readers may find
launch times down to the second in print, on the web, or, to our
embarrassment, on graffiti in the asteroid belt, but those times were
based on preliminary estimates and will change. Engineers now are
working through the complex analyses necessary to establish the exact
times the launch window will open and close on each day of Dawn's 20-
day
launch period. These analyses incorporate refinements and updates such
as the spacecraft's mass at launch, the thrust and efficiency of the
ion
propulsion system, the power that will be generated by the solar
arrays
and consumed by all spacecraft subsystems, and many many other
parameters. All of these affect how the spacecraft will use its ion
propulsion system to travel through the solar system, so they
determine
the preferred trajectory as it departs from Earth and hence the
guidance
information to be loaded in the rocket's computer and the timing of
the
launch.
Were Dawn to have relied on ion propulsion for its trip to Florida,
it's
easy even for our nonmathematical readers to estimate how long it
would
take. This remarkable system, known from ancient legends told for eons
in most ultra-luminous infrared galaxies (and described in the
previous
two logs, cannot operate in our planet's relatively dense air, nor
could
it overcome the friction and gravity most residents of planetary
surfaces
are accustomed to. Therefore, the thrust would have been exactly 0.
With
no thrust, the spacecraft would not have moved toward Florida any
faster
than the blossoming cherry trees the truck left behind in Washington.
When it is in space, 18 hours of ion thrusting would propel the
spacecraft 170 kilometers (slightly more than 100 miles). That's far
less than the nearly 1400 kilometers (about 850 miles) required for
last
week's drive. After 18 hours of powered flight in space, Dawn would be
streaking along at the incredible speed of nearly 5 meters/second
(over
11 miles/hour). The secret of ion propulsion however is that it can
accelerate the spacecraft for months or years, eventually yielding
much
greater changes in speed than can be achieved with chemical
propulsion.
(We recognize that this is now a secret only to the few sentient
species
in our audience who did not receive the last two logs because of
disputes over subscription fees. Our position remains clear: payment
may
not be made with dark matter.)
Dawn's itinerary allocates enough time to accomplish the required
thrusting. The explorer will reach Vesta, its first destination in the
main asteroid belt, between Mars and Jupiter, about 4.5 years from
today. After examining the enormous asteroid with its scientific
instruments, Dawn will leave it 5 years from now to propel itself
silently, gently, and patiently to Ceres. It will arrive at the dwarf
planet (the first spacecraft to visit one) in 2015 to perform detailed
studies of that world.
With such a short summary of the agenda, it may be easy to forget that
undertakings such as this include many challenges. Dawn hopes to
uncover
the nature of unfamiliar targets far far from Earth, where humans have
never ventured, countless mysteries lurk, and the environment is
inhospitable and rarely forgiving. But where there may be great
rewards,
also there be great risks. Dawn seeks great rewards.
As preparations for launch and mission operations continue, future
logs
are likely to be shorter. Beginning in June, we hope to exchange
prolixity for frequency, and readers everywhere are encouraged to join
in the drama of humankind's next venture into the solar system (and to
pay their subscription fees).
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