[meteorite-list] Dawn Journal - July 29, 2013

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 29 Jul 2013 16:37:17 -0700 (PDT)
Message-ID: <201307292337.r6TNbHc3005903_at_zagami.jpl.nasa.gov>


Dawn Journal
Dr. Marc Rayman
July 29, 2013

Dear Megalodawniacs,

Powering its way through the main asteroid belt between Mars and
Jupiter, Dawn continues on course and on schedule for its 2015
appointment with dwarf planet Ceres. After spending more than a year
orbiting and scrutinizing Vesta, the second most massive object in
the asteroid belt, the robotic explorer has its sights set on the
largest object between the sun and Neptune that a spacecraft
has not yet visited. This exotic expedition to unveil mysterious alien
worlds would be impossible without the probe's ion propulsion system.

Ion propulsion is not a source of power for this interplanetary
spaceship. Rather, the craft needs a great deal of power to operate its
ion propulsion system and all other systems. It needs so much that...

We crave power!!

The ion propulsion system is power-hungry. The process of ionizing xenon
and then accelerating it to high velocity consumes a significant amount
of electrical power, all of which is provided by the spacecraft's huge
solar arrays. With these two wings and its ion tail, Dawn resembles a
celestial dragonfly. But this extraterrestrial odonate is a giant, with
a wingspan of 19.7 meters (nearly 65 feet). When it was launched in 2007,
this was the greatest tip-to-tip length of any probe NASA had ever
dispatched on an interplanetary voyage. (Some such spacecraft have had
flexible wire-like antennas that reach to greater lengths.) The large
area of solar cells is needed to capture feeble sunlight in the remote
asteroid belt to meet all of the electrical needs. Each solar array wing
is the width of a singles tennis court, and the entire structure would
extend from a pitcher's mound to home plate on a professional baseball
field, although Dawn is engaged in activities considerably more
inspiring and rewarding than competitive sports.

To sail the ship to its intended destination, navigators plot a complex
course on the solar system sea. The thrust delivered by the ion engine
depends on the power level; higher power translates into higher (but
still ever so gentle) thrust. The farther Dawn is from the luminous sun,
the less power is available, so the thrust is lower. Therefore, to keep
it on its itinerary, mission planners need to know the thrust at all
times in the future. It would not be a recipe for success to propel the
spacecraft to a position in space from which it could not achieve enough
thrust to accomplish the rest of the carefully designed journey to Ceres.

To formulate the flight plan then requires knowing how much power will
be available even as the probe ventures farther from the sun. Engineers
make mathematical predictions of the power the solar arrays will
generate, but these calculations are surprisingly difficult. Well,
perhaps some readers would not be surprised, but it is more complicated
than simply reducing the power in proportion to the intensity of the
sunlight. As one example, at greater distances from the sun, the
temperature of the arrays in the cold depths of space would be even
lower, and the efficiency of the solar cells depends on their
temperature. In 2008, the operations team devised and implemented
a method to refine their estimates of the solar array performance, and
that work enabled the deep-space traveler to arrive at Vesta earlier and
depart later. Now they have developed a related but superior technique,
which the faithful spacecraft executed flawlessly on June 24.

The only way to measure the power generation capability of the arrays
is to draw power from them. With the ion thrust off, even with all other
systems turned on, the spacecraft cannot consume as much power as the
arrays can provide, so no meaningful measurement would be possible.

In typical operations, Dawn keeps its solar arrays pointed directly at
the sun. For this special calibration, it rotated them so the incident
sunlight came at a different angle. This reduced the total amount of
light falling on the cells, effectively creating the conditions the
spacecraft will experience when it has receded from the sun. As the
angle increased, corresponding to greater distances from the brilliant
star, the arrays produced less power, so the ion engine had to be
throttled down. (The engines can be operated at 112 different throttle
levels, each with a different input power and different thrust level.)

Engineers estimated what the maximum throttle level would be at each of
the angles as well as the total power all other systems would consume
during the test and then programmed it so the ion propulsion system
would throttle down appropriately as the solar array angle increased.
Of course, they could not know /exactly/ what the highest throttle level
at each angle would be; if they did, then they would already know the
solar array characteristics well enough that the calibration would be
unnecessary. Fortunately, however, they did not need to determine the
perfect levels in advance. The sophisticated robot is smart enough to
reduce by a few throttle levels if it detects that all systems combined
are drawing more power than the solar arrays generate.

Under normal circumstances, the spacecraft doesn't need to adjust the
ion throttle level on its own. Engineers know the solar array
performance well enough that they can predict the correct setting with
high accuracy for a typical four-week sequence of commands stored
onboard. It is only for the much greater distances from the sun in the
years ahead that the uncertainty becomes important. In addition, during
regular operations, if the spacecraft temporarily needs to use more
heaters than usual (more than 140 heaters are distributed around the
ship, each turning on and off as needed), thereby increasing the power
demand, its battery can make up for the difference. That avoids
unnecessary throttle changes.

Over the course of the exercise, the arrays were positioned at five
angles, each for an hour, and the main computer recorded their output
power and other pertinent measurements. Initially, when the wings were
pointing directly at the sun, a glowing orb 2.48 AU (371 million
kilometers, or 230 million miles) away, together they could generate
more than two kilowatts. The ion propulsion system then was thrusting
at level 53, consuming 1,368 watts. When the arrays were tipped to their
maximum angle of 47 degrees, the insolation was the same as it would be
at 3.00 AU (449 million kilometers, or 279 million miles), and the
system yielded more than 1,300 watts. By then, the program engineers had
stored onboard had throttled the ion drive down to level 24, where it
drew 753 watts, and the spacecraft autonomously reduced it still
further. When the activity was finished, the wings were turned back to
their usual orientation, facing the distant sun so they could generate
the maximum power possible, and the Brobdingnagian dragonfly could
resume its normal flight pattern.

The calibration will be repeated occasionally as Dawn proceeds on its
deep-space trek. Engineers will use the resulting data to continue to
refine their plans for reaching Ceres and for maneuvering in orbit once
there. Yet this is just one of the myriad details that must be worked
out with exquisite care to ensure that the exploration of that enigmatic
world is as richly productive, as tremendously rewarding, as
outstandingly successful as the investigation of Vesta.

It is thanks to the extraordinary scrupulousness of their work that
Dawn's human counterparts are able to accomplish this ambitious
adventure. And although they are responsible for ensuring that the craft
achieves its objectives, this endeavor extends far beyond the members
of the team. This is a mission of humankind. Everyone who ever gazes in
wonder at the night sky is part of it. Everyone who is curious about
nature and about the reality of the universe is part of it. Everyone who
hungers for knowledge and insight is part of it. Everyone who feels the
passion for pursuing bold dreams and the exhilaration of discovery is
part of it. Everyone who feels the lure of the unknown is part of it.
Everyone who appreciates the great challenges and the great rewards of
aiming beyond the horizon is part of it. So as Dawn continues its
audacious exploits, anyone can be part of it.

Dawn is 18 million kilometers (11 million miles) from Vesta and 50
million kilometers (31 million miles) from Ceres. It is also 3.47 AU
(519 million kilometers or 322 million miles) from Earth, or 1,310 times
as far as the moon and 3.42 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take
58 minutes to make the round trip.
Received on Mon 29 Jul 2013 07:37:17 PM PDT

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