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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sat, 30 Mar 2013 14:04:10 -0700 (PDT)
Message-ID: <201303302104.r2UL4A9O028240_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_03_29_13.asp

Dawn Journal
Dr. Marc Rayman
March 29, 2013

Dear Indawnstrious Readers,

In the depths of the main asteroid belt between Mars and Jupiter, far from Earth,
far even from any human-made object, Dawn remains in silent pursuit of dwarf planet
Ceres. It has been more than six months since it slipped gracefully away from the
giant protoplanet Vesta. The spacecraft has spent 95 percent of the time since then
gently thrusting with its ion propulsion system, using that blue-green beam of
high velocity xenon ions to propel itself from one alien world to another.

The ship set sail from Earth more than two thousand days ago, and its voyage
on the celestial seas has been wonderfully rewarding. Its extensive exploration
of Vesta introduced humankind to a complex and fascinating place that
had only been tantalizingly glimpsed from afar with telescopes beginning
with its discovery 206 years ago today. Thanks to the extraordinary capability
of ion propulsion, Dawn was able to spend 14 months orbiting Vesta, observing
dramatic landscapes and exotic features and collecting a wealth of measurements
that scientists will continue to analyze for many years.

When it was operating close to Vesta, the spacecraft was in frequent contact with
Earth. It took Dawn quite a bit of time to beam the 31,000 photos and
other precious data to mission control. In addition, engineers needed
to send a great many instructions to the distant adventurer to ensure
it remained healthy and productive in carrying out its demanding work
in the unforgiving depths of space.

Dawn is now more than 20 times farther from Vesta than the moon is from Earth.
Alone again and on its long trek to Ceres, it is not necessary for the ship to be
in radio contact as often. As we saw in November, the spacecraft now stops ion
thrusting only once every four weeks to point its main antenna to Earth. This
schedule conserves the invaluable hydrazine propellant the explorer will need at Ceres.
But communicating less frequently does not mean the mission operations team
is any less busy. Indeed, as we have explained before, "quiet cruise"
consists of a considerable amount of activity.

Each time Dawn communicates with Earth, controllers transmit a second-by-second schedule
for the subsequent four weeks. They also load a detailed flight profile with the ion
throttle levels and directions for that period. It takes about three weeks to calculate
and formulate these plans and to analyze, check, double check, and triple
check them to ensure they are flawless before they can be radioed to Dawn.

In addition to all the usual information Dawn needs to keep flying smoothly,
operators occasionally include some special instructions. As one example,
over the last few months, they have gradually lowered the temperatures
of some components slightly in order to reduce heater power. When Dawn
stretched out its solar array wings shortly after separating from the
Delta rocket on September 27, 2007, its nearly 65-foot wingspan was the
longest of any NASA interplanetary probe. The large area of solar cells
is needed to collect enough light from the distant sun to power the ion
propulsion system and all other spacecraft systems. Devoting a little
less power to heaters allows more power to be applied to ionizing and
accelerating xenon, yielding greater thrust. With two and a half years
of powered flight required to travel from Vesta to Ceres, even a little
extra power can make a worthwhile difference to a mission that craves
power.

Most temperature adjustments are only two degrees Celsius (3.8
degrees Fahrenheit) at a time, but even that requires careful analysis
and investigation, because lowering the temperature of one component may
affect another. Xenon and hydrazine propellants need to be maintained
in certain ranges, and the lines they flow through follow complicated
paths around the spacecraft, so the temperatures all along the way matter.
Most of the hardware onboard, from valves and switches to electronics
to structural mounts for sensitively aligned units, needs to be thermally
regulated to keep Dawn shipshape.

It can take hours for a component to
cool down and stabilize at a new setting, and sometimes the change won't
even occur until the spacecraft has turned away to resume thrusting, when
the faint warmth of the sun and the deep cold of black space affect different
parts of the complex robot. Then it will be another four weeks until engineers
will receive a comprehensive report on all the temperatures, so they need
to be cautious with each change.

In addition to the ongoing work to keep
Dawn flying true, some special activities are being developed for later
this year, each of which will serve two important purposes: they will
yield valuable experience in preparing for operations in orbit around
Ceres, and they will provide interesting material for you to read about
in future logs. Your correspondent has confidence both in the flight team
to design and execute these activities and in readers throughout the cosmos
to continue to follow this ambitious mission on its extraterrestrial exploits.

And to ensure that there is plenty to read about for years to come, Dawn's
human colleagues are working hard to prepare for exploring Ceres when
the spacecraft reaches that remote destination in 2015. As at Vesta, the
probe will take advantage of the unique maneuvering capability of ion
propulsion to fly to different orbits, each optimized for specific investigations
to reveal the complex character of the mysterious world, ensuring a rich
and gratifying experience for everyone who wonders about the nature of
the solar system. As the plans mature at the end of this year and in 2014,
we will delve into them here, just as we presented the Vesta strategy
in 2010 and 2011, leading up to the astounding achievements of 2011 and
2012.

Meanwhile, the spacecraft itself, loyally following carefully devised
and intricate plans, continues to make good progress, patiently and reliably
flying onward. Unknown challenges and unknown rewards lie ahead, and together
they promise that this bold mission in deep space will provide humankind
with still more inspiring and exciting cosmic adventures.

Dawn is 7.7 million kilometers (4.8 million miles) from Vesta and 56 million
kilometers (35 million miles) from Ceres. It is also 2.64 AU (395 million kilometers
or 246 million miles) from Earth, or 1075 times as far as the moon and
2.65 times as far as the sun today. Radio signals, traveling at the universal
limit of the speed of light, take 44 minutes to make the round trip.
Received on Sat 30 Mar 2013 05:04:10 PM PDT