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

[Ron Baalke]

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.