[meteorite-list] Dawn Journal - May 27, 2008

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_5_27_08.asp

Dawn Journal
Dr. Marc Rayman
May 27, 2008

Dear Pseudawnyms,

The Dawn mission continues smoothly, as the spacecraft reliably thrusts
with its ion propulsion system, demonstrating all the patience of a - well, 
of an ion-propelled spacecraft! In the 243 days since launch, the
probe has thrust a total of about 140 days. While only around 7% of the
total thrusting it will do in its mission, this figure represents vastly
more powered flight than any spacecraft that uses conventional chemical
propulsion would be capable of. In all this time, the ion thruster has
consumed only 37 kilograms (82 pounds) of xenon propellant but yielded a
change in the spacecraft's speed of 0.87 kilometers per second (1900
miles per hour).

In most months this year, Dawn's flight profile includes a few days of
coasting so mission controllers can conduct special activities. The
spacecraft was asked (and, of course, it politely complied) not to
resume thrusting after its weekly communications session with faraway
Earth on May 12. While some members of the team were conducting a 3-day
workshop to prepare for the complex program of scientific measurements
to be conducted at Vesta in 2011 and 2012, the spacecraft conducted
other work.

To ensure that certain backup memory locations remain uncorrupted by
space radiation (or by undesirable social practices on some planets far
from the plane of the Milky Way galaxy), engineers occasionally will
check the memory contents. If errors were found, they could be corrected
so that if the spacecraft ever had to call upon backup memory, the data
there would be in intact. As expected, all the tests verified that the
memory was in perfect condition.

Some of the time in this period was devoted to the mission's first
periodic maintenance on components of the attitude control system.
This system's name may be misleading, as it has
displayed a most decorous style in working both with its fellow
spacecraft systems and with its human colleagues. In this case,
"attitude" denotes the probe's orientation in the weightless conditions
of spaceflight.

The spacecraft carries 4 reaction wheels, gyroscope-like devices which,
when electrically spun faster or slower, rotate (or stop the rotation
of) the spacecraft. Only 3 wheels need to be operated at a time; as with
most components, Dawn has backups so the mission may continue even if a
unit experiences problems. Wheel #4 (known to the irreverent but
creative flight team as "wheel #4") was powered off during the initial
checkout phase of the mission last year, but now it is wheel #3's turn
to be the backup, and the switch was made during this coast period.

For most of the mission, the attitude control system's gyroscopes are
not powered, as they are not designed to operate for the duration of the
8-year flight. These devices, not to be confused with the reaction
wheels, help achieve the accurate pointing needed by the camera and the
visible and infrared spectrometer to uncover mysteries of asteroid Vesta
and dwarf planet Ceres. The intricate mechanisms have to be operated
occasionally, so they were activated and allowed to run for about 2 days.

All of the ion thrusting since the beginning of the long-term
interplanetary cruise phase has used ion thruster #3. Now thrust in a 
specific direction in space is required to reach Dawn's celestial targets. 
The ion thrusters point in different directions on the spacecraft, so the
orientation (if not the attitude) of the spacecraft during thrusting
depends on which thruster is selected. To point thruster #1 on the
bearing needed for thrust would cause the Sun to illuminate a part of
the spacecraft that has not yet been exposed to direct light from that
star. (Of course, many other stars have shone on that portion of the
craft, and while many of them are brighter than the Sun, we may resort
to the narrow perspective of our solar system readers and discount those
stars because of their extraordinary remoteness.) Flight controllers
have complex computer programs to predict the temperatures under such
conditions, but as sophisticated as these tools are, their accuracy is
not always sufficient. The typical duration of a set of thrust
instructions is 5 weeks, so before committing the spacecraft to spending
so long in this unfamiliar orientation, a half day was devoted to
measuring the temperatures at two orientations representative of what
would be required for thrusting with thruster #1. Engineers now are
using those data to refine the predictions for what the temperatures
would be when thrusting.

On May 14, having accomplished all its special activities, the
spacecraft resumed ion propulsion. Since then, it has continued with its
normal routine of only short suspensions of thrust each week for
pointing its main antenna to Earth.

As Dawn continued devoting its attention to its flight through deep
space, the operations team recognized some other noteworthy events
elsewhere in the very same solar system. Among the other spacecraft
conducting exciting and important investigations of the cosmos, Phoenix
had a particularly thrilling time on May 25 with its descent through the
Martian atmosphere and its wonderfully successful landing in the far
northern plains of the fourth planet from the Sun. Curious as it may
seem, while their spacecraft are 56 million kilometers (35 million
miles) apart, the Dawn and Phoenix teams at JPL are only 2 floors apart
in building 264. (We appreciate that you readers in a small, faint,
lenticular galaxy in Sculptor have a particular fondness for this
building's impressive designation). Now that Phoenix is beginning the
scientific part of its mission, along with space enthusiasts everywhere
we congratulate the Phoenix team, many of whom are our friends and
colleagues, on this superb accomplishment.

On the same day, we remembered Dr. Ernst Stuhlinger, who died at age 94.
He played an important role in the long development of ion propulsion,
making valuable contributions as early as the 1950s. He followed Deep
Space 1 (the first mission to use ion propulsion for interplanetary
flight) and Dawn with great interest and was most gracious in his
expressions of enthusiastic support for both missions. His many kind
words about these ambitious and challenging projects meant a great deal
to your correspondent, a lifelong space enthusiast, who knew of Dr.
Stuhlinger's work even as a youngster studying the space activities of
20th century humans.

Quite unaware of those news items, the spacecraft patiently travels
farther from both Earth and the Sun. During this part of its mission, it
recedes from its former home much faster than it does from the Sun.
Tomorrow, on May 28, it will be equidistant from the two. Dawn will be
244 million kilometers (152 million miles) from Earth and from the Sun.
With the planet 152 million kilometers (94 million miles) from the star,
the trio forms a aesthetically symmetrical triangle. If the distance
from the Sun to Earth were 1.00 units, the other two sides of the
triangle each would be 1.61 units. Geometers call such a shape an
"isosceles triangle," whereas some young male residents of the
constellation Triangulum call it a "hot babe."

To keep apprised of Dawn's current location, be sure to visit the cool
new feature "Where is Dawn Now?" at
http://dawn.jpl.nasa.gov/mission/live_shots.asp. The site includes
depictions not only of the craft's trajectory and location but also of
its attitude (that is, its orientation; visualizations of its behavioral
manner and emotional state have not been implemented yet), allowing
readers to achieve greater accuracy in their enactments of Dawn whenever
they have access to the World Wide Web.

Dawn is 243 million kilometers (151 million miles) from Earth, or 630
times as far as the moon and 1.60 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light, take 27
minutes to make the round trip.