[meteorite-list] Dawn Journal - July 26, 2010

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 28 Jul 2010 13:37:25 -0700 (PDT)
Message-ID: <201007282037.o6SKbPIq001590_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_07_26_10.asp

Dawn Journal
Dr. Marc Rayman
July 26, 2010

Dear Dawn Days of Summers,

Dawn is flying smoothly through the asteroid belt, now less than a
year from entering orbit around Vesta, the first of its two cosmic
destinations.
 
Earlier in July, while the spacecraft was devoting its time to
gentle thrusting with its ion propulsion system, members of the
mission control team spent some of their time in August 2011.
Impressively capable as NASA is, time travel is not within its
powers. (If it were, your correspondent could travel back in time
after this is posted to remove his controversial and inappropriate
comments above, thereby preventing anyone from ever having seen
them and avoiding the regrettable consequences of his poor
judgment. Alas, that awaits a future capability.) Instead, the
team simulated being in the future, when Dawn will be finishing
its approach to survey orbit around Vesta, where it will begin
its intensive scrutiny of the alien world. That will be a very
busy period not only for the spacecraft but also for the human
members of the team.


To account for details of the normal variations in the trajectory
(as illustrated in a log last year
<http://dawn.jpl.nasa.gov/missionjournal_1_27_09.asp#navigators>)
as well as the properties of the
protoplanet that will be determined as Dawn closes in on it,
controllers may adjust the sequences of commands that are being
developed this year for execution at Vesta . For example, once the
brightness of
different regions of the surface is known, the instructions for
the science instruments to perform their observations may need to
be changed accordingly. The procedures to make these modifications
reliably are complex, and the time available between receiving the
pertinent data from the spacecraft and radioing the refined
sequences back often will be only a few days. The team has
formulated detailed schedules for all the necessary work,
including the checks needed at every step. Engineers have
established the criteria for making revisions, determined exactly
what data must be presented and in what format for each meeting at
which a decision will need to be made, and developed the computer
programs to be used for verifying that no unintentional changes
are made along with the intentional ones.
 
For these complex operations involving many participants, the team
cannot wait until arrival at Vesta to verify that the plans are
sound, so they rehearse major elements of it. Such operational
readiness tests, or ORTs, also were conducted before launch.
This time, with great creativity and care,
some engineers had concocted data from the science instruments and
navigational data, all representing results from the approach
phase. The rest of the operations team treated the data as if they
were real and went through all the steps to be followed when Dawn
is nearly ready to begin surveying Vesta.
 
The ORT was successful, concluding with your correspondent
providing the final approval to transmit the fine-tuned sequences
to the spacecraft. The ORT allowed team members to identify
opportunities for improvements in their software tools and
procedures, such as parts of the schedule that allowed more time
than needed for some steps and not enough time for others. While
such details may seem prosaic, they are essential for the
accomplishment of a grand and challenging endeavor. All the
improvements will be incorporated into the final plans for how
operations will be conducted at Vesta.
 
Even as the team was simulating activities in the future, Dawn
remained committed to its present task of thrusting with its ion
propulsion system. Last month, it exceeded the greatest propulsive
change in speed by any spacecraft.
On July 8, it passed another milestone when its ion thrusters had
yielded 10,000 mph over the course of the mission. Such an
achievement may seem somewhat less noteworthy when expressed in
metric units, as the project does, but 4.47 kilometers per second
is just as great a velocity!
 
Although most of Dawn?s interplanetary travel is dedicated to
thrusting, the design of the flight profile included coasting
during most of the week of July 19 to accomplish some other work.
Each of the science instruments was activated and tested,
confirming that all remain healthy and ready to reveal Vesta?s
secrets to eager earthlings. A small software update was
transmitted to both the primary and the backup science cameras,
correcting a minor bug that would have
added some complexity to the acquisition of images at Vesta.
Subsequent tests showed the software ?patch? works perfectly. The
spacecraft also pointed the primary science camera to selected
stars as part of its regular calibration, as it has done before.
In addition, the camera imaged Ceres, the dwarf planet it will
study at close range in 2015. At a distance of 3.3 AU (almost 500
million kilometers, or nearly 310 million miles), the giant of the
asteroid belt appears only as a faint dot, no different from many
background stars, but the data are helpful in monitoring the
camera?s performance during its years of spaceflight before
reaching Ceres. (At the time of these observations, as each object
followed its own orbit around the sun, Ceres happened to be closer
to Earth, less than 2.0 AU, than to the spacecraft.)

Measurements were taken of the alignment between the primary
science camera and the visible and infrared mapping spectrometer
(VIR) to augment those made in April. All these will be of value in
combining data from the two complementary instruments at Vesta and
at Ceres.
 
The gamma ray and neutron detector (GRaND) sensed space radiation,
as it has during previous tests, including the first time it was
operated in space. Perhaps ironically, while the device
measures some kinds of radiation, one of the gamma-ray sensors in
the unit includes a crystal that deteriorates slowly under the
constant exposure to other kinds of radiation that originate far
outside the solar system. The changes can be reversed by carefully
controlled heating of the crystal, and GRaND includes the
electronics necessary to perform this function. While the
instrument remains in excellent condition, scientists expect to
implement this annealing prior to beginning measurements of Vesta
in order to ensure the device is at peak performance. By
conducting this test, engineers were able to verify that the
crystal attains the correct temperature.
 
As all readers who have flight software 9.0 commemorative tattoos
can readily attest, in June, engineers installed this new version
of software on the primary spacecraft computer. As we have seen before,
Dawn carries four copies of the software. In case the primary copy
is corrupted by radiation or any other problem, the computer has a
backup available. And if the primary computer suffers a problem
from which it cannot recover, a backup computer, with its own
primary and backup copies of the software, is ready to take over
operation of the ship. The activity in June was to load the
primary copy on the primary computer, and it has been running
smoothly ever since. On July 22, controllers stored the two copies
of the software on the backup computer. Dawn?s computer systems do
not allow copying programs from one location to another, so the
software was transmitted from Earth twice, once to each of the
required locations in computer memory. (The backup copy for the
primary computer will be radioed to the spacecraft during a future
opportunity.) Unlike the work last month, this did not require
rebooting the computer (with the consequent entry into safe mode),
because the software is not active.
 
As Dawn gradually closes in on Vesta, it continues to climb away
from the sun. On July 26, it was exactly 2 AU from the master of
the solar system, or twice Earth?s average distance from the sun.
In the northern hemisphere summer, Earth is at its farthest from
the sun, so it will not be until August 24 that Dawn will be
precisely twice as far from the sun as Earth is. Even then, as
most days, while our home world continues following its repetitive
loops around the sun, the spacecraft will persist in enlarging its
orbit. The region of the solar system occupied by our planet, its
inhabitants, and the companions it brings along under the grip of
its gravity is now alien to Dawn. The sun that burns bright and
large in our sky, although brilliant throughout the solar system,
grows dimmer and smaller as the ship sails on. Dawn is a permanent
denizen of the asteroid belt, the only spacecraft ever to take up
residence there. Far from the sun, far from Earth, the probe
carries with it the spirit of exploration and the quest for
knowledge that thrive in the home it left behind as it looks ahead
in its search for new insights into the dawn of the solar system.
 
Dawn is 0.26 AU (39 million kilometers or 24 million miles) from
Vesta, its next destination. It is also 2.55 AU (381 million
kilometers or 237 million miles) from Earth, or 940 times as far
as the moon and 2.51 times as far as the sun. Radio signals,
traveling at the universal limit of the speed of light, take 43
minutes to make the round trip.
Received on Wed 28 Jul 2010 04:37:25 PM PDT