Missione Dawn, aggiornamenti al 30 ottobre 2006

http://dawn.jpl.nasa.gov/mission/journal_10_06.asp

Dawn Journal
Dr. Marc D. Rayman
October 29, 2006

Dear Dawnvironments,

The Dawn spacecraft is in space! Well, not quite, but it is getting a
taste of the space environment courtesy of the team preparing it for
its
mission.

Although the individual components of the spacecraft have already been
tested, the point of the testing in Orbital Sciences Corporation’s
Environmental Test Facility is to verify that the fully assembled
spacecraft will survive the rigors of launch and be able to fulfill its
ambitious mission of exploration in deep space.

When Dawn is on the launch pad at Cape Canaveral and during the brief
(but exciting!) trip from there to space, many radio signals between
systems on the ground and between ground systems and the rocket will
impinge upon it. Some of the tests are designed to verify that these
signals will have no adverse effects on the spacecraft. Other tests
show
that Dawn’s electronics do not produce signals that might interfere
with
these other systems.

As one illustration of the importance of such tests, consider the
scientists eager for Dawn’s intimate portrait of the enormous asteroids
Ceres and Vesta, the team members who have invested years of their
lives
in creating this spaceship and the means to use it to explore distant
worlds, all people who thirst for greater knowledge of our solar system
and the thrill of discovery, and taxpayers who make it possible. One
may
reasonably expect members of all of those groups to find it
unsatisfying
if any of Dawn’s electronics produced signals that accidentally
activated the rocket’s self-destruct system. (That system is designed
to
be commanded with radio signals transmitted by range safety in the
event
of a serious malfunction during ascent.) Similarly, if Dawn’s radio
emissions interfered with the rocket’s reception of range safety’s
self-destruct command, the legions of Dawnophiles throughout the Milky
Way Galaxy, and the even greater number elsewhere, would no longer give
this project their loyal support.

In addition to testing Dawn’s compatibility with other systems,
engineers are testing its self-compatibility. It is essential to verify
that none of the subsystems, including the radio used for
interplanetary
communications, emits electromagnetic radiation that might interfere
with other subsystems.

Of course, Dawn’s designers and builders were well aware of these and
other concerns, and they have methods to make the probe satisfy the
many
associated requirements, but only through testing may we be confident
that the work was successful.

With the completion of testing in the electromagnetic
interference/electromagnetic compatibility facility, the spacecraft
will
be prepared for a series of mechanical tests. For the rocket’s control
systems to remain stable with Dawn perched at the top, it must be
accurately balanced and must meet certain criteria for how stable it is
when it is spun, and the next set of tests will help prepare for that.
(As we will discuss in an upcoming log in more detail, the spacecraft
will spin at about 50 rpm during a portion of the time it is on the
rocket.) Also on Dawn’s agenda during November are deafening noise and
powerful shaking that will show its readiness for the ride to space.
Sensors to measure the movements of certain parts of the spacecraft
will
be installed (after the balance measurements are complete) for these
tests.

As we know from earlier logs, between many of these environmental
tests,
other tests will be conducted on the spacecraft to ensure that its
systems remain intact, undamaged by previous environments and ready for
the next. At each stage, the health of the spacecraft will be verified.

Dawn’s busy autumn includes still another kind of test. Now that it is
a
complete spacecraft, it is scheduled for tests of its responses to many
of the complex sets of commands that will be sent to it while far from
Earth. These “mission scenario tests” exercise not only the spacecraft
but also many of the software systems used by mission control. While
this testing is invaluable, it does have some noteworthy limitations.
The spacecraft cannot respond now to all of these commands, because,
for
example, it cannot rotate itself while on Earth, it cannot see stars to
establish its orientation, and the ion propulsion system can thrust
only
in vacuum. (The ion propulsion system will be operated briefly with the
spacecraft in a vacuum chamber early next year.) Sophisticated
simulators connected to the spacecraft compensate for these and other
limitations of the terrestrial test program.

Important progress has already been made in these tests. The team has
demonstrated that commands can move smoothly through the complex path
from mission control at JPL, to the spacecraft’s main computer and then
to its engineering and science subsystems, and that responses can flow
back to mission control, to the Dawn Science Center at UCLA, and to the
institutions that built the science instruments.

The last log described the spacecraft’s engineering subsystems,
including a characterization of their relative importance, but what
about these science instruments? Some people would say they are more
important than any of the engineering subsystems. While others would
disagree, everyone would concur that without the science instruments,
the long and arduous journey to Vesta and then to Ceres would be of no
value without the information these instruments will gather. We’ll be
seeing much more about them in the years ahead, but let’s introduce
them
now.

Dawn is built and operated by humans who, in contrast to many of our
other readers, are very visual creatures. So it is no surprise that the
spacecraft carries cameras to share the sights with those who remain at
home. (There have been, and will continue to be, many space missions
that are fantastically productive and tremendously exciting even
without
cameras; nevertheless, the visceral appeal of pictures is undeniable.)
Besides satisfying our innate curiosity to know what Vesta and Ceres
look like, the cameras will provide important data essential to gaining
an understanding of the geological and physical properties of these
enigmatic bodies. And in the spirit of this mission representing all
humankind, and not only those who happen to reside in one portion of
one
continent, the cameras on Dawn are contributed by Germany. The
Max-Planck-Institut f?r Sonnensystemforschung (Max Planck Institute
for
Solar System Research) was responsible for their design and
fabrication,
in cooperation with the Institut f?r Planetenforschung (Institute for
Planetary Research) of the Deutsches Zentrum f?r Luft- und Raumfahrt
(German Aerospace Center) and the Institut f?r Datentechnik und
Kommunikationsnetze (Institute for Computer and Communication Network
Engineering) of the Technischen Universit?t Braunschweig (Technical
University of Braunschweig).

Because of the long duration of Dawn’s mission and the extraordinarily
remote locations in which it will operate (more than one million times
farther from Earth than the International Space Station), most of the
critical subsystems include backups, thus allowing Dawn to persevere
even in the event of a malfunction. The images of Vesta and Ceres are
so
essential that the probe carries two identical cameras. In addition to
the value for science and for the visually oriented fans of the
mission,
the images are critical for navigating the spacecraft in the vicinity
of
these bodies.

The cameras incorporate filters in 7 color ranges, chosen principally
to
help study the minerals on Vesta’s surface. In addition to detecting
the
visible light humans see, the cameras will register near infrared
light.

Another scientific instrument, contributed by Italy, covers a still
broader range of light, from shorter wavelengths in the ultraviolet
through the same wavelengths in which the cameras operate, to farther
in
the infrared. It is provided by Agenzia Spaziale Italiana (Italian
Space
Agency), and it was designed and built at Galileo Avionica, in
collaboration with the Istituto Nazionale di Astrofisica (National
Institute for Astrophysics). Because of its sensitivity across the
entire visible (V) spectrum and well into the infrared (IR), the team
that designed the instrument has named it VIR (“vir” is Latin for
“man”).

Each VIR picture records how strong the light is at more than 400
wavelength ranges in every pixel. Instruments such as this are known as
imaging spectrometers, and they see the world much as we might if we
looked through a prism, which breaks light into its component colors.
But this yields more than a beautifully surreal view of
extraterrestrial
landscapes. When scientists compare VIR’s observations of its targets
with laboratory measurements of minerals, they can determine what
minerals compose the surfaces of Vesta and Ceres.

With the data from the cameras and VIR, scientists will discover much
about the nature of those alien worlds, but these highly capable
instruments cannot reveal everything we would like to know. To learn
still more, Dawn will carry a device that measures the energy of gamma
rays and neutrons. Gamma rays are a form of light, not only higher
energy than visible, and even higher than ultraviolet, but more
energetic even than X-rays. Neutrons are particles that normally reside
in the nuclei of atoms (about half of your body weight is neutrons,
regardless of how much Halloween candy you eat). Some of the gamma rays
and neutrons emitted by Vesta and Ceres are produced by radioactive
elements and others are created by the bombardment of the surface
material by cosmic rays. As they emanate from the surface and travel
into space, some will be intercepted by Dawn’s gamma ray and neutron
detector (GRaND) which, despite its name, is very humble. (For the sake
of having an interesting appellation, it’s fortunate that GRaND detects
gamma rays and neutrons and not neutrons and gamma rays.) The complex
and impressive instrument was designed and built by a team at the Los
Alamos National Laboratory.

The gamma rays and neutrons reveal many of the important atomic
constituents to a depth of one meter (three feet) or so on Vesta and
Ceres, thereby adding to the detailed story Dawn will tell. As we know
from the first log, Ceres may be rich in water. If it is, the signature
of water may be contained in GRaND’s data.

Although there is not a special instrument for it, Dawn will make
another set of scientific measurements at its destinations. Using the
radio signals exchanged between the telecommunications system
(described
in the previous log) and NASA’s Deep Space Network, scientists can
detect subtle variations in the gravitational attraction between each
asteroid and Dawn. These variations reflect details of the internal
distribution of mass, so these tiny effects allow us to learn about the
interior structure of the massive bodies Dawn will orbit.

Dawn is well prepared to help scientists extract a wealth of
information
about Vesta and Ceres and thus teach us a great deal about the nature
of
the solar system when planets were forming. In less than 8 months, the
craft will be launched on the beginning of its journey to that distant
past. While much work remains, each step in the preparations brings us
closer to witnessing the thrilling discoveries it will make. I hope you
continue to share in the eager anticipation and ultimately in the
excitement of the rewards.