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April 18,
2006
Dear Dawnophile,
Coming in summer 2007 to a solar system near you (well, near most
of you, anyway): the Dawn mission!
NASA's next planned venture into the solar system, Dawn is a collaborative
effort of scientists, engineers, and people in other disciplines at
NASA/JPL, UCLA, Orbital Sciences Corporation, the space agencies of
Germany and Italy, and other universities and private companies in
the United States and elsewhere. But there is more to this mission
than the people working directly on it. I view this as an adventure
of humankind, with a spacecraft carrying not only a suite of sophisticated
scientific instruments and impressive engineering gadgetry, but the
dreams, aspirations, and most noble spirit of exploration of our still-young
space-faring species. For those of you who are members of that species
(and even those of you who aren't), I invite you to share in this extraordinary
adventure.
In what still seems like only yesterday (and note that I didn't sleep
at all last night), I enjoyed giving some of you an inside view of
the exciting flight of Deep
Space 1,
and I'm proud that those reports are still in circulation as a profitable
set of late-night reruns throughout much of the Virgo Supercluster
of galaxies. Now, by starting more than one year before launch, I am
looking forward to this opportunity to involve you in our preparations
for dispatching another of our planet's robotic emissaries. I hope
you will join me throughout the rest of Dawn's residence here on Earth
as well as on its journey to worlds we have yet to know.
The Dawn project is on course now for a launch into the cosmic void
in 14 months. Most of the project's work was put on hold in October
2005 while NASA reevaluated it, and last month NASA approved Dawn for
continuation. We are reassembling our team and formulating new and
detailed plans for completing the myriad tasks necessary to begin a
nearly decade-long mission in deep space. While the spacecraft is about
90% assembled in one of Orbital Sciences' environmentally controlled "clean
rooms," much work remains to finish the delicate job of installing
the rest of the components and to conduct extensive and rigorous testing
to verify the readiness of the entire spacecraft and the ground operations
system (consisting not only of the highly trained people, but also
all of their hardware, software, and procedures).
In the next log, I will provide some of the details of our new plan
for the next 14 months, but for the first of these logs, it seems more
appropriate to devote some attention to the overall mission. I will
offer more about this over the coming year, but let's start with a
broad overview of Dawn.
The fascinating process that is science has yielded remarkable insights
into the formation of our solar system, but many questions remain unanswered
and many details are yet to be filled in. In brief, about 4.6 billion
years ago, one of the Milky Way galaxy's vast nebulae of gas and dust
began to collapse. As it did so, most of the material fell to the center
of the cloud, eventually forming the Sun, where the majority of the
mass in our solar system remains concentrated. But as many residents
and visitors to it know, the solar system consists of more than the
Sun. Some of the tiny particles of dust accreted elsewhere in the condensing
cloud, gradually growing in size to become rocks and eventually building
up to planets. There is greater uncertainty about how the largest planets,
Jupiter and Saturn, formed, but apparently once Jupiter did achieve
its enormous bulk, its powerful gravity halted the assembly of nearby
matter into planets. Much of that material, deprived so long ago of
the opportunity to continue conglomerating, now forms the asteroid
belt, between Jupiter and Mars. The two most massive protoplanetary
remnants of that epoch are Ceres and Vesta, and they are Dawn's destinations.
While they seem to have formed at very similar distances from the
nascent Sun, and thus, one might expect, under similar conditions,
observations from distant Earth show these two bodies to be very different
from each other. Water seems to have played an important role in Ceres'
history, and there is reason to believe it might still harbor a substantial
inventory of that precious commodity, never having been hot enough
to drive the water away. Vesta, in contrast, displays the signatures
of minerals found in lava, indicating that different forces shaped
its history. Despite the impressive discoveries made so far, our ability
to learn about these asteroids from Earth, hundreds of millions of
kilometers away, is very limited indeed. By gathering information about
Ceres and Vesta from orbit around them, at distances of only hundreds
of kilometers, scientists can learn much much more and retrieve the
records the protoplanets hold about the very early solar system.
While some people may think of all asteroids as chips of space rock,
Ceres and Vesta in many ways are more like planets -- real worlds.
The largest asteroid yet encountered by a spacecraft is Mathilde, which
the remarkable NEAR-Shoemaker spacecraft glimpsed as they zipped past
each other in 1997. It has a very irregular shape, with its largest
dimension being about 66 kilometers (41 miles). In contrast, Vesta's
equatorial diameter is about 580 kilometers (360 miles). That is sometimes
compared to the size of Arizona in the United States. A tremendous
crater at Vesta's south pole is about 460 km (285 miles) in diameter.
How exciting it will be to use Dawn to see the rugged terrain and complex
geology of that enormous excavation, a window provided by nature to
let us peer deep into Vesta's interior. Ceres, which by itself contains
one quarter of all the mass in the asteroid belt, is about 975 km (605
miles) in diameter. The only states in the United States that are larger
are Texas and Alaska. But comparisons of the protoplanets' diameters
with terrestrial landforms fail to convey their real sizes, because
these orbs are three dimensional bodies. The surface area of Vesta
is more than three times that of Arizona, and Ceres' surface is as
large as Alaska plus Texas plus California. In fact, it is about one
third of the area of the United States, and almost 40% of the area
of the continental United States. These are big places, and
there certainly will be many beautiful and intriguing things to see
in their varied and alien landscapes. Part of the allure of Dawn is
that it is bound for some of the last unexplored worlds in the inner
solar system.
No spacecraft has ever attempted to orbit two targets after leaving
Earth. Such a feat would be far beyond our capabilities without the
use of ion propulsion, which Deep Space 1 proved to be the fantastically
efficient and reliable system generations of science fiction fans have
known it to be. Ion propulsion is also what allowed NASA to shift Dawn's
launch date from its original plan of 2006 to 2007 without having to
change the plans for the rich scientific investigations to be conducted.
Most missions beyond Earth orbit are restricted to short launch periods,
usually only a few weeks long. (Engineers distinguish the launch period
-- the range of days on which a launch can occur -- from the launch
window -- the span of time on any one day in which a launch can take
place.) With the extraordinary maneuvering capability of its ion propulsion
system, Dawn could conduct its planned mission with a launch any time
from May 2006 (or perhaps much earlier) to November 2007. This has
given us the flexibility to fit Dawn's launch in an opening in the
schedule at Cape Canaveral. Based upon that, and not the more interesting
science of celestial mechanics, we are targeting a launch in June or
July 2007.
The flexibility afforded by the ion propulsion system means that the
details of Dawn's itinerary may still change, but in the current plan
the spacecraft will fly past Mars in March 2009 on its way to the more
distant asteroid belt. Thrusting with its ion propulsion to ever-so-gently
shape its trajectory to match Vesta's path around the Sun, Dawn will
ease into orbit around Vesta in September 2011. It will spend about
seven months there, subjecting Vesta to intense scrutiny with its scientific
sensors. Leaving behind what will then be a familiar world, Dawn will
resume its interplanetary travels. Nearly three years later, following
its arrival at Ceres in February 2015, it will devote five months to
coaxing out the secrets that are stored there. At the end of the mission,
Dawn will remain in orbit, accompanying Ceres on its leisurely 4.6-year
revolutions around the Sun. Because of its heft, the gravity of Ceres
is too high for Dawn ever to make a controlled landing.
Travels far from Earth, exploration of new worlds, ion propulsion,
rocket science, amazing feats of engineering, new scientific understandings,
probably some disappointments and scares but certainly some drama and
thrills -- all this lies ahead on this futuristic mission. As the Dawn
team works hard to prepare for next year's launch and the voyage that
follows, I hope you will join me in this exciting journey through space
and time as we seek the dawn of the solar system. The future -- and
the past -- await us!
Dr. Marc D. Rayman
April 18, 2006
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