[meteorite-list] Dawn Journal - April 18, 2006

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Apr 26 17:26:46 2006
Message-ID: <200604262121.OAA14468_at_zagami.jpl.nasa.gov>

http://www-ssc.igpp.ucla.edu/dawn/outreach.html

Dawn Journal
Dr. Marc D. Rayman
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
<http://nmp.jpl.nasa.gov/ds1/archives.html>, 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!
Received on Wed 26 Apr 2006 05:21:40 PM PDT