[meteorite-list] Dawn Journal - September 27, 2012

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 27 Sep 2012 13:40:00 -0700 (PDT)
Message-ID: <201209272040.q8RKe0wR004449_at_zagami.jpl.nasa.gov>


Dawn Journal
Dr. Marc Rayman
September 27, 2012

Dear Dawnniversaries,

On the fifth anniversary of the beginning of its ambitious
interplanetary adventure, Dawn can look back with great satisfaction
on its spectacular exploration of the giant protoplanet Vesta and
forward with great eagerness to reaching dwarf planet Ceres. Today
Earth's robotic ambassador to the main asteroid belt is in quiet cruise,
gradually reshaping its orbit around the sun so it can keep its
appointment in 2015 with the mysterious alien world that lies ahead.

This anniversary resembles the first three more than the fourth. Its
first years in space were devoted to spiraling away from the sun,
ascending the solar system hill so it could gracefully slip into orbit
around Vesta in time for its fourth anniversary. One year ago, Dawn was
in the behemoth's gravitational grip and preparing to map its surface
in stereo and make other measurements. The subsequent year yielded
stunning treasures as Dawn unveiled the wondrous secrets of a world
that had only been glimpsed from afar for over two centuries. While at
Vesta, it spiraled around the massive orb to position itself for the
best possible perspectives. Its final spiral culminated in its
departure from Vesta earlier this month. Now
for its fifth anniversary, it is spiraling around the sun again,
climbing beyond Vesta so that it can reach Ceres.

For those who would like to track the probe's progress in the same terms
used on previous (and, we boldly predict, subsequent) anniversaries, we
present here the fifth annual summary, reusing the text from last year
with updates where appropriate. Readers who wish to cogitate about the
extraordinary nature of this deep-space expedition may find it helpful
to compare this material with the logs from its first, second, third,
and fourth anniversaries.

In its five years of interplanetary travels, the spacecraft has thrust
for a total of 1060 days, or 58 percent of the time (and about
0.000000021 percent of the time since the Big Bang). While for most
spacecraft, firing a thruster to change course is a special event, it is
Dawn's wont. All this thrusting has cost the craft only 267 kilograms
(587 pounds) of its supply of xenon propellant, which was 425 kilograms
(937 pounds) on September 27, 2007.

The fraction of time the ship has spent in powered flight is lower than
last year (when it was 68 percent), because Dawn devoted relatively
little of the past year to thrusting. Although it did change orbits
extensively at Vesta, most of the time it was focused on exactly what it
was designed and built to do: scrutinize the ancient world for clues
about the dawn of the solar system.

The thrusting so far in the mission has achieved the equivalent of
accelerating the probe by 7.14 kilometers per second (16,000 miles per
hour). As previous logs have described (see here for one of the more
extensive discussions), because of the principles of
motion for orbital flight, whether around the sun or any other
gravitating body, Dawn is not actually traveling this much faster than
when it launched. But the effective change in speed remains a useful
measure of the effect of any spacecraft's propulsive work. Having
accomplished slightly more than half of the thrust time planned for its
entire mission, Dawn has already far exceeded the velocity change
achieved by any other spacecraft under its own power. (For a comparison
with probes that enter orbit around Mars, refer to this earlier log.)

Since launch, our readers who have remained on or near Earth have
completed five revolutions around the sun, covering about 31.4 AU
(4.70 billion kilometers or 2.92 billion
miles). Orbiting farther from the sun, and thus moving at a more
leisurely pace, Dawn has traveled 23.4 AU (3.50 billion kilometers or
2.18 billion miles). As it climbed away from the sun to match its orbit
to that of Vesta, it continued to slow down to Vesta's speed. Since
Dawn's launch, Vesta has traveled only 20.4 AU (3.05 billion kilometers
or 1.90 billion miles) and the even more sedate Ceres has gone 18.9 AU
(2.82 billion kilometers or 1.75 billion miles).

Another way to investigate the progress of the mission is to chart how
Dawn's orbit around the sun has changed. This discussion will culminate
with a few more numbers than we usually include, and readers who prefer
not to indulge may skip this material, leaving that much more
for the grateful Numerivores. In order to
make the table below comprehensible (and to fulfill our commitment of
environmental responsibility), we recycle some more text here on the
nature of orbits.

Orbits are ellipses (like flattened circles, or ovals in which the ends
are of equal size). So as members of the solar system family follow
their paths around the sun, they sometimes move closer and sometimes
move farther from it.

In addition to orbits being characterized by shape, or equivalently by
the amount of flattening (that is, the deviation from being a perfect
circle), and by size, they may be described in part by how they are
oriented in space. Using the bias of terrestrial astronomers, the plane
of Earth's orbit around the sun (known as the /ecliptic/) is a good
reference. Other planets and interplanetary spacecraft may travel in
orbits that are tipped at some angle to that. The angle between the
ecliptic and the plane of another body's orbit around the sun is the
/inclination/ of that orbit. Vesta and Ceres do not orbit the sun in the
same plane that Earth does, and Dawn must match its orbit to that of its
targets. (The major planets orbit closer to the ecliptic, and part of
the arduousness of the journey is changing the inclination of its orbit,
an energetically expensive task.)

Now we can see how Dawn has been doing by considering the size and shape
(together expressed by the minimum and maximum distances from the sun)
and inclination of its orbit on each of its anniversaries.(Experts
readily recognize that there is more to describing an orbit than these
parameters. Our policy remains that we link to the experts' websites
when their readership extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft
had terminated thrusting on its anniversaries; the orbits of its
destinations, Vesta and Ceres, are included for comparison. Of course,
when Dawn was on the launch pad on September 27, 2007, its orbit around
the sun was exactly Earth's orbit. After launch, it was in its own solar

        Minimum distance from the Sun (AU) Maximum distance from the Sun
(AU) Inclination
Earth's orbit 0.98 1.02 0.0
Dawn's orbit on Sept. 27, 2007 (before launch) 0.98 1.02 0.0
Dawn's orbit on Sept. 27, 2007 (after launch) 1.00 1.62 0.6
Dawn's orbit on Sept. 27, 2008 1.21 1.68 1.4
Dawn's orbit on Sept. 27, 2009 1.42 1.87 6.2
Dawn's orbit on Sept. 27, 2010 1.89 2.13 6.8
Dawn's orbit on Sept. 27, 2011 2.15 2.57 7.1
Vesta's orbit 2.15 2.57 7.1
Dawn's orbit on Sept. 27, 2012 2.17 2.57 7.3
Ceres's orbit 2.54 2.99 10.6


For readers who are not overwhelmed by the number of numbers, the table
may help to demonstrate how Dawn has patiently
transformed its orbit during the course of
its mission. Note that last year, the spacecraft's path around the sun
was exactly the same as Vesta's. Achieving that perfect match was, of
course, the objective of the long flight that started in the same solar
orbit as Earth, and that is how Dawn managed to get into orbit around
Vesta. While simply flying by Vesta would have been far easier,
matching orbits with it required the
unique capability of the ion propulsion system. Without it, NASA's
Discovery Program would not have been able to afford a mission to
explore this fascinating world, and a mission to both Vesta and Ceres
would have been impossible.

Although the probe left Vesta only three weeks ago, the effect of the
ion thrusting is already evident. Dawn is no longer in the same orbit as
Vesta. It is propelling itself along a different path, forging its own
course through the asteroid belt. The journey will be long, and the
exploration of Ceres will not commence until well after Dawn's seventh
anniversary of venturing into space. Many exciting discoveries and many
daunting challenges lie ahead, and some of them have yet even to be
recognized. But this stalwart ship (supported by its crew on distant
Earth) has proven itself capable of accomplishing remarkable feats in
its quest to expand frontiers and reap the great rewards of new
knowledge and exciting new perspectives on the solar system for the bold
creatures whose passions and insightful creativity fuel its
extraordinary cosmic adventure.

Dawn is 160 thousand kilometers (99 thousand miles) from Vesta and 62
million miles (38 million miles kilometers) from Ceres. It is also 2.18
AU (325 million kilometers or 202 million miles) from Earth, or 840
times as far as the moon and 2.17 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 36
minutes to make the round trip.
Received on Thu 27 Sep 2012 04:40:00 PM PDT

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