[meteorite-list] Dawn Journal - September 27, 2013

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 27 Sep 2013 16:30:17 -0700 (PDT)
Message-ID: <201309272330.r8RNUHG2027044_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_09_27_13.asp

Dawn Journal
Dr. Marc Rayman
September 27, 2013

Dear Dawnniversaries,

On the sixth anniversary of leaving Earth to embark on a daring
deep-space expedition, Dawn is very, very far from its erstwhile
planetary residence. Now humankind's only permanent resident of
the main asteroid belt between Mars and Jupiter, the seasoned
explorer is making good progress toward the largest object in that
part of the solar system, the mysterious dwarf planet Ceres. The
voyage is long, and the intrepid but patient traveler will not
reach its next destination until half a year after its seventh
anniversary of departing Earth.

On its fifth anniversary, Dawn was still relatively close to Vesta,
the giant protoplanet that had so recently held the craft in its
gravitational grip. The only probe ever to orbit a main belt
asteroid, Dawn spent 14 months (including its fourth anniversary)
accompanying Vesta on its way around the sun. After more than two
centuries of appearing to astronomers as little more than a fuzzy
blob of light among the stars, the second most massive body in the
asteroid belt has been revealed as a fascinating, complex, alien
world more closely related to terrestrial planets (including Earth)
than to typical asteroids.

Most of the ship's first four years of spaceflight were devoted to using
its ion propulsion system to spiral away from the sun, ascending the solar
system hill from Earth to Vesta. Now it is working to climb still higher
up that hill to 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 sixth 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
<journal_9_27_08.asp>, second <journal_9_27_09.asp>, third
<journal_09_27_10.asp>, fourth <journal_09_27_11.asp>, and fifth
<journal_09_27_12.asp> anniversaries.

In its six years of interplanetary travels, the spacecraft has thrust
for a total of 1,410 days, or 64 percent of the time (and about
0.000000028 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 318 kilograms
(701 pounds) of its supply of xenon propellant, which was 425 kilograms
(937 pounds) on September 27, 2007.

The thrusting so far in the mission has achieved the equivalent of
accelerating the probe by 8.7 kilometers per second (19,500 mph). As
previous logs have described (see here <journal_02_28_13.asp#speed> 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 about three-quarters 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.
<journal_06_27_10.asp#resume> (For a comparison with probes that enter
orbit around Mars, refer to this earlier log
<journal_12_06.asp#perspective>.)

Since launch, our readers who have remained on or near Earth have
completed six revolutions around the sun, covering about 37.7 AU
(5.6 billion kilometers or 3.5 billion miles). Orbiting farther from
the sun, and thus moving at a more leisurely pace, Dawn has traveled
27.4 AU (4.1 billion kilometers or 2.5 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. It will have to slow down still more to
rendezvous with Ceres. Since Dawn's launch, Vesta has traveled only
24.2 AU (3.6 billion kilometers or 2.2 billion miles), and the even
more sedate Ceres has gone 22.8 AU (3.4 billion kilometers or 2.1
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
orbit.

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?
Dawn's orbit on Sept. 27, 2013 2.44 2.98 8.7?
Ceres's orbit 2.54 2.99 10.6?

 

For readers who are not overwhelmed by the number of numbers, investing
the effort in studying the table may help to demonstrate how Dawn has
patiently transformed its orbit during the course of its mission. Note
that two years ago, 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 slip into orbit around Vesta.
While simply flying by it would have been far easier, matching orbits
with Vesta required the extraordinary capability of the ion propulsion
system. Without that technology, NASA's Discovery Program would not have
been able to afford a mission to explore it in such detail. But now,
Dawn has gone even beyond that. Having discovered so many of Vesta's
secrets, the stalwart adventurer left the protoplanet behind. No other
spacecraft has ever escaped from orbit around one distant solar system
object to travel to and orbit still another extraterrestrial
destination. A true interplanetary spaceship, Dawn is enlarging,
reshaping and tilting its orbit again so that in 2015, it will be
identical to Ceres's. A mission to both Vesta and Ceres would have been
impossible without ion propulsion.

One way to chart Dawn's progress is with numbers. Another is to look
inside ourselves and feel the awe at an extraordinary enterprise
undertaken on our behalf. This robotic emissary from Earth to the cosmos
has journeyed far, swooping by Mars even as its sights were set on
distant, uncharted lands beyond. Powering its way through the solar
system with a blue-green beam of xenon ions, the ambitious explorer is
introducing humankind to ancient worlds, giant remnants from the dawn of
the solar system. With each passing year, we travel farther and see more
thanks to Dawn. It is amazing that creatures humble yet bold, who are
physically confined to the vicinity of their planet, not only take on
such daunting challenges but actually succeed in reaching so far and
attaining so much. As Dawn begins the seventh year of its interplanetary
odyssey, we can marvel at all that it has accomplished so far and look
forward with eager anticipation to more rewards that lie ahead: new
knowledge, new insights, new perspectives, and new fuel for those who
feel the burning passion to venture still farther.

Dawn is 23 million kilometers (14 million miles) from Vesta and 44
million kilometers (27 million miles) from Ceres. It is also 3.32 AU
(496 million kilometers or 308 million miles) from Earth, or 1,230 times
as far as the moon and 3.31 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 55
minutes to make the round trip.
Received on Fri 27 Sep 2013 07:30:17 PM PDT