http://dawnblog.jpl.nasa.gov/2014/09/27/dawn-journal-september-27/
Dawn Journal
by Marc Rayman
September 27, 2014
Dear Dawnniversaries,
On the seventh anniversary of embarking upon its extraordinary extraterrestrial
expedition, the Dawn spacecraft is far from the planet where its journey
began. While Earth has completed its repetitive loops around the sun seven
times, its ambassador to the cosmos has had a much more varied itinerary.
On most of its anniversaries, including this one, it reshapes its orbit
around the sun, aiming for some of the last uncharted worlds in the inner
solar system. (It also zipped past the oft-visited Mars, robbing the red
planet of some of its orbital energy to help fling the spacecraft on to
the more distant main asteroid belt.) It spent its fourth anniversary
exploring the giant protoplanet Vesta, the second most massive object
in the asteroid belt, revealing a fascinating, complex, alien place more
akin to Earth and the other terrestrial planets than to typical asteroids.
This anniversary is the last it will spend sailing on the celestial seas.
By its eighth, it will be at its new, permanent home, dwarf planet Ceres.
The mysterious world of rock and ice is the first dwarf planet discovered
(129 years before Pluto) and the largest body between the sun and Pluto
that a spacecraft has not yet visited. Dawn will take up residence there
so it can conduct a detailed investigation, recording pictures and other
data not only for scientists but for everyone who has ever gazed up at
the night sky in wonder, everyone who is curious about the nature of the
universe, everyone who feels the burning passion for adventure and the
insatiable hunger for knowledge and everyone who longs to know the cosmos.
Artist depiction of landmarks on Dawn's voyage.
Dawn is the only spacecraft ever to orbit a resident of the asteroid belt.
It is also the only ship ever targeted to orbit two deep-space destinations.
This unique mission would be quite impossible without its advanced ion
propulsion system, giving it capabilities well beyond what conventional
chemical propulsion provides. That is one of the keys to how such a voyage
can be undertaken.
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 seventh annual summary, reusing text from last year with
updates where appropriate. Readers who wish to reflect upon Dawn's ambitious
journey may find it helpful to compare this material with the logs from
its first, second, third, fourth, fifth and sixth anniversaries. On this
anniversary, as we will see below, the moon will participate in the celebration.
In its seven years of interplanetary travels, the spacecraft has thrust
for a total of 1,737 days, or 68 percent of the time (and about 0.00034
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 808 pounds (366 kilograms) of its
supply of xenon propellant, which was 937 pounds (425 kilograms) on Sep.
27, 2007.
Dawn launch, JSC, Sept. 27. 2007
Dawn launched at dawn (7:34 a.m. EDT) from Cape Canaveral Air Force Station,
Sep. 27, 2007. Its mission is to learn about the dawn of the solar system
by studying Vesta and Ceres. Credit: KSC/NASA
The thrusting so far in the mission has achieved the equivalent of accelerating
the probe by 22,800 mph (10.2 kilometers per second). 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 about seven-eighths 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
seven revolutions around the sun, covering 44.0 AU (4.1 billion miles,
or 6.6 billion kilometers). Orbiting farther from the sun, and thus moving
at a more leisurely pace, Dawn has traveled 31.4 AU (2.9 billion miles,
or 4.7 billion kilometers). 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
has been slowing down still more to rendezvous with Ceres. Since Dawn's
launch, Vesta has traveled only 28.5 AU (2.6 billion miles, or 4.3 billion
kilometers), and the even more sedate Ceres has gone 26.8 AU (2.5 billion
miles, or 4.0 billion kilometers). (To develop a feeling for the relative
speeds, you mig
