[meteorite-list] Dawn Journal - December 31, 2012

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 2 Jan 2013 10:00:00 -0800 (PST)
Message-ID: <201301021800.r02I004s013233_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_12_31_12.asp

Dawn Journal
Dr. Marc Rayman
December 31, 2012

Dear Auld Dawn Synes,

Dawn concludes 2012 almost 13,000 times farther from Vesta than it began
the year. At that time, it was in its lowest orbit, circling the alien
world at an average altitude of only 210 kilometers (130 miles),
scrutinizing the mysterious protoplanet to tease out its secrets about
the dawn of the solar system.

To conduct its richly detailed exploration, Dawn spent nearly 14 months
in orbit around Vesta, bound by the behemoth's gravitational grip. In
September they bid farewell, as the adventurer gently escaped from the
long embrace and slipped back into orbit around the sun. The spaceship
is on its own again in the main asteroid belt, its sights set on a 2015
rendezvous with dwarf planet Ceres. Its extensive ion thrusting is
gradually enlarging its orbit and taking it ever farther from its
erstwhile companion as their solar system paths diverge.

Meanwhile, on faraway Earth (and all the other locations throughout the
cosmos where Dawnophiles reside), the trove of pictures and other
precious measurements continue to be examined, analyzed, and admired by
scientists and everyone else who yearns to glimpse distant celestial
sights. And Earth itself, just as Vesta, Ceres, Dawn, and so many other
members of the solar system family, continues to follow its own orbit
around the sun.

Thanks to a coincidence of their independent trajectories, Earth and
Dawn recently reached their smallest separation in well over a year,
just as the tips of the hour hand and
minute hand on a clock are relatively near every 65 minutes, 27 seconds.
On Dec. 9, they were only 236 million kilometers (147 million miles)
apart. Only? In human terms, this is not particularly close. Take a
moment to let the immensity of their separation register. The
International Space Station, for example, firmly in orbit around Earth,
was 411 kilometers (255 miles) high that day, so our remote robotic
explorer was 575 thousand times farther. If Earth were a soccer ball,
the occupants of the orbiting outpost would have been a mere seven
millimeters (less than a third of an inch) away. Our deep-space traveler
would have been more than four kilometers (2.5 miles) from the ball. So
although the planet and its extraterrestrial emissary were closer than
usual, they were not in close proximity. Dawn remains extraordinarily
far from all of its human friends and colleagues and the world they inhabit.

As the craft reshapes its solar orbit to match Ceres's, it will wind up
farther from the sun than it was while at Vesta. (As a reminder, see the
table here <journal_09_27_12.asp#table> that illustrates Dawn's progress
to each destination on its long interplanetary voyage.) We saw recently,
however, that the route is complex, and the spacecraft is temporarily
approaching the sun. Before the ship has had time to swing back out to a
greater heliocentric range, Earth will have looped around again, and the
two will briefly be even a little bit closer early in 2014. After that,
however, they will never be so near each other again, as Dawn will climb
higher and higher up the solar system hill, its quest for new and exciting
knowledge of distant worlds taking it farther from the sun and hence
from Earth.

Although our cosmic ambassador is much, much too remote to be discerned
with our humble eyes, our far more powerful minds' eyes can locate it.
As a convenient guide to begin, you can use the moon on Jan. 21. The
details of the geometry will be somewhat dependent on your terrestrial
position, which determines when the moon is above your horizon and how
it aligns with the more distant cosmic landscape. Nevertheless, if you
look at the moon that day, Dawn will appear to be nearby in the sky
(although more than 670 times farther away). For observers in the
continental United States, as the sun sets, the probe will be about five
degrees to the east (left) of the moon, or about the width of three
fingers held at arm's length. (Your correspondent has found that the
measurement works best if you use not only your own fingers, but also
your own arm.) As the evening progresses, they will draw closer and
closer together. By the time the moon sinks below the western horizon at
around 3:30 a.m. on Jan. 22, they will be separated by less than two
degrees for observers on the east coast, and those on the west coast
will find Dawn less than one degree away.

After using the moon to guide your eyes to the general location of the
intrepid ship in the sky, let your mind take over. Allow it to transport
you far, far into space, beyond all the human-made satellites around
Earth, beyond the moon, beyond the orbit of Mars, and continue out
farther than the sun (although in a different direction). Deep in the
main asteroid belt, where no other spacecraft has ever taken up
permanent residence and farther than all but a handful of probes have
ever ventured, you can espy Dawn. Its long solar array wings, spanning
nearly 20 meters (almost 65 feet) are pointing at the sun, capturing its
light. A lovely beam of xenon ions glows blue-green as it propels the
craft toward Ceres. Almost like a living creature, it is intensely active
within; computers and myriad electronic circuits,
heaters, and other components are working together to keep the ship
flying true. Out there, in that direction, alone, far away from any
firm surface or familiar ground, this Brobdingnagian celestial dragonfly
flies gracefully, silently, and patiently toward its next planetary perch.

Dawn is 2.7 million kilometers (1.7 million miles) from Vesta and 57
million kilometers (38 million miles) from Ceres. It is also 1.65 AU
(247 million kilometers or 153 million miles) from Earth, or 625 times
as far as the moon and 1.68 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 27
minutes to make the round trip.
Received on Wed 02 Jan 2013 01:00:00 PM PST