[meteorite-list] Dawn Journal - March 29, 2012

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_03_29_12.asp

Dawn Journal
Dr. Marc Rayman
March 29, 2012

Dear Dawnscoverers,

Vesta is spending the 205th anniversary of its discovery by treating
Dawn to more spectacular vistas When Heinrich Wilhelm Matthäus
Olbers first spotted Vesta, he could hardly have imagined that the power
of the noble human spirit for adventure and the insatiable hunger for
knowledge would propel a ship from Earth to that mysterious point of
light among the stars. And yet today our spacecraft is conducting a
detailed and richly rewarding exploration of the world that Olbers found.

Dawn is continuing its intensive low-altitude mapping orbit (LAMO)
campaign, scrutinizing the protoplanet 210
kilometers (130 miles) beneath it with all instruments. The primary
objectives of the craft's work here are to measure the atomic
composition and the interior distribution of mass in this geologically
complex world. In addition, this low orbit provides the best vantage
point for high resolution pictures and visible and infrared spectra to
reveal the nature of the minerals on the surface.

Ever since it left its home planet behind in September 2007,
the robotic adventurer has pursued its own independent course through 
the solar system. As Earth and its orbiting retinue (including the moon 
and many artificial satellites) followed their repetitive annual loop 
around the sun, Dawn used its ion propulsion system to spiral outward 
to rendezvous with Vesta in July 2011. When the gigantic asteroid's 
gravity gently took hold of the visiting craft, the two began traveling 
together around the sun, taking the same route Vesta has since long 
before humans gazed in wonder at the nighttime sky.

As we have discussed before, the speed of an
object in orbit, whether around Earth, the sun, the Milky Way (either my
cat or the galaxy of the same name) or anything else, decreases as its
orbital altitude increases. Farther from the sun than Earth is, and
hence bound to it by a weaker gravitational grip, Vesta moves at a more
leisurely pace, taking more than 3.6 years per revolution. When Dawn
travels to the more remote Ceres, it will orbit the sun even more
slowly, eventually matching Ceres' rate of 4.6 years for each loop.

Just as the hour hand and minute hand of a clock occasionally are near
each other and at other times are on opposite sides of the clock face,
Earth and Dawn sometimes are relatively close and other times are much
farther apart. Now their orbits are taking them to opposite sides of the
sun, and the distance is staggering. They have been on opposite sides of
the sun twice before (albeit not as far apart as this time), in November
2008 and November 2010 We used both occasions to explain more about the
nature of the alignment as well as to contemplate the profundity of such
grand adventures.

On April 18, Dawn will attain its greatest separation yet from Earth,
nearly 520 million kilometers (323 million miles) or more than 3.47
astronomical units (AU). Well beyond Mars,
fewer than a dozen spacecraft have ever operated so far from Earth.
Those interested in the history of space exploration (such as your
correspondent) will enumerate them, but what should be more rewarding is
marveling at the extent of humanity's reach. At this extraordinary
range, Dawn will be nearly 1,400 times farther than the average distance
to the moon (and 1,300 times farther than the greatest distance attained
by Apollo astronauts 42 years ago). The deep-space ship will be well
over one million times farther from Earth than the International Space
Station and Tiangong-1. 

Vesta does not orbit the sun in the same plane that Earth does. Indeed,
a significant part of the challenge in matching Dawn's orbit to Vesta's
was tipping the plane of its orbit from
Earth's, where it began its journey, to Vesta's, where it is now. As a
result, when they are on opposite sides of the sun this time, Dawn will
not appear to go directly behind the sun but rather will pass a little
south of it. In addition, because the orbits are not perfectly circular,
the greatest separation does not quite coincide with the time that Dawn
and the sun appear to be most closely aligned. The angular separation
will be at its minimum of less than five degrees (about 10 times the
angular size of the sun itself) on April 9, but the sun and Dawn appear
to be within ten degrees of each other from March 23 until April 27. For
our human readers, that small angle is comparable to the width of your
palm at arm's length, providing a handy way to find the approximate
position of the spacecraft in the sky. Earth's robotic ambassador to the
cosmos began east of the salient celestial signpost and progresses
slowly to the west over the course of those five weeks. Readers are
encouraged to step outside and join your correspondent in raising a
saluting hand to the sun, Dawn, and what we jointly accomplish in our
efforts to gain a perspective on our place in the universe.

For those awestruck observers who lack the requisite superhuman visual
acuity to discern the faraway spacecraft amidst the dazzling light of
the sun, this alignment provides a convenient occasion to reflect once
again upon missions deep into space. Formed at the dawn of the solar
system, Vesta, arguably the smallest of the terrestrial planets, has
waited mostly in patient inconspicuousness for a visit from the largest
terrestrial planet. For the entire history of life on Earth, the
inhabitants remained confined to the world on which they have lived. Yet
finally, one of the millions upon millions of species, inspired by the
majesty of the universe, applied its extraordinary talents and
collective knowledge to overcome the limitations of planetary life and
strove to venture outward. Dawn is the product of creatures fortunate
enough to be able to combine their powerful curiosity about the workings
of the cosmos with their impressive abilities to explore, investigate
and ultimately understand. While its builders remain in the vicinity of
the planet upon which they evolved, their emissary now is passing on the
far side of the sun! This is the same sun that is more than 100 times
the diameter of Earth and a third of a million times its mass. This is
the same sun that has been the unchallenged master of our solar system
for more than 4.5 billion years. This is the same sun that has shone
down on Earth throughout that time and has been the ultimate source of
so much of the heat, light and other energy upon which the planet's
residents have been so dependent. This is the same sun that has so
influenced human expression in art, literature, mythology and religion
for uncounted millennia.  This is the same sun that has motivated
scientific studies for centuries. This is the same sun that is our
signpost in the Milky Way galaxy. And humans have a spacecraft on the
far side of it. We may be humbled by our own insignificance in the
universe, yet we still undertake the most valiant adventures in our
attempts to comprehend its majesty.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.45 AU (516
million kilometers or 321 million miles) from Earth, or 1,290 times as
far as the moon and 3.45 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 57 minutes
to make the round trip.