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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 1 Jun 2012 12:27:29 -0700 (PDT)
Message-ID: <201206011927.q51JRTlG008708_at_zagami.jpl.nasa.gov>


Dawn Journal
Dr. Marc Rayman
May 31, 2012

Dear Readers of all Dawnominations,

Far from Earth, on the opposite side of the sun, deep in the asteroid
belt, Dawn is gradually spiraling around the giant protoplanet Vesta.
Under the gentle pressure of its uniquely efficient ion propulsion
system, the explorer is scaling the gravitational mountain from its
low-altitude mapping orbit (LAMO) to its second high-altitude mapping
orbit (HAMO2).

Dawn spent nearly five months in LAMO, circling the rocky world at an
average altitude of 210 kilometers (130 miles) as it acquired a fabulous
bounty of pictures; visible, infrared, neutron, and gamma ray spectra;
and measurements of the gravity field. As we saw last month,
the probe was far more productive in each
investigation than the ambitious team members had expected or had ever
dared hope it would be. With that outstanding success behind it, it is
looking ahead and up to its work in HAMO2, about 680 kilometers (420
miles) high.

Dawn is the first spacecraft to explore Vesta, the second most massive
resident of the main asteroid belt between Mars and Jupiter. Indeed,
this is the only craft ever to orbit a body in the asteroid belt. No
other missions are currently on the books to visit this remote, exotic
world, which is now appreciated to be more closely related to the
terrestrial planets (including Earth) than to typical asteroids. And now
Dawn is receding from it. On May 1, it began the slow ascent to its next
observation orbit. It may well be decades before another robotic
ambassador from Earth comes as close to Vesta as this bold traveler has.

Humankind's first exploration of Vesta has been exceptionally rewarding.
A simple measure of that can be seen with just two photographs. More
than two centuries after its discovery, this giant asteroid was first
glimpsed by the approaching spaceship from Earth on May 3, 2011.
>From a distance of 1.2 million kilometers (750
thousand miles), or more than three times the separation between Earth
and the moon, Dawn's mapping camera perceived Vesta as only five pixels
across Each pixel spanned more than 110 kilometers (70 miles), revealing
nothing new compared to what astronomers' most powerful telescopes had
shown (but the image was of importance for navigation purposes).
Nevertheless, at the time, it was
tremendously exciting to obtain the first views of a distant, unfamiliar
shore after a voyage of more than 2.6 billion kilometers (1.6 billion
miles) on the interplanetary ocean. Sighting our first celestial port of
call more than three and a half years after this cosmic adventure began
was thrilling indeed. But now, with more than 25 thousand spectacular
photos in hand from much smaller distances, it is even more gratifying
to acknowledge that first picture as one of the worst ever taken of
Vesta. The Image of the Day from one year later was acquired in
October 2011 from 1,700 times closer; and most of the images have been
obtained from LAMO, about 5,700 times nearer than that first one. Dawn
has rapidly transformed Vesta from a mere fleck among the stars into a
fascinating, complex and splendidly detailed world.

Keeping the remote vessel on the planned spiraling course from one
mapping orbit to another presents the crew with a set of formidable
challenges, but this team has accomplished the maneuvers to successively
reach survey orbit, the first high-altitude mapping orbit (HAMO1) and
LAMO. The current orbital transfer is complex and demanding, but it is
proceeding very well. Controllers update the flight profile every few days
to ensure the probe stays close to the carefully designed trajectory to
HAMO2. To gain a sense of the progress, go here <status.asp> for your
correspondent's atypically succinct weekly summaries of the spiral status.

As in the previous transits between orbits, there is more involved than
only changing the altitude. Thanks to the tremendous flexibility
provided by ion propulsion, Dawn can adjust each orbit to optimize its
observations. In November we described how the plane of the orbit is
tilted for survey, HAMO1 and LAMO. Now let's summarize that and add HAMO2.
Readers incurious about the specifics of the orbital geometry may safely
skip the next five paragraphs (thus also saving 10 seconds of reading).

Imagine a globe of Vesta 30 centimeters (1 foot) in diameter. For the
purpose of this illustration, you may be confident that no inhabitants
(permanent or temporary) of the massive orb will object if we pretend
that it does not rotate. We will use this to demonstrate the alignments
of the orbits.

First, let's chose the position of the sun, because the orbits were
chosen on the basis of their angles relative to its location. Even in
this miniaturized cosmos, the sun today is 213 kilometers (134 miles)
away. (Space is big!) What matters more, however, is the direction, so
we will place the luminous master of the solar system over (albeit very,
very far over) the prime meridian, the 0 degree longitude line on our
stationary Vesta. Now we recall that Vesta, like Earth, has seasons
because its axis is tipped. It is southern hemisphere summer there, so
the sun is not over the equator; rather, it is currently at about 8
degrees south latitude. (On Nov. 29, 2011, when we last used the analogy
of the globe, the sun was at 25 degrees south latitude. Since then, it
has moved north because of the progression of seasons.) Although Earth's
location is not pertinent to this discussion, we can accurately position
it 285 kilometers (180 miles) away, high above a point at 5 degrees
south latitude and 10 degrees east longitude.

Now with the sun over the 0 degree longitude line, we can orient Dawn's
orbits. Think of each orbit as a ring encircling Vesta, going over both
poles and crossing the equator at a right angle. Globes of Earth often
are supported within a ring like that, and it may be helpful to have a
terrestrial globe in mind, or even in sight, as you ponder the celestial
arrangement. Because our imaginary Vesta is not rotating, a ring that is
aligned with a longitude line represents one of Dawn's orbits. (Of
course, Vesta really does rotate, so as the spacecraft loops from pole
to pole and back and the protoplanet turns beneath it, all parts come
within view of its sensors.)

Survey orbit is a little more than 1.5 meters (5 feet) above the 15
degree west longitude line (and, to make a complete circle, it goes over
the 165 degree east longitude line as well). It was from that vantage
that the first thorough mapping was conducted in August. The ring
representing HAMO1 is twisted to 30 degrees west (and 150 degrees east
on the other side of the globe), only about 38 centimeters (15 inches)
over the surface. The lower altitude of HAMO1 afforded much better views
of the great variety of geological features than the reconnaissance from
survey orbit. In addition, because the orbit was shifted farther from
the sun, the angle of light on the landscape beneath the spacecraft was
different, aiding in formulating a more complete portrait of the
terrain. LAMO is rotated still farther from the sun, at 46 degrees west
(and 134 degrees east), and is less than 12 centimeters (only 4.7
inches) high. The adventurer spent more time in this low orbit than
anywhere else at Vesta.

Now the ship is on its way to HAMO2, which will be at exactly the same
altitude as HAMO1 but not the same orientation. When its scientific
scrutiny resumes on June 15, the orbit will be approximately aligned
with the 35 degree west (and 145 degree east) longitude line on our
globe. There is another important difference however. The HAMO2 ring
does not quite extend to the poles this time; rather, it is tilted a
little so that it goes only to 86 degrees north and south latitude.
(Those familiar with orbital mechanics would describe the orbit as
having an inclination of 94 degrees; those unfamiliar with orbital
mechanics would not describe it that way. You all know who you are.)
This tip allows the spacecraft to take advantage of Vesta's gravity
field, which navigators have mapped with great accuracy, to gradually
turn the orbit by about one degree every five days. As a consequence, by
the time Dawn completes its observations in late July, the orbit will be
above 27 degrees west (and 153 degrees east).

Using different orientations of the orbits relative to the sun is a
crucial element of the strategy for gathering such a wealth of
scientifically valuable data on Vesta. Each orbit provides views of the
ground with different illumination angles.

In LAMO, the lighting was less important, as the primary objectives of
that phase were to measure the protoplanet's nuclear radiation
and changing gravitational tug as Dawn circled it, and neither of those
depended on sunlight. (Although it was purely bonus, the operations team
still managed to photograph most of the surface at high resolution.) But
the LAMO angle was chosen in large part to ensure that the desired plane
for HAMO2 would be within reach when the time came to undertake the
orbital ascent. Moving the plane of an orbit is energetically very very
expensive, and even with Dawn's extraordinary capabilities, only limited
changes are practicable.

In contemplating the Vesta-centric universe we have just described, it
may be evident that Dawn is not only enlarging its orbit from LAMO to
HAMO2 but also twisting and tilting it. As with the descent to LAMO,
described in more detail here, the team has
designed a flight profile that relies principally on the extraordinary
capability of ion propulsion but also rides Vesta's gravitational
currents to help accomplish some of the shifts in the orbit plane.

The location of the sun described above suggests why HAMO2 is valuable.
Orbiting farther from the sun than Earth, Vesta's year is equivalent to
more than 3.6 terrestrial years. The seasons pass correspondingly
slowly, lasting an average of 11 months each. In the time that will have
passed from HAMO1 in October 2011 to HAMO2 in June and July, the sun
will have moved northward, thus revealing some terrain that was in the
deep shadow of northern winter during HAMO1. It is that landscape that
is the principal target of HAMO2. As we will see in the next log,
however, this phase will present opportunities for other investigations
as well.

HAMO2 will be the final intensive campaign of observing Vesta. When it
is complete, the craft will once again resume powered flight. It will
escape from Vesta's gravitational grip in August and begin the next
stage of its interplanetary voyage, aiming for dwarf planet Ceres in
2015 -- a new world explored, another world awaits!

Dawn is 610 kilometers (380 miles) from Vesta. It is also 3.37 AU (503
million kilometers or 313 million miles) from Earth, or 1,385 times as
far as the moon and 3.32 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 56 minutes
to make the round trip.
Received on Fri 01 Jun 2012 03:27:29 PM PDT

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