[meteorite-list] Dawn Journal - July 25, 2012

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_07_25_12.asp

Dawn Journal
Dr. Marc Rayman
July 25, 2012

Dear Dawnpartures,

Dawn has completed the final intensive phase of its extraordinary exploration 
of Vesta, and it has now begun its gradual departure. Propelled by its uniquely 
efficient ion propulsion system, the probe is spiraling ever higher, reversing the 
winding path it followed into orbit last year.

In the previous log (which gained prominence last month by making it into the 
list of the top 78 logs ever written on this ambitious interplanetary adventure), we 
saw the plan for mapping Vesta from an altitude of 680 kilometers (420 miles). 
In this second high-altitude mapping orbit (HAMO2), the spacecraft circled 
the alien world beneath it every 12.3 hours. On the half of each orbit 
that it was on the day side, it photographed the dramatic scenery. As 
it passed over the night side, it beamed the precious pictures to the 
distant planet where its human controllers (and many of our readers) reside. 
Tirelessly repeating this strategy while Vesta rotated allowed Dawn's 
camera to observe the entirety of the illuminated land every five days.

The robot carried out its complex itinerary flawlessly, completely mapping 
the surface six times. Four of the maps were made not by pointing the 
camera straight down at the rocky, battered ground but rather at an angle. 
Combining the different perspectives of each map, scientists have a rich 
set of stereo images, allowing a full three dimensional view of the terrain 
that bears the scars of more than 4.5 billion years in the main asteroid 
belt between Mars and Jupiter.

Dawn also mapped Vesta six times during 
the first high-altitude mapping orbit (HAMO1) in September and October 
2011. The reason for mapping it again is that Vesta has seasons, and they 
progress more slowly than on Earth. Now it is almost northern hemisphere 
spring, so sunlight is finally reaching the high latitudes, which were 
under an impenetrable cloak of darkness throughout most of Dawn's residence 
here.

For most of the two centuries this mysterious orb had been studied 
from Earth, it was perceived as little more than a small fuzzy blob in 
the night sky. With the extensive imaging from HAMO1 and HAMO2, as well 
as from the low-altitude mapping orbit (LAMO), earthlings now know virtually 
all of the protoplanet's landscape in exquisite detail.

Among the prizes for the outstanding performance in HAMO2 are more than 
4,700 pictures. In addition to the comprehensive mapping, Dawn collected 
nearly nine million spectra with its visible and infrared mapping spectrometer 
(VIR) to help scientists determine more about the nature of the minerals. This 
phenomenal yield is well over twice that of HAMO1, illustrating the great benefit 
of dedicating valuable observation time in HAMO2 to VIR before the mapping.

Dawn's measurements of the peaks and valleys, twists and turns of Vesta's gravity 
field, from which scientists can map the distribution of material in the 
interior of the behemoth, were at their best in LAMO. That low altitude 
also was where the gamma ray and neutron detector (GRaND) obtained its 
finest data, revealing the atomic constituents of the surface and subsurface. 
Indeed, the motivation for undertaking the challenging descent to LAMO 
was for those investigations, although the bonus pictures and spectra 
greatly enhanced the reward. Even in HAMO2, however, gravity and GRaND 
studies continued, adding to an already fabulous bounty.

Mission controllers have continued to keep the distant spacecraft very busy, 
making the most of its limited time at Vesta. Pausing neither to rest nor to 
marvel or delight in its own spectacular accomplishments, when the robot finished 
radioing the last of its HAMO2 data to Earth, it promptly devoted its 
attention to the next task: ion thrusting.

Missions that use conventional propulsion coast almost all of the time, but 
long-time readers know that Dawn has spent most of its nearly five years in 
deep space thrusting with its advanced ion propulsion system, the exotic and 
impressive technology it inherited from NASA's Deep Space 1. Without ion 
propulsion, the exploration already accomplished would have been unaf
fordable for NASA's Discovery Program and the unique exploit to orbit both 
Vesta and dwarf planet Ceres would have been quite impossible. Ion propulsion 
not only enables the spacecraft to orbit residents of the main asteroid belt, 
something no other probe has attempted, but it also allows the interplanetary 
spaceship to maneuver extensively while at each destination, thus tailoring the 
orbits for the different investigations.

On July 25 at 9:45 a.m. PDT, as it has well over 500 times before, the 
sophisticated craft began emitting a beam of high-velocity xenon ions. In 
powered flight once again, it is now raising its orbital altitude. On August 26, 
the ship will be too far and traveling too fast for Vesta's gravity to maintain 
its hold. Dawn will slip back into orbit around the sun with its sights set on 
Ceres.

Although HAMO2 is complete, the spacecraft will suspend thrusting four times to 
direct its instruments at Vesta during the departure phase, much as it 
did in the approach phase. The approach pictures aided in navigation and 
provided tantalizing views of the quarry we had been seeking for so long. 
This time, however, we will see a familiar world receding rather than 
an unfamiliar one approaching. But as the sun creeps north, advancing 
by about three quarters of a degree of latitude per week, the changing 
illumination around the north pole will continue to expose new features.

On August 15, the craft will interrupt its ascent for four and a half days. 
By then, Dawn will be at an altitude of about 5,000 kilometers (3,100 
miles), but it will still be in orbit. Before it resumes thrusting, it 
will coast to as high as 6,400 kilometers (4,000 miles) and then descend 
again. Meanwhile, four times during this period it will photograph the 
giant asteroid throughout a full Vestan day of 5 hours, 20 minutes. This 
is a familiar activity for the spacecraft, as it watched Vesta rotate 
beneath it from a similar vantage point during its spiral descent in July 
2011. With Vesta's weak gravitational grip at this distance, Dawn would 
take more than a week to complete one revolution, so it will be almost 
as if the probe hovers in place as Vesta pirouettes before its camera. 
The itinerary is planned so the explorer will begin its observations while 
flying over the highest northern latitudes, and subsequently it will take 
the opportunity to observe lower latitudes as it sails down to the equator. 
The ship will circle so slowly that there will be time between acquiring 
each set of rotation images to point its main antenna to Earth to transmit 
its findings. After the third session, while waiting for the orbit to 
carry it to the latitude needed for the final one, mission planners are 
squeezing in a routine calibration of the camera and VIR. Dawn will turn 
to aim them at Jupiter. It is much too far away to reveal any new or interesting 
details, because the sensors are designed for mapping from close orbit. 
The planet will appear to be little more than a speck. (Terrestrial observers 
can gain a better view with binoculars.) But Jupiter is bright and easily 
seen from there, and it is so well studied that it is a useful reference 
source to verify that the instruments are still performing in top condition 
as they continue their discoveries at Vesta.

On August 22, nearly 6,000 kilometers (3,700 miles) over the night side, the 
probe will halt thrusting again. With the sun on the other side of the protoplanet, 
Dawn will see only a thin glowing crescent against the deep blackness of space, like 
a new moon. This is a perspective we have not yet had for Vesta, and although 
not much of the terrain will be visible, a few pictures to measure the 
strength of the sunlight's reflection at this extreme angle will be useful 
for understanding certain properties of the surface material. As a bonus, 
the view may prove to be quite aesthetically appealing.

Dawn will be patiently and gently thrusting at the moment of escape from Vesta on 
August 26 and will not even notice a change. It will be as serene and uneventful 
for the spacecraft (and operations team) as the moment of capture was. 
Shortly after, when it is around 17,000 kilometers (over 10,000 miles) 
away, it will watch Vesta rotate once again. On September 1, at a distance 
of 38,000 kilometers (almost 24,000 miles), it will gaze upon Vesta for 
the last time. By then, the world it has scrutinized for more than a year 
will be shrinking rapidly and few details will be visible. Although scientists 
will spend many years delving into the data the probe has returned, learning 
more and more not only about Vesta but also what it reveals about the 
dawn of the solar system, Dawn will leave it behind as it journeys deeper 
into the main asteroid belt in search of another uncharted world to explore.

Dawn is 740 kilometers (460 miles) from Vesta. It is also 2.94 AU (439 million 
kilometers or 273 million miles) from Earth, or 1,185 times as far as 
the moon and 2.89 times as far as the sun today. Radio signals, traveling 
at the universal limit of the speed of light, take 49 minutes to make 
the round trip.