http://dawnblog.jpl.nasa.gov/2015/08/21/dawn-journal-august-21/
Dawn Journal
by Dr. Marc Rayman
August 21, 2015
Dear Unhesidawntingly Enthusiastic Readers,
An ambitious explorer from Earth is gaining the best views ever of dwarf
planet Ceres. More than two centuries after its discovery, this erstwhile
planet is now being mapped in great detail by Dawn.
The spacecraft is engaged in some of the most intensive observations of
its entire mission at Ceres, using its camera and other sensors to scrutinize
the alien world with unprecedented clarity and completeness. At an average
altitude of 915 miles (1,470 kilometers) and traveling at 400 mph (645
kilometers per hour), Dawn completes an orbit every 19 hours. The pioneer
will be here for more than two months before descending to its final orbit.
The complex spiral maneuver down from the second mapping orbit at 2,700
miles (4,400 kilometers) went so well that Dawn arrived in this third
mapping orbit on Aug. 13, which was slightly ahead of schedule. (Frequent
progress of its descent, and reports on the ongoing work in the new orbit,
are available here and on Twitter @NASA_Dawn.) It began this third mapping
phase on schedule at 9:53:40 p.m. PDT on Aug. 17.
Map of Ceres with named craters
We had a detailed preview of the plans last year when Dawn was more than
six thousand times farther from Ceres than it is today. (For reasons almost
as old as Ceres itself, this phase is also known as the high altitude
mapping orbit, or HAMO, although we have seen that it is the second lowest
of the four mapping orbits.) Now let's review what will happen, including
a change mission planners have made since then.
The precious pictures and other data have just begun to arrive on Earth,
and it is too soon to say anything about the latest findings, but stand
by for stunning new discoveries. Actually, you could get pictures about
as good as Dawn's are now with a telescope 217 times the diameter of
Hubble Space Telescope. An alternative is to build your own interplanetary
spaceship, travel through the depths of space to the only dwarf planet
in the inner solar system, and look out the window. Or go to the Ceres
image gallery.
Dawn has already gained fabulous perspectives on this mysterious world
from its first and second mapping orbits. Now at one third the altitude
of the mapping campaign that completed in June, its view is three times
as sharp. (Exploring the cosmos is so cool!) That also means each picture
takes in a correspondingly smaller area, so more pictures are needed now
to cover the entire vast and varied landscape. At this height, Dawn's
camera sees a square about 88 miles (140 kilometers) on a side, less than
one percent of the more than one million square miles (nearly 2.8 million
square kilometers). The orbital parameters were chosen carefully so that
as Ceres rotates on its axis every nine hours (one Cerean day), Dawn will
be able to photograph nearly all of the surface in a dozen orbital loops.
his image, taken by NASA's Dawn spacecraft, shows the brightest spots
on dwarf planet Ceres from an altitude of 2,700 miles (4,400 kilometers).
The image, with a resolution of 1,400 feet (410 meters) per pixel, was
taken on June 24, 2015.
When Dawn explored the giant protoplanet Vesta from comparable orbits
(HAMO1 in 2011 and HAMO2 in 2012), it pointed its scientific instruments
at the illuminated ground whenever it was on the dayside. Every time its
orbit took it over the nightside, it turned to point its main antenna
at Earth to radio its findings to NASA's Deep Space Network. As we explained
last year, however, that is not the plan at Ceres, because of the failure
of two of the ship's reaction wheels. (By electrically changing the
speed at which these gyroscope-like devices rotate, Dawn can turn or stabilize
itself in the zero-gravity conditions of spaceflight.)
We discussed in January that the flight team has excogitated innovative
methods to accomplish and even exceed the original mission objectives
regardless of the condition of the wheels, even the two operable ones
(which will not be used until the final mapping orbit). Dawn no longer
relies on reaction wheels, although when it left Earth in 2007, they were
deemed indispensable. The spacecraft's resilience (which is a direct
result of the team's resourcefulness) is remarkable!
One of the many ingredients in the recipe for turning the potentially
devastating loss of the wheels into a solid plan for success has been
to rotate the spacecraft less frequently. Therefore, sometimes Dawn will
wait patiently for half an orbit (almost 9.5 hours) as it flies above
ground cloaked in the deep darkness of night, its instruments pointed
at terrain they cannot detect. Other times, it will keep its antenna fixed
on Earth without even glancing at the sunlit scenery below, because it
can capture the views on other revolutions. This strategy conserves hydrazine,
the
