[meteorite-list] Dawn Journal - October 31, 2013

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 3 Nov 2013 19:30:58 -0800 (PST)
Message-ID: <201311040330.rA43UwYP029083_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_10_31_13.asp

Dawn Journal
Dr. Marc Rayman
October 31, 2013

Dear All Hallows' Dawns,

Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing
its smooth, silent flight toward dwarf planet Ceres. Far behind it now
is the giant protoplanet Vesta, which the spacecraft transformed from
a tiny splotch in the night sky to an exotic and richly detailed world.

The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years.
The great majority of spacecraft coast most of the time (just as planets
and moons do), each one following a trajectory determined principally
by whatever momentum they started with (usually following release from
a rocket) and the gravitational fields of the sun and other nearby, massive
bodies. In contrast, Dawn spends most of its time thrusting with its ion
propulsion system. The gentle but efficient push from the high velocity
xenon ions gradually reshapes its orbit around the sun. In September 2012,
as it departed Vesta after 14 months of scrutinizing the second most massive
resident of the asteroid belt, Dawn's heliocentric orbit was the same
as the rocky behemoth's. Now they are very far apart, and by early 2015,
the robotic explorer's path will be close enough to Ceres's that they
will become locked in a gravitational embrace.

Without ion propulsion, Dawn's unique mission to orbit two extraterrestrial
destinations would be impossible. No other spacecraft has attempted such
a feat. To accomplish its interplanetary journey, the spaceship has thrust
more than 96 percent of the time since propelling itself away from Vesta
last year. Whenever it points its ion engine in the direction needed to
rendezvous with Ceres, its main antenna cannot also be aimed at Earth.
Dawn functions very well on its own, however, communicating only occasionally
with its terrestrial colleagues. Once every four weeks, it interrupts
thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish
contact with NASA's Deep Space Network, receiving new instructions from
the Dawn operations team at JPL and transmitting a comprehensive report
on all its subsystems. Then it turns back to the orientation needed for
thrusting and resumes its powered flight.

During its years of interplanetary travel, Dawn has reliably followed
a carefully formulated flight plan from Earth past Mars to Vesta and now
from Vesta to Ceres. We discussed some of the principles underlying the
development of the complex itinerary in a log written when Dawn was still
gravitationally anchored to Earth. To carry out its ambitious adventure,
Dawn should thrust most of the time, but not all of the time. Indeed,
at some times, thrusting would be unproductive.

We will not delve into the details here, but remember that Dawn is doing
more than ascending the solar system hill, climbing away from the sun.
More challenging than that is making its orbit match the orbit of its
targets so that it does not fly past them for a brief encounter as some
other missions do. Performing its intricate interplanetary choreography
requires exquisite timing with the grace and delicacy of the subtly powerful
ion propulsion.

Of course Dawn does not thrust much of the time it is in orbit at Vesta
and Ceres; rather, its focus there is on acquiring the precious pictures
and other measurements that reveal the detailed nature of these mysterious
protoplanets. But even during the interplanetary flight, there are two
periods in the mission in which it is preferable to coast. Sophisticated
analysis is required to compute the thrusting direction and schedule,
based on factors ranging from the physical characteristics of the solar
system (e.g., the mass of the sun and the masses and orbits of Earth,
Mars, Vesta, Ceres and myriad other bodies that tug, even weakly, on Dawn)
to the capabilities of the spacecraft (e.g., electrical power available
to the ion thrusters) to constraints on when mission planners will not
allow thrusting (e.g., during spacecraft maintenance periods).

The first interval that interplanetary trajectory designers designated
as "optimal coast" was well over four years and 1.8 billion miles (2.8
billion kilometers) ago. Dawn coasted from October 31, 2008, to June 8,
2009. During that time, the ship took some of Mars's orbital energy to
help propel itself toward Vesta. (In exchange for boosting Dawn, Mars
slowed down by an amount equivalent to about 1 inch, or 2.5 centimeters,
in 180 million years.)

The second and final interval when coasting is better than thrusting begins
next month. From Nov. 11 to Dec. 9, Dawn will glide along in its orbit
around the sun without modifying it. The timing of this coast period is
nearly as important to keeping the appointment with Ceres as is the timing
of the thrusting. In next month's log, we will describe some of the special
assignments the sophisticated robot will perform instead of its usual
quiet cruise routine of accelerating and emitting xenon ions. We also
will look ahead to some interesting celestial milestones and alignments
in December.

While the spacecraft courses through the asteroid belt, the flight team
continues refining the plans for Ceres. In logs in December and several
months in 2014, we will present extensive details of those plans so that
by the time Dawn begins its mission there, you will be ready to ride along
and share in the experience.

In the meantime, as the stalwart ship sails on, it is propelled not only
by ions but also by the promise of exciting new knowledge and the prospects
of a thrilling new adventure in exploring an uncharted alien world.

Dawn is 16 million miles (26 million kilometers) from Vesta and 25 million
miles (39 million kilometers) from Ceres. It is also 3.07 AU (286 million
miles, or 460 million kilometers) from Earth, or 1,200 times as far as
the moon and 3.10 times as far as the sun today. Radio signals, traveling
at the universal limit of the speed of light, take 51 minutes to make
the round trip.
Received on Sun 03 Nov 2013 10:30:58 PM PST


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