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

[Ron Baalke]

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.