[meteorite-list] Dawn Journal - January 31, 2014

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_01_31_14.asp

Dawn Journal
Dr. Marc Rayman
January 31, 2014

Dear Rendawnvous,

Dawn is continuing its trek through the main asteroid belt between Mars
and Jupiter. Leaving behind a blue-green wake of xenon from its ion
propulsion system, its sights are set on dwarf planet Ceres ahead. The 
journey has been long, but the veteran space traveler (and its support 
team on distant Earth) is making good progress for its rendezvous early 
next year.

The final part of Dawn's approach trajectory to Ceres

[Image]
The final part of Dawn's approach trajectory to Ceres, including when
the dwarf planet captures the spacecraft. Dawn continues ion thrusting
to its first observational orbit at an altitude of 8,400 miles (13,500
kilometers). Credit: JPL/NASA

Last month, we had a preview of many of the activities the probe will 
execute during the three months that culminate in settling into the first 
observational orbit at Ceres in April 2015.  At that orbit, about 8,400 
miles (13,500 kilometers) above the alien landscapes of rock and ice, 
Dawn will begin its intensive investigations. Nevertheless, even during 
the "approach phase," it will often observe Ceres with its camera and 
one of its spectrometers to gain a better fix on its trajectory and to 
perform some preliminary characterizations of the mysterious world prior 
to initiating its in-depth studies. The discussion in December did not 
cover the principal activity, however, which is one very familiar not 
only to the spacecraft but also to readers of these logs. The majority 
of the time in the approach phase will be devoted to continuing the 
ion-powered flight. We described this before Vesta, but for those few 
readers who don't have perfect recall (we know who you are), let's take 
another look at how this remarkable technology is used to deliver the 
adventurer to the desired orbit around Ceres.

Thrusting is not necessary for a spacecraft to remain in orbit, just as
the moon remains in orbit around Earth and Earth and other planets
remain in orbit around the sun without the benefit of propulsion. All
but a very few spacecraft spend most of their time in space coasting,
following the same orbit over and over unless redirected by a
gravitational encounter with another body. In contrast, with its
extraordinarily efficient ion propulsion system, Dawn's near-continuous
thrusting gradually changes its orbit. Thrusting since December 2007
has propelled Dawn from the orbit in which the Delta rocket deposited it 
after launch to orbits of still greater distance from the sun. The flight 
profile was carefully designed to send the craft by Mars in February 2009, 
so our celestial explorer could appropriate some of the planet's orbital
energy for the journey to the more distant asteroid belt, of which it is
now a permanent resident. In exchange for Mars raising Dawn's heliocentric 
orbit, Dawn lowered Mars's orbit, ensuring the solar system's energy account 
remained balanced.

While spacecraft have flown past a few asteroids in the main belt
(although none as large as the gargantuan Vesta or Ceres, the two most
massive objects in the belt), no prior mission has ever attempted to
orbit one, much less two. For that matter, this is the first mission
ever undertaken to orbit any two extraterrestrial destinations. Dawn's
exclusive assignment would be quite impossible without its uniquely
capable ion propulsion system. But with its light touch on the
accelerator, taking nearly four years to travel from Earth past Mars to
Vesta, and more than two and a half years from Vesta to Ceres, how will
it enter orbit around Ceres? As we review this topic in preparation for
Ceres, bear in mind that this is more than just a cool concept or neat
notion. This is real. The remarkable adventurer actually accomplished
the extraordinary feats at Vesta of getting into and out of orbit using
the delicate thrust of its ion engines.

Whether conventional spacecraft propulsion or ion propulsion is
employed, entering orbit requires accompanying the destination on its
own orbit around the sun. This intriguing challenge was addressed in
part in February 2007. In February 2013, we considered another aspect 
of what is involved in climbing the solar system hill, with the sun at 
the bottom, Earth partway up, and the asteroid belt even higher. We saw 
that Dawn needs to ascend that hill, but it is not sufficient simply 
to reach the elevation of each target nor even to travel at the same 
speed as each target; the explorer also needs to travel in the same 
direction. Probes that leave Earth to orbit other solar system bodies 
traverse outward from (or inward toward) the sun, but then need to turn 
in order to move along with the body they will orbit, and that is 
difficult.

Those of you who have traveled around the solar system before are
familiar with the routine of dropping into orbit. The spacecraft
approaches its destination at very high velocity and fires its powerful
engine for some minutes or perhaps even about an hour, by the end of
which it is traveling slowly enough that the planet's gravity can hold
it in orbit and carry it around the sun. These exciting events may range
from around 1,300 to 3,400 mph (0.6 to 1.5 kilometers per second). With
ten thousand times less thrust than a typical propulsion system on an
interplanetary spacecraft, Dawn could never accomplish such a rapid 
maneuver. As it turns out, however, it doesn't have to.

Dawn's method of getting into orbit is quite different, and the key is
expressed in an attribute of ion propulsion that has been referred to 63
times (trust or verify; it's your choice) before in these
logs: it is gentle. (This example <journal_12_06.asp#farther> shows just
how gentle the acceleration is.) With the gradual trajectory
modifications inherent in ion propulsion, sharp changes in direction and
speed are replaced by smooth, gentle curves. The thrust profiles for
Dawn's long interplanetary flights are devoted to the gradual reshaping
of its orbit around the sun so that by the time it is in the vicinity of
its target, its orbit is nearly the same as that of the target. Rather
than hurtling toward Vesta or Ceres, Dawn approaches with grace and
elegance. Only a small trajectory adjustment is needed to let its new
partner's gravity capture it, so even that gentle ion thrust will be
quite sufficient to let the craft slip into orbit. With only a nudge, it
transitions from its large, slow spiral away from the sun to an inward
spiral centered around its new gravitational master.

[Image]
This graphic shows the planned trek of NASA's Dawn spacecraft from its
launch in 2007 through its arrival at the dwarf planet Ceres in early
2015. Note how Dawn spirals outward to Vesta and then still more to
Ceres. Credit: JPL/NASA

To get into orbit, a spacecraft has to match speed, direction and
location with its target. A mission with conventional propulsion first
gets to the location and then, using the planet's gravity and its own
fuel-guzzling propulsion system, very rapidly achieves the required
speed and direction. By spiraling outward from the sun, first to the
orbit of Vesta and now to Ceres, Dawn works on its speed, direction and
location all at the same time, so they all gradually reach the needed
values at just the right time.

To illustrate this facet of the difference between how the different
systems are applied to arrive in orbit, let's imagine you want to drive
your car next to another traveling west at 60 mph (100 kilometers per
hour). The analogy with the conventional technology would be similar to
speeding north toward an intersection where you know the other car will
be. You arrive there at the same time and then execute a screeching,
whiplash-inducing left turn at the last moment using the brakes,
steering wheel, accelerator and adrenaline. When you drive an ion
propelled car (with 10 times higher fuel efficiency), you take an
entirely different path from the start, one more like a long, curving
entrance ramp to a highway. As you enter the ramp, you slowly (perhaps
even gently) build speed. You approach the highway gradually, and by the
time you have reached the far end of the ramp, your car is traveling at
the same speed and in the same direction as the other car. Of course, to
ensure you are there when the other car is, the timing is very different
from the first method, but the sophisticated techniques of orbital
navigation are up to the task.

In March or April 2015, as the probe follows its approach trajectory to
Ceres, their paths will be so similar they will be racing around the sun
at nearly the same speed (38,500 mph, or 17.2 kilometers per second) and
in the same direction. But what matters is their relative velocity.
When at a range of 30,000 miles (48,000 kilometers), the spacecraft will
be closing in on its destination at less than 85 mph (37 meters per
second). The combination of distance and velocity will allow Ceres to
take Dawn in its grasp. The spacecraft will not even notice the
difference, but it will be in orbit around its second and final
celestial target, even as it continues ion thrusting to spiral to its
first planned orbital altitude two and a half weeks later.

Unlike missions that use conventional chemical propulsion, there is no
sudden change on the spacecraft and no nail-biting on Earth. If you were
in space watching the action, you probably would be hungry, cold and
hypoxic, but you would not notice anything unusual about the scene as
Ceres smoothly and tenderly takes Dawn into an invisible gravitational
embrace.

If instead of being in deep space, you had been in Dawn mission control
watching the action when the spacecraft entered orbit around Vesta in
July 2011 you would have been in the dark and all alone (until JPL 
Security arrived to escort you away). Your correspondent was out dancing, 
and other members of the team were engaged in activities similarly 
unrelated to controlling a probe hundreds of times farther away than the 
moon.  There was no need to have radio contact with the reliable spaceship. 
It had already been thrusting for 70 percent of its time in space, so it
was performing a very familiar function. It should be no different at
Ceres (although the dance program may not be exactly the same). When
Dawn enters orbit, no one is tense or anxious; rather, all the drama is
in the promise of the spectacular discoveries in exploring uncharted
worlds, the rewards of new knowledge, and the thrill of knowing that
humankind is reaching far, far from home in a grand effort to know the
cosmos.

Dawn is 16 million miles (26 million kilometers) from Ceres. It is also
2.05 AU (191 million miles, or 307 million kilometers) from Earth, or
855 times as far as the moon and 2.08 times as far as the sun today.
Radio signals, traveling at the universal limit of the speed of light,
take 34 minutes to make the round trip.