[meteorite-list] Dawn Journal - February 24, 2010

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 25 Feb 2010 12:47:11 -0800 (PST)
Message-ID: <201002252047.o1PKlB52017019_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_02_24_10.asp

Dawn Journal
Dr. Marc Rayman
February 24, 2010

Dear Dawnthropoids,
 
Pushing ever farther into space, deeper into the asteroid belt,
Dawn is continuing to progress smoothly on its solar system journey.

The spacecraft spends most days climbing away from the Sun atop
its pillar of blue-green xenon ions. A day's thrusting is enough
to change the spacecraft's speed by a very modest 7.3
meters/second (16.3 miles/hour). While such an effect would be
entirely inadequate for an interplanetary mission as ambitious as
Dawn's, the extraordinary efficiency of ion propulsion
allows the probe to thrust for much more
than a day. Although almost all spacecraft coast most of the time,
as do planets, moons, and asteroids, this explorer usually
maintains a gentle pressure on its orbit, constantly changing it
so that it can rendezvous with Vesta next year, leave in 2012, and
then rendezvous with Ceres in 2015. Dawn has spent 60% of its time
since launch patiently accelerating with the ion propulsion
system. It has already managed to change its speed by more than
3.6 kilometers/second (8100 miles/hour), far exceeding the
capability of most spacecraft, yet it has a great deal more
thrusting ahead. (For a comparison with probes that enter orbit
around Mars, visit a previous log.)
 
In contrast to conventional chemical propulsion systems, ion
propulsion achieves its astonishingly high performance by using
electrical power to create the thrust. Outfitted with the most
powerful solar arrays ever
carried on an interplanetary probe, Dawn converts sunlight into
the electricity consumed by the ion thrusters. And yet even as its
travels take it ever more distant from the luminous source of its
electrical power, the effect of the ion thrust becomes greater
each day, not less. At the beginning of the mission, a day of
thrust yielded only about 6.5 meters/second (14.5 miles/hour).
Now, more than 1.8 times farther from the Sun, the acceleration is
greater, and by this summer, when Dawn is still farther from the
Sun, it will climb to 7.6 meters/second (17.0 miles/hour) every 24
hours. The reason for the paradoxical increase is deceptive, yet
simple.
 
Dawn's solar arrays are so large
that they can produce enough power to operate an ion thruster at
the maximum throttle level (as well as all other spacecraft
systems) even when twice as far from the Sun as Earth is.
Therefore, the propulsion system will not have to be throttled to
lower power until the probe is more than 2 astronomical units (AU)
from the Sun, a distance it will reach
this summer. In the meantime, because the arrays produce excess
power, the thrust is independent of the distance to the Sun.

The acceleration depends on more than the thrust, however; the
thruster pushes against the spacecraft, so the change in speed
depends on the spacecraft's total mass. A rocket engine (whether
powerful and inefficient or soft and efficient) imparts a lower
velocity to a more massive craft than to a less massive one. (Gravity,
or the absence of it, is not relevant.) This is quite familiar from
terrestrial experience. If you throw a baseball with the same
force as you throw a shot put, the baseball will depart with a
higher speed.

When Dawn began its mission in September 2007, it was about 1218
kilograms (2685 pounds). Since then, it has expended 140 kilograms
(309 pounds) of xenon plus about 5 kilograms (11 pounds) of
hydrazine from the reaction control system
(the system that uses small conventional thrusters to aid in
orienting the spacecraft in the zero-gravity of spaceflight). The
ion thruster now is pushing against less mass, so the /effect/ of
the thrust is greater. As Dawn continues to expel its propellants,
it will become still less resistant to the thruster's efforts to
change its speed. In summary, with the thrust staying constant and
the mass decreasing, the acceleration is increasing.

The mass will always go down when the ion propulsion or reaction
control systems are operated. Once the spacecraft is far enough
from the Sun that it needs to reduce the throttle level, the
availability of power, and hence the thrust, generally will
decline faster than the mass, so the effect of the thrust will
diminish. By the end of the mission, a day of thrusting will
provide less than half of the change in speed that it does now.

Of course, engineers have been accounting for this since they
began designing the project. The entire flight plan from Earth to
Vesta (via Mars) and from Vesta to Ceres is based on how much the
craft can accelerate throughout its mission.

While the spacecraft will not reach Vesta until July 2011, the
Dawn team has been hard at work developing the detailed plans for
what it will do there. This month, they initiated the long process
of formulating the specific instructions that will be radioed to
the probe to carry out those plans, guiding it through all the
steps it must follow to get into orbit, to perform the myriad
scientific measurements that are planned, to transmit the results
to Earth, and to remain healthy and productive in that distant and
forbidding environment. The team is beginning with the "approach
phase," which commences in May 2011 and concludes when Dawn has
completed thrusting its way to the first orbit from which it will
conduct intensive observations in August 2011. (Of course, it will
stop occasionally to peer at Vesta as it closes in on the enormous
asteroid during the approach phase.) As work on each of the Vesta
phases is completed, the team will turn its focus to the next, so
by the time Dawn begins its approach, most of the instructions for
its year at Vesta will have been prepared.

The commands will be checked and double checked just as they are
for carrying out the interplanetary flight. For operating at Vesta
so long from now, however, they also are being designed so that
shortly before it is time to transmit them to the spacecraft,
controllers can update them to account for the exact trajectory
the spacecraft is on and other details that may change slightly.

In upcoming logs, we will describe some of the highlights of the
plans that are being readied for when the ship reaches its first
celestial destination.

To help refine preparations for flying near Vesta and studying it
from Dawn's vantage point, scientists are taking advantage of the
convenient alignment
<http://www.jpl.nasa.gov/news/news.cfm?release=2010-051> between
Earth and the protoplanet to observe it with the Hubble Space
Telescope on February 25 and 28. The venerable 200-inch
(5.1-meter) Hale Telescope on Palomar Mountain in California will
be used for other Vesta measurements in April.

While engineers and scientists focus their attentions on Dawn, the
ship continues to grow more remote. As we saw in the previous log,
after closing in on each other for 14
months, Earth and the spacecraft are now separating again, their
independent orbits around the Sun carrying them farther apart. On
February 28, Earth will be equidistant from Dawn and the Sun.
Readers on that planet will be at the apex of a broad cosmic
isosceles triangle, 0.99 AU from both the tremendous star that has
governed the solar system for 4.6 billion years and the tiny probe
that is quietly and patiently making its way to investigate
unexplored alien worlds to help us understand the dawn of the
solar system. The third leg, between Dawn and the Sun, will be
1.84 AU long.

Although quite undetectable with all but the most sensitive radio
receivers of the Deep Space Network, those who are share in the
profundity and the passion for the exploration of the cosmos may
wish to gaze upon the spacecraft with their minds' eyes. It has
been farther from Earth before, and other spacecraft have been
much farther still, but while it is at the same distance as the
Sun, it presents an occasion to reflect upon humankind's
achievements. Dawn's milestone represents much more than the
opportunity to gain fascinating new insights into the solar system
and an exciting adventure to reveal vistas previously unseen. At
the same distance as the Sun, it symbolizes the extraordinary
success of science and engineering. At the same distance as the
Sun, it compels us to mediate upon what humankind can accomplish
when we are inspired to translate our grand ambitions into action.
At the same distance as the Sun, it reminds us that some feats
that once were even beyond imagination may be achieved. While
physically we are confined to the vicinity of our home planet, the
power of uncounted millennia of thoughts, hopes, and dreams
combined with persistence, discipline, and tremendous cognitive
effort allows us to have an extension of ourselves as far as the
Sun. A spacecraft as far as the Sun is a triumph of humble
creatures bold enough to reach out into the universe.

To aid in contemplating the nature of such grand undertakings,
some readers may wish to peer in the direction of the invisible,
distant craft. It is 5.5 degrees northwest of Mars, an easily
identified ruby among the gems of the evening sky. (For reference,
5.5 degrees is 11 times the diameter of the Moon or about the
width of 3 fingers held together at arm's length.) It is roughly
2/3 of the way from Mars to the bright star Pollux. As the month
ends, your correspondent (reporting on location from Earth) plans
to contemplate his view of the Sun during the day and the sky near
Dawn in the evening.

This will be the last chance to peer toward the spacecraft while
it is this close. For the rest of its mission, and effectively
forever, Dawn will be farther from Earth than the Sun. Yet it will
remain eternally tied to the planet by virtue of the unique human
thirst for knowledge, spirit of adventure, and insatiable yearning
to know the cosmos, all of which propel it beyond distant horizons.

Dawn is 0.96 AU (144 million kilometers or 90 million miles) from
Earth, or 395 times as far as the Moon and 0.97 times as far as
the Sun. Radio signals, traveling at the universal limit of the
speed of light, take 16 minutes to make the round trip.
Received on Thu 25 Feb 2010 03:47:11 PM PST