[meteorite-list] Dawn Journal - September 27, 2009

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 28 Sep 2009 17:19:19 -0700 (PDT)
Message-ID: <200909290019.n8T0JJ6h008156_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_9_27_09.asp

Dawn Journal
Dr. Marc Rayman
September 27, 2009
 
Dear Dawnniversaries,

Dawn is celebrating the second anniversary of leaving its home planet
by engaging in the same function it has performed
most of its time in space: with the utmost patience, it is using its ion
propulsion system to gradually modify its orbit around the Sun.

In its interplanetary travels, the spacecraft has thrust for a total of
about 389 days, or 53% of the time (and about 0.000000008% of the time
since the Big Bang). While for most spacecraft, firing a thruster to
change course is a special event, it is Dawn's wont. All this thrusting
has cost the craft only 103 kilograms (228 pounds) of its supply of
xenon propellant, which was 425 kilograms (937 pounds) on September 27,
2007.

The thrusting so far in the mission has achieved the equivalent of
accelerating the probe by 2.62 kilometers per second (5870 miles per
hour). As previous logs have described, because of
the principles of motion for orbital flight, whether around the Sun or
any other gravitating body, Dawn is not actually traveling this much
faster than when it launched. But the effective change in speed remains
a useful measure of the effect of any spacecraft's propulsive work.
Having accomplished only one-fifth of the thrust time planned for its
entire mission, Dawn has already far exceeded the velocity change
achieved by most spacecraft. (For a comparison with probes that enter
orbit around Mars, refer to a previous log
<journal_12_06.asp#perspective>.)

Since launch, our readers who have remained on or near Earth have
completed 2 revolutions around the Sun, covering about 1.88 billion
kilometers (1.17 billion miles). Orbiting farther from the Sun, and
moving at a more leisurely pace, Dawn has traveled 1.57 billion
kilometers (980 million miles). As it climbs away from the Sun to match
its orbit to that of Vesta, it will continue to slow down to Vesta's
speed. Since Dawn's launch, Vesta has traveled only 1.18 billion
kilometers (730 million miles).

Readers with nothing better to do have already discovered that much of
the text in the 3 preceding paragraphs is taken verbatim from the log
that commemorated Dawn's first anniversary of being in space,
with the principal changes being that the numbers
are updated here. (In addition, most of the humor was removed to comply
with a request from the Glum Legion of Ardent Dawnniversaries). This is
not a result of any more otiosity than normally displayed by your
correspondent; rather, comparing the beginning of this log with last
year's may be helpful for measuring the progress in the intervening
time. Of course, most of the last 12 months was devoted to coasting,
and the gravitational boost from Mars is not reflected in the effect of the
ion thrusting, but the comparison may be illuminating for some readers.
This also provides a handy preview of the beginning of the September 27,
2010 log. [Note to self: Perhaps there really is an option here for
greater lassitude. Think about that after taking a nap.]

Another way to investigate the progress of the mission is to chart how
Dawn's orbit around the Sun has changed. This discussion will culminate
with a few more numbers than we usually include, and readers who prefer
not to indulge may skip this material, leaving that much more
for the grateful Numerivores.

Orbits are ellipses (like flattened circles, or ovals in which the ends
are of equal size). So as members of the solar system family follow
their paths around the Sun, they sometimes move closer and sometimes
move farther from it. For thinking about these distances, we may remind
ourselves once again of the convenient unit of measure in the solar
system, the astronomical unit (AU).

In addition to orbits being characterized by shape, or equivalently by
the amount of flattening (that is, the deviation from being a perfect
circle), and by size, they may be described in part by how they are
oriented in space. Using the bias of terrestrial astronomers, the plane
of Earth's orbit around the Sun is a good reference. Other planets and
interplanetary spacecraft travel in orbits that are tipped at some angle
to that. Vesta and Ceres do not orbit the Sun in the same plane that
Earth does, and Dawn must match its orbit to that of its targets. (The
major planets orbit closer to the plane of Earth's orbit, and no
spacecraft has had to venture as far out of that plane to orbit another
body as Dawn will.)

Now we can see how Dawn has been doing by considering the size and shape
(together expressed by the minimum and maximum distances from the Sun)
and the angle of its orbit on its anniversaries. (Experts readily
recognize that there is more to describing an orbit than these
parameters. Our policy is to link to the experts' sites when their
readership extends to 1 more elliptical galaxy than ours does.)

The table below shows what the orbit would be if the spacecraft
terminated thrusting on its anniversaries; the orbits of its
destinations, Vesta and Ceres, are included for comparison. Of course,
when Dawn was on the launch pad on September 27, 2007, its orbit around
the Sun was exactly Earth's orbit. After launch, it had its own orbit.


 

        

                          Minimum distance Maximum distance Angle from
                          from the Sun (AU) from the Sun (AU) Earth's orbit


Dawn's orbit on
Sept. 27, 2007 0.98 1.02 0.0?
(before launch)

Dawn's orbit
on Sept. 27, 2007 1.00 1.62 0.6?
(after launch)

Dawn's orbit
on Sept. 27, 2008 1.21 1.68 1.4?

Dawn's orbit
on Sept. 27, 2009 1.42 1.87 6.2?

Vesta's orbit 2.15 2.57 7.1?

Ceres's orbit 2.54 2.99 10.6?

Readers may disregard the table or gaze into it for insight or
inspiration for as long as they like. The point of it, however, is to
illustrate both that Dawn has come a long way since the launch pad, and
it has a long journey ahead before it begins its exploration of Vesta.

But the trek will be a little shorter than mission planners had
anticipated until quite recently. As we have seen in a previous log,
the plan for thrusting depends on how
much electrical power will be available to the ion propulsion system,
which converts electrical power into thrusting power. Greater electrical
power translates into higher (but still exceptionally gentle) thrust.

Last year Dawn's engineers, who remain on distant Earth, devised a
method to calibrate the solar arrays, and the spacecraft dutifully
carried it out. The resulting data, combined with an extensive refinement
of the mathematical model that predicts solar array power, allowed the team to
be confident in increasing the prediction of the future availability of
power by up to 10%. Equipped with this crucial information, they could
update the plan for thrusting.

Many other factors affect the design of the thrust profile as well. As
one example, how effective the thrust is depends on how massive the
spacecraft is. Although weightless, Dawn still has mass (the resistance
to a change in its velocity), and the greater the mass, the lower the
acceleration provided by the ion thruster. This phenomenon is no
different from what readers experience frequently even in the gravity of
their home planet. The heavier the load you carry, the more gradually
you will accelerate, whether the effort is exerted by the muscles in
your legs (or wings or tentacles, depending on your species) or the
engine in your car (or spaceship). Dawn's mass decreases as the mission
progresses because the ion propulsion system expends xenon and the
reaction control system expends hydrazine. By
refining predictions for how much of these propellants will be onboard
at all times for the rest of the mission, engineers could predict how
long it will take Dawn to propel itself into the same orbit around the
Sun as Vesta and then later into the same orbit as Ceres.

After an extended set of analyses late in 2008 and the first half of
2009, all the elements needed to update the thrust plan were in place.
The seemingly modest improvement in solar array power is by far the
dominant one. When all were combined, the result revealed that Dawn's
remarkable maneuvering capability over the course of the mission will be
even better than engineers had been counting on. The probe will be able
to reach Vesta about 6 weeks earlier than had previously been planned.
Moreover, the newfound capability will enable the craft to travel from
Vesta to Ceres more quickly, so the deadline for leaving the first world
to reach the second on schedule in 2015 is about 6 weeks later.

Together, these changes allow the explorer to increase its planned
9-month stay at Vesta to 12 months. This is of extraordinary benefit to
the project. Vesta promises to be a fascinating place to visit, and we
know quite well from other solar system adventures that no matter how
much data we collect, there is always still more to learn. Mission
planners had been working hard to squeeze as much as possible into the
precious time they expected Dawn could spend at Vesta, so being able to
increase the duration of its residency there by a third makes a
tremendous difference. As details continue to be formulated for all the
activities necessary to operate at and study this alien world, the
additional time will prove extremely valuable in allowing the team to
accommodate the glitches that are inevitable in such a complex
expedition and to uncover as much of Vesta's intriguing story as possible.

Dawn is already following the new flight plan, targeting where Vesta
will be in July 2011. It is not enough, though, just for them to be in
(nearly) the same place at the same time. That would result in a flyby,
but our probe will enter orbit around the protoplanet, accompanying it
on its orbit around the Sun, just as satellites of Earth remain close by
throughout the planet's solar orbit. The craft will remain with Vesta
until July 2012, when it will begin thrusting to Ceres. We have
discussed before why flying by (providing
only a glimpse of each body) is significantly less challenging than
matching orbits (enabling more extensive explorations), a capability
that would be essentially impossible without the ion propulsion system.
A subsequent log will delve further into this issue, as it is a
fundamental feature of this ambitious mission.

As Dawn begins its third year in space, now on its new and better
course, much work remains before it can return the scientific bounty it
seeks. We hope readers will continue to follow the progress of this bold
adventure in the exciting years to come.

Dawn is 1.50 AU (225 million kilometers or 140 million miles) from
Earth, or 555 times as far as the moon and 1.50 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of light,
take 25 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 am PDT September 27, 2009

P.S. The astronomical unit has been mentioned in these logs frequently
enough that we will include that convenient unit of measurement from now
on in the famously unimaginative concluding paragraph. It might appear
redundant to present the distance from Earth both in astronomical units
and in terms of how many times as far as the Sun it is. Isn't that
simply 2 different ways to describe exactly the same quantity? Well, no
it is not; they are different, although they are close. An astronomical
unit is the /average/ distance between Earth and the Sun and hence does
not change. The /actual/ distance varies slightly throughout the year,
so Earth's distance from the Sun at any given time may not be precisely
the average value of 1.00000000 AU (149,597,871 kilometers or 92,955,629
miles). This would be more apparent if your correspondent did not round
off the numbers as dramatically. The details on that closing text are
that Dawn is 1.50456971 AU (225,080,425 kilometers or 139,858,224 miles)
from Earth. At the same time, Earth is 1.00222102 AU (149,928,510
kilometers or 93,161,078 miles) from the Sun, very close to the average,
but not exactly equal to it. So Dawn is 1.50123544 times as far from
Earth as the Sun is, given the distance to the Sun now. When rounded
off, the distance in astronomical units and the distance in terms of how
far the Sun is both come out to 1.50, but we see they are not really
equal. Other times of the year, when the actual distance to the Sun is
farther from the average, the difference will be apparent. As long as
these secrets of the final paragraph are being revealed, here are the
rest: the distance relative to the moon is rounded to the nearest
multiple of 5, and the travel time for radio signals to the nearest
minute. But just for this special occasion: Dawn is 556.865373 times as
far as the moon right now, and radio signals take 25 minutes 1.574966
seconds. Approximately. Best regards to the Numerivores.
Received on Mon 28 Sep 2009 08:19:19 PM PDT