[meteorite-list] Dawn Journal - September 27, 2014

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2014/09/27/dawn-journal-september-27/

Dawn Journal 
by Marc Rayman
September 27, 2014

Dear Dawnniversaries,

On the seventh anniversary of embarking upon its extraordinary extraterrestrial 
expedition, the Dawn spacecraft is far from the planet where its journey 
began. While Earth has completed its repetitive loops around the sun seven 
times, its ambassador to the cosmos has had a much more varied itinerary. 
On most of its anniversaries, including this one, it reshapes its orbit 
around the sun, aiming for some of the last uncharted worlds in the inner 
solar system. (It also zipped past the oft-visited Mars, robbing the red 
planet of some of its orbital energy to help fling the spacecraft on to 
the more distant main asteroid belt.) It spent its fourth anniversary 
exploring the giant protoplanet Vesta, the second most massive object 
in the asteroid belt, revealing a fascinating, complex, alien place more 
akin to Earth and the other terrestrial planets than to typical asteroids. 
This anniversary is the last it will spend sailing on the celestial seas. 
By its eighth, it will be at its new, permanent home, dwarf planet Ceres.

The mysterious world of rock and ice is the first dwarf planet discovered 
(129 years before Pluto) and the largest body between the sun and Pluto 
that a spacecraft has not yet visited. Dawn will take up residence there 
so it can conduct a detailed investigation, recording pictures and other 
data not only for scientists but for everyone who has ever gazed up at 
the night sky in wonder, everyone who is curious about the nature of the 
universe, everyone who feels the burning passion for adventure and the 
insatiable hunger for knowledge and everyone who longs to know the cosmos.
Artist depiction of landmarks on Dawn's voyage.

Dawn is the only spacecraft ever to orbit a resident of the asteroid belt. 
It is also the only ship ever targeted to orbit two deep-space destinations. 
This unique mission would be quite impossible without its advanced ion 
propulsion system, giving it capabilities well beyond what conventional 
chemical propulsion provides. That is one of the keys to how such a voyage 
can be undertaken.

For those who would like to track the probe's progress in the same terms 
used on previous (and, we boldly predict, subsequent) anniversaries, we 
present here the seventh annual summary, reusing text from last year with 
updates where appropriate. Readers who wish to reflect upon Dawn's ambitious 
journey may find it helpful to compare this material with the logs from 
its first, second, third, fourth, fifth and sixth anniversaries. On this 
anniversary, as we will see below, the moon will participate in the celebration.

In its seven years of interplanetary travels, the spacecraft has thrust 
for a total of 1,737 days, or 68 percent of the time (and about 0.000000034 
percent 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 808 pounds (366 kilograms) of its 
supply of xenon propellant, which was 937 pounds (425 kilograms) on Sep. 
27, 2007.
Dawn launch, JSC, Sept. 27. 2007

Dawn launched at dawn (7:34 a.m. EDT) from Cape Canaveral Air Force Station, 
Sep. 27, 2007. Its mission is to learn about the dawn of the solar system 
by studying Vesta and Ceres. Credit: KSC/NASA

The thrusting so far in the mission has achieved the equivalent of accelerating 
the probe by 22,800 mph (10.2 kilometers per second). As previous logs 
have described (see here for one of the more extensive discussions), 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 about seven-eighths of the thrust time planned for its entire 
mission, Dawn has already far exceeded the velocity change achieved by 
any other spacecraft under its own power. (For a comparison with probes 
that enter orbit around Mars, refer to this earlier log.)

Since launch, our readers who have remained on or near Earth have completed 
seven revolutions around the sun, covering 44.0 AU (4.1 billion miles, 
or 6.6 billion kilometers). Orbiting farther from the sun, and thus moving 
at a more leisurely pace, Dawn has traveled 31.4 AU (2.9 billion miles, 
or 4.7 billion kilometers). As it climbed away from the sun to match its 
orbit to that of Vesta, it continued to slow down to Vesta's speed. It 
has been slowing down still more to rendezvous with Ceres. Since Dawn's 
launch, Vesta has traveled only 28.5 AU (2.6 billion miles, or 4.3 billion 
kilometers), and the even more sedate Ceres has gone 26.8 AU (2.5 billion 
miles, or 4.0 billion kilometers). (To develop a feeling for the relative 
speeds, you might reread this paragraph by paying attention to only one 
set of units, whether you choose AU, miles, or kilometers. Ignore the 
other two scales so you can focus on the differences in distance among 
Earth, Dawn, Vesta and Ceres over the seven years. You will see that as 
the strength of the sun's gravitational grip weakens at greater distance, 
the corresponding orbital speed decreases.)

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. (If you prefer not to skip it, click here.) In order 
to make the table below comprehensible (and to fulfill our commitment 
of environmental responsibility), we recycle some more text here on the 
nature of orbits.

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.

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 (known as the ecliptic) is a good reference. Other 
planets and interplanetary spacecraft may travel in orbits that are tipped 
at some angle to that. The angle between the ecliptic and the plane of 
another body's orbit around the sun is the inclination of that orbit. 
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 ecliptic, and part of the arduousness of the journey 
is changing the inclination of its orbit, an energetically expensive task.)

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 inclination of its orbit on each of its anniversaries. (Experts readily 
recognize that there is more to describing an orbit than these parameters. 
Our policy remains that we link to the experts' websites when their readership 
extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft 
had terminated ion 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 Sep. 27, 2007, its orbit around the sun was exactly 
Earth's orbit. After launch, it was in its own solar orbit.

	Minimum distance from the Sun (AU) 	Maximum distance from the Sun (AU) 
	Inclination
Earth's orbit 	0.98 	1.02 	0.0°
Dawn's orbit on Sep. 27, 2007 (before launch) 	0.98 	1.02 	0.0°
Dawn's orbit on Sep. 27, 2007 (after launch) 	1.00 	1.62 	0.6°
Dawn's orbit on Sep. 27, 2008 	1.21 	1.68 	1.4°
Dawn's orbit on Sep. 27, 2009 	1.42 	1.87 	6.2°
Dawn's orbit on Sep. 27, 2010 	1.89 	2.13 	6.8°
Dawn's orbit on Sep. 27, 2011 	2.15 	2.57 	7.1°
Vesta's orbit 	2.15 	2.57 	7.1°
Dawn's orbit on Sep. 27, 2012 	2.17 	2.57 	7.3°
Dawn's orbit on Sep. 27, 2013 	2.44 	2.98 	8.7°
Dawn's orbit on Sep. 27, 2014 	2.46 	3.02 	9.8°
Ceres' orbit 	2.56 	2.98 	10.6°

For readers who are not overwhelmed by the number of numbers, investing 
the effort to study the table may help to demonstrate how Dawn has patiently 
transformed its orbit during the course of its mission. Note that three 
years ago, the spacecraft?s path around the sun was exactly the same as 
Vesta's. Achieving that perfect match was, of course, the objective of 
the long flight that started in the same solar orbit as Earth, and that 
is how Dawn managed to slip into orbit around Vesta. While simply flying 
by it would have been far easier, matching orbits with Vesta required 
the exceptional capability of the ion propulsion system. Without that 
technology, NASA's Discovery Program would not have been able to afford 
a mission to explore it in such detail. But now, Dawn has gone even beyond 
that. Having discovered so many of Vesta's secrets, the stalwart adventurer 
left the protoplanet behind. No other spacecraft has ever escaped from 
orbit around one distant solar system object to travel to and orbit still 
another extraterrestrial destination. A true interplanetary spaceship, 
Dawn is enlarging, reshaping and tilting its orbit again so that in 2015, 
it will be identical to Ceres'.

It may surprise you that if Dawn stopped thrusting today, it would sail 
out farther from the sun than where it is headed, as shown in the table. 
We can understand that, however, by thinking carefully about how the craft 
reaches its target. It has been propelling itself up the solar system 
hill so it can fly to the vicinity of Ceres, and its own momentum now 
is sufficient to carry it even beyond. This is little different from driving 
to a destination with the recognition that near the end of your trip, 
you need to slow down or you will overshoot. While trajectories that use 
ion propulsion are much more complicated, that fundamental principle applies. 
Indeed, Dawn's speed toward Ceres has been declining since December 2013. 
In addition to the recent and future ion thrusting that guides the ship 
smoothly into its new port, the gravity of Ceres itself will help tug 
Dawn in. We will see more about that next month when we present the revised 
approach plan.
Dawn mission trajectory

[Graphic]
Dawn's interplanetary trajectory (in blue). The dates in white show Dawn?s 
location every Sep. 27, starting on Earth in 2007. Note that Earth returns 
to the same location, taking one year to complete each revolution around 
the sun. As Dawn climbs farther from the sun, it orbits more slowly. Credit: 
NASA/JPL

On Sep. 11, as the spacecraft was engaged in routine ion thrusting, a 
high-energy particle of space radiation struck an electrical component 
onboard. That triggered a chain of events that halted thrusting and required 
the team of flight controllers on distant Earth to leap into action to 
resume normal operations. Their swift and expert response was successful, 
and by Sep. 15 the robot was back on duty. In the next log, we will describe 
what happened on the spacecraft and in mission control. We will also see 
how navigators take advantage of the tremendous flexibility provided by 
ion propulsion to devise a new path into Ceres orbit following this interruption 
in thrust. (As we also will see, the rest of the intricate plans for exploring 
the dwarf planet will be unchanged. The logs from December 2013 through 
last month have previews of those plans.)

As Dawn begins the eighth year of its trek through the solar system, Earthlings 
have a convenient opportunity today to locate the distant spacecraft thanks 
to the moon. That celestial orb serves as a guidepost to Dawn, which will 
be well over one thousand times farther away. When the moon rises in the 
United States later this morning, it will be leading Dawn by less than 
nine lunar diameters. The sun, moon, planets and stars all appear to move 
west as Earth rotates on its axis, but the moon itself travels eastward 
in its orbit quickly enough that it falls behind noticeably over the course 
of a day. Throughout most of the day today, our natural satellite's progression 
will slowly shrink the distance to Dawn. For observes in the eastern part 
of the country, by the time the moon sets, it will be about one lunar 
diameter from the spacecraft. For those on the west coast, the moon will 
be less than its own width from Dawn around sunset. By the time they see 
the moon setting, Dawn will have passed it and will lead it down to the 
horizon, the pair still within two lunar diameters of each other. The 
details are not so important, however. For observers anywhere today, the 
moon allows us to get a sense of where in the vast sky our faithful explorer 
is.

Of course Dawn is much, much, much too far away to be seen with our humble 
eyes. The spacecraft is more than 1.2 million times farther from Earth 
than the International Space Station is. It is more remote than Mars ever 
is. The most powerful optical telescopes on high mountains or in orbit 
could not detect anything nearly as faint as Dawn in the depths of space. 
Yet readers have ready access to vision far more acute. We can turn our 
mind's eye to that part of the sky near the moon. Out there, in that direction, 
is a probe from Earth, an emissary to the cosmos, silently streaking through 
the distant void, conducting an ambitious and exciting mission of discovery 
on behalf of curious and ingenious humans who yearn for new knowledge 
and new insight and who have an insatiable passion for grand adventures.

Dawn is 2.1 million miles (3.5 million kilometers) from Ceres. It is also 
3.37 AU (313 million miles, or 504 million kilometers) from Earth, or 
1,290 times as far as the moon and 3.36 times as far as the sun today. 
Radio signals, traveling at the universal limit of the speed of light, 
take 56 minutes to make the round trip.