[meteorite-list] Dawn Journal - April 30, 2013

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 1 May 2013 08:54:37 -0700 (PDT)
Message-ID: <201305011554.r41FsbIc025414_at_zagami.jpl.nasa.gov>


Dawn Journal
Dr. Marc Rayman
April 30, 2013

Dear Dawnscerning Readers,

Nearly three times as far from Earth as the sun is, the Dawn spacecraft
is making very good progress on its ambitious trek from Vesta to Ceres.
After a spectacular adventure at the second most massive resident of the
main asteroid belt between Mars and Jupiter, Dawn used its extraordinary
ion propulsion system to leave it behind and undertake the long journey
to a dwarf planet.

Ceres orbits the sun outside Vesta's orbit, yet Dawn is now closer to
the sun than both of these alien worlds. How can it be that as the probe
climbs from one to the other, it seems to be falling inward? Perhaps the
answer lies in the text below; let's venture on and find out!

On Halloween we discussed why Dawn is heading in toward the sun, but this
question is different. Vesta also is getting closer to the sun, but what's
of interest now is that Dawn, despite its more remote destination, has been
approaching the sun more quickly. That earlier log stands out as the best
one ever written on this exciting mission in the entire history of October
2012, but if you prefer not to visit it now, we can summarize here the
explanation for the spacecraft moving toward the sun. Like all members of
the sun's entourage, Vesta and Ceres follow elliptical orbits, their
distances from the master of the solar system growing and shrinking as they
loop around it. Even Earth's orbit, although nearly round, certainly is not
perfectly circular. Our planet is a little closer to the sun in the northern
hemisphere winter (southern hemisphere summer) than it is in the summer
(southern hemisphere winter). Dawn's orbit is elliptical as well, so it
naturally moves nearer to the sun sometimes, and now is such a time. But
that does not address why it is currently closer to the sun than Vesta,
even though it is seeking out the more distant Ceres.

Because it will orbit Ceres, and not simply fly past it (which would be
significantly easier but less valuable), Dawn must make its own orbit
around the sun be identical to its target's. But that is not the entire
story. After spending 14 months orbiting Vesta, Dawn's challenge is more
than to change the shape of its orbit to match Ceres's. The spacecraft
also must be at the same place in Ceres's heliocentric orbit that
Ceres itself is.

It would not be very rewarding to follow the same looping path around
the sun but always be somewhere else on that path. You can visualize
this if you have one of the many defective -- er, exotic clocks from the
Dawn gift shop on your planet that have two minute hands. If the clock
starts with one hand pointed at 12 and another pointed at 1, they will
take the same repetitive route, but neither hand will ever catch up with
the other. For Dawn's goal of exploring Ceres, this would not prove
satisfying. Therefore, part of the objective of the ion thrusting is to
ensure the spacecraft arrives not only on the same heliocentric course
as Ceres but is there when Ceres is also.

This is a problem familiar to all readers who have maneuvered in orbit,
where the principles of orbital mechanics are the rules of the road. To
solve it, we rely on one of the laws that we have addressed many times
in these logs: objects in a lower orbit travel faster. We described this
in more detail in February, and we can recall the essential idea here. The
gravitational attraction of any body, whether it is the sun, Earth, a black
hole, or anything else, is greater at shorter ranges. So to balance that
strong inward pull, an orbiter is compelled to race around quickly. At higher
orbits, where gravity is weaker, a more leisurely orbital pace suffices.

We can take advantage of this characteristic of orbits. If we drop to a
slightly lower orbit, we travel along more swiftly. That is precisely
what Dawn needs to do in order to ensure that when it finishes expanding
and tilting its orbit in 2015 so that it is the same as Ceres's, it
winds up at the same location as its target. This would be like speeding
up the minute hand that had begun at the 12, allowing it to catch up
with the hand that would otherwise always be leading it.

Dawn's orbital maneuvering is a little bit more complicated than that of
clock hands, but thanks to the ingenuity and creativity of the
operations team and the unique capability of its ion propulsion system,
the interplanetary ship is sailing on a carefully plotted course to its
next celestial port. As soon as it departed from Vesta's gravitational
embrace in September, it slipped in closer to the sun. Today, Vesta is
2.53 AU from the sun, and Dawn is 2.51 AU, so the spacecraft is three
million kilometers (1.9 million miles) nearer to the sun. (Dawn is farther
from Vesta than that, because they are not aligned with the sun. The
spacecraft has also moved ahead of the rocky behemoth.)

Of course, eventually Dawn will climb to higher altitudes from the sun
than Vesta, because its destination lies beyond. As they progress on
their own independent orbits, with Dawn constantly reshaping its, they
will be at the same solar distance on July 31, 2013. After that, the
robotic explorer will never again be as close to the sun as Vesta. By
then, they will be 18 million kilometers (11 million miles) apart. But
they will always be connected. Dawn was Earth's first probe to take up
residence in the main asteroid belt, and Vesta was its first target. The
exotic world had beckoned to humankind for over two centuries before the
spacecraft obtained its richly detailed view. Now what was little more
than an indistinct point of light is known as a complex and fascinating
place with a unique character. And as it follows its repetitive orbit
around the sun, its erstwhile companion seeks to reveal the secrets of
another extraterrestrial enigma, Ceres. Great treasures await Dawn as it
patiently continues its extraordinary deep-space expedition.

Dawn is 10 million kilometers (6.3 million miles) from Vesta and 56
million kilometers (35 million miles) from Ceres. It is also 2.99 AU
(448 million kilometers or 278 million miles) from Earth, or 1,215 times
as far as the moon and 2.97 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 50
minutes to make the round trip.
Received on Wed 01 May 2013 11:54:37 AM PDT

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