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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 1 Apr 2014 09:38:25 -0700 (PDT)
Message-ID: <201404011638.s31GcPST013813_at_zagami.jpl.nasa.gov>


Dawn Journal
by Marc Rayman
March 31, 2014

Dear Correspondawnts,

Powering its way through deep space, Dawn draws ever closer to dwarf
planet Ceres. To reach its destination, the interplanetary spaceship
gently reshapes its path around the sun with its extraordinary ion
propulsion system. In about a year, the spacecraft will gracefully
slip into orbit so it can begin to unveil the nature of the mysterious
world of rock and ice, an intriguing protoplanetary remnant from the
dawn of the solar system.

Even as Dawn ascends the solar system hill, climbing farther and
farther from the sun, penetrating deeper into the main asteroid belt
between Mars and Jupiter, its distance to Earth is shrinking. This
behavior may be perplexing for readers with a geocentric bias, but
to understand it, we can take a broader perspective.

The sun is the conductor of the solar system symphony. Its gravity
dictates the movements of everything that orbits it: Earth as well as
the other planets, Vesta, Ceres, and myriad smaller objects, including
asteroids and Dawn. (Actually, the gravity of every single body affects
how all of the others move, but with more than 99 percent of the entire
solar system's mass concentrated in the gargantuan sun, it dominates the
gravitational landscape.)

Whether it is for a planet or Dawn orbiting the sun, a spacecraft or
moon orbiting a planet, the sun or other stars orbiting the Milky Way
(the Milky Way galaxy, that is, not your correspondent's cat Milky Way),
or the Milky Way galaxy orbiting the Virgo supercluster of galaxies
(home to an appreciable fraction of our readership), any orbit is the
perfect balance between the inward tug of gravity and the inexorable
tendency of objects to travel in a straight line. If you attach a weight
to a string and swing it around in a circle, the force you use to pull
on the string mimics the gravitational force the sun exerts on the
bodies that orbit it. The effort you expend in keeping the weight
circling serves constantly to redirect its course, forcing it to curve;
if you release the string, the weight's natural motion would take it
away in a straight line (we are ignoring here the effect of Earth's
gravity on the weight).

The force of gravity dwindles as the distance increases, so the sun
pulls harder on a nearby body than on a farther one. Therefore, to
remain in orbit, to balance the relentless gravitational lure, the
closer object must travel at higher speed, resisting the stronger
attraction. The same effect applies at Earth. Satellites that orbit very
close (including, for example, the International Space Station, 250
miles, or 400 kilometers, above the surface) must streak around the
planet at about 17,000 mph (7.6 kilometers per second) to avoid being
drawn down. The moon, orbiting almost a thousand times farther above,
needs only to travel at less than 2300 mph (about 1.0 kilometers per
second) to balance Earth's weaker hold at its remote location.

For that reason, Mercury zips around the sun faster than any of the
other planets. Mars travels more slowly than Earth, and the still more
distant residents of the asteroid belt, whether natural (all of them but
one) or a product of human ingenuity (one: Dawn), proceed at an even
more leisurely pace. As Earth makes its relatively rapid annual trip
around the sun, the distance to the spacecraft that left it behind in
2007 alternately shrinks and grows.

We can visualize this with one of the popular models of clocks available
in the Dawn gift shop on your planet, in which the hour hand is longer
than the minute hand. Imagine the sun as being at the center of the
clock. The tip of the short minute hand represents Earth, and the end of
the hour hand represents Dawn. Some of the time (such as between noon
and shortly after 12:30), the distance between the ends of the hands
increases. Then the situation reverses as the faster minute hand begins
moving closer and closer to the hour hand as the time approaches about 1:05.

Earth and Dawn are exhibiting the same repetitive behavior. Of course,
their relative motion is more complicated than that of the clock hands,
because Dawn's ion thrusting is constantly changing its solar orbit (and
so the distance and speed at which it loops around the sun), but the
principle is the same. They have been drawing closer since August 2013.
Earth, coming from behind, is now about to pass Dawn and move ahead. The
stalwart probe will not even take note however, as its sights remain
firmly set on an unexplored alien world.

Dawn's interplanetary trajectory

Dawn's interplanetary trajectory from launch through its arrival at
Ceres next year. The positions of the spacecraft and Earth are shown on
April 10, 2014, when their independent orbits bring them relatively
close together. Credit: JPL/NASA

On April 10, the separation will be 1.56 AU (1.56 times the average
distance between Earth and the sun, which means 145 million miles, or
233 million kilometers), an almost inconceivably large distance (well
in excess of half a million times farther than the International Space
Station, which orbits Earth, not the sun) but less than it has been since
September 2011. (The skeptical reader may verify this by reviewing the
concluding paragraph of each log in the intervening months.) Enjoy the
upcoming propinquity while you can! As the ship sails outward from the
sun toward Ceres, it will never again be this close to its planet of origin.
The next time Earth, taking an inside track, overtakes it, in July 2015
(by which time Dawn will be orbiting Ceres), they will only come within
1.94 AU (180 million miles, or 290 million kilometers) of each other.

By the way, Vesta, the endlessly fascinating protoplanet Dawn unveiled
in 2011-2012, will be at its smallest separation from Earth of 1.23 AU
(114 million miles, or 183 million km) on April 18. Ceres, still awaiting
a visitor from Earth, despite having first been glimpsed from there in 1801,
will attain its minimum distance on April 15, when it will be 1.64 AU (153
million miles, or 246 million km) away. It should not be a surprise that
Dawn's distance is intermediate; it is between them as it journeys from
one to the other.

Not only is each one nearly at its shortest geocentric range, but from
Earth's point of view, they all appear to be near each other in the
constellation Virgo. In fact, they also look close to Mars, so you can
locate these exotic worlds (and even the undetectably small spacecraft)
in the evening sky by using the salient red planet as a signpost. In
June, the coincidental celestial alignment will make Vesta and Ceres
appear to be separated by only one third the diameter of the full Moon,
although these behemoths of the asteroid belt will be 0.57 AU (52
million miles, or 85 million kilometers) from each other.

We mentioned above that by constantly modifying its orbit under the
persistent pressure of its ion engine, Dawn complicates the simple
clock-like behavior of its motion relative to Earth. On Halloween 2012,
we were treated to the startling fact that to rendezvous with Ceres, at
a greater distance from the sun, Dawn had to come in toward the sun for
a portion of its journey - quite a trick! In that memorable log (which
is here <http://dawn.jpl.nasa.gov/mission/journal_10_31_12.asp>, for
those readers who didn't find every detail to be so memorable), we
observed that it would not be until May 2014 that Dawn would be as far
from the sun as it was on Nov. 1, 2012. Sure enough, having faithfully
performed all of the complex and intricate choreography since then, it
will fly to more than 2.57 AU from the solar system's star in May, and
it will continue heading outward.

With the sun behind it and without regard to where Earth or most other
residents of the solar system are in their orbits, Dawn rises to ever
greater heights on its extraordinary expedition. Distant though it is,
the celestial ambassador is propelled by the burning passion for
knowledge, the powerful yearning to reach beyond the horizon, and the
noble spirit of adventure of the inhabitants of faraway Earth. The
journey ahead presents many unknowns, promising both great challenges
and great rewards. That, after all, is the reason for undertaking it,
for such voyages enrich the lives of all who share in the grand quest to
understand more about the cosmos and our humble place in it.

Dawn is 11 million miles (18 million kilometers) from Ceres. It is also
1.57 AU (146 million miles, or 235 million kilometers) from Earth, or
625 times as far as the moon and 1.57 times as far as the sun today.
Radio signals, traveling at the universal limit of the speed of light,
take 26 minutes to make the round trip.
Received on Tue 01 Apr 2014 12:38:25 PM PDT

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