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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 1 Dec 2009 18:04:26 -0800 (PST)
Message-ID: <200912020204.nB224QTc027291_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_11_27_09.asp

Dawn Journal
Dr. Marc Rayman
November 27, 2009

Dear Dawnticlimaxes,

Dawn continues to make steady progress through the solar system as it
maintains a gentle pressure on its orbit around the Sun. It has spent
95% of the time since the last log thrusting with
its ion propulsion system, stopping only briefly each week to
communicate with the mission control team on distant Earth.

The probe is on an exciting journey to unlock secrets from the dawn of
the solar system ensconced in the mysterious worlds Vesta and Ceres. And
yet there is one aspect of this expedition that likely is much less
exciting than some readers may expect.

Dawn entered the main asteroid belt on November 13. As it ventures ever
deeper into this vast
collection of material between Mars and Jupiter, it may be tempting to
think of the spacecraft constantly dodging asteroids. In some science
fiction movies, the huge rocky bodies are so close together that highly
skilled piloting is required to avoid catastrophes. Now Dawn is guided
by some of the most proficient interplanetary fliers this side of Pluto,
but the reality is that accidental impacts are exceedingly unlikely.
Space is big, and as plentiful as asteroids are, the distances between
them are tremendous.

After crossing the threshold of the belt earlier this month, Dawn will
travel 7.7 astronomical units (AU), or nearly
1.2 billion kilometers (almost 720 million miles), to its July 2011
rendezvous with Vesta. Yet in all that time, and across all that
distance, the closest the probe will come to a catalogued asteroid is
1.0 million kilometers (greater than 600 thousand miles), or more than
2.5 times the distance between Earth and the moon. Certainly travelers
on Earth would not consider something that far away to be a hazard
(especially compared to what many Dawn team members regularly experience
on the freeways in Los Angeles), and neither would our intrepid explorer.

To bring this down to a more tractable scale, we can imagine Dawn's
journey through the asteroid belt to Vesta as a trip from New York City
to Los Angeles, with rocks littered along the way. In this case, along
the entire route to a bizarre and forbidding land, the nearest we would
come to one of these rocks would be 3.4 kilometers (2.1 miles) -- hardly
a close call. At that distance, it would be difficult even to detect the
rock, as it would be a mere 1.5 centimeters (less than 5/8 of an inch)
in diameter; this corresponds to an asteroid less than 5 kilometers
(under 3 miles) across. Even looking out to 20 kilometers (12 miles)
during our trek, the largest object we would pass would be just 3.4
centimeters (1.3 inches), representing a 10-kilometer (6-mile) asteroid
Dawn will miss by 15 times the distance between Earth and the moon.

Dawn is bound for the giants of the asteroid belt. Vesta's equatorial
diameter is about 580 kilometers (360 miles), and Ceres is 975 km (605
miles) across. (Remember that when thinking about three-dimensional
worlds such as these, the diameter may fail to illustrate how large they
really are.) Together these two
behemoths contain more than a third of all the mass in the main asteroid
belt. On the scale of our cross-country drive, Vesta would be 2.0 meters
(6.5 feet) wide and Ceres would be 3.3 meters (11 feet). Rather than
missing them by great distances, we would move to within 0.6 meters (2
feet) of the first target and 2.4 meters (8 feet) of the second.

Dawn's science instruments are optimized for studying these immense
bodies in detail from orbit around them, just as many Earth-observation
spacecraft peer down constantly on our planet. Diverting the probe to
zip past a chunk of rock for a very brief view would be possible, but
doing so would take precious time away from the far richer and more
valuable investigations planned for Vesta. That is where Dawn will find
the rewards of the next 20 months of travel.

While astronomers observe members of the asteroid belt as small as about
a kilometer (a mile), what about still smaller rocks that are large
enough to damage the spacecraft? Because available telescopes generally
are not powerful enough to detect such objects from Earth, mathematical
models are used to predict their prevalence and thus Dawn's likelihood
of encountering them. Although far more abundant than the larger
asteroids, there still are too few pebbles distributed over the enormous
volume of space through which the ship sails to pose a serious threat.

The spacecraft was designed so that the tiniest particles, which are
sufficiently plentiful that some likely will strike it, cannot inflict
significant damage. Dawn's largest area is in its solar arrays, and
asteroidal dust cracking a few of the 11,480 cells is inconsequential.
More sensitive components are covered with protective materials that
will cause the high-speed grains to break up and slow down before they
reach the vulnerable elements. There is good reason to believe Dawn's
travels in the asteroid belt will be safe.

Even as Dawn recedes from the Sun, Earth (moving faster in its tighter
solar orbit) is approaching the spacecraft; indeed, the distance has
been decreasing for more than a year (and
will continue to do so for another 2 months). On December 5, the craft
and the star will be equidistant from the planet. We saw instances of
these 3 members of the solar system family forming a triangle with 2
equal sides, known as an /isosceles/ triangle, on May 28, 2008 and again
on September 18 of this year. In those cases however, the equal sides
were those between Dawn and Earth and between Dawn and the Sun. Next
month, it will be Earth at the apex of the astronomical triangle, with
both the spacecraft and the Sun at a distance of 0.99 AU. The third leg
of the triangle, from Dawn to the Sun, will be 1.70 AU.

To illustrate the geometry, let's use one of the new clocks that have
just reached the shelves of the Dawn gift shop on your planet. (And note
that for any purchase through the end of 2009, we will donate a used
xenon ion to the charity of your choice.) With Earth at the center of
the clock face, if the Sun were at the 10, Dawn would be the same
distance but at the 2. (The clock hands are not important here; the
objective is to illustrate the relative lengths and the angles of the
isosceles triangle. Ignoring the hands also lets us offer the clock at a
very low price!)

Any readers who happen to reside on or be visiting Earth on December 5
may find this arrangement a convenient opportunity to contemplate
something of the nature of an interplanetary voyage. Dawn is quite
invisible even to the most powerful telescopes, but it will be at the
same distance as the most easily detectable extraterrestrial body, the
Sun. The spacecraft has been more remote (as have other pro
bes) and will be again later in the mission, but on that
day it will be just as far from Earth as the star that rules from the
center of the solar system. While the Sun has seemed - indeed, has
been - unreachably distant for the overwhelming majority of human history,
farther even than any horizon travelers could set their sights on, a
craft that we set sail upon the cosmic ocean will be exactly that far away.

To add more dimensions to our mental imagery of Dawn's location, we can
take advantage of another celestial reference on December 6, before the
triangular alignment of the previous day has changed noticeably. At
about 8:30 am PST, the spacecraft will appear just over 2 degrees (or a
little more than 4 times the moon's diameter) north of the moon. As the
moon's orbit carries it around Earth, it will be less than twice that
far from the apparent position of the spacecraft for the 6 hours before
and after that time, so anyone who can see the moon during that interval
can get a rough fix on Dawn's location. For readers in North America,
the alignment occurs when the moon is the western sky after dawn (yes!).
>From the vantage point of the center of the clock, observers may be able
to see both the Sun and the approximate location of the spacecraft at
the same distance, letting their imaginations take over where their eyes
leave off. Out there, in /that/ direction, as far as the Sun, will be
Dawn, patiently, reliably, silently continuing its bold voyage of
exploration.

Dawn is 1.03 AU (154 million kilometers or 96 million miles) from Earth,
or 395 times as far as the moon and 1.05 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light, take 17
minutes to make the round trip.
Received on Tue 01 Dec 2009 09:04:26 PM PST