[meteorite-list] Dawn Journal - May 27, 2011

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_05_27_11.asp

Dawn Journal
Dr. Marc Rayman
May 27, 2011

Dear Dependawnble Readers,

Dawn remains healthy and on course as it continues to approach Vesta. 
Thrusting with its ion propulsion system, as it has for most of its 
interplanetary journey so far, the spacecraft is gradually matching 
its solar orbit to that of the protoplanet just ahead.

As these two residents of the asteroid belt, one very new and one quite 
ancient, travel around the sun, they draw ever closer. Vesta follows its 
own familiar path, repeating it over and over, just as Earth and many other 
solar system bodies do. Dawn has been taking a spiral route, climbing away
from the sun atop a blue-green pillar of xenon ions. With an accumulated
total in excess of two and a half years of ion thrusting, providing an
effective change in velocity of more than 6.5 kilometers per second 
(14,500 mph), the probe is close to the end of the first leg of its 
interplanetary trek. On July 16 Vesta's gravity will capture the ship as it 
smoothly transitions from spiraling around the sun to spiraling around Vesta, 
aiming for survey orbit in August. For several reasons, the date for the 
beginning of the intensive observations there has not yet been set exactly.

Astronomers have estimated Vesta's mass , principally by measuring how it 
occasionally perturbs the orbits of some of its neighbors in the asteroid belt 
and even the orbit of Mars, but this method yields only an approximate value. 
Because the mass is not well known, there is some uncertainty in the precise 
time that Dawn will become gravitationally bound to the colossal asteroid. As 
we have seen before, entry into orbit is quite unlike the highly suspenseful 
and stressful event of missions that rely on conventional chemical propulsion. 
Dawn simply will be thrusting, just as it has for 70 percent of its time in 
space. Orbit entry will be much like a typical day of quiet cruise. That Vesta
will take hold at some point will matter only to the many Dawnophiles
throughout the cosmos following the mission. The ship will continue to
sail along a gently curving arc to survey orbit.

The bending of Dawn's path will depend on exactly what Vesta's mass is,
so navigators will continue to refine the flight profile as they measure
the strength of the pull it exerts. As a result, the exact date of
arrival in survey orbit will not be known until the mass is determined
more accurately. Indeed, although the altitudes of survey orbit,
the high altitude mapping orbit, and the low altitude mapping orbit
have been presented in previous logs, mission planners may target somewhat 
different altitudes depending on what they discover Vesta's gravity to be.

There is another reason that the beginning of survey orbit cannot be
specified precisely. Ion propulsion tends to afford much greater
flexibility to missions than conventional propulsion does. One example
of that was evident in the original schedule for the mission. When the
first log (whose current price on the black market is reportedly well in 
excess of 2.4 percent of its original price) was written, the launch was 
planned for June 2007. Because of schedule changes, including those in the 
preparation of the rocket and the inflexibility of another deep-space mission 
(which did not use ion propulsion), Dawn's launch moved to September of that 
year.  This unique mission to Vesta and Ceres, ambitious
as it is, would have been possible with a launch on any day from 2005
through late 2007 because of the ion propulsion. A typical
interplanetary mission has a period of a few weeks in which it must
depart Earth.

In many cases, controllers use this flexibility to allow the dates of
key events to move based on details of the progress of the thrusting
and other subtleties of spaceflight. That choice permits Dawn to squeeze
still more out of the mission by, for example, spending a little extra
time at Vesta. So instead of pinning down the date of survey orbit
(assuming a particular value of Vesta's mass), they let it change as the
mission proceeds. This strategy also makes it easier to update the
flight profile as the spacecraft closes in on its destination.

Navigators steer the ship toward a survey orbit with certain geometrical
characteristics, such as its altitude (depending on Vesta's mass) and
the angle of the orbit relative to the sun. They do not choose a
specific entry point in the orbit when performing their trajectory
calculations; rather, they let the trajectory calculations determine the
arrival point. With each refinement, that location shifts slightly. The
survey orbit observations are designed to begin after the craft has
attained the correct orbit and then passes over the north pole,
traveling from the night side to the day side of Vesta. Regardless of
where the spacecraft arrives along the circle of the intended orbit,
controllers will program it to begin surveying at that moment. Because
one revolution will take Dawn almost three days, the freedom given to
the mathematics of the trajectory design computer programs to determine
where the probe will enter orbit can introduce a shift of that much in
the timing of survey orbit.

To picture this, let's take a look at one of the new clocks now
available in the Dawn gift shop on a planet near you. Imagine Vesta in
the middle of the clock face and survey orbit as the perimeter, where
the numbers are. Dawn spirals in from a greater distance and eventually
reaches that orbit, circling around as if on the tip of a clock hand.
But if the sun is far to the right of the 3, then the observations of
the protoplanet at the center begin when the spacecraft loops past the
12. Now if it enters the orbit at the 11, it only has a short distance
to go before it is ready to initiate the observations. If it enters at
the 1, we will have to wait for it to travel all the way around to the
12. So until we finalize where it enters survey orbit, we cannot
specify when it will undertake its activities there. (This complex
problem is a result of flying in from deep space to Vesta orbit. When
traveling from one orbit to another, such as from survey to the high
altitude mapping orbit, engineers will not let the mathematics establish
where in the orbit the spacecraft will arrive; rather, they will tell
the trajectory programs where they want it to arrive.)

Based on the current approach trajectory, survey orbit will begin some
time from August 8 to 11. The date and time will be established firmly
in July. The sequences of commands to operate in Vesta orbit were
designed last year with that in mind (just as they were for the approach
phase), so they can easily be adjusted once the exact initial time is known.

To help target the probe for survey orbit, controllers have commanded it
to observe Vesta once a week since the beginning of the approach phase
on May 3.  As we saw that day, the pictures allow navigators to gain a 
better fix on Dawn's trajectory relative to Vesta.  So far, the images 
reveal little more than the desired important information of where Vesta 
appears against the background of stars. And yet, in a sense they show 
much more. After its long and lonely voyage through the vast emptiness of 
interplanetary space, most of the time far from anything but bits of dust 
and the occasional insignificant rock, an alien world is finally coming into
view. Although too far now to do more than illuminate a handful of
pixels in the camera, the small disc of Vesta stands out as the
brightest and largest object visible to the explorer except the master
of the solar system, the sun. The pictures are visible proof of Dawn's
progress from an intriguing concept not so many years ago to a distant
spaceship about to orbit an uncharted protoplanet, the second most
massive body between Mars and Jupiter.

Dawn has traveled 2.7 billion kilometers (1.7 billion miles) since
leaving Earth. Today it is only 580 thousand kilometers (360 thousand
miles) from Vesta, just 1.5 times the distance between Earth and the
moon. (Note: If you are in doubt about these numbers, you may confirm
them by seeing the standard closing paragraph below.) Yet as the
spacecraft continues to thrust during the approach phase, making the
final adjustments to its solar orbit, it will travel more than 88
million kilometers (55 million miles) around the sun before Vesta
captures it in a month and a half.

Dawn's orbit is already so similar to Vesta's that today it is closing
in at only 240 meters per second (540 mph), not even as fast as many
commercial aircraft fly. That is unusually slow for the speeds typical
of interplanetary travel. Meanwhile the two of them rush around the sun
at nearly 21 kilometers per second (more than 46,000 mph). This is
similar to what you would experience if you tried to match velocities
with a car on the freeway (you probably would want to use a car yourself
for such a demonstration). The two cars may be traveling at high speed,
but their /relative/ speed could be quite modest.

Early in the evening of June 6, if you gaze at Earth's moon before it
sinks below the horizon, you might consider that the moon is about as
far from you as Dawn is from Vesta. On its way out into the solar system
after lifting off from Cape Canaveral, Dawn passed the orbit of the moon
in less than 29 hours. The tremendous push imparted by the Delta rocket to 
start the probe on its mission is quite unlike the gentle approach to Vesta. 
As you enjoy the sight of the moon, Dawn will have two months of flight ahead 
of it to cover that same distance to survey orbit, where it will begin reaping 
the rewards of its long journey.

Dawn is 580 thousand kilometers (360 thousand miles) from Vesta, or 1.5
times the average distance between Earth and the moon. (See?) It is also
1.64 AU (246 million kilometers or 153 million miles) from Earth, or 610
times as far as the moon and 1.62 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 28
minutes to make the round trip.