[meteorite-list] Dawn Spacecraft Creeping Up on Vesta

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 9 Mar 2011 16:02:13 -0800 (PST)
Message-ID: <201103100002.p2A02DWN014335_at_zagami.jpl.nasa.gov>

http://blogs.jpl.nasa.gov/2011/03/dawn-spacecraft-creeping-up-on-vesta/

Dawn Spacecraft Creeping Up on Vesta
By Marc Rayman
March 9, 2011

NASA's Dawn spacecraft is a mere four months away from getting into
orbit around its first target, the giant asteroid Vesta. Each month,
Marc Rayman, Dawn's chief engineer, shares an update on the mission's
progress.

Dear Pleasant Dawnversions,

Deep in the asteroid belt, Dawn continues thrusting with its ion
propulsion system. The spacecraft is making excellent progress in
reshaping its orbit around the sun to match that of its destination, the
unexplored world Vesta, with arrival now less than five months away.

We have considered before the extraordinary differences between Dawn's
method of entering orbit and that of planetary missions employing
conventional propulsion. This explorer will creep up on Vesta, gradually
spiraling closer and closer. Because the probe and its target already are
following such similar routes around the sun, Dawn is now approaching Vesta
relatively slowly compared to most solar system velocities. The benefit of
the more than two years of gentle ion thrusting the spacecraft has completed
so far is that now it is closing in at only 0.7 kilometers per second
(1600 mph). Each day of powered flight causes that speed to decrease by
about 7 meters per second (16 mph) as their orbital paths become still more
similar. Of course, both are hurtling around the sun much faster,
traveling at more than 21 kilometers per second (47,000 mph), but for
Dawn to achieve orbit around Vesta, what matters is their relative velocity.

It may be tempting to think of that difference from other missions as
somehow being a result of the destination being different, but that is
not the case. The spiral course Dawn will take is a direct consequence
of its method of propelling itself. If this spacecraft were entering
orbit around any other planetary body, it would follow the same kind of
flight plan. This unfamiliar kind of trajectory ensues from the long
periods of thrusting (enabled by the uniquely high fuel efficiency of
the ion propulsion system) with an extremely gentle force.

Designing the spiral trajectories is a complex and sophisticated
process. It is not sufficient simply to turn the thrust on and expect to
arrive at the desired destination, any more than it is sufficient to
press the accelerator pedal on your car and expect to reach your goal.
You have to steer carefully (and if you don't, please don't drive near
me), and so does Dawn. As the ship revolves around Vesta in the giant
asteroid???s gravitational grip, it has to change the pointing of the
xenon beam constantly to stay on precisely the desired winding route to
the intended science orbits.

Dawn will scrutinize Vesta from three different orbits, known somewhat
inconveniently as survey orbit
<http://dawn.jpl.nasa.gov/mission/journal_05_27_10.asp>, high altitude
mapping orbit (HAMO)
<http://dawn.jpl.nasa.gov/mission/journal_09_27_10.asp>, and low
altitude mapping orbit (LAMO)
<http://dawn.jpl.nasa.gov/mission/journal_12_30_10.asp>. Upon concluding
its measurements in each phase, it will resume operating its ion
propulsion system, using the mission control team's instructions for
pointing its thruster to fly along the planned spiral to the next orbit.

Those who have navigated around the solar system, as well as others who
have contemplated the nature of orbits without having practical
experience, recognize that the lower the orbital altitude, the faster
the orbital motion. This important principle is a consequence of
gravity???s strength increasing as the distance between the massive body
and the orbiting object decreases. The speed has to increase to balance
the stronger gravitational pull. (For a reminder of some of the details,
be sure to go here
<http://dawn.jpl.nasa.gov/mission/journal_8_24_08.asp> before you go out
for your next orbital expedition.)

Dawn's winding orbital path obeys the same rules. The lower the orbit,
the faster and tighter the spirals, because the orbital velocity is
greater. The first few coiled routes around Vesta this summer will be
long and slow <http://dawn.jpl.nasa.gov/mission/journal_04_28_10.asp>,
taking days to complete. When it is at the lowest altitude, where each
orbit takes only four hours, the spirals will be that much faster, so
the craft will have to steer with greater agility to synchronize its ion
thrusting with its rapidly changing location.

In its interplanetary travels, spiraling outward from the sun to reach
Vesta, each loop takes years to complete, so Dawn has not yet had to
steer through any tight turns. The direction it aims the operating
thruster hardly moves at all during a full week of thrusting.

Pointing a thruster in the direction needed to spiral around Vesta
requires turning the entire spacecraft. Each thruster is mounted on its
own gimbal with only a very limited range of motion. In normal
operation, the gimbal is aimed so that the line of thrust goes through
the center of the ship. When the gimbal is swiveled to a different
direction, the gentle force of the thruster causes the ship to rotate
slowly. This is similar to the use of an outboard motor on a boat. When
it is aligned with the centerline of the boat, the craft travels
straight ahead. When the motor is turned, it continues to propel the
boat but it also turns it. (A jet, in contrast, does not alter the
direction of its thrust to turn but rather uses other means.) In
essence, Dawn's steering of its thrust is accomplished in large part by
pivoting the thruster itself.

A crucial difference between the boat and our interplanetary ship is
that with the former, the farther the engine is turned, the tighter the
curving course. For our craft, the gimballing of the thruster needs to
be carefully coordinated with the orbital motion, as if the motorboat
operator needed to compensate for a constantly curving current. This has
important implications at Vesta. Sophisticated as it is, Dawn knows
where it is in orbit only by virtue of information mission controllers
install onboard to predict where it will be at any time. That is based
on their best computations of Vesta's gravity, the planned operation of
the ion propulsion system, and many other considerations, but it will
never be perfectly accurate. Let's take a look at some of the reasons.

Vesta is a member of the elite family of rocky, terrestrial planets
that live in the inner solar system. Just as its kin, Mercury, Venus,
Earth, the moon, and Mars, have complex gravity fields, it is likely
Vesta does as well. The distribution of materials of different densities
within the interior creates variations in the strength of the gravitational
force, so Dawn will feel a slightly changing tug from Vesta as it travels
in orbit. Our ship will be traversing unknown, choppy waters.

In December we saw that by sensing the irregularities in the gravity
field <http://dawn.jpl.nasa.gov/mission/journal_12_30_10.asp>, Dawn will
reveal the nature of Vesta's internal structure. Until those detailed
measurements have been made and accounted for in the design of the
flight plan, however, the subtle effects of the gravity field will cause
deviations from the planned trajectory. Therefore, as the spacecraft
travels from one science orbit to another, it will thrust for a few days
and then stop to allow navigators to get a new fix on its position. As
it points its main antenna to Earth, the Doppler shift of its radio
signal will reveal its speed, and the time for radio signals (traveling,
as all readers know so well, at the universal limit of the speed of
light) to make the round trip will yield its distance. Combining those
results with other data, mission controllers will update the plan for
where to point the thruster at each instant during the next phase of the
spiral travel, check it, double check it, and transmit it to the distant
explorer which will put it into action. This intensive process will be
repeated every few days as Dawn maneuvers between science orbits.

The as-yet uncharacterized details of the gravity field are not the only
reason the flight plan will require regular adjustments. As the ion
propulsion system will be changing the orbit, even tiny deviations from
the calculated thrust eventually will build up to have a significant
effect. This is no different from any realistic electrical or mechanical
system, which is sure to have imperfections. If you planned a trip in
which you will drive 100 kilometers (62 miles) at 100 kilometers per
hour (62 mph), you could expect you would arrive in exactly 60 minutes.
But even if you maintained the speedometer as close to 100 as possible,
it would not be accurate enough to indicate the true speed. If the
actual speed averaged 101 kilometers per hour (63 mph), you would arrive
about 36 seconds early. Perhaps that difference wouldn't matter to you
(and if it did, you might consider replacing your car with a spaceship),
but such tiny errors, when compounded by Dawn's repeated spirals around
Vesta, will make a difference in achieving its carefully chosen orbit.

Still other phenomena contribute to minor differences between the flight
plan controllers send to the spacecraft and what actually occurs. Two of
these, the slight force of sunlight on the probe and the larger
perturbation from the occasional firing of the small jets to reduce the
spin rate of the reaction wheels, were explained in some detail in a
previous log <http://dawn.jpl.nasa.gov/mission/journal_1_27_09.asp>, and
both will play a role at Vesta.

The mission control team has devised strategies to accommodate all these
tiny contributors (and others) to deviations from the plan. An
additional component of preparing for the intricacies of Vesta
operations is establishing how accurately Dawn can perform the team's
masterful choreography. It has repeatedly proved that it can execute the
slowly changing profile of interplanetary cruise. For the more
challenging case of orbiting its protoplanetary destinations, engineers
have developed mathematical models and conducted studies with the
spacecraft simulator at JPL, but to verify that the results are valid, a
test this month of the ship's ability to steer through some maneuvers
was deemed worthwhile. Just as with the activities it practiced in
January <http://dawn.jpl.nasa.gov/mission/journal_01_30_11.asp>, the
robotic explorer performed very well indeed on its latest demonstration.

Although it is not in orbit around Vesta now, operators commanded Dawn
to aim its thruster as it will near the end of the transfer from HAMO to
LAMO <http://dawn.jpl.nasa.gov/mission/journal_12_30_10.asp>. For the
equivalent of one and a half spiral revolutions (the duration being
adequate to assess all the pertinent aspects of the maneuvering), the
spacecraft rotated using its thruster, changing the direction of its ion
beam in much the same way it will when it is lowering its orbit. Because
Dawn will be farther from the sun at that time in the mission than it is
now, this trial run used a lower ion throttle level (and hence lower
thrust), reflecting the reduced solar power that will be available.

The results confirmed that the spacecraft's operation matches the
mathematical predictions and that the ongoing preparations for these
elaborate flight profiles are sound. Although more work remains, the
success of this test is a valuable step in becoming ready for reaching
the intended orbits around Vesta.

When Dawn has completed its work in LAMO, it will reverse its spirals
and begin climbing away from the world it has been studying. It has an
appointment with dwarf planet Ceres, so it cannot linger at Vesta
indefinitely. Nevertheless, the itinerary allows for the traveler to
stop for three weeks at an altitude of about 660 kilometers (410 miles).
At the same height as HAMO, this orbit is innovatively named HAMO2.
Although there are some differences in the orbital geometry, the
principal distinction between HAMO and HAMO2 is that they are separated
by about eight months, during which Vesta (with Dawn in tow) will have
progressed in its orbit around the sun. As we noted during Earth's most
recent northern hemisphere autumn
<http://dawn.jpl.nasa.gov/mission/journal_09_27_10.asp>, Vesta has
seasons, and the changing angle of the sunlight on the surface of that
alien world during Dawn's residence there affects its appearance and how
much of it is visible to some of the science instruments. Because more
of the northern hemisphere will be illuminated, HAMO2 affords the
opportunity to see previously hidden landscapes and to gain a new
perspective on some terrain observed earlier.

>From Dawn's perspective now, its destination already glows bright. By
the middle of March, Vesta will easily outshine all the objects in the
adventurer's sky save the sun. If you take a moment to enjoy the view of
Jupiter low in your western evening sky, it will appear about as bright
these days as Vesta would for Dawn. But even as Jupiter sinks toward the
sun and becomes more difficult for terrestrial observers to see, the
point of light in our remote probe's sky grows ever more luminous as
their separation shrinks.

For those on Earth who want their own perspective on the locations of
Dawn and Vesta, the changing solar system alignments will help. This
summer, just as our planet's robotic emissary is getting settled in
orbit and beginning its survey, Vesta will be easily visible with
binoculars and may even be detected by keen, unaided eyes under dark
skies, although its visitor from Earth will be quite imperceptible.

In a few weeks, when the moon will be visible at dawn and for much of
the first half of the day (regardless of your time zone), you can use it
as a guide to the approximate location of Dawn and Vesta. Between about
2:00 PM PDT on March 27 and 10:00 AM PDT on March 28 (the day before the
204th anniversary of Vesta's discovery), both distant inhabitants of the
asteroid belt will be less than 5 degrees from the moon. (For reference,
5 degrees is 10 times the diameter of the moon or about the width of
three fingers held together at arm's length.) You won't need binoculars
or a telescope to see them; you need only your imagination to reveal a
distant ship, far from the port from which it set sail more than three
years ago. In the silent depths of space, with a faint blue-green trail
of xenon ions behind it, the craft will be closing steadily on a
mysterious, ancient world that soon will reveal exciting and fascinating
new vistas as it bears witness to the very dawn of the solar system.

Dawn is 4.2 million kilometers (2.6 million miles) from Vesta, or 11
times the average distance between Earth and the moon. It is also 2.56
AU (383 million kilometers or 238 million miles) from Earth, or 970
times as far as the moon and 2.59 times as far as the sun. Radio
signals, traveling at the universal limit of the speed of light, take 43
minutes to make the round trip.
Received on Wed 09 Mar 2011 07:02:13 PM PST