[meteorite-list] Dawn Journal - July 18, 2011

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 18 Jul 2011 15:59:39 -0700 (PDT)
Message-ID: <201107182259.p6IMxd6K027119_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_07_18_11.asp

Dawn Journal
Dr. Marc Rayman
July 18, 2011

Dear Residawnts of Vesta,

Dawn has arrived!!

After covering 2.8 billion kilometers (1.7 billion miles) on its own,
after traveling for nearly four years through the lonely emptiness of
interplanetary space, after being bound by the gravity only of the sun,
Dawn is finally in orbit around Vesta. To get here, it gently propelled
itself with its ion propulsion system for 70% of its journey, or more than
2.6 years. Deep in the asteroid belt, far from its planet of origin, well
beyond Mars (which it visited ever so briefly more than two years ago),
where no spacecraft has ever been before, Dawn now resides with a giant.

While more detailed navigational analyses will be required to determine
the exact time, around 10:00 pm PDT on July 15, as the spacecraft performed
its familiar routine of ion thrusting, its orbit around the sun finally was
so close to that of Vesta that the protoplanet's gravity could take hold of
it. Dawn was only about 16,000 kilometers (9,900 miles) above the ancient,
scarred surface of the alien world. Traveling together around the sun at more
than 20.5 kilometers per second (46,000 mph), their orbits were so
similar that the cosmic craft was closing in at the leisurely speed of
only 27 meters per second (60 mph). The last time it approached a nearby
destination so slowly was in April 2007. At that time, it used more
conventional propulsion technology: it rode on a truck from Washington, DC
to Cape Canaveral, Florida.

That may be too many numbers for some readers (and too few for our good
friends the Numerivores). But it all reduces to one cool fact: humankind has
succeeded in delivering an interplanetary spaceship to orbit around one of the
largest objects in the main asteroid belt between Mars and Jupiter. Indeed,
Dawn is the first spacecraft to orbit any object in the main belt.

The probe slipped gently into orbit with the same grace it has displayed
during its nearly 1000 days of ion thrusting through the solar system.
Although the unusual nature of the spiral capture has been explained in
detail before, there is one important difference (in addition to some minor ones)
from previous descriptions: now it is history.

Dawn has orbited two other bodies. Shortly after it left Cape Canaveral
atop a fiery rocket, the spacecraft spent about 45 minutes in Earth orbit, waiting
for the proper orbital alignment to begin its ambitious deep-space voyage. Once
the rocket gave it enough energy to leave the planet behind, Dawn orbited the sun
as surely as Earth and the other planets do, although, of course, it spent most of
its time reshaping that orbit. Now it is orbiting Vesta, as surely as
the moon orbits Earth.

Entering orbit around the protoplanet is essential to Dawn's plans for
comprehensive studies of this exotic world, but simply being in orbit is
not adequate. The craft did not miss a beat as it flew into Vesta's
grasp; it is spiraling around its new master as it aims for its first
science orbit at an altitude of 2700 kilometers (1700 miles). The
intensive scrutiny of Vesta from survey orbit will begin in the second week of
August.

It's a noteworthy coincidence that Earth and Vesta will happen to be very well
aligned then. As they follow their independent orbits around the sun, occasionally
their paths bring them relatively near to each other. So just as Dawn begins
looking closely at Vesta, so too can residents of Earth. The protoplanet is the
brightest object in the asteroid belt, and the only one ever visible to unaided
terrestrial eyes, although binoculars or a telescope make it much easier
to spot, especially under skies that are brightened by the lights of
cities.

Even when their separation is at its minimum, Earth and Vesta will come
only to within about 1.23 AU (184 million kilometers or 114 million miles) of
each other. While their closest approach is late at night on July 31, the geometry
changes slowly enough that there are good viewing opportunities well before and after.
Go here <http://dawn.jpl.nasa.gov/news/observing_vesta.asp> for guidance on how to
find Vesta in the constellation Capricornus. And if you are fortunate enough to
glimpse that distant point of light, let your imagination add to the scene the
recent immigrant from Earth, representing you and the rest of humankind
on its mission of exploration. There, far from its erstwhile home and
the beings who urge it on, this ambitious adventurer is translating that
dot of light among the myriad stars into an exciting and fascinating
account of the dawn of the solar system.

Dawn has spent most of its time since the last log thrusting as usual. The thrusting
even at the time it was captured by Vesta's gravity was no different. We have seen
before that, in stark contrast to the tension when other missions enter orbit, with
ion propulsion, the process is very calm indeed. For that matter, since May 2010,
Dawn has thrust with its radio transmitter turned off, devoting that precious power to
accelerating xenon ions rather than generating radio waves. The ship continued in
silence when it went into orbit on Friday night. Mission control was empty, there
being no need to monitor the probe's operation. In fact, your
correspondent was dancing, confident that the pas de deux being
performed 188 million kilometers (117 million miles) away would be
executed with graceful beauty and flawless precision.

Confirmation that Dawn was in orbit came shortly before 11:30 pm PDT on
July 16 (more than 24 hours after it glided into orbit) when its radio
signals were received at the Deep Space Network.
Following its preprogrammed sequence of instructions, the spacecraft
acquired more images of Vesta earlier in the evening and then initiated
communications with Earth right on schedule. Observing that it was in
good health and continuing to perform all of its functions demonstrated
that it had achieved orbit. The choreography was beautiful!

Reliable as Dawn is, it did experience an unexpected interruption in
thrust recently. On June 27, a cosmic ray, a high energy subatomic
particle traveling through space, apparently managed to strike an
electrical component on the spacecraft in an especially effective way.
The component is used by the ion propulsion system computer controller
to operate valves in the complex
plumbing that transports xenon from the main tank to the operating
thruster. The propellant needs to be delivered at just the right rate
for optimal performance. When the cosmic ray deposited its energy in
that device, it deprived the circuit of the ability to send signals to
the valves, even when directed to do so by the computer. (A cosmic ray
is the most likely culprit, but other explanations for the circuit's
inaction are still being considered.) As a result, when it was time to
open valves to feed a little more xenon into the thruster, the
controller was unable to comply. The computer detected the problem,
followed the appropriate procedure for terminating thrust, and alerted
the main spacecraft computer. That computer correctly responded by
canceling other planned activities and commanding the ship into one of
its safe modes. In this case, because all other systems were healthy, it
was not necessary to invoke the normal safe mode. Rather, the robot
properly chose to make fewer reconfigurations. It pointed its main
antenna to Earth and transmitted its status, awaiting a response from
controllers.

The Deep Space Network began a routine communications session early on
June 28, and the Dawn team immediately understood the spacecraft
condition. Before the end of the day, they had restored it to its normal
flight mode and made preparations to activate the other controller.

Dawn had been using controller #1 and ion thruster #3 since December.
With the controller unable to operate valves, engineers instructed the ship
to switch to controller #2, which was in command for most of the thrusting in
2010. Its ability to operate the valves was
not compromised. That unit can be used with thruster #2 and #3, but it
was faster to formulate commands to use thruster #2, so in the interest
of time, that was the choice.

Later this summer, engineers will conduct tests with controller #1 to
assess its health and determine whether its valve signals can be
restored. That controller operates thruster #1 and #3. Mission planners
had long ago decided not to use venerable #1 for the rest of the
mission, as it requires slightly more power than its siblings, so
whether controller #1 will be fully functional or not, Dawn's
extraterrestrial expedition can be completed as planned with controller #2.

Once the spacecraft had deviated from its intended flight plan by not
thrusting, navigators had to devise a new plan to fly to Vesta. To
ensure there would be enough time to make up for the lost thrust, they
removed one of the navigation imaging
sessions (and the communications period that followed it) from the
schedule and another routine communications session. Of course, as
experienced interplanetary explorers, Dawn's mission team had always
recognized that glitches could interfere with any activity, so more
imaging and more communications had been planned than truly were
required. Doing without a few to allow time for some compensatory
thrusting was easily accommodated.

In order to resume thrusting quickly, controllers chose not to optimize
the plan but rather simply to devise a plan that was adequate. The
consequence was that they ended up giving Dawn /more/ time to thrust
than it really even needed. The entire episode beginning with the balky
controller cost 1.2 days of thrust, and the revised plan added 1.8 days
of thrust at other times. As a result, the insertion into orbit shifted
15 hours earlier. Such flexibility is another of the many differences
between missions that use ion propulsion and those that use conventional
propulsion.

Before restarting its powered flight, however, the team was eager to
allow Dawn to conduct its first planned observation of Vesta throughout
one full rotation of the protoplanet on its axis, a Vestian day of 5
hours 20 minutes. (This and other activities during the approach phase
were described last year.) Thanks to the fleet and flawless work of the
team, that was carried out on schedule on June 29-30, and all the planned
images were acquired. The visible and infrared mapping spectrometer (VIR)
also peered at Vesta to provide additional information for use in setting
instrument parameters for the science observations in survey orbit.
After it acquired two excellent sets of data, its internal computer
detected an unexpected condition, so it did not complete the rest of its
activities. As the camera's images were beaming back to Earth on June
30, engineers verified that VIR was in good condition, and they will
study its telemetry further as they continue to plan for its important
measurements of the minerals that compose Vesta's surface.

In the original itinerary, ion thrusting would recommence after the
communications session on June 30. And that is exactly what occurred,
even with the unplanned thrusting hiatus in the preceding days. Dawn
continued closing in on Vesta with the gentle pressure of thruster #2,
just as it still is today.

As a reminder, an easy way you can have the same otherworldly view of Vesta
as Dawn is to visit here <http://dawn.jpl.nasa.gov//multimedia/vesta_dawn_gallery.asp>.
These logs generally will not provide interpretations of the rich bounty of images
(but they are fantastic, aren't they?) or other fascinating measurements. As the
data are assessed by Dawn's team of planetary scientists from four countries,
news of the results will be distributed by NASA's and JPL's news
organizations. And for more frequent updates on the progress of the
mission than are provided in these logs, readers may want to go here
<http://dawn.jpl.nasa.gov/mission/status.asp>, where your correspondent abandons
his idiolect to provide extremely brief reports much more often (with much less,
ahem, color).

On July 9-10, the spacecraft's agenda included another pause in
thrusting. This time, in addition to acquiring its second set of images
while Vesta completed a full rotation, Dawn photographed the space
around Vesta in search of moons. Remote observations with the Hubble
Space Telescope and other observatories on Earth had not found any, but
that did not rule out their presence. As no moons had been detected yet,
however, they would have to be small and therefore faint. In order to
try to discover whether there might be any, the camera used different
exposures, some as long as 4.5 minutes. (For photographers, the
effective shutter speed for the pictures of Vesta that reveal its
surface features is 1/125 of a second.) The spacecraft pointed its
camera around Vesta and acquired 72 images. Three hours later, it imaged
the same locations, and then another nine hours after that, it repeated
the sequence once again. The pictures are being scrutinized for points
of light that shift position from one set of images to another,
betraying the orbital motion of natural satellites of Vesta.

Although those results are not yet available, we now know with certainty
that Vesta does have a moon. Its name is Dawn!

Dawn is 11,000 kilometers (6,800 miles) from Vesta, closer than many
terrestrial satellites are to Earth. It is also 1.25 AU (187 million
kilometers or 116 million miles) from Earth, or 470 times as far as the
moon and 1.23 times as far as the sun today. Radio signals, traveling at
the universal limit of the speed of light, take 21 minutes to make the
round trip.
Received on Mon 18 Jul 2011 06:59:39 PM PDT