[meteorite-list] Dawn Journal - August 11, 2011
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 11 Aug 2011 16:46:34 -0700 (PDT)
Dr. Marc Rayman
August 11, 2011
Dawn is now beginning intensive observations of the alien world it
orbits. The approach phase, which began on May 3, is complete. Today Dawn
is in its survey orbit around Vesta.
Following the previous log, the spacecraft continued using its ion
propulsion system to spiral around Vesta, gradually descending to its present
altitude of 2700 kilometers (1700 miles). Its flight plan included more
observations of Vesta, each one producing incredible views more exciting
than the last. Every image revealed new and exotic landscapes. Vesta is
unlike any other place humankind's robotic ambassadors have visited. To
continue to share in the thrill of discovery, remember to visit here
<http://dawn.jpl.nasa.gov/multimedia/vesta_dawn_gallery.asp> to see a new
image every day during survey orbit. Your correspondent, writing with
atypical brevity, also will continue to provide progress reports here
http://dawn.jpl.nasa.gov/mission/status.asp at least once a week.
As the ship sailed ever closer to the massive protoplanet during the
approach phase, the gravitational attraction grew stronger. We saw in
previous logs that astronomers had
estimated Vesta's mass by observing the effect of the 530-kilometer
(330-mile) diameter behemoth on distant bodies, including smaller
residents of the asteroid belt and even Mars. Now that navigators can
detect its pull on nearby Dawn, they are improving that value. Before
the explorer's arrival, Vesta's mass was calculated to be about 262
billion billion kilograms (289 million billion tons). Now it is measured
to be about 259 billion billion kilograms (286 million billion tons),
well within the previous margin of error. It is impressive how
accurately astronomers had been able to determine the heft of what had
appeared as little more than a point of light among the myriad stars.
Nevertheless, even this small change of 1.2 percent is important for
planning the rest of Dawn's mission.
With this superior knowledge of the strength of Vesta's gravity,
navigators now estimate that the giant asteroid took Dawn tenderly in
its hold at 9:48 p.m. PDT on July 15. They will spend the next year
>From Vesta's point of view, its visitor approached from the south.
Shortly after the previous log, Dawn passed over the south pole and then
arced north as its orbit carried it over the unilluminated side of
Vesta. It continued its graceful, spiraling descent until July 22 when
it was about 5200 kilometers (3200 miles) above the rocky surface and
coming within view of the day side in the northern hemisphere. As it
curved south and flew over Vesta's illuminated face, it stopped
thrusting to conduct its most extensive set of observations yet.
At that altitude, it would take a full week for the spacecraft to
complete one orbit, so the pace was not rushed. During four sessions
over the next three days, Dawn viewed the world beneath it, turning
after each one to beam its findings back to Earth.
The first occurred in the northern hemisphere, which was of particular
interest because the preceding pictures were centered deep in the
southern hemisphere. In the original plan for this time late in
approach, Dawn would observe Vesta three times during its passage over
the bright side. The second of the sessions would be devoted to
collecting images throughout a complete Vestan day of 5 hours, 20
minutes, just as the probe had done twice before during the approach
phase from greater distances. Because watching Vesta for an entire
rotation was so important, and recognizing the many challenges in
acquiring such data, mission planners decided to schedule a backup
observation, bringing the total to four during this coast period on the
way to survey orbit. The first rotation observation was performed with
Dawn near Vesta's equator. The backup occurred 15 hours later, by which
time the spacecraft had moved to latitudes about 30 degrees farther
south. That was followed more than 30 hours later on July 25 by
additional photography over the far southern regions.
All planned observations were completed successfully, providing
magnificent views of the complex surface and showing once again that
further exploration promises many more exciting rewards. The rotation
observations included not only the standard black and white images but
also the use of all of the science camera's color filters, providing
tantalizing hints of how varied the surface is. In addition, the visible
and infrared mapping spectrometer (VIR) collected a rich set of spectra
throughout both Vestan days.
Spectacular as the pictures and spectra are, they are not the objective
of the mission; still better views are desired and will be obtained
soon. With that in mind, thrusting resumed on July 28.
Since June 30, Dawn has been propelling itself with ion thruster #2. On
June 27, while operating with thruster #3, an ion propulsion system
electronics unit that controls the valves used to regulate the flow of
xenon to the thruster stopped working.
The operations team responded with swift professionalism to keep the
mission on course and schedule. In addition to reconfiguring the
spacecraft from its safe mode, swapping to another control unit and
thruster #2, and devising a new flight plan, the team conducted a
thorough analysis of the circuit that misbehaved. The best explanation
for its inability to send electrical signals to the valves, as well as
for all the other detailed telemetry the spacecraft provided to
engineers, was that a component in the circuit had been struck by a
cosmic ray, a high energy particle of space radiation. Experts also
concluded that the circuit would operate correctly again once the unit
was powered off and powered back on.
After careful consideration of whether there might be any risk to the
electronics or to other systems onboard in reactivating the controller,
the team devised a plan to turn it on and test it. On July 20, when Dawn
was conducting a routine communications session over the night side of
Vesta, they transmitted the instructions to the spacecraft and were
rewarded with a fully functional control unit that operated as well as
ever. Although the mission certainly could have been completed without
restoring this device, having it available again provides greater
robustness and makes flying the spacecraft easier. When ion thrusting
resumes on August 31 to begin spiraling from survey orbit to the next
science orbit, this controller will be used again with thruster #3.
At 12:07 a.m. PDT on August 2, Dawn reached its targeted orbit. Although
entering orbit around Vesta was an extremely
important milestone for the mission, reaching survey orbit is of even
greater significance. It is here that Dawn will focus on doing what it
was conceived for: exploring an ancient protoplanet to provide new
insight into the dawn of the solar system and its present nature.
To get here from Earth, the spacecraft traveled more than 2.8 billion
kilometers (1.7 billion miles) in nearly four years. It accumulated two
years, eight months of ion thrusting, during which it expended 252
kilograms (556 pounds) of its xenon propellant. That enabled the
spacecraft to achieve the equivalent of 6.8 kilometers per second
(15,200 mph) after leaving its Delta rocket
behind. This is almost two thirds more than any other spacecraft has
achieved under its own power.
Orbiting Vesta, which Dawn has now accomplished, would have been
unaffordable within NASA's Discovery Program with conventional
propulsion, and a mission to both Vesta and Ceres would be impossible.
The exquisitely gentle touch of the ion thruster that allowed the patient
spacecraft to reshape its orbit around the sun, and then around Vesta, was
as silent as space itself, but if we imagined that it made a sound, it would be
the faintest of whispers, the softest of sighs. Yet it tells us the
secret of making an interplanetary spaceship that can travel to and
explore distant, alien worlds, carrying with it the dreams of those on
Earth who long to know the cosmos.
Even after the spacecraft arrived in the planned orbit, mission
controllers continued preparing for the survey orbit phase. Navigators
tracked the probe as it looped slowly around Vesta, refining their
measurements of the orbit for use in timing the observation sequences
and telling the spacecraft exactly where to aim its sensors. With the
latest approach results in hand, engineers and scientists updated
parameters for the camera and VIR, essentially tuning the instruments to
ensure they will perform at their best. For example, a few areas of
Vesta were found to be somewhat brighter than anticipated, so the
effective shutter speed of the camera needed to be reduced slightly.
While controllers were putting the finishing touches on the survey
sequences, Dawn acquired its final set of approach phase observations
with the camera and VIR on August 5 - 6, returning the best images and
spectra yet. On August 8 the team set the spacecraft's main clock to the
correct time. Reflecting the more casual nature of operations during
interplanetary cruise and approach, they had allowed the clock to drift
off by 0.37 seconds, but such insouciance is no longer appropriate.
The detailed surveying of Vesta was designed to begin when Dawn passed
over the boundary between night and day in the northern hemisphere,
known as the terminator. Following the completion of all the exacting
work by the mission control team, the sequences were transmitted to the
spacecraft, where they waited until the probe was correctly positioned
at 9:13 a.m. PDT on August 11 to delve into its work.
Each survey orbit lasts about 69 hours, or nearly three days. Dawn will
spend most of the time over the lit half aiming its instruments at the
surface to acquire data. While over the dark half, it will point its
main antenna to Earth to radio its findings back to the Deep Space
Network. When the plan was described last year, the survey orbit phase
lasted for six revolutions, which included acquiring much more data than
needed. As with the rotation observations, mission planners had scheduled backups
so that even if some are not obtained, it is likely there will be enough
to avoid complex and rapid replanning in the event of glitches. The
principal change since the previous description is that they have added
one orbit, ensuring an even more resilient plan for this vitally
important phase of the mission.
Dawn is beginning to fulfill its destiny in service of humankind's
collective passion for knowledge. With the amazing images and other data
returned so far, the Dawn project is gratified to say, "Earth, meet
Vesta!" Now we are about to really get to know our new acquaintance.
Dawn is 2700 kilometers (1700 miles) from Vesta. It is also 1.24 AU (186
million kilometers or 115 million miles) from Earth, or 480 times as far
as the moon and 1.22 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 Thu 11 Aug 2011 07:46:34 PM PDT