[meteorite-list] Dawn Journal - February 12, 2009

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 13 Feb 2009 17:11:30 -0800 (PST)
Message-ID: <200902140111.RAA01223_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_2_12_09.asp

Dawn Journal
Dr. Marc Rayman
February 12, 2009

Dear Charles Dawnwins,
 
Dawn continues to close in on Mars, ready for the gravitational
slingshot that will help it on its expedition to the asteroid belt
and its quest to gain insights into the evolution of the solar
system. The ship remains on course, with the latest calculations
of its trajectory being very close to those described in the last
log. The spacecraft will streak about 549 kilometers (341 miles)
above the planet at 4:28 p.m. PST on February 17.
 
While Dawn and Mars move in their separate orbits around the Sun,
they are approaching each other at the stately pace of 2.56
kilometers/second (5720 miles/hour). Gradually, the gravitational
pull of the planet will grow as the distance shrinks, and the
spacecraft's path will start to change, beginning the boost we
seek. By about noon PST on February 14, the tug from Mars will
have grown to be the same as the famously gentle thrust from Dawn's
ion propulsion system. When Dawn is closest to Mars, speeding past
it at 5.31 kilometers/second (11,900 miles/hour), the red planet
will be exerting 34,000 times greater force than the blue-green
xenon beam generates.
 
Just as a swing speeds up as it approaches the bottom of its arc
and slows down as it rises again, Dawn accelerates as it gains on
the planet and decelerates as it climbs away. Unlike a swing,
though (at least the kind your correspondent was allowed to play
on as a youngster on Earth), Dawn will not retrace its path; it
will not descend again after ascending from the vicinity of Mars.
It is in its own orbit around the Sun and will move too swiftly by
Mars for the planet to capture it into orbit.
 
To illustrate Dawn's arc, let's use the somewhat arbitrary speed
of 4 kilometers/second (9000 miles/hour) as a reference. Over the
coming days, Mars will attract Dawn, and the craft will have
accelerated to that speed about 34 minutes before its closest
point, still 5500 kilometers (3400 miles) away from the planet.
After dipping still lower and falling still faster while it
approaches Mars, the spacecraft will slow down as it departs. It
will have decelerated to the same speed of 4 kilometers/second at
34 minutes after its closest encounter. Within a few days, the
speed will have dropped to today's value of 2.56 kilometers/second
again. The arc of Dawn's acceleration toward Mars matches the arc
of its deceleration, displaying the same kind of symmetrical
motion as a swing.
 
Where then is the effect of the gravity assist? Because Dawn is in
orbit around the Sun, it is the distortion of that solar orbit
caused by Mars that provides the advantage to the mission. In the
previous log, we saw how the benefits
of the gravitational interaction with Mars could be described as
changes in the spacecraft speed. The 1.1 kilometers/second (2500
miles/hour) that represented the change in the shape of the orbit
means that Dawn's incoming speed and outgoing speed relative to
the Sun are not the same, even though, as we saw above, they are
the same relative to Mars. Before it reaches Mars, the probe will
be traveling around the Sun at less than 25.5 kilometers/second
(57,000 miles/hour). Thanks to the boost from Mars, the speed
after the encounter will be more than 26.6 kilometers/second
(59,500 miles/hour). The difference is precisely the effect
described in the previous log.
 
The reshaping of Dawn's orbit is different from the reorienting of
the plane of the orbit, also explained in the previous log.
The plane change from the
gravity assist, of vital importance to the mission, does not
manifest itself as a difference in the speed of the spacecraft
around the Sun; it is a difference in the direction of motion.
Nevertheless, the effect can be described as being equivalent to a
change in speed (actually, in velocity), as it was in the last
log. (For interested readers for whom these points are not already
evident, please click here to go to the log that clarifies it.
[Editor, I have not written such a log yet, although I will write
an explanation in the future. In the meantime, please insert an
acausal hyperlink for readers who do not want to wait.])
 
As Dawn plunges toward Mars, it will be coming in over the
northern hemisphere, reaching above 60? latitude. As Mars bends
the orbit, changing the direction the craft travels around the
Sun, Dawn will depart over the southern hemisphere. Leaving the
planet behind on its new path around the Sun, it will be above 60?
south latitude.
 
The focus of the Dawn team's work for the Mars encounter has been
to achieve a trajectory that would deliver the probe to the target
above Mars at the right time and traveling at the correct angle to
accomplish the needed gravitational boost. As long as the
spacecraft will be in the vicinity of such a familiar solar system
site, albeit briefly, the team decided to take advantage of the
occasion to calibrate the instruments that are designed to
elucidate the nature of Vesta and Ceres. As mentioned at the end
of the previous log, there is
little Dawn can learn about Mars that is new. Indeed, it is flying
higher and faster than spacecraft there right now. In addition,
its instruments are intended for the initial examination of
previously unexplored worlds, not the detailed investigation of a
planet that already has been scrutinized by spacecraft for
decades. That very scrutiny, however, means that there is a
valuable database for use in comparing observations by Dawn's
instruments, helping to prepare them for their assigned tasks in
the asteroid belt. Dawn's observations at Mars, like the other,
more distant measurements it has made of stars and planets since
launch, help prepare for the real rewards of the mission. (To
recapture the thrill of some of the previous in-flight
observations, visit the logs of October 24, 2007, December 17, 2007,
February 29 of any year, or April 22, 2008.)

Each instrument will acquire data to be used in comparisons with
observations made by similar instruments on spacecraft dedicated
to the study of Mars. Rather than using Dawn to learn about Mars,
our focus is on using Mars to learn about the performance of our
instruments. This will be helpful in doing science at Vesta and
Ceres and in navigating there. As we will see in subsequent logs,
the positions of Vesta and Ceres are not known accurately enough
that Dawn could rendezvous with them using conventional radio
navigation techniques alone. To improve the navigation, the craft
will take images of the bodies as it is closing in on them, and
analysis of those images by the Dawn navigation team will help pin
down the location of the target. Tests at Mars will contribute to
characterizing the camera not only for science, but for this
separate function as well.
 
If Dawn's encounter had represented a unique opportunity to
conduct vital new science, the plans would have been different. As
just one example, Dawn will not attempt to acquire the highest
resolution visible images that it might be capable of achieving.
Instead, by smearing the view of Mars across the camera's line of
sight, engineers will provide a relatively uniform illumination
for the camera's detectors, providing interesting engineering data
that cannot be obtained when observing the pinpoint light of stars.
 
In preparation for the encounter, on January 20 controllers
activated the gamma-ray and neutron detector (GRaND), which, despite
its name, is a very modest (but wonderfully capable) member of the
on-board instrumentation. The last time GRaND was operated was in
April. It remains healthy and is continuing to work well, sensing
the effects of cosmic radiation impinging on the spacecraft.
 
Dawn is approaching Mars from outside the planet's orbit around
the Sun. Therefore, from the spacecraft's point of view, Mars and
the Sun are close together, and Mars would appear to be a thin
crescent. This geometry precludes directing its instruments toward
the planet well before it arrives. Designed to operate in the more
distant asteroid belt, the instruments would not be able to
tolerate the heating from the Sun. Therefore, although GRaND is
able to detect space radiation, there would be nothing for the
other instruments to do while awaiting their calibrations at Mars.
 
These instruments will be activated on February 17 using
instructions already stored onboard. Dawn will be pointing its
main antenna to Earth, so the mission control team can observe
telemetry, but there is no plan to send additional instructions
then. The visible and infrared mapping spectrometer will be
powered on at about 9:47 a.m. PST, and the primary science camera
will be commanded on at about 1:21 p.m. PST. (The backup science
camera has been used for other tests in
flight, but it will not participate in the activities at Mars.)
All the times presented here are as measured on the spacecraft.
Mission controllers will have to wait more than 19 minutes, as
radio signals traverse the great distance to Earth, to observe the
associated telemetry. In essence, their entire view of events will
be delayed by this "one-way light time." (The Dawn project remains
ready to advise the FCC on the use of such a system to provide a
guaranteed delay in live broadcasting.)
 
At 1:57 p.m. PST, the spacecraft will begin turning to prepare for
its calibration activities. The maneuver will move Earth out of
the radio beam from the main antenna, so the spacecraft will
switch to 1 of its 3 auxiliary antennas. Each of these antennas
can emit a much broader beam, allowing communications over a wider
range of orientations. The cost of spreading the signal over a
much greater area is that when it is received at Earth, it is
significantly weaker, so only a very limited amount of telemetry
can be sent. For the subsequent day and a half, controllers will
use this reduced flow of information to monitor Dawn's work.
Meanwhile, the instruments will attempt to record neutrons,
gamma-rays, and ultraviolet, visible, and infrared light, all from
Mars, all providing a bonus to the mission. (Dawn has previously
conducted infrared observations of Mars. At that time
the spacecraft was about 100,000 times farther from the planet
than it will be when it swoops by next week.)
 
At 3:09 a.m. PST on February 19, Dawn will begin turning back to
sight its main antenna on Earth once again. For more than a day,
it will radio engineering data it stored during the time it could
not transmit at high speed. At first, a Deep Space Network (DSN)
antenna near Madrid, Spain, will receive
the signals. As Earth rotates, the 34-meter (112-foot) dish will
no longer be able to point to Dawn (as viewed from Madrid, Dawn
will set in the west, just as the Sun, the moon, stars, and other
celestial objects do), so a DSN antenna near Goldstone,
California, will take over. After about 8 hours, the Goldstone
facility will hand the responsibility over to a DSN antenna still
farther west, near Canberra, Australia, which eventually will pass
the baton once again to Madrid.
 
At 7:48 a.m. on February 20, the spacecraft will rotate again to
bring its camera to bear on Mars. By then, the view of distant
Mars will be similar to what the spacecraft will have as it
navigates to Vesta, once again providing an opportunity to prepare
for the visit to that mysterious world. At 11:28 a.m., it will
turn away from Mars for the last time and resume transmitting data
back to Earth 45 minutes later. The precious capabilities of the
DSN are shared among all interplanetary spacecraft, so Dawn will
return to more intermittent tracking on Friday. The return of data
will be completed the following week.
 
All data that Dawn collects at Mars are considered a bonus of the
mission. Indeed, when the mission was conceived, its launch was to
be in 2006, and the mission to Vesta and Ceres then did not
require the assistance of Mars. It was only in 2005, when the
launch was rescheduled to 2007, that trajectory designers added
Mars to the itinerary. The gravitational deflection is essential
to the success of the mission, but the activities during the visit
to Mars are secondary to the mission's principal objectives.
 
Even before Dawn has paid a visit to Mars, engineers are already
busy planning the details of the spacecraft's next assignments.
Following its brief divertissement at the planet, it will continue
to coast in its orbit around the Sun until June. With Mars helping
to reshape its orbit, there is no need for ion thrusting for a
while. In the next few logs, we will follow along as Dawn engages
in other work to be completed before the resumption of its most
familiar function of reaching for the asteroid belt atop a
blue-green pillar of xenon ions.

Dawn is 1.1 million kilometers (680 thousand miles) from Mars. It is 348
million kilometers (216 million miles) from Earth, or 910 times as far
as the moon and 2.36 times as far as the Sun. Radio signals, traveling
at the universal limit of the speed of light, take 39 minutes to make
the round trip.
Received on Fri 13 Feb 2009 08:11:30 PM PST