[meteorite-list] Dawn Journal - November 28, 2016

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 19 Dec 2016 16:06:40 -0800 (PST)
Message-ID: <201612200006.uBK06eeP005039_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_11_28_16.html

Dawn Journal
Dr. Marc Rayman
November 28, 2016

Dear Decadawnt Readers,

Blue rope lights adorn Dawn mission control at JPL, but not because the
flight team is in the holiday spirit (although they are in the holiday
spirit). The felicitous display is more than decorative. The illumination
indicates that the interplanetary spacecraft is thrusting with one of
its ion engines, which emit a lovely, soft bluish glow in the forbidding
depths of space. Dawn is completing another elegant spiral around dwarf
planet Ceres, maneuvering to its sixth science orbit.

Dawn's ion propulsion system has allowed the probe to accomplish
a mission unlike any other, orbiting two distant extraterrestrial destinations.
Even more than that, Dawn has taken advantage of the exceptional efficiency
of its ion engines to fly to orbits at different altitudes and orientations
while at Vesta and at Ceres, gaining the best perspectives for its photography
and other scientific investigations.
Occator on Ceres' Limb

Dawn has thrust for a total of 5.7 years during its deep-space adventure.
All that powered flight has imparted a change in the ship's velocity
of 25,000 mph (40,000 kilometers per hour). As we have seen, this is not
the spacecraft's actual speed, but it is a convenient measure of
the effect of its propulsive work. Reaching Earth orbit requires only
about 17,000 mph (less than 28,000 kilometers per hour). In fact, Dawn's
gentle ion engines have delivered almost 98 percent of the change in speed
that its powerful Delta 7925H-9.5 rocket provided. With nine external
rocket engines and a core consisting of a first stage, a second stage
and a third stage, the Delta boosted Dawn by 25,640 mph (41,260 kilometers
per hour) from Cape Canaveral out of Earth orbit and onto its interplanetary
trajectory, after which the remarkable ion engines took over. No other
spacecraft has accomplished such a large velocity change under its own
power. (The previous record holder, Deep Space 1, achieved 9,600 mph,
or 15,000 kilometers per hour.)

Early this year, we were highly confident Dawn would conclude its operational
lifetime in its fourth orbit at Ceres (and remain there long after). But
unexpectedly healthy and with an extension from NASA, Dawn is continuing
its ambitious mission. After completing all of its tasks in its fifth
scientific phase at Ceres, Dawn is pursuing new objectives by flying to
another orbit for still more discoveries. Although we never anticipated
adding a row to the table of Dawn's orbits, last presented in December
2015, we now have an updated version.

[Table]

As with the obscure Dawn code names for other orbits, this fifth orbit's
name requires some explanation. The extended mission is devoted to undertaking
activities not envisioned in the prime mission. That began with two extra
months in the fourth mapping orbit performing many new observations, but
because it was then the extended mission, that orbit was designated extended
mission orbit 1, or XMO1. (It should have been EMO1, of course, but the
team's spellchecker was offline on July 1, the day the extended mission
started.) Therefore, the next orbit was XMO2. Dawn left XMO2 on Nov. 4,
and we leave it to readers' imaginations to devise a name for the
orbit the spacecraft is now maneuvering to.

Surprisingly, Dawn is flying higher to enhance part of the scientific
investigation that motivated going to the lowest orbit. We have explained
before that Dawn's objective in powering its way down to the fourth
mapping orbit was to make the most accurate measurements possible of gravity
and of nuclear radiation emitted by the dwarf planet.

For more than eight months, the explorer orbited closer to the alien world
than the International Space Station is to Earth, and the gamma ray spectra
and neutron spectra it acquired are outstanding, significantly exceeding
all expectations. But ever-creative scientists have recognized that even
with that tremendous wealth of data, Dawn can do still better. Let's
look at this more carefully and consider an example to resolve the paradox
of how going higher can yield an improvement.

The gamma ray and neutron detector (GRaND) reveals some of Ceres'
atomic constituents down to about a yard (meter) underground. The principal
limitation in analyzing these spectra is "noise." In fact, noise limits
the achievable accuracy of many scientific measurements. It isn't
necessarily the kind of noise that you hear from loud machinery (nor from
the mouth of your unhelpful parent, inattentive progeny or boring and
verbose coworker), but all natural systems have something similar. Physical
processes other than the ones of interest make unwanted contributions
to the measurements. The part of a measurement scientists want is called
the "signal." The part of a measurement scientists don't want is
called the "noise." The quality of a measurement may be characterized
by comparing the strength of the signal to the strength of the noise.
(This metric is called the "signal to noise ratio" by people who like
to use jargon like "signal to noise ratio.")

We have discussed that cosmic rays, radiation that pervades space, strike
atomic nuclei on Ceres, creating the signals that GRaND measures. Remaining
at low altitude would have allowed Dawn to enhance its measurement of
the Cerean nuclear signal. But scientists determined that an even better
way to improve the spectra than to increase the signal is to decrease
the noise. GRaND's noise is a result of cosmic rays impinging directly
on the instrument itself and on nearby parts of the spacecraft. With a
more thorough measurement of the noise from cosmic rays, scientists will
be able to mathematically remove that component of the low altitude measurements,
leaving a clearer signal.

For an illustration of all this, suppose you want to hear the words of
a song. The words are the signal and the instruments are the noise. (This
is a scientific discussion, not a musical one.) It could be that the instruments
are so loud and distracting that you can't make the words out easily.

You might try turning up the volume, because that increases the signal,
but it increases the noise as well. If the performance is live, you might
even try to position yourself closer to the singer, perhaps making the
signal stronger without increasing the noise too much. (Other alternatives
are simply to Google the song or ask the singer for a copy of the lyrics,
but those methods would ruin this example.)

If you're doing this in the 21st century (or later), there's
another trick you can employ, taking advantage of computer processing.
Suppose you had a recording of the singing with the instruments and then
obtained separate recordings of the instruments. You could subtract the
muscial sounds that constitute the noise, removing the contributions from
both guitars, the drums, the harp, both ukuleles, the kazoo and all the
theremins. And when you eliminate the noise of the instruments, what remains
is the signal of the words, making them much more intelligible.

To obtain a better measure of the noise, Dawn needs to go to higher altitude,
where GRaND will no longer detect Ceres. It will make detailed measurements
of cosmic ray noise, which scientists then will subtract from their measurements
at low altitude, where GRaND observed Ceres signal plus cosmic ray noise.
The powerful capability to raise its orbit so much affords Dawn the valuable
opportunity to gain greater insight into the atomic composition. Of course,
it's not quite that simple, but essentially this method will help
Dawn hear Ceres' nuclear song more clearly.

 To travel from one orbit to another, the sophisticated explorer has followed
complex spiral routes. We have discussed the nature of these trajectories
quite a bit, including how the operations team designs and flies them.
But now they are using a slightly different method.

Those of you at Ceres who monitor the ship's progress probably wouldn't
notice a difference in the type of trajectory. And the rest of you on
Earth and elsewhere who keep track through our mission status updates
also would not detect anything unusual in the ascent profile (to the extent
that a spacecraft using ion propulsion to spiral around a dwarf planet
is usual). But celestial navigators are now enjoying their use of a method
they whimsically call local maximal energy spiral feedback control.

The details of the new technique are not as important for our discussion
here as one of the consequences: Dawn's next orbit will not be nearly
as circular as any of its other orbits at Ceres (or at Vesta). Following
the conclusion of this spiral ascent on Dec. 5, navigators will refine
their computations of the orbit, and we will describe the details near
the end of the month. We will see that as the spacecraft follows its elliptical
loops around Ceres, each taking about a week, the altitude will vary smoothly,
dipping below 4,700 miles (7,600 kilometers) and going above 5,700 miles
(9,200 kilometers). Such a profile meets the mission's needs, because
as long as the craft stays higher than about 4,500 miles (7,200 kilometers),
it can make the planned recordings of the cacophonous cosmic rays. We
will present other plans for this next phase of the mission as well, including
photography, in an upcoming Dawn Journal.

As Dawn continues its work at Ceres, the dwarf planet continues its stately
4.6-year-long orbit around the sun, carrying Earth's robotic ambassador
with it. Ceres follows an elliptical path around the sun (see, for example,
this discussion, including the table). In fact, all orbits, including
Earth's, are ellipses. Ceres' orbit is more elliptical than
Earth's but not as much as some of the other planets. The shape of
Ceres' orbit is between that of Saturn (which is more circular) and
Mars (which is more elliptical). (Of course, Ceres' orbit is larger
than Mars' and smaller than Saturn's, but here we are describing
how much each orbit deviates from a perfect circle.)

When Ceres tenderly took Dawn into its gravitational embrace in March
2015, they were 2.87 AU (267 million miles, or 429 million kilometers)
from the sun. In January 2016, we mentioned that Ceres had reached its
aphelion, or greatest distance from the sun, at 2.98 AU (277 million miles,
or 445 million kilometers). Today at 2.85 AU (265 million miles, or 427
million kilometers), Ceres is closer to the sun than at any time since
Dawn arrived, and the heliocentric distance will gradually decrease further
throughout the extended mission. (If the number of numbers is overwhelming
here, you might reread this paragraph while paying attention to only one
set of units, whether you choose AU, miles or kilometers. Ignore the other
two scales so you can focus on the relative distances.)
Another consequence of orbiting the sun is the progression of seasons.
Right on schedule, as we boldly predicted in August 2015, Nov. 13 was
the equinox on Ceres, marking the beginning of northern hemisphere autumn
and southern hemisphere spring. Although it is celebrated on Ceres with
less zeal than on Earth, it is fundamentally the same: the sun was directly
over the equator that day, and now it is moving farther south. It takes
Ceres so long to orbit the sun that this season will last until Dec. 22,
2017.

A celebration that might occur on Ceres (and which you, loyal Dawnophile,
are welcome to attend) would honor Dawn itself. Although the spacecraft
completed its ninth terrestrial year of spaceflight in September, on Dec.
12, it will have been two Cerean years since Dawn left Earth for its interplanetary
journey. Be sure to attend in order to learn how a dawnniversary is commemorated
in that part of the solar system.

Although a year on Ceres lasts much longer than on Earth, 2016 is an unusually
long year on our home planet. Not only was a leap day included, but a
leap second will be added at the very end of the year to keep celestial
navigators' clocks in sync with nature. (Clocks used for other purposes
will not have the added second. The extra second applies in the specialized
timekeeping system known as "ephemeris time.") The Dawn team already has
accounted for the leap second in the intricate plans formulated for the
spacecraft. And at that second, on Dec. 31 at 23:59:60, we will be able
to look back on 366 days and one second, an especially full and gratifying
year in this remarkable deep-space expedition. But we needn't wait.
Even now, as mission control is bathed in a lovely glow, the members of
the team as well as space enthusiasts everywhere are aglow with the thrill
of new knowledge, the excitement of a daring, noble adventure and the
anticipation of more to come.

Dawn is 3,150 miles (5,070 kilometers) from Ceres. It is also 2.08 AU
(194 million miles, or 312 million kilometers) from Earth, or 770 times
as far as the moon and 2.11 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 35 minutes
to make the round trip.
Received on Mon 19 Dec 2016 07:06:40 PM PST