[meteorite-list] Dawn Journal - March 6, 2015

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 8 Mar 2015 23:38:56 -0700 (PDT)
Message-ID: <201503090638.t296cuWX014366_at_zagami.jpl.nasa.gov>

http://dawnblog.jpl.nasa.gov/2015/03/06/dawn-journal-march-6/

Dawn Journal
by Marc Rayman
March 6, 2015
 
Dear Unprecedawnted Readers,

Since its discovery in 1801, Ceres has been known as a planet, then as
an asteroid, and later as a dwarf planet. Now, after a journey of 3.1
billion miles (4.9 billion kilometers) and 7.5 years, Dawn calls it "home."
Earth's robotic emissary arrived at about 4:39 a.m. PST today. It will
remain in residence at the alien world for the rest of its operational
life, and long, long after.

Before we delve into this unprecedented milestone in the exploration of
space, let's recall that even before reaching orbit, Dawn started taking
pictures of its new home. Last month we presented the updated schedule
for photography. Each activity to acquire images (as well as visible spectra
and infrared spectra) has executed smoothly and provided us with exciting
and tantalizing new perspectives.

While there are countless questions about Ceres, the most popular now
seems to be what the bright spots are. It is impossible not to be mesmerized
by what appear to be glowing beacons, shining out across the cosmic seas
from the uncharted lands ahead. But the answer hasn't changed: we don't
know. There are many intriguing speculations, but we need more data, and
Dawn will take photos and myriad other measurements as it spirals closer
and closer during the year. For now, we simply know too little.

For example, some people ask if those spots might be lights from an alien
city. That's ridiculous! At this early stage, how could Dawn determine
what kinds of groupings Cereans live in? Do they even have cities? For
all we know, they may live only in rural communities, or perhaps they
only have large states.

What we already know is that in more than 57 years of space exploration,
Dawn is now the only spacecraft ever to orbit two extraterrestrial destinations.
A true interplanetary spaceship, Dawn left Earth in Sep. 2007 and traveled
on its own independent course through the solar system. It flew past Mars
in Feb. 2009, robbing the red planet of some of its own orbital energy
around the sun. In July 2011, the ship entered orbit around the giant
protoplanet Vesta, the second most massive object in the main asteroid
belt between Mars and Jupiter. (By the way, Dawn's arrival at Vesta
was exactly one Vestan year ago earlier this week.) It conducted a spectacular
exploration of that fascinating world, showing it to be more closely related
to the terrestrial planets (including Earth, home to many of our readers)
than to the typical objects people think of as asteroids. After 14 months
of intensive operations at Vesta, Dawn climbed out of orbit in Sep. 2012,
resuming its interplanetary voyage. Today it arrived at its final destination,
Ceres, the largest object between the sun and Pluto that had not previously
been visited by a spacecraft. (Fortunately, New Horizons is soon to fly
by Pluto. We are in for a great year!)

What was the scene like at JPL for Dawn's historic achievement? It's
easy to imagine the typical setting in mission control. The tension is
overwhelming. Will it succeed or will it fail? Anxious people watch their
screens, monitoring telemetry carefully, frustrated that there is nothing
more they can do now. Nervously biting their nails, they are thinking
of each crucial step, any one of which might doom the mission to failure.
At the same time, the spacecraft is executing a bone-rattling, whiplash-inducing
burn of its main engine to drop into orbit. When the good news finally
arrives that orbit is achieved, the room erupts! People jump up and down,
punch the air, shout, tweet, cry, hug and feel the tremendous relief of
overcoming a huge risk. You can imagine all that, but that's not what
happened.

If you had been in Dawn mission control, the scene would have been different.
You would mostly be in the dark. (For your future reference, the light
switches are to the left of the door.) The computer displays would be
off, and most of the illumination would be from the digital clock and
the string of decorative blue lights that indicate the ion engine is scheduled
to be thrusting. You also would be alone (at least until JPL Security
arrived to escort you away, because you were not cleared to enter the
room, and, for that matter, how did you get past the electronic locks?).
Meanwhile, most of the members of the flight team were at home and asleep!
(Your correspondent was too, rare though that is. When Dawn entered orbit
around Vesta, he was dancing. Ceres' arrival happened to be at a time
less conducive to consciousness.)

Why was such a significant event treated with somnolence? It is because
Dawn has a unique way of entering orbit, which is connected with the nature
of the journey itself. We have discussed some aspects of getting into
orbit before (with this update to the nature of the approach trajectory).
Let's review some of it here.

It may be surprising that prior to Dawn, no spacecraft had even attempted
to orbit two distant targets. Who wouldn't want to study two alien worlds
in detail, rather than, as previous missions, either fly by one or more
for brief encounters or orbit only one? A mission like Dawn's is an
obvious kind to undertake. It happens in science fiction often: go somewhere,
do whatever you need to do there (e.g., beat someone up or make out with
someone) and then boldly go somewhere else. However, science fact is not
always as easy as science fiction. Such missions are far, far beyond the
capability of conventional propulsion.

Deep Space 1 (DS1) blazed a new trail with its successful testing of ion
propulsion, which provides 10 times the efficiency of standard propulsion,
showing on an operational interplanetary mission that the advanced technology
really does work as expected. (This writer was fortunate enough to work
on DS1, and he even documented the mission in a series of increasingly
wordy blogs. But he first heard of ion propulsion from the succinct Mr.
Spock and subsequently followed its use by the less logical Darth Vader.)

Dawn's ambitious expedition would be truly impossible without ion propulsion.
(For a comparison of chemical and ion propulsion for entering orbit around
Mars, an easier destination to reach than either Vesta or Ceres, visit
this earlier log.) So far, our advanced spacecraft has changed its own
velocity by 23,800 mph (38,400 kilometers per hour) since separating from
its rocket, far in excess of what any other mission has achieved propulsively.
(The previous record was held by DS1.)

Dawn is exceptionally frugal in its use of xenon propellant. In this phase
of the mission, the engine expends only a quarter of a pound (120 grams)
per day, or the equivalent of about 2.5 fluid ounces (75 milliliters)
per day. So although the thrust is very efficient, it is also very gentle.
If you hold a single sheet of paper in your hand, it will push on your
hand harder than the ion engine pushes on the spacecraft at maximum thrust.
At today's throttle level, it would take the distant explorer almost
11 days to accelerate from zero to 60 mph (97 kilometers per hour). That
may not evoke the concept of a drag racer. But in the zero-gravity, frictionless
conditions of spaceflight, the effect of this whisper-like thrust can
build up. Instead of thrusting for 11 days, if we thrust for a month,
or a year, or as Dawn already has, for more than five years, we can achieve
fantastically high velocity. Ion propulsion delivers acceleration with
patience.

Most spacecraft coast most of the time, following their repetitive orbits
like planets do. They may use the main engine for a few minutes or perhaps
an hour or two throughout the entire mission. With ion propulsion, in
contrast, the spacecraft may spend most of its time in powered flight.
Dawn has flown for 69% of its time in space emitting a cool blue-green
glow from one of its ion engines. (With three ion engines, Dawn outdoes
the Star Wars TIE - twin ion engine - fighters.)

As Dawn maneuvers into orbit, its trajectory takes it to the opposite
side of Ceres from the sun, providing these crescent views. These pictures
(part of the OpNav 5 activity), were taken on March 1 at a distance of
30,000 miles (49,000 kilometers). Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
The robotic probe uses its gentle thrust to gradually reshape its path
through space rather than simply following the natural course that a planet
would. After it escaped from Vesta's gravitational clutches, it slowly
spiraled outward from the sun, climbing the solar system hill, making
its heliocentric orbit more and more and more like Ceres'. By the time
it was in the vicinity of the dwarf planet today, both were traveling
around the sun at more than 38,600 mph (62,100 kilometers per hour). Their
trajectories were nearly identical, however, so the difference in their
speeds was only 100 mph (160 kilometers per hour), or less than 0.3 percent
of the total. Flying like a crackerjack spaceship pilot, Dawn elegantly
used the light touch of its ion engine to be at a position and velocity
that it could ease gracefully into orbit. At a distance of 37,700 miles
(60,600 kilometers), Ceres reached out and tenderly took the newcomer
from Earth into its permanent gravitational embrace.

If you had been in space watching the event, you would have been cold,
hungry and hypoxic. But it would not have looked much different from the
1,885 days of ion thrust that had preceded it. The spacecraft was perched
atop its blue-green pillar of xenon ions, patiently changing its course,
as it does for so much of quiet cruise. But now, at one moment it was
flying too fast for Ceres' gravity to hang on to it, and the next moment
it had slowed just enough that it was in orbit. Had it stopped thrusting
at that point, it would have continued looping around the dwarf planet.
But it did not stop. Instead, it is working now to reshape its orbit around
Ceres. As we saw in November, its orbital acrobatics first will take it
up to an altitude of 47,000 miles (75,000 kilometers) on March 19 before
it swoops down to 8,400 miles (13,500 kilometers) on April 23 to begin
its intensive observations in the orbit designated RC3.

In fact, Dawn's arrival today really is simply a consequence of the
route it is taking to reach that lower orbit next month. Navigators did
not aim for arriving today. Rather, they plotted a course that began at
Vesta and goes to RC3 (with a new design along the way), and it happens
that the conditions for capture into orbit occurred this morning. As promised
last month, we present here a different view of the skillful maneuvering
by this veteran space traveler.

[animation]
This animation gives a three-dimensional view of Dawn's complex approach
to Ceres. The spacecraft deftly maneuvers into orbit with its ion propulsion
system, flying to RC3 orbit, which is achieved when the thrust is turned
off. (The size of Ceres is exaggerated compared to the size of the orbit
here.) At the end, the viewpoint shifts to provide another perspective
on the unique trajectory.

If Dawn had stopped thrusting before Ceres could exert its gravitational
control, it wouldn't have flown very far away. The spacecraft had already
made their paths around the sun very similar, and the ion propulsion system
provides such exceptional flexibility to the mission that controllers
could have guided it into orbit some other time. This was not a one-time,
all-or-nothing event.

So the flight team was not tense. They had no need to observe it or make
a spectacle out of it. Mission control remained quiet. The drama is not
in whether the mission will succeed or fail, in whether a single glitch
could cause a catastrophic loss, in whether even a tiny mistake could
spell doom. Rather, the drama is in the opportunity to unveil the wonderful
secrets of a fascinating relict from the dawn of the solar system more
than 4.5 billion years ago, a celestial orb that has beckoned for more
than two centuries, the first dwarf planet discovered.

Dawn usually flies with its radio transmitter turned off (devoting its
electricity instead to the power-hungry ion engine), and so it entered
orbit silently. As it happened, a routine telecommunications session was
scheduled about an hour after attaining orbit, at 5:36 a.m. PST. (It's
only coincidence it was that soon. At Vesta, it was more than 25 hours
between arrival and the next radio contact.) For primary communications,
Dawn pauses thrusting to point its main antenna to Earth, but other times,
as in this case, it is programmed to use one of its auxiliary antennas
to transmit a weaker signal without stopping its engine, whispering just
enough for engineers to verify that it remains healthy.

The Deep Space Network's exquisitely sensitive 230-foot (70-meter) diameter
antenna in Goldstone, Calif., picked up the faint signal from across the
solar system on schedule and relayed it to Dawn mission control. One person
was in the room (and yes, he was cleared to enter). He works with the
antenna operator to ensure the communications session goes smoothly, and
he is always ready to contact others on the flight team if any anomalies
arise. In this case, none did, and it was a quiet morning as usual. The
mission director checked in with him shortly after the data started to
trickle in, and they had a friendly, casual conversation that included
discussing some of the telemetry that indicated the spacecraft was still
performing its routine ion thrusting. The determination that Dawn was
in orbit was that simple. Confirming that it was following its flight
plan was all that was needed to know it had entered orbit. This beautifully
choreographed celestial dance is now a pas de deux.

As casual and tranquil as all that sounds, and as logical and systematic
as the whole process is, the reality is that the mission director was
excited. There was no visible hoopla, no audible fanfare, but the experience
was powerful fuel for the passionate fires that burn within.
As soundlessly as a spacecraft gliding through the void, the realization
emerges...

Dawn made it!!

It is in orbit around a distant world!!

Yes, it's clear from the technical details, but it is more intensely
reflected in the silent pounding of a heart that has spent a lifetime
yearning to know the cosmos. Years and years of hard work devoted to this
grand undertaking, constant hopes and dreams and fears of all possible
futures, uncounted challenges (some initially appearing insurmountable)
and a seeming infinitude of decisions along the way from early concepts
through a real interplanetary spacecraft flying on an ion beam beyond
the sun.

And then, a short, relaxed chat over a few bits of routine data that report
the same conditions as usual on the distant robot. But today they mean
something different.

They mean we did it!!

Everyone on the team will experience the news that comes in a congratulatory
email in their own way, in the silence and privacy of their own thoughts.
But it means the same to everyone.

We did it!!

And it's not only the flight team. Humankind!! With our relentless curiosity,
our insatiable hunger for knowledge, our noble spirit of adventure, we
all share in the experience of reaching out from our humble home to the
stars.

Together, we did it!!!

It was a good way to begin the day. It was Dawn at Ceres.

[Video]
This video overview of the mission at Ceres is a great way to start your
day, but you can enjoy it at any time.

Let's bring into perspective the cosmic landscape on which this remarkable
adventure is now taking place. Imagine Earth reduced to the size of a
soccer ball. On this scale, the International Space Station would orbit
at an altitude of a bit more than one-quarter of an inch (seven millimeters).
The moon would be a billiard ball almost 21 feet (6.4 meters) away. The
sun, the conductor of the solar system orchestra, would be 79 feet (24
meters) across at a distance of 1.6 miles (2.6 kilometers). But even more
remote, Dawn would be 5.3 miles (8.6 kilometers) away. (Just a few months
ago, when the spacecraft was on the opposite side of the sun from Earth,
it would have been more than six miles, or almost 10 kilometers, from
from the soccer ball.) Tremendously far now from its erstwhile home, it
would be only a little over a yard (a meter) from its new residence. (By
the end of this year, Dawn will be slightly closer to it than the space
station is to Earth, a quarter of an inch, or six millimeters.) That distant
world, Ceres, the largest object between Mars and Jupiter, would be five-eighths
of an inch (1.6 centimeters) across, about the size of a grape. Of course
a grape has a higher water content than Ceres, but we can be sure that
exploring this intriguing world of rock and ice will be much sweeter!
As part of getting to know its new neighborhood, Dawn has been hunting
for moons of Ceres. Telescopic studies had not revealed any, but if there
were a moon smaller than about half a mile (one kilometer), it probably
would not have been discovered. The spacecraft's unique vantage point
provides an opportunity to look for any that might have escaped detection.
Many pictures have been taken specifically for this purpose, and scientists
scrutinize them and all of the other photographs for any indication of
moons. While the search will continue, so far, no picture has shown evidence
of companions orbiting Ceres.

And yet we know that as of today, Ceres most certainly does have one.
Its name is Dawn!

Dawn is 37,800 miles (60,800 kilometers) from Ceres, or 16 percent of
the average distance between Earth and the moon. It is also 3.33 AU (310
million miles, or 498 million kilometers) from Earth, or 1,230 times as
far as the moon and 3.36 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 55 minutes
to make the round trip.
Received on Mon 09 Mar 2015 02:38:56 AM PDT