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

[Ron Baalke]

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.