[meteorite-list] Dawn Journal: Getting Down to Science at Ceres

[Ron Baalke]

http://www.jpl.nasa.gov/blog/2015/4/getting-down-to-science-at-ceres

Dawn Journal
Getting Down to Science at Ceres
April 29, 2015
By Marc Rayman

Let's get Dawn to business, Dear Readers,

Dawn's assignment when it embarked on its extraordinary extraterrestrial 
expedition in 2007 can be described quite simply: explore the two most 
massive uncharted worlds in the inner solar system. It conducted a spectacular 
mission at Vesta, orbiting the giant protoplanet for 14 months in 2011-2012, 
providing a wonderfully rich and detailed view. Now the sophisticated 
probe is performing its first intensive investigation of dwarf planet 
Ceres. Dawn is slowly circling the alien world of rock and ice, far from 
Earth and far from the sun, executing its complex operations with the 
prowess it has demonstrated throughout its ambitious journey.

Following an interplanetary trek of 7.5 years and 3.1 billion miles (4.9 
billion kilometers), Earth's ambassador arrived in orbit on March 6, answering 
Ceres' two-century-old celestial invitation. With its advanced ion propulsion 
system and ace piloting skills, it has maneuvered extensively in orbit. 
Traveling mostly high over the night side of Ceres, arcing and banking, 
thrusting and coasting, accelerating and decelerating, climbing and diving, 
the spaceship flew to its first targeted orbital altitude, which it reached 
on April 23.

Dawn is at an altitude of about 8,400 miles (13,600 kilometers) above 
the mysterious terrain. This first mapping orbit is designated RC3 by 
the Dawn team and is a finalist in the stiff competition for the coveted 
title of Most Confusing Name for a Ceres Mapping Orbit. (See this table 
for the other contestants.) Last month we described some of the many observations 
Dawn will perform here, including comprehensive photography of the alien 
landscapes, spectra in infrared and visible wavelengths, a search for 
an extremely tenuous veil of water vapor and precise tracking of the orbit 
to measure Ceres' mass.

[Image]
Dawn's four mapping orbits, shown to scale in altitude with the size of 
Ceres, which is about 590 miles (950 kilometers) in diameter. See this 
table for descriptions of the orbits and links to the activities scheduled 
for each. Image credit: NASA/JPL-Caltech

On the way down to this orbit, the spacecraft paused ion thrusting twice 
earlier this month to take pictures of Ceres, as it had seven times before 
in the preceding three months. (We presented and explained the schedule 
for photography during the three months leading up to RC3 here.) Navigators 
used the pictures to measure the position of Ceres against the background 
of stars, providing crucial data to guide the ship to its intended orbit. 
The Dawn team also used the pictures to learn about Ceres to aid in preparing 
for the more detailed observations.

We described last month, for example, adjusting the camera settings for 
upcoming pictures to ensure good exposures for the captivating bright 
spots, places that reflect significantly more sunlight than most of the 
dark ground. Scientists have also examined all the pictures for moons 
of Ceres (and many extra pictures were taken specifically for that purpose). 
And thanks to Dawn's pictures, everyone who longs for a perspective on 
the universe unavailable from our terrestrial home has been transported 
to a world one million times farther away than the International Space 
Station.

The final pictures before reaching RC3 certainly provide a unique perspective. 
(You can see Dawn's pictures of Ceres here.) On April 10 and April 14-15, 
Dawn peered down over the northern hemisphere and watched for two hours 
each time as Ceres turned on its  axis, part of the unfamiliar cratered 
terrain bathed in sunlight, part in the deep dark of night. This afforded 
a very different view from what we are accustomed to in looking at other 
planets, as most depictions of planetary rotations are from nearer the 
equator to show more of the surface. (Indeed, Dawn acquired views like 
that in its February "rotation characterizations.") The latest animations 
of Ceres rotating beneath Dawn are powerful visual reminders that this 
capable interplanetary explorer really is soaring around in orbit about 
a distant, alien world. Following the complex flight high above the dark 
hemisphere, where there was nothing to see, the pictures also show us 
that the long night's journey into day has ended.

[Animation]
This animated sequence of images from NASA's Dawn spacecraft shows northern 
terrain on the sunlit side of dwarf planet Ceres. Dawn took these images 
on April 14 and 15 from a vantage point 14,000 miles (22,000 kilometers) 
above Ceres' northern hemisphere. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA 

Gradually descending atop its blue-green beam of high velocity xenon ions, 
Dawn crossed over the terminator -- the boundary between the dark side 
and the lit side -- on April 15 almost directly over the north pole. On 
April 20, on final approach to RC3, it flew over the equator at an altitude 
of about 8,800 miles (14,000 kilometers).

The spacecraft completed its ion thrusting shortly after 1:00 a.m. PDT 
on April 23. What an accomplishment this was! From the time Dawn left 
its final mapping orbit at Vesta in July 2012, this is where it has been 
headed. The escape from Vesta's gravitational clutches in September 2012, 
the subsequent two and a half years of interplanetary travel and entering 
into orbit around Ceres on March 6, as genuinely exciting and important 
as it was, all really occurred as consequences of targeting this particular 
orbit.

In September 2014, the aftereffects of being struck by cosmic radiation 
compelled the operations team to rapidly develop a complex new approach 
trajectory because they still wanted to achieve this very orbit, where 
Dawn is now. And the eidetic reader will note that even when the innovative 
flight profile was presented five months ago (with many further details 
in subsequent months), we explained that it would conclude on April 23. 
And it did! Here we are! All the descriptions and figures plus a cool 
video elucidated a pretty neat idea, but it's also much more than an idea: 
it's real!! A probe from Earth is in a mapping orbit around a faraway 
dwarf planet.

When it had accomplished the needed ion thrusting, the veteran space traveler 
turned to point its main antenna to Earth so mission controllers could 
prepare it for the intensive mapping observations. The first task was 
to measure the orbital parameters so they could be transmitted to the 
spacecraft.

A few readers (you and I both know who you are) may have noted that in 
Dawn Journals during the last year, we have described the altitude of 
RC3 as 8,400 miles and 13,500 kilometers. Above, however, it is 13,600 
kilometers. This is not a mistake. (It would be a mistake if the previous 
sentence were written, "Above, howevr, it is 13,600 kilometers.") This 
subtle difference belies several important issues about the orbits at 
Ceres. Let's take a further look.

As we explained when Dawn resided at Vesta, the orbital altitude we present 
is always an average (and rounded off, to avoid burdening readers with 
too many unhelpful digits). Vesta, Ceres, Earth and other planetary bodies 
are not perfect spheres, so even if the spacecraft traveled in a perfect 
circle, its altitude would change. They all are somewhat oblate, being 
wider across the equator than from pole to pole. In addition, they have 
more localized topography. Think of flying in a plane over your planet. 
If the pilot maintains a constant altitude above sea level, the distance 
above the ground changes because the elevation of the ground itself varies, 
coming closer to the aircraft on mountains and farther in valleys. In 
addition, as it turns out, orbits are not perfect circles but tend to 
be slightly elliptical, as if the plane flies slightly up and down occasionally, 
so the altitude changes even more.

In their exquisitely detailed planning, the Dawn team has had to account 
for the unknown nature of Ceres itself, including its mass and hence the 
strength of its gravitational pull. Dawn is the only spacecraft to orbit 
large, massive planetary bodies that were not previously visited by flyby 
spacecraft. Mercury, Venus, the moon, Mars, Jupiter and Saturn all were 
studied by spacecraft that flew past them before subsequent missions were 
sent to orbit them. The first probes to each provided an initial measurement 
of the mass and other properties that were helpful for the arrival of 
the first orbiters. At Vesta and Ceres, Dawn has had to discover the essential 
characteristics as it spirals in closer and closer. For each phase, engineers 
make the best measurements they can and then use them to update the plans 
for the subsequent phases. As a result, however, plans are based on impressive 
but nevertheless imperfect knowledge of what will be encountered at lower 
altitudes. So even if the spacecraft executes an ion thrust flight profile 
perfectly, it might not wind up exactly where the plan had specified.

There are other reasons as well for small differences between the predicted 
and the actual orbit. One is minor variations in the thrust of the ion 
propulsion system, as we discussed here. Another is that every time the 
spacecraft fires one of its small rocket thrusters to rotate or to stabilize 
its orientation in the zero-gravity conditions of spaceflight, that also 
nudges the spacecraft, changing its orbit a little. (See here for a related 
example of the effect of the thrusters on the trajectory.)

The Dawn flight team has a deep understanding of all the sources of orbit 
discrepancies, and they always ensure that their intricate plans account 
for them. Even if the RC3 altitude ended up more than 300 miles (500 kilometers) 
higher or lower than the specified value, everything would still work 
just fine to yield the desired pictures and other data. In fact, the actual 
RC3 orbit is within 25 miles (40 kilometers), or less than one tenth what 
the plan was designed to accommodate, so the spacecraft achieved a virtual 
cosmic bullseye!

In the complex preparations on April 23, one file was not radioed to Dawn 
on time, so late that afternoon when the robot tried to use this file, 
it could not find it. It responded appropriately by running protective 
software, stopping its activities, entering "safe mode" and beaming a 
signal back to distant Earth to indicate it needed further instructions. 
After the request arrived in mission control at JPL, engineers quickly 
recognized what had occurred. That night they reconfigured the spacecraft 
out of safe mode and back to its normal operational configuration, and 
they finished off the supply of ice cream in the freezer just outside 
the mission control room. Although Dawn was not ready to begin its intensive 
observation campaign in the morning of April 24, it started later that 
same day and has continued to be very productive.

Dawn is a mission of exploration. And rather than be constrained by a 
fast flight by a target for a brief glimpse, Dawn has the capability to 
linger in orbit for a very long time at close range. The probe will spend 
more than a year conducting detailed investigations to reveal as much 
as possible about the nature of the first dwarf planet discovered, which 
we had seen only with telescopes since it was first glimpsed in 1801. 
The pictures Dawn has sent us so far are intriguing and entrancing, but 
they are only the introduction to this exotic world. They started transforming 
it from a smudge of light into a real, physical place and one that a sophisticated, 
intrepid spacecraft can even reach. Being in the first mapping orbit represents 
the opportunity now to begin developing a richly detailed, intimate portrait 
of a world most people never even knew existed. Now, finally, we are ready 
to start uncovering the secrets Ceres has held since the dawn of the solar 
system.

Dawn is 8,400 miles (13,600 kilometers) from Ceres. It is also 2.66 AU 
(247 million miles, or 398 million kilometers) from Earth, or 985 times 
as far as the moon and 2.64 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 44 minutes 
to make the round trip.

P.S. Our expert outreach team has done a beautiful job modernizing the 
website, and blog comments are no longer included. I appreciate all the 
very kind feedback, expressions of enthusiasm, interesting questions and 
engaging discussions that the community of Dawnophiles has posted over 
the last year or so. Now I will devote the time I had been spending responding 
to comments to providing more frequent mission status updates as well 
as fun and interesting tidbits you can follow on Twitter @NASA_Dawn.