[meteorite-list] Dawn Journal - August 31, 2014

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2014/08/31/dawn-journal-august-31/

Dawn Journal 
by Marc Rayman
August 31, 2014

Dear Omnipodawnt Readers,

Dawn draws ever closer to the mysterious Ceres, the largest body between 
the sun and Pluto not yet visited by a probe from Earth. The spacecraft 
is continuing to climb outward from the sun atop a blue-green beam of 
xenon ions from its uniquely efficient ion propulsion system. The constant, 
gentle thrust is reshaping its solar orbit so that by March 2015, it will 
arrive at the first dwarf planet ever discovered. Once in orbit, it will 
undertake an ambitious exploration of the exotic world of ice and rock 
that has been glimpsed only from afar for more than two centuries.

An important characteristic of this interplanetary expedition is that 
Dawn can linger at its destinations, conducting extensive observations. 
Since December, we have presented overviews of all the phases of the mission 
at Ceres save one. (In addition, questions posted by readers each month, 
occasionally combined with an answer, have helped elucidate some of the 
interesting features of the mission.) We have described how Dawn will 
approach its gargantuan new home (with an equatorial diameter of more 
than 600 miles, or 975 kilometers) and slip into orbit with the elegance 
of a celestial dancer. The spacecraft will unveil the previously unseen 
sights with its suite of sophisticated sensors from progressively lower 
altitude orbits, starting at 8,400 miles (13,500 kilometers), then from 
survey orbit at 2,730 miles (4,400 kilometers), and then from the misleadingly 
named high altitude mapping orbit (HAMO) only 910 miles (1,470 kilometers) 
away. To travel from one orbit to another, it will use its extraordinary 
ion propulsion system to spiral lower and lower and lower. This month, 
we look at the final phase of the long mission, as Dawn dives down to 
the low altitude mapping orbit (LAMO) at 230 miles (375 kilometers). We 
will also consider what future awaits our intrepid adventurer after it 
has accomplished the daring plans at Ceres.

It will take the patient and tireless robot two months to descend from 
HAMO to LAMO, winding in tighter and tighter loops as it goes. By the 
time it has completed the 160 revolutions needed to reach LAMO, Dawn will 
be circling Ceres every 5.5 hours. (Ceres rotates on its own axis in 9.1 
hours.) The spacecraft will be so close that Ceres will appear as large 
as a soccer ball seen from less than seven inches (17 centimeters) away. 
In contrast, Earth will be so remote that the dwarf planet would look 
to terrestrial observers no larger than a soccer ball from as far as 170 
miles (270 kilometers). Dawn will have a uniquely fabulous view.

As in the higher orbits, Dawn will scrutinize Ceres with all of its scientific 
instruments, returning pictures and other information to eager Earthlings. 
The camera and visible and infrared mapping spectrometer (VIR) will reveal 
greater detail than ever on the appearance and the mineralogical composition 
of the strange landscape. Indeed, the photos will be four times sharper 
than those from HAMO (and well over 800 times better than the best we 
have now from Hubble Space Telescope). But just as in LAMO at Vesta, the 
priority will be on three other sets of measurements which probe even 
beneath the surface.

All of the mass within Ceres combines to hold Dawn in orbit, exerting 
a powerful gravitational grip on the ship. But as the spacecraft moves 
through its orbit, any variations in the internal structure of Ceres from 
one place to another will lead to slight perturbations of the orbit. If, 
for example, there is a large region of unusually dense material, even 
if deep underground, the craft will speed up slightly as it travels toward 
it. After Dawn passes overhead, the same massive feature will slightly 
retard its progress, slowing it down just a little.

Dawn will be in almost constant radio contact with Earth during LAMO. 
When it is pointing its payload of sensors at the surface, it will broadcast 
a faint radio signal through one of its small auxiliary antennas so exquisitely 
sensitive receivers on a planet far, far away can detect it. At other 
times, in order to transmit its findings from LAMO, it will aim its main 
antenna directly at Earth. In both cases, the slightest changes in speed 
toward or away from Earth will be revealed in the Doppler shift, in which 
the frequency of the radio waves changes, much as the pitch of a siren 
goes up and then down as an ambulance approaches and then recedes. Using 
this and other remarkably powerful techniques mastered for traveling throughout 
the solar system, navigators will carefully plot the tiny variations in 
Dawn's orbit and from that determine the distribution of mass throughout 
the interior of the dwarf planet.

The spacecraft will use its sophisticated gamma ray and neutron detector 
(GRaND) to determine the atomic constituents of the material on the surface 
and to a depth of up to about a yard (a meter). Gamma rays are a very, 
very high frequency form of electromagnetic radiation, beyond visible 
light, beyond ultraviolet, beyond even X-rays. Neutrons are very different 
from gamma rays. They are the electrically neutral particles in the nuclei 
of atoms, slightly more massive than protons, and in most elements, neutrons 
outnumber them too. It would be impressive enough if GRaND only detected 
these two kinds of nuclear radiation, but it also measures the energy 
of each kind. (Unfortunately, that description doesn't lend itself to 
such a delightful acronym).

Most of the gamma rays and neutrons are byproducts of the collisions between 
cosmic rays (radiation from elsewhere in space) and the nuclei of atoms 
in the ground. (Cosmic rays don't do this very much at Earth; rather, 
most are diverted by the magnetic field or stopped by atoms in the upper 
atmosphere.) In addition, some gamma rays are emitted by radioactive elements 
near the surface. Regardless of the source, the neutrons and the gamma 
rays that escape from Ceres and travel out into space carry a signature 
of the type of nucleus they came from. When GRaND intercepts the radiation, 
it records the energy, and scientists can translate those signatures into 
the identities of the atoms.

The radiation reaching GRaND, high in space above the surface, is extremely 
faint. Just as a camera needs a long exposure in very low light, GRaND 
needs a long exposure to turn Ceres' dim nuclear glow into a bright 
picture. Fortunately, GRaND's pictures do not depend on sunlight; regions 
in the dark of night are no fainter than those illuminated by the sun.

For most of its time in LAMO, Dawn will point GRaND at the surface beneath 
it. The typical pattern will be to make 15 orbital revolutions, lasting 
about 3.5 days, staring down, measuring each neutron and each gamma ray 
that encounters the instrument. Simultaneously, the craft will transmit 
its broad radio signal to reveal the gentle buffeting by the variations 
in the gravitational field. On portions of its flights over the lit terrain, 
it will take photos and will collect spectra with VIR. Then the spacecraft 
will rotate to point its main antenna to distant Earth, and while it makes 
five more circuits in a little more than a day, it will beam its precious 
discoveries to the 230-foot (70-meter) antennas at NASA's Deep Space 
Network.

Dawn will spend more time in each successive observational phase at Ceres 
than the ones before. After two months in HAMO, during which it will complete 
about 80 orbits, the probe will devote about three months to LAMO, looping 
around more than 400 times. That is more than enough time to collect the 
desired data. Taxpayers have allocated sufficient funds to operate Dawn 
until June 2016, allowing some extra time for the flight team to grapple 
with the inevitable glitches that arise in such a challenging undertaking. 
As in all phases, mission planners recognize that complex operations in 
that remote and hostile environment probably will not go exactly according 
to plan, but even if some of the measurements are not completed, enough 
should be to satisfy all the scientific objectives.

The indefatigable explorer will work hard in LAMO. Aiming its sensors 
at the surface beneath it throughout its 5.5-hour orbits does not happen 
naturally. Dawn needs to keep turning to point them down. When it is transmitting 
its scientific bounty, it needs to hold steady enough to maintain Earth 
in the sights of its radio antenna. An essential element of the design 
of the spacecraft to achieve these and related capabilities was the use 
of three reaction wheels. By electrically changing the speed at which 
these gyroscope-like devices rotate, the probe can turn or stabilize itself. 
Because they are so important, four were included, ensuring that if any 
one encountered difficulty, the ambitious mission could continue with 
the other three.

As long-time readers know, one did falter in June 2010. Another stopped 
operating in August 2012. The failure of two such vital devices could 
have proven fatal for a mission, but thanks to the expertise, creativity, 
swiftness, and persistence of the members of the Dawn flight team, the 
prospects for completing the exploration of Ceres are bright. The two 
remaining reaction wheels are powered off now and will not be used for 
the higher altitude orbits. Rather, the conventional rocket propellant 
hydrazine, squirted out through the tiny thrusters of the reaction control 
system, controls the ship's orientation. It is quite remarkable that 
the team was able to stretch the small supply to cover all the activities 
needed from departure from Vesta in 2012 to the end of the mission in 
LAMO nearly four years later.

When Dawn arrives in LAMO, operators will power the two operable wheels 
on and use them for as long as the pair lasts. Given the unexpectedly 
early loss of the other two (as well as the failures of similar units 
on other spacecraft), engineers do not have high confidence that will 
be very long. But LAMO is the most hydrazine-expensive part of the mission, 
so any useable lifetime will lower (but not stop) the hydrazine expenditure. 
Regardless, with or without functioning reaction wheels, the reliable 
Dawn spacecraft should be able to conduct a fully rewarding, exciting 
campaign at the enigmatic world.

What fate awaits our stalwart adventurer following the completion of its 
primary assignment? There are several possibilities, but they all conclude 
the same way. If hydrazine remains at the end, and if the spacecraft is 
still healthy, NASA will decide whether to invest further in Dawn. NASA 
has many exciting and important activities to choose among - after all, 
there's a vast universe to explore! If it provides further funds, Dawn 
will perform further investigations in LAMO, making GRaND's gamma ray 
and neutron pictures even sharper, refining the gravitational measurements, 
collecting still more photos of the expansive surface, and acquiring even 
more spectra with VIR.

There is no intention to fly to a lower orbit. Even if the two remaining 
reaction wheels operate, hydrazine will be running very low, so time will 
be short. Following another spiral to a different altitude would not be 
wise. There will be no below-LAMO (BLAMO) or super low altitude mapping 
orbit (SLAMO) phase of the mission.

There is another issue as well. As we will describe in December, there 
is good reason to believe Ceres has a substantial inventory of water, 
mostly as ice but perhaps some as liquid.The distant sun and the gradual 
decay of radioactive elements provide a little warmth. Telescopic studies 
suggest the probable presence of organic chemicals. As a result of these 
and other considerations, scientists recognize that Ceres might display 
"prebiotic chemistry," or the ingredients and conditions that, on 
your planet, led to the origin of life. This could present important clues 
to help advance our understanding of how life can arise.

We want to protect that special environment from contamination by the 
great variety of terrestrial materials in the spacecraft. As responsible 
citizens of the solar system, NASA conforms to "planetary protection" 
protocols which specify that Dawn may not reach the surface for at least 
50 years after arrival. (The reasoning behind the limited duration is 
that if our data indicate that Ceres really does need special protection, 
half a century would be long enough to mount another mission if need be.) 
Extensive analyses by engineers and scientists show that for any credible 
detail of the dwarf planet's gravitational field, the orbit will remain 
relatively stable for much longer than that, perhaps even millennia. The 
ship will not make landfall.

Despite some romantic notions of a controlled landing, it would not be 
physically possible, even if there were no planetary protection prohibitions. 
Ceres is entirely unlike the little chunks of rock most people think of 
as asteroids. The behemoth's surface gravity is nearly three percent 
of Earth's. At 800 kilograms, Dawn would weigh the equivalent of about 
50 pounds there. The famously efficient but gentle thrust of the ion propulsion 
system, providing a force equivalent to the weight of a sheet of paper 
on Earth, would be quite insufficient for slowing the spaceship down as 
it approached the hard ground.

The best place for Dawn, should it be asked to continue its work, will 
be in LAMO. And when the last puffs of hydrazine are expelled, it will 
no longer be able to aim its instruments at the surface, any of its ion 
engines in the direction required to maneuver, its antenna at Earth, or 
its solar arrays at the sun. The battery will be depleted in a matter 
of hours. The spacecraft will remain in orbit as surely as the moon remains 
in orbit around Earth, but it will cease operating.

Long after its final controlled actions, indeed long after you, faithful 
reader, and your correspondent and everyone else involved in the mission 
(whether directly or by virtue of sharing in the excitement and the wonder 
of such a grand undertaking) are gone, Dawn's successes will still be 
important. Its place in the annals of space exploration will be secure: 
the first spacecraft to orbit an object in the asteroid belt, the first 
spacecraft to visit a dwarf planet (indeed, the first spacecraft to visit 
the first dwarf planet that was discovered), the first spacecraft to orbit 
a dwarf planet, the first spacecraft to orbit any two extraterrestrial 
destinations, and more. And with Ceres and Vesta being, by far, the two 
most massive of the millions of objects between Mars and Jupiter, Dawn 
will have single-handedly examined about 40 percent of the mass of the 
main asteroid belt. Its scientific legacy will be secure, having revealed 
myriad fascinating and exciting insights into two such different and exotic 
alien bodies, introducing Earth to some of the last uncharted worlds in 
the inner solar system. Leaving the remarkable craft in orbit around the 
distant colossus will be a fitting and honorable conclusion to its historic 
journey of discovery. This interplanetary ambassador from Earth will be 
an inert celestial monument to the power of human ingenuity, creativity, 
and curiosity, a lasting reminder that our passion for bold adventures 
and our noble aspirations to know the cosmos can take us very, very far 
beyond the confines of our humble home.

Dawn is 3.0 million miles (4.8 million kilometers) from Ceres. It is also 
3.07 AU (285 million miles, or 459 million kilometers) from Earth, or 
1,185 times as far as the moon and 3.04 times as far as the sun today. 
Radio signals, traveling at the universal limit of the speed of light, 
take 51 minutes to make the round trip.