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.
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.
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.
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.
Dr. Marc D. Rayman
6:00 p.m. PDT August 31, 2014