Archive for August, 2014


31 Aug
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | August 31

by Marc Rayman

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.


Dawn’s spiral transfer from HAMO to LAMO. The trajectory turns from blue to red as time progresses during the two months. Red dashed sections are where ion thrusting is stopped so the spacecraft can point its main antenna to Earth. Credit: NASA/JPL

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 3.5 inches (less than nine 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.

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