SCIENCE CORNER
The right side of Dantu crater on Ceres as pictured in November (Photo – Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA).
A veteran interplanetary traveler is writing the closing chapter in its long and storied expedition.
By Dr. Marc D. Rayman
In its final orbit, where it will remain even beyond the end of its mission, at its lowest altitude, Dawn is circling dwarf planet Ceres, gathering an album of spellbinding pictures and other data to reveal the nature of this mysterious world of rock and ice.
Ceres turns on its axis in a little more than nine hours (one Cerean day). Meanwhile, its new permanent companion, a robotic emissary from Earth, revolves in a polar orbit, completing a loop in slightly under 5.5 hours.
Now that Dawn is only about 240 miles high, its images are four times as sharp, revealing new details of the strange and beautiful landscapes.
Our spaceship is closer to Ceres than the International Space Station is to Earth. At that short range, it takes a long time to capture all of the vast territory, because each picture covers a relatively small area.
Excerpt from an extensive animation of Occator crater, with its famous bright regions, on Ceres. It is made with the color and stereo pictures Dawn collected in its third mapping orbit 915 miles (Photo – NASA/JPL-Caltech/UCLA/MPS/DLR/IDA).
The ground beneath it
Most of the time, Dawn is programmed to turn at just the right rate to keep looking at the ground beneath it as it travels, synchronizing its rotation with its revolution around Ceres. It photographs the passing scenery, storing the pictures for later transmission to Earth. But some of the time, it cannot take pictures, because to send its bounty of data, it needs to point its main antenna at earth. Dawn spends about three and a half days (nine Cerean days) with its camera and other sensors pointed at Ceres. Then it radios its findings home for a little more than one day (almost three Cerean days). During these communications sessions, even when it soars over lit terrain, it does not observe the sights below.
Sometimes controllers program Dawn to take a few more pictures after it stops aiming its instruments down, while it starts to turn to aim its antenna to Earth. This clever idea provides bonus views of whatever happens to be in the camera’s sights as it slowly rotates from the point beneath the spacecraft off to the horizon. Who doesn’t feel the attraction of the horizon and long to know what lies beyond?
During the course of 2015, Ceres grew from a small and unremarkable disc to a complex and intriguing world in this selection of Dawn’s pictures (Photo – NASA/JPL-Caltech/UCLA/MPS/DLR/IDA).
Ceres’ only satellite
…for the benefit
of all those
who remain with Earth
but who still look
to the sky
with wonder…
Earth and Ceres are so far from each other that their motions are essentially independent. The planet and the dwarf planet follow their own separate repetitive paths around the sun. And each carries its own retinue: Earth has thousands of artificial satellites and one prominent natural one, the moon. Ceres has one known satellite. It arrived there in March 2015, and its name is Dawn.
Far, far from the planet where its deep-space voyage began, Dawn is now bound to Ceres, held in a firm but gentle gravitational embrace. The spacecraft continues to unveil new and fascinating secrets there for the benefit of all those who remain with Earth but who still look to the sky with wonder, who feel the lure of the unknown, who are thrilled by new knowledge, and who yearn to know the cosmos.
Dawn is 240 miles from Ceres. It is also 3.87 AU (360 million miles, or 580 million kilometers) from Earth, or 1,440 times as far as the moon and 3.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and four minutes to make the round trip.
Dr. Marc D. Rayman is the Dawn Mission Director and Chief Engineer at JPL. Marc greatly enjoys sharing the thrill of interplanetary adventures with the public.
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