Dawn

Dawn is a spacecraft launched by the U.S. whose primary mission is to investigate the asteroid Vesta and the asteroid and dwarf planet Ceres. Dawn will investigate these two asteroids in particular, because they are large, and supposedly have remained intact for billions of years. Also, the ways in which Ceres and Vesta were very different, one formed with a "wet" or icy composition, and one farther out and closer to Jupiter, which formed with a "dry" or rocky composition. The contrast of these two asteroids makes the information collected very beneficial to an understanding of the formation of the Solar System.

Dawn was launched on September 27, 2007, after having been delayed several times. Dawn's orbit continued as roughly an outward spiral. The spacecraft completed an orbit around the Sun, and had a flyby of Mars on February 17, 2009 to put in on track to reach Vesta. On May 3, 2011, the first images of Vesta were captured.

The first image of Vesta taken by Dawn at a distance of approximately 750,000 miles. Another image was taken of Vesta on July 9, only about a week before entering orbit (below).

The gravity assist at Mars slowed the spacecraft down enough to orbit Vesta until 2012. The probe successfully entered Vesta orbit on July 15, 2011, and has begun conducting scientific experiments. The probe used an array of spectrometers and detectors to determine the surface composition of Vesta. Further analysis of Vesta's gravitational field also revealed clues concerning the asteroid's inner structure.

After orbital insertion, Dawn continued to decrease its orbital altitude, mapping the surface in broad swaths during the month of August 2011, and later spiraled into an orbit less than 500 miles from Vesta, from where it began more detailed surface analyses.

One significant feature of Vesta is the difference between the northern and southern hemispheres. The northern is littered with craters and the surface is as old as the Solar System itself, over 4 billion years! However, by dating estimates, the southern hemisphere's surface only has 1-2 billion years' worth of craters, suggesting that a very large impact by another asteroid may have changed the surface.



A view of Vesta showing the northern hemisphere (top) and southern (bottom). Many long scores in the surface are present near the equator, further supporting the idea of a large impact on the asteroid. The scores are probably a result of internal fracturing. Dawn also characterized the temperatures of various areas of the surface of Vesta, and the climate was found to be such that there may be frozen water beneath the surface in the colder regions, despite the asteroid's reputation as "dry". Also, later data indicated an unexpected abundance of hydrated minerals, supporting the possibility that asteroid impacts may have fed Earth's oceans.

Observation of the surface of Vesta on different wavelengths records a wider range of emitted radiation. This radiation, in turn, indicates the surface composition and structure. In the final two images above, false color imaging highlights the differences in material along the surface. Much of the surface is composed of iron and magnesium-rich dust, probably from the accumulation of material and not reflecting the internal composition. This point of view is confirmed when observing craters, where an impact has exposed lower layers of the asteroid, and these have been found to be composed of different minerals. In early 2012, Dawn revealed the unexpected intricacy of Vesta's composition, including a many-layered structure and an iron-rich core, a scenario characteristic of much larger bodies, including many moons.

Having spent almost year in orbit, the spacecraft adjusted its orbit outward in June 2012 to record final data before Vesta departure. This data underwent continued analysis, yielding even more insight. For example, the distribution of hydrated (incorporating water) minerals was different than expected and in turn changed our understanding of how planetary bodies, including the earth, gather water. Vesta showed evidence of receiving water from a steady bombardment of small dust particles very early in the history of the solar system, rather than by large impacts. Also, Dawn found evidence that Vesta is in effect a "mini-planet" as far as internal structure is concerned; there are layers corresponding to crust, mantle, and core in its interior. However, the composition of the asteroid suggests that the formation process of Vesta is more complex than previously thought.

The spacecraft propelled itself away from Vesta in early September 2012, beginning its spiral outward to reach Ceres. By December 27, 2013, Dawn was closer to Ceres than Vesta. By early 2015, the probe was beginning its approach towards Ceres. In mid-January, it began to resolve surface features, as in the image below.

On March 6, 2015, Dawn entered orbit around Ceres at a distance of about 30,000 miles. The insertion represented two historic milestones in spaceflight: Dawn became the first spacecraft ever to visit (or orbit) a dwarf planet, and the first spacecraft to successfully orbit two extraterrestrial targets. Most approaches to objects in the solar system by other spacecraft have been flybys, but the use of ion thrusters allowed Dawn to repeatedly accelerate and decelerate and orbit multiple bodies.

Dawn's arrival trajectory brought it around the side of Ceres facing away from the Sun. The first images taken in orbit (two are shown above) reveal crescents of Ceres from a distance of 30,000 miles. Over the following months, the spacecraft performed several more maneuvers to spiral in towards Ceres in preparation for entering its science orbit in late April.

During the month of May, Dawn returned numerous images of Ceres from its first mapping orbit. In particular, it captured in great resolution the mysterious "bright spots" on Ceres (see below).

The unusually reflective spots are suspected to be ice, but the spacecraft's data had not yet established this definitively. In late May, Dawn began to spiral inward to an altitude of 2,700 miles where it will enter its second mapping orbit. Further thrusts subsequently brought the orbiter to its final science orbit at an altitude of only 235 miles in October of that year. This allowed images to be taken with resolutions as high as 120 ft/pixel.

Further study revealed that the bright spots were primarily due to the presence of a salt compound, sodium carbonate. Its presence had paradigm-shifting ramifications for our understanding of Ceres's interior, namely that this material must have reached the surface due to hydrothermal activity underneath it. This in turn implies that the asteroid's interior is warmer and more dynamic than previously anticipated.

As the analysis of Ceres continued into 2016, the Dawn mission pursued other techniques of analyzing its interior, including through its gravitational field. More orbital maneuvers were making Dawn's orbit about Ceres larger over time, offering global views. By using radio signals to measure precisely how the spacecraft was responding to Ceres's gravitational pull, scientists could infer the distribution of mass in the asteroid. They concluded that its interior was fairly low in density and was differentiated into layers, as with other large Solar System bodies such as planets.

Another significant discovery occurred in February 2017, when Dawn detected organic molecules on Ceres near a crater known as Ernutet. This was the first discovery of its kind for a main belt asteroid and bolstered theories that meteorites on Earth harboring such materials could trace their origins to these objects. The Dawn mission was extended once again by NASA in October 2017.

The next year, the Dawn team planned further maneuvers to lower its closest approach to Ceres to an even lower altitude. The above photo, taken in May 2018, reveals surface features from an altitude of 270 miles. At the completion of the orbit adjustment (in early June), the spacecraft's highly elliptical orbit brought it as close as 22 miles from the surface before retreating to 2,500 miles on every circuit.

Finally, on October 31, 2018, the spacecraft went silent. After a day of studying the problem, the Dawn team concluded that the probe had finally run out of the hydrazine fuel that enabled it to control its orientation. After over 11 years in space, this concluded the highly successful mission. The image of Ceres below was among the last transmitted by the Dawn mission.

Dawn was the first spacecraft ever to orbit two different extraterrestrial objects and the wealth of data it returned will be invaluable to our future understanding of the birth of our own Solar System.
For more information, see the NASA page on Dawn.

Images from wikipedia, and Dawn website, at http://dawn.jpl.nasa.gov/