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What Was Dawn's Mission on Vesta?
On September 27, 2007 Dawn launched on top of a Delta II rocket from Cape Canaveral just after sunrise, thus beginning its 3.2 billion mile-long trip to Vesta. Principal investigator Chris Russel had some time to kill, for the first year was uneventful but in July 2008 it began to slow down so that Mars could capture it. As Dawn fell into the gravity well of Mars, it was able to use some of the angular momentum the planet had to boost Dawn’s speed, cut some time off the mission duration, and increase the angle it has to the ecliptic by 5 degrees, putting it on the same plane as Vesta. This gravity maneuver also saved Dawn money, for if it had not done the boost then an additional 230 pounds of xenon would have been required to increase Dawn’s speed by 5,800 miles per hour. Dawn also used the fly-by to calibrate its instruments by cross-referencing with other prober that were already in Mars orbit (Guterl 49, NASA “Spacecraft Falling”).
Arrival at Vesta and Investigations
Finally, On July 16, 2011, Dawn entered Vesta orbit and began a series of orbital maneuvers to document the asteroid at three main orbital levels. The spectrometer took data from a 680 kilometer orbit and also after Dawn moved to a 210 kilometer orbit on December 12 to help determine the chemical composition and also what was molten and what was just debris on the surface. Dawn spotted breccia, which are formed when rocks impact at high velocities. Some of them are iron an magnesium-rich, known as pyroxene, very similar to Earth volcano rocks. This is partial evidence for molten activity on Vesta in the past. Some smooth areas are also visible on Vesta, possibly because of dust settling on the surface after impacts. While all of this was intriguing, it seemed to hint that Vesta’s inner layers may be undiscernible, hidden from view or simply melted away, according to Carol Raymond (Dawn's deputy principal investigator). Further observations from the gravity probe and GRaND revealed the lattermost was likely. A deep crater would be needed to help determine more of Vesta’s properties. (NASA “Dawn Reveals”, Dunbar “NASA’s Dawn,” Kruesi "Dawn," Ferron "Dawn").
The Tarpeia crater, near the south pole of Vesta, fit the bill. It allowed scientists to look at the layering and determine what was new and what was ancient. But two even bigger craters awaited Vesta for further investigation. Rheasilvia, 314 miles wide (9/10 of Vesta’s diameter, happened 1 billion years ago, while Veneneia, 245 miles wide (3/4 of Vesta’s diameter), happened 2 billion years ago. It is hard to imagine that kind of devastation upon a body, but Vesta weathered it and survived (mostly intact). Remember those HED meteorites mentioned earlier? Rheasilvia is the remnant of the event that helped create them. Interestingly, when you compare the crater height to the width, they are taller than those on the Moon and also have a greater variety of color than their lunar counterparts, making Vesta more like the moons of Saturn and Jupiter (NASA “Dawn Reveals”, Redd, NASA/JPL “NASA’s Dawn,” Ferron "Dawn").
As Dawn continued to orbit Vesta, more and more discoveries were made, many because of those craters. Vesta seems more like a planet than it does an asteroid, with a crust and mantle surrounding an iron core that is about 68 miles in diameter. This iron core was determined based on density measurements as well as the gravity field of Vesta. The layering was based on the depth of Rheasilvia and Veneneia .That magma on the surface may be a result of the collisions that formed those two large craters liquefying the crust, causing it to become thicker. Temperatures on Vesta range from -10 degrees F to potentially more than -150 degrees F (for this was the lowest temperature range Dawn could measure). This wide range demonstrated the lack of an atmosphere regulating the temperature fluctuations (NASA/JPL “NASA’s Dawn,” Ferron "Dawn").
More evidence for a layered Vesta may have been found in some linear features on the surface of the asteroid. Scientists now think that they are analogous to graben, or the gap between faults that we see here on Earth's crust, based on their similar U-shape (while most gaps on asteroids form a V-shape). Models indicate that a large hit taken by Vesta would have created the graben, but some scientists want more evidence before they make their call, for they want to see the features go through craters and other permanent structures. An alternate theory states that the gaps on Vesta were caused by one of the giant collisions to the asteroid's south pole, which would have increased its rotation rate and bulged the equator out, causing gaps on the surface. If Vesta is layered, then it causes the distinction of planet to become even murkier than it currently is (American Geophysical Union).
Additionally, Dawn data indicates that minerals having exposure to water may have been found around the equator of Vesta. There, markings on the surface indicate potential places where water could have boiled off. The instrument that brought it there was space rocks that collided as a rate sufficient for the hydrogen they brought to merge with oxygen and become water. But because of the location of the water near the equator it quickly disappeared (NASA/JPL "Dawn Spacecraft," Betz).
Dawn was making so much headway that it was given 40 days of bonus time to make even better measurements of Vesta. This was financially possible because of the good fiscal skills the team utilized. The extra time was spent at the 210 kilometer range, allowing GRaND to continue mapping elements and refining the gravity field. It also allowed Dawn to orbit more of the northern hemisphere which was in darkness upon Dawn’s arrival. But all good things must come to an end, and so Dawn left Vesta in the beginning of September 2012. It slowly spiraled out of its orbit using its ion engines and set a course towards Ceres (JPL “Dawn”, NASA/JPL “NASA’s Dawn Ready," NASA/JPL "Dawn Has Departed").
Even after Dawn left Vesta the science it gathered was being analyzed and put to use against computer models that try to show how Vesta formed. According to the simulation, 20 mile-wide rocks hit Vesta and caused the surface to liquefy, causing the crust to become thicker than before. If it did not liquefy then the crust is thin so some of the mantle material would have been brought to the surface. Since the mantle is made of olivine, Dawn should have seen it on the surface or in traces of the 60 mile-deep craters. But Dawn did not find any signs of olivine. This hints at the thicker crust scenario (up to 80 miles deep), although it is possible that Dawn just missed it (because olivine is hard to find with spectrometers) or that it is buried underneath the surface debris on Vesta. On top of this, lots of aluminum-26 has been found on the surface, hinting at an early solar system formation (for 26 is a daughter of a radioactive decay parent). If any of this is confirmed, then planetary models may need to be updated to include more complicated formations accounting for rock formations forming in the mantle and rising to the surface to build up the crust further (Redd, Ecole, Betz). Who knows what other surprises await us on this new tour of outer space.
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© 2015 Leonard Kelley