Does Asteroid Vesta Have a Moon?   
       
   July 6, 2011: NASA's Dawn spacecraft is closing in on Vesta, and from now   
   until the ion-powered spacecraft goes into orbit in mid-July, every picture of   
   the giant asteroid will be the best one ever taken. What will researchers do   
   with this unprecedented clarity?   
       
   "For starters," says Dawn chief engineer Marc Rayman, "we're going to look for   
   an asteroid moon."   
       
   You might think of asteroids as isolated bodies tumbling alone through space,   
   but it's entirely possible for these old "loners" to have companions. Indeed,   
   19-mile-wide Ida, 90-mile-wide Pulcova, 103-mile-wide Kalliope, and   
   135-mile-wide Eugenia each have a moon. And 175-mile-wide Sylvia has two   
   moons. Measuring 330 miles across, Vesta is much larger than these other   
   examples, so a "Vesta moon" is entirely possible.   
   [...]   
   NASA's Galileo spacecraft took this image of asteroid Ida and its moon Dactyl   
   in 1994. The image was the first conclusive evidence that natural satellites   
   of asteroids exist. We still don't know whether Vesta has a moon. [more]   
   Where do such moons come from?   
       
   Rayman suggests one source: "When another large body collides with an   
   asteroid, the resulting debris is sprayed into orbit around the asteroid and   
   can gradually collapse to form a moon."   
       
   Another possibility is "gravitational pinball": A moon formed elsewhere in the   
   asteroid belt might, through complicated gravitational interactions with   
   various bodies, end up captured by the gravity of one of them.1   
       
   Hubble and ground based telescopes have looked for Vesta moons before, and   
   seen nothing. Dawn is about to be in position for a closer look. This   
   Saturday, July 9th, just one week before Dawn goes into orbit around Vesta,   
   the moon hunt will commence.2 The cameras will begin taking images of the   
   space surrounding the asteroid, looking for suspicious specks.   
       
   "If a moon is there, it will appear as a dot that moves around Vesta in   
   successive images as opposed to remaining fixed, like background stars," says   
   Dawn Co-investigator Mark Sykes, who is also director of the Planetary Science   
   Institute. "We'll be able to use short exposures to detect moons as small as   
   27 meters in diameter. If our longer exposures aren't washed out by the glare   
   of nearby Vesta, we'll be able to detect moons only a few meters in diameter."   
   [...]   
   Approaching Vesta: Dawn took this picture on June 24, 2011. [more] While you   
   won't see "find a moon" among the mission's science goals, a moon-sighting   
   would be a nice feather in Dawn's cap. Not that it will need more feathers.   
   The probe is already primed to build global maps and take detailed images of   
   the asteroid's surface, reveal the fine points of its topography, and catalog   
   the minerals and elements present there.3   
       
   Besides, Dawn will become a moon itself when it enters orbit around Vesta. And   
   the probe's motions as it circles will provide a lot of information about the   
   rocky relic.   
       
   Sykes explains: "We'll use the spacecraft's radio signal to measure its motion   
   around Vesta. This will give us a lot of detailed information about the   
   asteroid's gravitational field. We'll learn about Vesta's mass and interior   
   structure, including its core and potential mascons (lumpy concentrations of   
   mass)."   
       
   As you read this, the spacecraft is gently thrusting closer to its target. And   
   with the navigation images alone we're already watching a never-before-seen   
   world grow ever larger and clearer.   
       
   "The pictures are beginning to reveal the surface of this battered, alien   
   world," says Rayman. "They're more than enough to tantalize us. We've been in   
   flight for four years, we've been planning the mission for a decade, and   
   people have been looking at Vesta in the night sky for two centuries. Now,   
   finally, we're coming close up to it, and we'll be getting an intimate view of   
   this place."   
       
   This is not only the first time a spacecraft has visited this alien world,   
   it's also the first time a spacecraft has visited a massive body we haven't   
   approached previously. In the past, rocket ships have orbited Earth, the moon,   
   Mars, Venus, Jupiter, Saturn, and Mercury.   
       
   "In each case, flyby missions occurred first, providing a good estimate of the   
   target's gravity along with information on other aspects of its physical   
   environment, including whether any moons are present. This time we're much   
   less certain what we'll find."   
       
   At a recent press conference, NASA Planetary Science Deputy Director Jim Adams   
   told reporters that Dawn will "paint a face on a world seen only as a 'fuzzy   
   blob' up to now." What does Rayman think Vesta's face will look like?   
       
   "Wrinkled, ancient, wizened, with a tremendous amount of character that bears   
   witness to some fascinating episodes in the solar system's history."   
       
   If a new moon is among the episodes, Rayman has a name in mind.   
       
   "How about 'Dawn'?"   
       
       
   Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA   
       
   More Information   
   Dawn Journal -- penned by Dawn's chief engineer Marc Rayman   
       
   Footnotes:   
       
   (1) In addition to having moons, asteroids can also be double: Binary   
   asteroids sometimes form when a spinning parent body splits. The body is spun   
   up by a phenomenon called YORP that occurs when the body absorbs photons from   
   the sun and reradiates them as heat: more.   
       
   (2) Dawn will perform dedicated observations in search of moons for about 15   
   hours. However, if no moon is found at Vesta on July 9th, that doesn't mean   
   there isn't one. Rayman says: "If there is a moon, it might show up by   
   coincidence in other observations, but we have no other observations dedicated   
   in this mission to finding a moon. There is just so much to learn about Vesta   
   itself, that that is where we are focusing our time."   
       
   (3) From NASA press release: When Vesta captures Dawn into its orbit on July   
   16, there will be approximately 9,900 miles (16,000 kilometers) between them.   
   When orbit is achieved, they will be approximately 117 million miles (188   
   million kilometers) away from Earth. During the initial reconnaissance orbit,   
   at approximately 1,700 miles (2,700 kilometers), the spacecraft will get a   
   broad overview of Vesta with color pictures and data in different wavelengths   
   of reflected light. The spacecraft will move into a high-altitude mapping   
   orbit, about 420 miles (680 kilometers) above the surface to systematically   
   map the parts of Vesta's surface illuminated by the sun; collect stereo images   
   to see topographic highs and lows; acquire higher-resolution data to map rock   
   types at the surface; and learn more about Vesta's thermal properties. Dawn   
   then will move even closer, to a low-altitude mapping orbit approximately 120   
   miles (200 kilometers) above the surface. The primary science goals of this   
   orbit are to detect the byproducts of cosmic rays hitting the surface and help   
   scientists determine the many kinds of atoms there, and probe the   
   protoplanet's internal structure. As Dawn spirals away from Vesta, it will   
   pause again at the high-altitude mapping orbit. Because the sun's angle on the   
   surface will have progressed, scientists will be able to see previously hidden   
   terrain while obtaining different views of surface features.   
       
   Credits: Dawn's mission to Vesta and Ceres is managed by JPL for NASA's   
   Science Mission Directorate in Washington. Dawn is a project of the   
   directorate's Discovery Program, managed by NASA's Marshall Space Flight   
   Center in Huntsville, Ala. UCLA is responsible for overall Dawn mission   
   science. Orbital Sciences Corp. of Dulles, Va., designed and built the   
   spacecraft. The German Aerospace Center, the Max Planck Institute for Solar   
   System Research, the Italian Space Agency and the Italian National   
   Astrophysical Institute are part of the mission team. JPL is managed for NASA   
   by the California Institute of Technology in Pasadena. For more information   
   about Dawn, visit: http://www.nasa.gov/dawn and http://dawn.jpl.nasa.gov . You   
   can also follow Dawn on Twitter at: http://www.twitter.com/NASA_Dawn .   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.62   
    * Origin: NCS BBS (1:3828/7)