Read more at www.nasa.gov
Though generally thought to be quite dry, roughly half of the giant asteroid Vesta is expected to be so cold and to receive so little sunlight that water ice could have survived there for billions of years, according to the first published models of Vesta’s average global temperatures and illumination by the sun.
“Near the north and south poles, the conditions appear to be favorable for water ice to exist beneath the surface,” says Timothy Stubbs of NASA’s Goddard Space Flight Center in Greenbelt, Md., and the University of Maryland, Baltimore County. Stubbs and Yongli Wang of the Goddard Planetary Heliophysics Institute at the University of Maryland published the models in the January 2012 issue of the journal Icarus. The models are based on information from telescopes including NASA’s Hubble Space Telescope.
Vesta, the second-most massive object in the asteroid belt between Mars and Jupiter, probably does not have any significant permanently shadowed craters where water ice could stay frozen on the surface all the time, not even in the roughly 300-mile-diameter (480-kilometer-diameter) crater near the south pole, the authors note. The asteroid isn’t a good candidate for permanent shadowing because it is tilted on its axis at about 27 degrees, which is even greater than Earth’s tilt of roughly 23 degrees. In contrast, the moon, which does have permanently shadowed craters, is tilted at only about 1.5 degrees. As a result of its large tilt, Vesta has seasons, and every part of the surface is expected to see the sun at some point during Vesta’s year.
The presence or absence of water ice on Vesta tells scientists something about the tiny world’s formation and evolution, its history of bombardment by comets and other objects, and its interaction with the space environment. Because similar processes are common to many other planetary bodies, including the moon, Mercury and other asteroids, learning more about these processes has fundamental implications for our understanding of the solar system as a whole. This kind of water ice is also potentially valuable as a resource for further exploration of the solar system.
Though temperatures on Vesta fluctuate during the year, the model predicts that the average annual temperature near Vesta’s north and south poles is less than roughly minus 200 degrees Fahrenheit (145 kelvins). That is the critical average temperature below which water ice is thought to be able to survive in the top 10 feet or so (few meters) of the soil, which is called regolith.
Near Vesta’s equator, however, the average yearly temperature is roughly minus 190 degrees Fahrenheit (150 kelvins), according to the new results. Based on previous modeling, that is expected to be high enough to prevent water from remaining within a few meters of the surface. This band of relatively warm temperatures extends from the equator to about 27 degrees north and south in latitude.
“On average, it’s colder at Vesta’s poles than near its equator, so in that sense, they are good places to sustain water ice,” says Stubbs. “But they also see sunlight for long periods of time during the summer seasons, which isn’t so good for sustaining ice. So if water ice exists in those regions, it may be buried beneath a relatively deep layer of dry regolith.”
The modeling also indicates that relatively small surface features, such as craters measuring around 6 miles (10 kilometers) in diameter, could significantly affect the survival of water ice. “The bottoms of some craters could be cold enough on average — about 100 kelvins — for water to be able to survive on the surface for much of the Vestan year [about 3.6 years on Earth],” Stubbs explains. “Although, at some point during the summer, enough sunlight would shine in to make the water leave the surface and either be lost or perhaps redeposit somewhere else.”
So far, Earth-based observations suggest that the surface of Vesta is quite dry. However, the Dawn spacecraft is getting a much closer view. Dawn is investigating the role of water in the evolution of planets by studying Vesta and Ceres, two bodies in the asteroid belt that are considered remnant protoplanets – baby planets whose growth was interrupted when Jupiter formed.
Dawn is looking for water using the gamma ray and neutron detector (GRaND) spectrometer, which can identify hydrogen-rich deposits that could be associated with water ice. The spacecraft recently entered a low orbit that is well suited to collecting gamma ray and neutron data.
“Our perceptions of Vesta have been transformed in a few months as the Dawn spacecraft has entered orbit and spiraled closer to its surface,” says Lucy McFadden, a planetary scientist at NASA Goddard and a Dawn mission co-investigator. “More importantly, our new views of Vesta tell us about the early processes of solar system formation. If we can detect evidence for water beneath the surface, the next question will be is it very old or very young, and that would be exciting to ponder.”
The modeling done by Stubbs and Wang, for example, relies on information about Vesta’s shape. Before Dawn, the best source of that information was a set of images taken by NASA’s Hubble Space Telescope in 1994 and 1996. But now, Dawn and its camera are getting a much closer view of Vesta.
“The Dawn mission gives researchers a rare opportunity to observe Vesta for an extended period of time, the equivalent of about one season on Vesta,” says Stubbs. “Hopefully, we’ll know in the next few months whether the GRaND spectrometer sees evidence for water ice in Vesta’s regolith. This is an important and exciting time in planetary exploration.”…
Read more: www.nasa.gov
NASA’s Dawn spacecraft has returned the first close-up image after beginning its orbit around the giant asteroid Vesta. On Friday, July 15, Dawn became the first probe to enter orbit around an object in the main asteroid belt between Mars and Jupiter.
The image taken for navigation purposes shows Vesta in greater detail than ever before. When Vesta captured Dawn into its orbit, there were approximately 9,900 miles (16,000 kilometers) between the spacecraft and asteroid. Engineers estimate the orbit capture took place at 10 p.m. PDT Friday, July 15 (1 a.m. EDT Saturday, July 16).
Vesta is 330 miles (530 kilometers) in diameter and the second most massive object in the asteroid belt. Ground- and space-based telescopes have obtained images of Vesta for about two centuries, but they have not been able to see much detail on its surface. “We are beginning the study of arguably the oldest extant primordial surface in the solar system,” said Dawn principal investigator Christopher Russell from the University of California, Los Angeles. “This region of space has been ignored for far too long. So far, the images received to date reveal a complex surface that seems to have preserved some of the earliest events in Vesta’s history, as well as logging the onslaught that Vesta has suffered in the intervening eons.”
Vesta is thought to be the source of a large number of meteorites that fall to Earth. Vesta and its new NASA neighbor, Dawn, are currently approximately 117 million miles (188 million kilometers) away from Earth. The Dawn team will begin gathering science data in August. Observations will provide unprecedented data to help scientists understand the earliest chapter of our solar system. The data also will help pave the way for future human space missions.
After traveling nearly four years and 1.7 billion miles (2.8 billion kilometers), Dawn also accomplished the largest propulsive acceleration of any spacecraft, with a change in velocity of more than 4.2 miles per second (6.7 kilometers per second), due to its ion engines. The engines expel ions to create thrust and provide higher spacecraft speeds than any other technology currently available. “Dawn slipped gently into orbit with the same grace it has displayed during its years of ion thrusting through interplanetary space,” said Marc Rayman, Dawn chief engineer and mission manager at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “It is fantastically exciting that we will begin providing humankind its first detailed views of one of the last unexplored worlds in the inner solar system.”
Although orbit capture is complete, the approach phase will continue for about three weeks. During approach, the Dawn team will continue a search for possible moons around the asteroid; obtain more images for navigation; observe Vesta’s physical properties; and obtain calibration data.
In addition, navigators will measure the strength of Vesta’s gravitational tug on the spacecraft to compute the asteroid’s mass with much greater accuracy than has been previously available. That will allow them to refine the time of orbit insertion.
Dawn will spend one year orbiting Vesta, then travel to a second destination, the dwarf planet Ceres, arriving in February 2015. The mission to Vesta and Ceres is managed by JPL for the agency’s Science Mission Directorate in Washington. Dawn is a project of the directorate’s Discovery Program, which is managed by NASA’s Marshall Space Flight Center in Huntsville, Ala.
UCLA is responsible for 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’s team.
This movie shows surface details beginning to resolve as NASA’s Dawn spacecraft closes in on the giant asteroid Vesta. The framing camera aboard NASA’s Dawn spacecraft obtained the images used for this animation on June 1, 2011, from a distance of about 300,000 miles (483,000 kilometers).
Vesta’s jagged shape, sculpted by eons of cosmic impacts in the main asteroid belt, is apparent. Variations in surface brightness and hints of surface features can be seen. Vesta’s south pole is to the lower right at about the 5 o’clock position.
Vesta is 330 miles (530 kilometers) in diameter and the second most massive object in the asteroid belt. It is also the only large asteroid with a basaltic surface formed due to volcanic processes early in the solar system’s history. Vesta is considered a protoplanet because it is a large body that almost formed into a planet.
The video presents 20 frames, looped five times, that span a 30-minute period. During that time, Vesta rotates about 30 degrees. The images included here are used by navigators to fine-tune Dawn’s trajectory during its approach to Vesta, with arrival expected on July 16, 2011.
The Dawn mission to Vesta and Ceres is managed by the Jet Propulsion Laboratory, for NASA’s Science Mission Directorate, Washington, D.C. It is a project of the Discovery Program managed by NASA’s Marshall Space Flight Center, Huntsville, Ala. UCLA, is responsible for overall Dawn mission science. Orbital Sciences Corporation of Dulles, Va., designed and built the Dawn spacecraft.