Comet Hartley 2 Leaves a Bumpy Trail

New findings from NEOWISE, the asteroid- and comet-hunting portion of NASA's Wide-field Infrared Survey Explorer mission, show that comet Hartley 2 leaves a pebbly trail as it laps the sun, dotted with grains as big as golf balls.

Previously, NASA's EPOXI mission, which flew by the comet on Nov. 4, 2010, found golf ball- to basketball-sized fluffy ice particles streaming off comet Hartley 2. NEOWISE data show that the golf ball-sized chunks survive farther away from the comet than previously known, winding up in Hartley 2's trail of debris. The NEOWISE team determined the size of these particles by looking at how far they deviated from the trail. Larger particles are less likely to be pushed away from the trail by radiation pressure from the sun.

The observations also show that the comet is still actively ejecting carbon dioxide gas at a distance of 2.3 astronomical units from the sun, which is farther away from the sun than where EPOXI detected carbon dioxide jets streaming from the comet. An astronomical unit is the average distance between Earth and the sun.

"We were surprised that carbon dioxide plays a significant role in comet Hartley 2's activity when it's farther away from the sun," said James Bauer, the lead author of a new paper on the result in the Astrophysical Journal.

JPL manages and operates the Wide-field Infrared Survey Explorer for NASA's Science Mission Directorate, Washington. The principal investigator, Edward Wright, is at UCLA. The mission was competitively selected under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Md.

The science instrument was built by the Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

Dawn Nears Start of Year-Long Stay at Giant Asteroid

NASA's Dawn spacecraft obtained this image on its approach to the protoplanet Vesta, the second-most massive object in the main asteroid belt.

PASADENA, Calif. – NASA's Dawn spacecraft is on track to begin the first extended visit to a large asteroid. The mission expects to go into orbit around Vesta on July 16 and begin gathering science data in early August. Vesta resides in the main asteroid belt and is thought to be the source of a large number of meteorites that fall to Earth.

"The spacecraft is right on target," said Robert Mase, Dawn project manager at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. "We look forward to exploring this unknown world during Dawn's one-year stay in Vesta's orbit."

After traveling nearly four years and 1.7 billion miles (2.7 billion kilometers), Dawn is approximately 96,000 miles (155,000 kilometers) away from Vesta. 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.

These views of the protoplanet Vesta were obtained by NASA's Dawn spacecraft and NASA's Hubble Space Telescope.

After Dawn enters Vesta's orbit, engineers will need a few days to determine the exact time of capture. Unlike other missions where a dramatic, nail-biting propulsive burn results in orbit insertion around a planet, Dawn has been using its placid ion propulsion system to subtly shape its path for years to match Vesta's orbit around the sun.

Images from Dawn's framing camera, taken for navigation purposes, show the slow progress toward Vesta. They also show Vesta rotating about 65 degrees in the field of view. The images are about twice as sharp as the best images of Vesta from NASA's Hubble Space Telescope, but the surface details Dawn will obtain are still a mystery.

"Navigation images from Dawn's framing camera have given us intriguing hints of Vesta, but we're looking forward to the heart of Vesta operations, when we begin officially collecting science data," said Christopher Russell, Dawn principal investigator, at UCLA. "We can't wait for Dawn to peel back the layers of time and reveal the early history of our solar system."

WISE Image Reveals Strange Specimen in Starry Sea

A new image from NASA's Wide-field Infrared Survey Explorer shows what looks like a glowing jellyfish floating at the bottom of a dark, speckled sea. In reality, this critter belongs to the cosmos -- it's a dying star surrounded by fluorescing gas and two very unusual rings.

"I am reminded of the jellyfish exhibition at the Monterey Bay Aquarium -- beautiful things floating in water, except this one is in space," said Edward (Ned) Wright, the principal investigator of the WISE mission at UCLA, and a co-author of a paper on the findings, reported in the Astronomical Journal.

The object, known as NGC 1514 and sometimes the "Crystal Ball" nebula, belongs to a class of objects called planetary nebulae, which form when dying stars toss off their outer layers of material. Ultraviolet light from a central star, or in this case a pair of stars, causes the gas to fluoresce with colorful light. The result is often beautiful -- these objects have been referred to as the butterflies of space.

NGC 1514 was discovered in 1790 by Sir William Herschel, who noted that its "shining fluid" meant that it could not be a faint cluster of stars, as originally suspected. Herschel had previously coined the term planetary nebulae to describe similar objects with circular, planet-like shapes.

Planetary nebulae with asymmetrical wings of nebulosity are common. But nothing like the newfound rings around NGC 1514 had been seen before. Astronomers say the rings are made of dust ejected by the dying pair of stars at the center of NGC 1514. This burst of dust collided with the walls of a cavity that was already cleared out by stellar winds, forming the rings.

"I just happened to look up one of my favorite objects in our WISE catalogue and was shocked to see these odd rings," said Michael Ressler, a member of the WISE science team at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and lead author of the Astronomical Journal paper. Ressler first became acquainted with the object years ago while playing around with his amateur telescope on a desert camping trip. "It's funny how things come around full circle like this."

WISE was able to spot the rings for the first time because their dust is being heated and glows with the infrared light that WISE can detect. In visible-light images, the rings are hidden from view, overwhelmed by the brightly fluorescing clouds of gas.

"This object has been studied for more than 200 years, but WISE shows us it still has surprises," said Ressler.

Infrared light has been color-coded in the new WISE picture, such that blue represents light with a wavelength of 3.4 microns; turquoise is 4.6-micron light; green is 12-micron light; and red is 22-micron light. The dust rings stand out in orange. The greenish glow at the center is an inner shell of material, blown out more recently than an outer shell that is too faint to be seen in WISE's infrared view. The white dot in the middle is the central pair of stars, which are too close together for WISE to see separately.

Ressler says NGC 1514's structure, though it looks unique, is probably similar in overall geometry to other hour-glass nebulae, such as the Engraved Hourglass Nebula (http://hubblesite.org/newscenter/archive/releases/1996/07). The structure looks different in WISE's view because the rings are detectable only by their heat; they do not fluoresce at visible wavelengths, as do the rings in the other objects.

Serendipitous findings like this one are common in survey missions like WISE, which comb through the whole sky. WISE has been surveying the sky in infrared light since January 2010, cataloguing hundreds of millions of asteroids, stars and galaxies. In late September, after covering the sky about one-and-a-half times, it ran out of the frozen coolant needed to chill its longest-wavelength detectors. The mission, now called NEOWISE, is still scanning the skies with two of its infrared detectors, focusing primarily on comets and asteroids, including near-Earth objects, which are bodies whose orbits pass relatively close to Earth's orbit around the sun.

The WISE science team says that more oddballs like NGC 1514 are sure to turn up in the plethora of WISE data -- the first batch of which will be released to the astronomical community in spring 2011.

Other study authors are Martin Cohen of the Monterey Institute for Research in Astronomy, Marina, Calif.; Stefanie Wachter and Don Hoard of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena; and Amy Mainzer of JPL.

JPL manages and operates the Wide-field Infrared Survey Explorer for NASA's Science Mission Directorate, Washington. The mission was competitively selected under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at Caltech.

NASA's LRO Exposes Moon's Complex, Turbulent Youth

The moon was bombarded by two distinct populations of asteroids or comets in its youth, and its surface is more complex than previously thought, according to new results from NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft featured in three papers appearing in the Sept. 17 issue of Science.



In the first paper, lead author James Head of Brown University in Providence, R.I., describes results obtained from a detailed global topographic map of the moon created using LRO's Lunar Orbiter Laser Altimeter (LOLA). "Our new LRO LOLA dataset shows that the older highland impactor population can be clearly distinguished from the younger population in the lunar 'maria' -- giant impact basins filled with solidified lava flows," says Head. "The highlands have a greater density of large craters compared to smaller ones, implying that the earlier population of impactors had a proportionally greater number of large fragments than the population that characterized later lunar history."

Meteorite impacts can radically alter the history of a planet. The moon, Mars, and Mercury all bear scars of ancient craters hundreds or even thousands of miles across. If Earth was subjected to this assault as well -- and there's no reason to assume our planet was spared -- these enormous impacts could have disrupted the initial origin of life. Large impacts that occurred later appear to have altered life's evolution. The approximately 110-mile-diameter, partially buried crater at Chicxulub, in the Yucatan Peninsula of Mexico, is from an impact about 65 million years ago that is now widely believed to have led or contributed to the demise of the dinosaurs and many other lifeforms.

Scientists trying to reconstruct the meteorite bombardment history of Earth face difficulty because impact craters are eroded by wind and water, or destroyed by the action of plate tectonics, the gradual movement and recycling of the Earth's crust. However, a rich record of craters is preserved on the moon, because it has only an extremely thin atmosphere – a vacuum better than those typically used for experiments in laboratories on Earth. The moon’s surface has no liquid water and no plate tectonics. The only source of significant erosion is other impacts.

"The moon is thus analogous to a Rosetta stone for understanding the bombardment history of the Earth," said Head. "Like the Rosetta stone, the lunar record can be used to translate the 'hieroglyphics' of the poorly preserved impact record on Earth."

Even so, previous lunar maps had different resolutions, viewing angles, and lighting conditions, which made it hard to consistently identify and count craters. Head and his team used the LOLA instrument on board LRO to build a map that highlights lunar craters with unprecedented clarity. The instrument sends laser pulses to the lunar surface, measures the time that it takes for them to reflect back to the spacecraft, and then with a very precise knowledge of the orbit of the LRO spacecraft, scientists can convert this information to a detailed topographic map of the moon, according to Head.

Objects hitting the moon can be categorized in different “impactor populations,” where each population has its own set of characteristics. Head also used the LOLA maps to determine the time when the impactor population changed. "Using the crater counts from the different impact basins and examining the populations making up the superposed craters, we can look back in time to discover when this transition in impactor populations occurred. The LRO LOLA impact crater database shows that the transition occurred about the time of the Orientale impact basin, about 3.8 billion years ago. The implication is that this change in populations occurred around the same time as the large impact basins stopped forming, and raises the question of whether or not these factors might be related. The answers to these questions have implications for the earliest history of all the planets in the inner solar system, including Earth," says Head.

In the other two Science papers, researchers describe how data from the Diviner Lunar Radiometer Experiment instrument on LRO are showing that the geologic processes that forged the lunar surface were complex as well. The data have revealed previously unseen compositional differences in the crustal highlands, and have confirmed the presence of anomalously silica-rich material in five distinct regions.

Every mineral, and therefore every rock, absorbs and emits energy with a unique spectral signature that can be measured to reveal its identity and formation mechanisms. For the first time ever, LRO's Diviner instrument is providing scientists with global, high-resolution infrared maps of the moon, which are enabling them to make a definitive identification of silicate minerals commonly found within its crust. "Diviner is literally viewing the moon in a whole new light," says Benjamin Greenhagen of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., lead author of one of the Diviner Science papers.

Lunar geology can be roughly broken down into two categories – the anorthositic highlands, rich in calcium and aluminium, and the basaltic maria, which are abundant in iron and magnesium. Both of these crustal rocks are what’s deemed by geologists as 'primitive'; that is, they are the direct result of crystallization from lunar mantle material, the partially molten layer beneath the crust.

Diviner’s observations have confirmed that most lunar terrains have spectral signatures consistent with compositions that fall into these two broad categories. However they have also revealed that the lunar highlands may be less homogenous than previously thought.

In a wide range of terrains, Diviner revealed the presence of lunar soils with compositions more sodium rich than that of the typical anorthosite crust. The widespread nature of these soils reveals that there may have been variations in the chemistry and cooling rate of the magma ocean which formed the early lunar crust, or they could be the result of secondary processing of the early lunar crust.

Most impressively, in several locations around the moon, Diviner has detected the presence of highly silicic minerals such as quartz, potassium-rich, and sodium-rich feldspar - minerals that are only ever found in association with highly evolved lithologies (rocks that have undergone extensive magmatic processing).

The detection of silicic minerals at these locations is a significant finding for scientists, as they occur in areas previously shown to exhibit anomalously high abundances of the element thorium, another proxy for highly evolved lithologies.

"The silicic features we've found on the moon are fundamentally different from the more typical basaltic mare and anorthositic highlands," says Timothy Glotch of Stony Brook University in Stony Brook, N.Y., lead author of the second Diviner Science paper. "The fact that we see this composition in multiple geologic settings suggests that there may have been multiple processes producing these rocks."

One thing not apparent in the data is evidence for pristine lunar mantle material, which previous studies have suggested may be exposed at some places on the lunar surface. Such material, rich in iron and magnesium, would be readily detected by Diviner.

However, even in the South Pole Aitken Basin (SPA), the largest, oldest, and deepest impact crater on the moon -- deep enough to have penetrated through the crust and into the mantle -- there is no evidence of mantle material.

The implications of this are as yet unknown. Perhaps there are no such exposures of mantle material, or maybe they occur in areas too small for Diviner to detect.

However it's likely that if the impact that formed this crater did excavate any mantle material, it has since been mixed with crustal material from later impacts inside and outside SPA. "The new Diviner data will help in selecting the appropriate landing sites for potential future robotic missions to return samples from SPA. We want to use these samples to date the SPA-forming impact and potentially study the lunar mantle, so it's important to use Diviner data to identify areas with minimal mixing," says Greenhagen.

The research was funded by NASA's Exploration Systems Missions Directorate at NASA Headquarters in Washington. LRO was built and is managed by NASA's Goddard Space Flight Center in Greenbelt, Md. LOLA was built by NASA Goddard. David E. Smith from the Massachusetts Institute of Technology and NASA Goddard is the LOLA principal investigator. The Diviner instrument was built and is managed by NASA’s Jet Propulsion Laboratory in Pasadena, Calif. UCLA is the home institution of Diviner’s principal investigator, David Paige.