Navigators and mission controllers for NASA’s EPOXI mission communicated with the spacecraft for a trajectory correction maneuver on October 27, 2010..

In one of its final mission trajectory correction maneuvers, the EPOXI mission spacecraft has refined its orbit, preparing it for the flyby of comet Hartley 2 on Nov. 4. The time of closest approach to the comet on that day is expected to be about 7:02 a.m. PDT (10:02 a.m. EDT).

Today's trajectory correction maneuver began at 11 a.m. PDT (2 p.m. EDT), when the spacecraft burned its engines for 60 seconds, changing its velocity by 1.59 meters per second (3.6 miles per hour).

On Nov. 4, the spacecraft will fly past Hartley 2 at a distance of about 700 kilometers (435 miles). It will be only the fifth time in history that a spacecraft has been close enough to image a comet's nucleus.

EPOXI is an extended mission that uses the already "in-flight" Deep Impact spacecraft to explore distinct celestial targets of opportunity. The name EPOXI itself is a combination of the names for the two extended mission components: the extrasolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh); and the flyby of comet Hartley 2, called the Deep Impact Extended Investigation (DIXI). The spacecraft will continue to be referred to as "Deep Impact."

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the EPOXI mission for NASA's Science Mission Directorate, Washington. The University of Maryland, College Park, is home to the mission's principal investigator, Michael A'Hearn. Drake Deming of NASA's Goddard Space Flight Center, Greenbelt, Md., is the science lead for the mission's extrasolar planet observations. The spacecraft was built for NASA by Ball Aerospace & Technologies Corp., Boulder, Colo.

For more information about EPOXI, visit: http://epoxi.umd.edu/ .

The sun isn't kind to objects without atmospheres. Bombarded by solar radiation, the surfaces of some comets, for example, tend to be a charred carbon-black. But the 1,000 objects so far directly imaged in the Kuiper Belt – that swath of icy bodies circling around the sun with Pluto – appear to be a wide range of colors: red, blue, and white.

With scant observations to go on – most of the Kuiper belt objects are just a single pixel of light to the Hubble Space Telescope – few hypotheses have been developed to explain the colors. But a new computer model maps out the right combination of materials and space environment that could produce some of those lovely hues. The model suggests that these objects have many layers, and that the red colors of one particularly interesting group of these objects -- the so-called Cold Classical Kuiper Belt -- could come from organic materials in the layer just under the crust.

"This multi-layer model provides a more flexible approach to understanding the diversity of colors," says John Cooper, a Heliospheric physicist at NASA's Goddard Space Flight Center in Greenbelt, Md. "The model calculates the rate at which energy comes in from radiation and could be causing changes at different depths. So we can define different layers based on that."

The layers may have different colors, and could also be dynamic. For example, a deeper layer of relatively pure water ice could erupt upwards to form a new uppermost layer, perhaps accounting for the bright icy surface of Eris, the largest of the known Kuiper Belt objects.

Just how these bodies are composed has been a mystery ever since the first observed Kuiper Belt member, a red Cold Classical named 1992 QB1, was discovered in1992, says Cooper, who presented his model in October at the Division for Planetary Sciences meeting of the American Astronomical Society in Pasadena, Calif. Subsequent discoveries of many more objects created instant buzz not only because they helped demote Pluto from a planet to just-another-Kuiper-Belt-object, but because of the Kuiper Belt's mysterious diversity. These bodies sport not just coats of many colors, but also have different sizes and different orbits.

"There's a group called the Cold Classicals that move in relatively circular orbits, and are nearly aligned in the same plane as the orbits of the other planets," says Cooper. "These are all consistently reddish. Other objects, which might range from red to blue to white, tend to move in more elliptical or inclined orbits, which suggest they came from a different location within the solar system early in its history. So, it's possible that the uniformly red Cold Classicals represent a more pristine sample, showing the original composition of the Kuiper Belt with minimal disturbances."

The first thing Cooper had to do was explain why the objects don't have a black crust like, for example, Halley's Comet, since Kuiper Belt bodies are made of hydrocarbons and water ice, and "from lab experiments we know that usually when you take a mixture of ice and carbon and overexpose it to radiation, you get new, dark, tarry materials," says Cooper.

Cooper calculated how the space radiation constantly flowing past the Kuiper Belt should affect different objects depending on where they're located. He believes that the Cold Classicals formed in a sweet spot where plasma ions from the Sun aren't intense enough to overcook the outermost surface to a dark crust.

Instead, the plasma ions have the right amount of energy to simply "sandblast" the topmost layer of the surface -- which is perhaps a millimeter thick -- right off. The sandblasting is partly due to what's known as "ion sputtering" where an incoming plasma ion causes a mini-explosion on the surface, blowing away molecules. Additional erosion could come from impacts of tiny dust grains ejected into the Kuiper Belt region when nearby larger objects collide. Over time, the combined effects of plasma sputtering and meteoritic dust erode away the top layer.

That means that what we see as red must actually be from the exposed second layer. Cooper explains that this second layer is gently cooked by radiation from interstellar space. The radiation can penetrate deeply into the object but also is not overly intense because the sun's magnetic field protects the solar system from its strongest effects. This radiation passes through the crust right into the "shelf" layer where it can induce simple chemical reactions, turning water ice, carbon, methane, nitrogen and ammonia – the basic substances believed to be on these bodies – into organic molecules containing oxygen and carbon like formaldehyde, acetylene and ethane. "Cooking" by radiation can make these molecules appear red to our eyes.

"So if there wasn't any cooking at all, we would just see primordial ice, and the object would appear bright and white," says Cooper. "And if there was too much radiation we would just see black crust, but instead we see a moderately processed shelf layer, which under these circumstances is red."

Cooper's layer model accounts for bright white Kuiper belt objects as well. Further beneath the red shelf would be less-processed water ice in a deep mantle layer that could volcanically erupt through the crust onto the surface, leaving visible global layers or localized patches of bright white ice. "Some of these objects in the Kuiper Belt like Eris are quite bright," he says. "So these may not be dead icy objects, they may be volcanically active over billions of years."

At this point, the layer model is based on limited data from the Voyager mission that has provided information on the energy levels of radiation beyond Neptune. NASA's New Horizons mission will pass through the Kuiper Belt region beyond Neptune's orbit in 2014, getting a good look at Pluto and its largest moon Charon in 2015, and later, if all goes well, one or two other objects. Cooper hopes it will pass close enough to another object to make detailed observations of its surface, which would help confirm what materials are present. New Horizons can provide additional verification simply by confirming that the energy distribution and particles in this region of the solar system jibe with what the model requires.

Not only would such data help explain the many-colored mystery of the Kuiper Belt, but it would support current theories that organic materials might be common in the universe.

"When you take the right mix of materials and radiate them, you can produce the most complex species of molecules," says Cooper. In some cases you may be able to produce the components of life -- not just organic materials, but biological molecules such as amino acids. We're not saying that life is produced in the Kuiper Belt, but the basic chemistry may start there, as could also happen in similar Kuiper Belt environments elsewhere in the universe and that is a natural path which could lead toward the chemical evolution of life."

For more information visit http://www.nasa.gov/topics/solarsystem/sunearthsystem/main/kuiper-colors.html

The ground where NASA's Mars Exploration Rover Spirit became stuck last year holds evidence that water, perhaps as snow melt, trickled into the subsurface fairly recently and on a continuing basis.

Stratified soil layers with different compositions close to the surface led the rover science team to propose that thin films of water may have entered the ground from frost or snow. The seepage could have happened during cyclical climate changes in periods when Mars tilted farther on its axis. The water may have moved down into the sand, carrying soluble minerals deeper than less soluble ones. Spin-axis tilt varies over timescales of hundreds of thousands of years.

The relatively insoluble minerals near the surface include what is thought to be hematite, silica and gypsum. Ferric sulfates, which are more soluble, appear to have been dissolved and carried down by water. None of these minerals are exposed at the surface, which is covered by wind-blown sand and dust.

"The lack of exposures at the surface indicates the preferential dissolution of ferric sulfates must be a relatively recent and ongoing process since wind has been systematically stripping soil and altering landscapes in the region Spirit has been examining," said Ray Arvidson of Washington University in St. Louis, deputy principal investigator for the twin rovers Spirit and Opportunity.

Analysis of these findings appears in a report in the Journal of Geophysical Research published by Arvidson and 36 co-authors about Spirit's operations from late 2007 until just before the rover stopped communicating in March.

The twin Mars rovers finished their three-month prime missions in April 2004, then kept exploring in bonus missions. One of Spirit's six wheels quit working in 2006.

In April 2009, Spirit's left wheels broke through a crust at a site called "Troy" and churned into soft sand. A second wheel stopped working seven months later. Spirit could not obtain a position slanting its solar panels toward the sun for the winter, as it had for previous winters. Engineers anticipated it would enter a low-power, silent hibernation mode, and the rover stopped communicating March 22. Spring begins next month at Spirit's site, and NASA is using the Deep Space Network and the Mars Odyssey orbiter to listen if the rover reawakens.

Researchers took advantage of Spirit's months at Troy last year to examine in great detail soil layers the wheels had exposed, and also neighboring surfaces. Spirit made 13 inches of progress in its last 10 backward drives before energy levels fell too low for further driving in February. Those drives exposed a new area of soil for possible examination if Spirit does awaken and its robotic arm is still usable.

"With insufficient solar energy during the winter, Spirit goes into a deep-sleep hibernation mode where all rover systems are turned off, including the radio and survival heaters," said John Callas, project manager for Spirit and Opportunity at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "All available solar array energy goes into charging the batteries and keeping the mission clock running."

The rover is expected to have experienced temperatures colder than it has ever before, and it may not survive. If Spirit does get back to work, the top priority is a multi-month study that can be done without driving the rover. The study would measure the rotation of Mars through the Doppler signature of the stationary rover's radio signal with enough precision to gain new information about the planet's core. The rover Opportunity has been making steady progress toward a large crater, Endeavour, which is now approximately 8 kilometers (5 miles) away.

Spirit, Opportunity, and other NASA Mars missions have found evidence of wet Martian environments billions of years ago that were possibly favorable for life. The Phoenix Mars Lander in 2008 and observations by orbiters since 2002 have identified buried layers of water ice at high and middle latitudes and frozen water in polar ice caps. These newest Spirit findings contribute to an accumulating set of clues that Mars may still have small amounts of liquid water at some periods during ongoing climate cycles.

JPL, a division of the California Institute of Technology in Pasadena, manages the rovers for the agency's Science Mission Directorate in Washington.

For more information visit http://www.nasa.gov/mission_pages/mer/news/mer20101028.html