NASA's Goddard, Glenn Centers Look to Lift Space Astronomy out of the Fog

A fogbank is the least useful location for a telescope, yet today's space observatories effectively operate inside one. That's because Venus, Earth and Mars orbit within a vast dust cloud produced by comets and occasional collisions among asteroids. After the sun, this so-called zodiacal cloud is the solar system's most luminous feature, and its light has interfered with infrared, optical and ultraviolet observations made by every astronomical space mission to date.
To put it simply, it has never been night for space astronomers," said Matthew Greenhouse, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md. Light from zodiacal dust can be a thousand times brighter than the sources astronomers actually target, limiting sensitivity in much the same way that bright moonlight hampers ground-based observatories. The dust and its unwanted illumination are greatest in the plane of Earth's orbit, the same plane in which every space telescope operates.
Placing future astronomy missions on more tilted orbits would let spacecraft spend significant amounts of time above and below the thickest dust and thereby reduce its impact on observations. So Greenhouse teamed with Scott Benson at NASA's Glenn Research Center in Cleveland, Ohio, to investigate how these "dark sky" or extra-zodiacal orbits might improve mission science and to develop a means of cost-effectively reaching them.
"Just by placing a space telescope on these inclined orbits, we can improve its sensitivity by a factor of two in the near-ultraviolet and by 13 times in the infrared," Greenhouse explained. "That's a breakthrough in science capability with absolutely no increase in the size of the telescope's mirror."
Greenhouse, Benson and the COllaborative Modeling and Parametric Assessment of Space Systems (COMPASS) study team at NASA Glenn designed a mission that utilizes new developments in solar arrays, electric propulsion and lower-cost expendable launch vehicles. Their proof-of-concept mission is the Extra-Zodiacal Explorer (EZE), a 1,500-pound EX-class observatory that could accommodate a telescope in the size range of the recently completed WISE mission -- all within the cost and schedule constraints of NASA's Explorer Program.
Launched on a SpaceX Falcon 9 rocket, EZE would use a powerful new solar-electric drive as an upper stage to direct the spacecraft on a gravity-assist maneuver past Earth or Mars. This flyby would redirect the mission into an orbit inclined by as much as 30 degrees to Earth's.
The result, the scientists say, will be the highest-performance observatory ever achieved in the decades-long history of NASA's Explorer program.
"We see EZE as a game-changer, the first step on a new path for NASA Explorers that will yield major science goals despite limited resources," said Benson, who previously managed the new electric propulsion technology project.
Named NASA's Evolutionary Xenon Thruster (NEXT), the engine is an improved type of ion drive. Using electric power supplied by solar panels, the NEXT engines operate by removing electrons from atoms of xenon gas, then accelerating the charged ions through an electric field to create thrust. While these types of engine provide much less thrust at any given time than traditional chemical rockets, they are much more fuel efficient and can operate for years.
Built by Aerojet, an aerospace company based in Sacramento, Calif., each 6.9-kilowatt NEXT engine delivers two and a half times the thrust of the NSTAR ion engines now flying on NASA's Dawn spacecraft.
"We've run one NEXT thruster for over 40,000 hours in ground testing, more than twice the thruster operating lifetime needed to deliver the EZE spacecraft to its extra-zodiacal orbit," Benson explained. "This is mature technology that will enable much more cost-effective space missions across both the astrophysics and planetary science disciplines."
"Development of this solar-electric upper stage for Falcon 9, which the Goddard/Glenn EZE team is advocating, will make extra-zodiacal orbits available to any astronomer proposing to NASA's Explorer program. This will enable unprecedented science capability for astrophysics Explorers," Greenhouse said.
The EZE upper stage would carry two NEXT engines, their xenon gas propellant and two 18-foot-wide UltraFlex solar arrays built by Alliant Techsystems in Goleta, Calif. These arrays, which were originally developed for NASA's Orion Crew Exploration Vehicle, would see their first deep-space application with EZE. Once the spacecraft achieves the desired orbit, the transfer stage would separate and the science mission would begin.
The Goddard/Glenn study also showed that the EZE mission concept could be the lowest-cost option for a planned flight demonstration of a high-power solar-electric propulsion stage. A standardized solar-electric upper stage for the Falcon 9 that can be used by any mission will set the small-payload Explorer program on a new path to achieve science goals that rival the capability of larger, more expensive systems.
"Undertaking a project like this will provide key flight experience toward developing the higher-power systems needed to enable NASA's human exploration objectives in deep space while providing immediate scientific return on the investment," Greenhouse added.

Flying Through a Geomagnetic Storm

Glowing green and red, shimmering hypnotically across the night sky, the aurora borealis is a wonder to behold. Longtime sky watchers say it is the greatest show on Earth.
It might be the greatest show in Earth orbit, too. High above our planet, astronauts onboard the International Space Station (ISS) have been enjoying an up-close view of auroras outside their windows as the ISS flys through geomagnetic storms.
"We can actually fly into the auroras," says eye-witness Don Pettit, a Flight Engineer for ISS Expedition 30. "It's like being shrunk down and put inside of a neon sign."
Auroras are caused by solar activity. Gusts of solar wind and coronal mass ejections strike Earth's magnetic field, rattling our planet's protective shell of magnetism. This causes charged particles to rain down over the poles, lighting up the atmosphere where they hit. The physics is akin to what happens in the picture tube of a color TV.
Incoming particles are guided by Earth's magnetic field to a pair of doughnut-shaped regions called "auroral ovals." There's one around the North Pole and one around the South. Sometimes, when solar activity is high, the ovals expand, and the space station orbits right through them.
That's exactly what happened in late January 2012, when a sequence of M-class and X-class solar flares sparked a light show that Pettit says he won't soon forget. "The auroras could be seen [as brightly as] city lights on the Earth below -- and even in the day-night terminator of the rising and setting sun. It was simply amazing."
Pettit is a skilled astrophotographer. He and other members of the crew video-recorded the displays, producing footage that officials say is some of the best-ever taken from Earth orbit.
The videos capture the full range of aurora colors -- red, green, and many shades of purple. These hues correspond to different quantum transitions in excited atoms of oxygen and nitrogen. The precise color at any altitude depends on the temperature and density of the local atmosphere.
"Red auroras reach all the way up to our altitude 400 km above Earth," says Pettit. "Sometimes you feel like you can reach out and touch them."
"Green emissions, on the other hand, tend to stay below the space station," he says. They move like a living 'shag carpet' of lights. "We fly right over them."
Surprisingly, Pettit does not find this unsettling. "It is not disorienting to see auroras underfoot," he says. "Perhaps it is because I have been up here so long."
What he does find disorienting is the meteors.
"Occasionally we see a meteor burning up in the atmosphere below -- and this does look strange. You should be looking up for meteors not down."
As marvelous as these sights are, Petit has seen better. He was the science officer for ISS expedition 6 back in 2003 when the auroras were even stronger than they were now.
"But this expedition is not over yet," he points out hopefully.
Indeed, more auroras are in the offing. Following some recent years of deep quiet, the sun is waking up again. Solar activity is now trending upward with a maximum expected in early 2013.
This means the greatest show on Earth -- and in Earth orbit -- is about to get even better.

Cassini Captures New Images of Icy Moon

New raw, unprocessed images of Saturn's second largest moon, Rhea, were taken on March 10, 2012, by NASA's Cassini spacecraft. This was a relatively distant flyby with a close-approach distance of 26,000 miles (42,000 kilometers), well suited for global geologic mapping.
During the flyby, Cassini captured these distinctive views of the moon's cratered surface, creating a 30-frame mosaic of Rhea's leading hemisphere and the side of the moon that faces away from Saturn. The observations included the large Mamaldi (300 miles, or 480 kilometers, across) and Tirawa (220 miles, or 360 kilometers, across) impact basins and the 29-miles (47-kilometers) ray crater Inktomi, one of the youngest surface features on Rhea (about 950 miles, or 1,530 kilometers, across).
All of Cassini's raw images can be seen at http://saturn.jpl.nasa.gov/photos/raw/ .
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory in Pasadena manages the mission for the agency's Science Mission Directorate in Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations team is based at the Space Science Institute in Boulder, Colo. JPL is a division of Caltech.

Cassini Spies Wave Rattling Jet Stream On Jupiter

New movies of Jupiter are the first to catch an invisible wave shaking up one of the giant planet's jet streams, an interaction that also takes place in Earth's atmosphere and influences the weather.
The movies, made from images taken by NASA's Cassini spacecraft when it flew by Jupiter in 2000, are part of an in-depth study conducted by a team of scientists and amateur astronomers led by Amy Simon-Miller at NASA's Goddard Space Flight Center in Greenbelt, Md., and published in the April 2012 issue of Icarus.
"This is the first time anyone has actually seen direct wave motion in one of Jupiter's jet streams," says Simon-Miller, the paper's lead author. "And by comparing this type of interaction in Earth's atmosphere to what happens on a planet as radically different as Jupiter, we can learn a lot about both planets."
Like Earth, Jupiter has several fast-moving jet streams that circle the globe. Earth's strongest and best known jet streams are those near the North and South Poles; as these winds blow west to east, they take the scenic route, wandering north and south. What sets these jet streams on their meandering paths-and sometimes makes them blast Florida and other warm places with frigid air-are their encounters with slow-moving waves in Earth's atmosphere, called Rossby waves.
In contrast, Jupiter's jet streams "have always appeared to be straight and narrow," says co-author John Rogers, who is the Jupiter Section Director of the British Astronomical Association, London, U.K., and one of the amateur astronomers involved in this study.
Rossby waves were identified on Jupiter about 20 years ago, in the northern hemisphere. Even so, the expected meandering winds could not be traced directly, and no evidence of them had been found in the southern hemisphere, which puzzled planetary scientists.
To get a more complete view, the team analyzed images taken by NASA's Voyager spacecraft, NASA's Hubble Space Telescope, and Cassini, as well as a decade's worth of observations made by amateur astronomers and compiled by the JUPOS project.
The movies zoom in on a single jet stream in Jupiter's southern hemisphere. A line of small, dark, v-shaped "chevrons" has formed along one edge of the jet stream and zips along west to east with the wind. Later, the well-ordered line starts to ripple, with each chevron moving up and down (north and south) in turn. And for the first time, it's clear that Jupiter's jet streams, like Earth's, wander off course.
"That's the signature of the Rossby wave," says David Choi, the postdoctoral fellow at NASA Goddard who strung together about a hundred Cassini images to make each time-lapse movie. "The chevrons in the fast-moving jet stream interact with the slower-moving Rossby wave, and that's when we see the chevrons oscillate."
The team's analysis also reveals that the chevrons are tied to a different type of wave in Jupiter's atmosphere, called a gravity inertia wave. Earth also has gravity inertia waves, and under proper conditions, these can be seen in repeating cloud patterns.
"A planet's atmosphere is a lot like the string of an instrument," says co-author Michael D. Allison of the NASA Goddard Institute for Space Studies in New York. "If you pluck the string, it can resonate at different frequencies, which we hear as different notes. In the same way, an atmosphere can resonate with different modes, which is why we find different kinds of waves."
Characterizing these waves should offer important clues to the layering of the deep atmosphere of Jupiter, which has so far been inaccessible to remote sensing, Allison adds.
Crucial to the study was the complementary information that the team was able to retrieve from the detailed spacecraft images and the more complete visual record provided by amateur astronomers. For example, the high resolution of the spacecraft images made it possible to establish the top speed of the jet stream's wind, and then the amateur astronomers involved in the study looked through the ground-based images to find variations in the wind speed.
The team also relied on images that amateur astronomers had been gathering of a large, transient storm called the South Equatorial Disturbance. This visual record dates back to 1999, when members of the community spotted the most recent recurrence of the storm just south of Jupiter's equator. Analysis of these images revealed the dynamics of this storm and its impact on the chevrons. The team now thinks this storm, together with the Great Red Spot, accounts for many of the differences noted between the jet streams and Rossby waves on the two sides of Jupiter's equator.
"We are just starting to investigate the long-term behavior of this alien atmosphere," says co-author Gianluigi Adamoli, an amateur astronomer in Italy. "Understanding the emerging analogies between Earth and Jupiter, as well as the obviously profound differences, helps us learn fundamentally what an atmosphere is and how it can behave."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the mission for NASA's Science Mission Directorate, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena.

Thermonuclear Behavior of Unique Neutron Star Captured

A neutron star is the closest thing to a black hole that astronomers can observe directly, crushing half a million times more mass than Earth into a sphere no larger than a city. In October 2010, a neutron star near the center of our galaxy erupted with hundreds of X-ray bursts that were powered by a barrage of thermonuclear explosions on the star's surface. NASA's Rossi X-ray Timing Explorer (RXTE) captured the month-long fusillade in extreme detail. Using this data, an international team of astronomers has been able to bridge a long-standing gap between theory and observation. "In a single month from this unique system, we have identified behavior not seen in observations of nearly 100 bursting neutron stars during the past 30 years," said Manuel Linares, a postdoctoral researcher at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology in Cambridge. He led a study of the RXTE data that will be published in the March 20 issue of The Astrophysical Journal. On Oct. 10, 2010, the European Space Agency's INTEGRAL satellite detected a transient X-ray source in the direction of Terzan 5, a globular star cluster about 25,000 light-years away toward the constellation Sagittarius. The object, dubbed IGR J17480-2446, is classed as a low-mass X-ray binary system, in which the neutron star orbits a star much like the sun and draws a stream of matter from it. As only the second bright X-ray source to be found in the cluster, Linares and his colleagues shortened its moniker to T5X2. Three days after the source's discovery, RXTE targeted T5X2 and detected regular pulses in its emission, indicating that the object was a pulsar -- a type of neutron star that emits electromagnetic energy at periodic intervals. The object's powerful magnetic field directs infalling gas onto the star's magnetic poles, producing hot spots that rotate with the neutron star and give rise to X-ray pulses. At NASA's Goddard Space Flight Center in Greenbelt, Md., RXTE scientists Tod Strohmayer and Craig Markwardt showed that T5X2 spins at a sedate -- for neutron stars -- rate of 11 times a second. And because the pulsar's orbital motion imparts small but regular changes in the pulse frequency, they showed that the pulsar and its sun-like companion revolve around each other every 21 hours. That same day, RXTE observed its first burst from the system: an intense spike in X-rays lasting nearly 3 minutes and caused by a thermonuclear explosion on the neutron star's surface. Ultimately, RXTE cataloged some 400 events like this between Oct. 13 and Nov. 19, with additional bursts observed by INTEGRAL and NASA's Swift and Chandra observatories. NASA decommissioned RXTE on Jan. 5, 2012. In the T5X2 system, matter streams from the sun-like star to the neutron star, a process called accretion. Because a neutron star packs more than the sun's mass into a sphere between 10 and 15 miles across -- about the size of Manhattan or the District of Columbia -- its surface gravity is extremely high. The gas rains onto the pulsar's surface with incredible force and ultimately coats the neutron star in a layer of hydrogen and helium fuel. When the layer builds to a certain depth, the fuel undergoes a runaway thermonuclear reaction and explodes, creating intense X-ray spikes detected by RXTE and other spacecraft. The bigger the blast, the more intense its X-ray emission. Models designed to explain these processes made one prediction that had never been confirmed by observation. At the highest rates of accretion, they said, the flow of fuel onto the neutron star can support continuous and stable thermonuclear reactions without building up and triggering episodic explosions. At low rates of accretion, T5X2 displays the familiar X-ray pattern of fuel build-up and explosion: a strong spike of emission followed by a long lull as the fuel layer reforms. At higher accretion rates, where a greater volume of gas is falling onto the star, the character of the pattern changes: the emission spikes are smaller and occur more often. But at the highest rates, the strong spikes disappeared and the pattern transformed into gentle waves of emission. Linares and his colleagues interpret this as a sign of marginally stable nuclear fusion, where the reactions take place evenly throughout the fuel layer, just as theory predicted. "We see T5X2 as a 'model burster,' the one that's doing everything expected of it," said Diego Altamirano, an astrophysicist at the University of Amsterdam in The Netherlands and a co-author on the paper describing the findings. The question now before the team is why this system is so different from all others studied in previous decades. Linares suspects that T5X2's slow rotation may hold the key. Faster rotation would introduce friction between the neutron star's surface and its fuel layers, and this frictional heat may be sufficient to alter the rate of nuclear burning in all other bursting neutron stars previously studied.