The nebula is the wreckage of an exploded star that emitted light which reached Earth in the year 1054. It is located 6,500 light-years away in the constellation Taurus. At the heart of an expanding gas cloud lies what is left of the original star's core, a superdense neutron star that spins 30 times a second. With each rotation, the star swings intense beams of radiation toward Earth, creating the pulsed emission characteristic of spinning neutron stars (also known as pulsars).

Apart from these pulses, astrophysicists believed the Crab Nebula was a virtually constant source of high-energy radiation. But in January, scientists associated with several orbiting observatories, including NASA's Fermi, Swift and Rossi X-ray Timing Explorer, reported long-term brightness changes at X-ray energies.

"The Crab Nebula hosts high-energy variability that we're only now fully appreciating," said Rolf Buehler, a member of the Fermi Large Area Telescope (LAT) team at the Kavli Institute for Particle Astrophysics and Cosmology, a facility jointly located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University.

Since 2009, Fermi and the Italian Space Agency's AGILE satellite have detected several short-lived gamma-ray flares at energies greater than 100 million electron volts (eV) -- hundreds of times higher than the nebula's observed X-ray variations. For comparison, visible light has energies between 2 and 3 eV.

On April 12, Fermi's LAT, and later AGILE, detected a flare that grew about 30 times more energetic than the nebula's normal gamma-ray output and about five times more powerful than previous outbursts. On April 16, an even brighter flare erupted, but within a couple of days, the unusual activity completely faded out.

"These superflares are the most intense outbursts we've seen to date, and they are all extremely puzzling events," said Alice Harding at NASA's Goddard Space Flight Center in Greenbelt, Md. "We think they are caused by sudden rearrangements of the magnetic field not far from the neutron star, but exactly where that's happening remains a mystery."

The Crab's high-energy emissions are thought to be the result of physical processes that tap into the neutron star's rapid spin. Theorists generally agree the flares must arise within about one-third of a light-year from the neutron star, but efforts to locate them more precisely have proven unsuccessful so far.

Since September 2010, NASA's Chandra X-ray Observatory routinely has monitored the nebula in an effort to identify X-ray emission associated with the outbursts. When Fermi scientists alerted astronomers to the onset of a new flare, Martin Weisskopf and Allyn Tennant at NASA's Marshall Space Flight Center in Huntsville, Ala., triggered a set of pre-planned observations using Chandra.

"Thanks to the Fermi alert, we were fortunate that our planned observations actually occurred when the flares were brightest in gamma rays," Weisskopf said. "Despite Chandra's excellent resolution, we detected no obvious changes in the X-ray structures in the nebula and surrounding the pulsar that could be clearly associated with the flare."

Scientists think the flares occur as the intense magnetic field near the pulsar undergoes sudden restructuring. Such changes can accelerate particles like electrons to velocities near the speed of light. As these high-speed electrons interact with the magnetic field, they emit gamma rays.

To account for the observed emission, scientists say the electrons must have energies 100 times greater than can be achieved in any particle accelerator on Earth. This makes them the highest-energy electrons known to be associated with any cosmic source. Based on the rise and fall of gamma rays during the April outbursts, scientists estimate that the size of the emitting region must be comparable in size to the solar system.

NASA's Fermi is an astrophysics and particle physics partnership managed by NASA's Goddard Space Flight Center in Greenbelt, Md., and developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

The Marshall Space Flight Center manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.

For more information visit http://www.nasa.gov/mission_pages/GLAST/news/crab-flare.html

For many of them, long before they are ever launched into space, they are tested from NASA airplanes. One of the objectives of the NASA Airborne Science Program is to test new instruments in space-like environments. Testing future satellite instruments from airplanes is the next best thing to actually testing them in space.

Over the past three weeks, a team from NASA's Goddard Space Flight Center, Greenbelt, Md., led by Bill Heaps has been testing a new broadband lidar instrument on NASA’s DC-8 flying laboratory that they hope will fly on the ASCENDS satellite mission. ASCENDS, an acronym for Active Sensing of Carbon dioxide Emissions over Nights, Days and Seasons, is an upcoming NASA satellite expected to be launched in 2018-2020. The goal of the ASCENDS mission is to measure the sources, distribution and variations in carbon dioxide gas with very high precision all over the Earth. Mapping carbon dioxide is important for understanding the global carbon cycle and for modeling global climate change.

How is carbon dioxide measured from space?

Carbon dioxide makes up a very small fraction of the gas in Earth’s atmosphere. In addition, the majority of the carbon dioxide variability occurs in the first 100 feet above the surface of the Earth. In order to measure the abundance of carbon dioxide from a satellite, any instrument must therefore look through Earth’s entire atmosphere in order to detect the variations in carbon dioxide occurring near the surface.

Heaps’ broadband lidar – an acronym for light detection and ranging -- uses an infrared laser beam aimed at the surface of the Earth. As the laser passes through the atmosphere and bounces off the ground, carbon dioxide molecules in the atmosphere absorb some of the light from the laser. Measuring the amount of absorption that occurs as the instrument passes over different locations on the Earth will allow the team to build global carbon dioxide maps.

Typical lidar systems have lasers that emit light at very specific colors, or wavelengths. The carbon dioxide molecule, however, absorbs light at a several different infrared wavelengths. The broadband laser used in Heaps’ instrument emits light with a broader range of wavelengths, and thus has the advantage of being able to detect carbon dioxide absorption in multiple wavelength bands with one laser. The wavelength control requirements are also less strict than for a more conventional narrowband laser, which may make the system easier to implement on a satellite.

The Goddard team worked for over two weeks to install and test their instrument in the belly of the DC-8 at the NASA Dryden Aircraft Operations Facility in Palmdale, Calif.

The team then flew with their instrument on two four-hour flights on the converted jetliner during the week of May 2 – 6 over northern and central California. During the flights, they tested the instrument’s performance at variety of altitudes and over different types of surfaces – deserts, agricultural fields, mountainous terrain, the ocean and the flat waters of Lake Tahoe. The team was very pleased with the performance of the instrument.

“The system definitely measured CO2 on both flights, even transmitting a very small amount of laser power. I believe the broadband technique has excellent potential to be scaled up for measurements from space,” Heaps said.

This July, several instrument teams, all vying to have their instrument fly on ASCENDS, will test their instruments side-by-side on the DC-8. With data from the test flights of the broadband lidar instrument in hand, Heaps’ team will return to Goddard to make refinements and improvements in the hope that their instrument will be chosen to fly on the ASCENDS satellite mission.

The NASA Earth Science Technology Office Instrument Incubator program provided funding for the Goddard broadband lidar.

For more information visit http://www.nasa.gov/centers/dryden/Features/broadband_lidar_tested.html

A combination of heavy rains and a large snow melt has put parts of the central U.S. at risk for record flooding this spring with several locations along the Mississippi already at or near record levels. One likely culprit is La Niña. Despite the fact that the current La Niña appears to be winding down, its effects in the atmosphere can persist for a while. Furthermore, although not every La Niña brings major flooding to the region, La Niña's are conducive for above-normal rainfall from East Texas and northern Louisiana up through Arkansas and the Tennessee and Ohio Valleys with below-normal rainfall across Texas, southern Louisiana and Florida.

During La Niña, below-normal sea surface temperatures occur in the equatorial East Pacific and above-normal temperatures in the West Pacific. This pattern leads to enhanced tropical thunderstorm activity over the West Pacific, which in turn can influence the weather in middle latitudes by shifting the jet stream pattern. On average, La Niña's favor an upper-level trough over the Midwest with the jet stream dipping down out of the northern Rockies and flowing west-to-east across the central Mississippi and Ohio Valleys before heading back up over the Northeast. This pattern steers developing low pressure systems across the Plains and central Mississippi into the Tennessee and Ohio Valleys. These areas of low pressure provide the focus for showers and storms while drawing warm moist air up from the Gulf of Mexico, resulting in enhanced rainfall across the central part of the country.

The main objective of the Tropical Rainfall Measuring Mission or TRMM satellite is to measure rainfall over the global Tropics. TRMM measures rainfall using a combination of passive microwave and active radar sensors. For expanded coverage, TRMM can be used to calibrate rainfall estimates from other satellites. The TRMM-based, near-real time Multi-satellite Precipitation Analysis (TMPA) at the NASA Goddard Space Flight Center, Greenbelt, Md. provides rainfall estimates over the global Tropics.

TMPA rainfall anomalies were created in a rainfall map for the period April 4 to May 4, 2011 for the eastern two thirds of the country. The anomalies were constructed by computing the average rainfall rate over the period and then subtracting the 10-year average rate for the same period. The resulting pattern shows a broad area of above-normal rainfall (shown in green and blue) stretching from eastern Oklahoma across the central Mississippi Valley and up into the lower Ohio Valley with below-normal rainfall along the northern Gulf Coast. This rainfall pattern is consistent with a La Niña.

In addition to rainfall, this type of jet stream pattern can lead to strong storms by allowing strong jet stream winds to override warm moist air from the Gulf as was evidenced by the recent tornado outbreak. In fact, some of the biggest tornado outbreaks, including the previous record "Super Outbreak" in 1974, have occurred during La Niña's.

TRMM is a joint mission between NASA and the Japanese space agency JAXA.

For more information visit http://www.nasa.gov/topics/earth/features/spring-rainfall.html

ASTER combines infrared, red, and green wavelengths of light to make false-color images that distinguish between water and land. Water is blue. Buildings and paved surfaces are blue-gray. Vegetation is red.

The images to the right are from an observation that occurred on May 4, 2011 at 11:45 A.M. local time (1645 UTC), near Tuscaloosa, Ala.

The physical principle guiding the use of satellite data to detect tornado damage is based on the premise that the strong winds associated with a tornado will change the physical characteristics of the surface in such a way as to alter the visible and infrared energy reflected. These characteristics could be a change in the orientation of surface features, such as the complete destruction of a house in a residential area, the snapping of trees in a forest region, the uprooting of crops in an agriculture area, or minimal damage to grassland in a pasture or field.

Images from NASA satellites will aid in damage assessment, determining the tornado width and path length. Further scientific analysis using satellite imagery is planned.

Terra/ASTER is a joint activity between NASA's Science Mission Directorate Earth Science Division and Japan's Ministry of Economy, Trade and Industry. Terra is one of 14 NASA satellites that look at the Earth to study and understand changes in the Earth system and provide societal benefits.

The NASA image created by the Short-term Prediction and Research Transition or SPoRT project at the Marshall Space Flight Center in Huntsville, using data provided courtesy of NASA Goddard Space Flight Center, the Land Processes Distributed Active Archive Center, Japan’s Earth Remote Sensing Data Analysis Center, the Ministry of Economy, Trade and Industry, along with the Japan Research Observation System Organization.

For more information visit http://www.nasa.gov/topics/earth/features/tuscaloosa_tornado.html