Calling from the Roosevelt Room in the White House, the President congratulated the two crews on their continuing successful mission, saying the work on board is a testimony to why exploration is so important and that his commitment to NASA is unwavering.

Endeavour and its crew launched Feb. 8 on the STS-130 mission to the space station. During the mission, astronauts installed the Tranquility node and a cupola with seven windows that provide a panoramic view of Earth, celestial objects and visiting spacecraft. Tranquility and its cupola are the final major U.S. portions of the station.

President Obama also told the crew that his administration is “very excited about putting research dollars into the technologies” that will get humans to Mars and beyond.

The President also asked the crew about the experiments being performed on the space station saying that some of the experiments are in line with where his administration wants to see NASA go.

At the conclusion of the event, the President repeated that he is proud and excited about the work being done on the space station and told the crew that he is committed to continuing human space exploration and complimented the crew on being “great role models.”

“Be sure and tell your families that we appreciate them letting you float up in space like this,” said Obama.

The students participating in the event were in Washington D.C. as part of 39 teams competing for the “Future City” engineering competition hosted by National Engineers Week that builds on the President’s "Educate to Innovate" campaign that emphasizes inspiring students to pursue excellence in science, technology, engineering and math (STEM).

The schools represented were Birney Middle School in Detroit, Elkhorn Middle School in Omaha, Neb., St. Thomas the Apostle in Miami, and Davidson IB Middle School in Davidson, N.C. The North Carolina team was the overall winner of the competition.

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"These are such critical objects in understanding the universe," said Marat Gilfanov of the Max PlanckInstitute for Astrophysics in Germany and lead author of the study that appears in the Feb. 18 edition of the journal Nature. "It was a major embarrassment that we did not know how they worked. Now we are beginning to understand what lights the fuse of these explosions."

Most scientists agree a Type 1a supernova occurs when a white dwarf star -- a collapsed remnant of an elderly star -- exceeds its weight limit, becomes unstable and explodes. Scientists have identified two main possibilities for pushing the white dwarf over the edge: two white dwarfs merging or accretion, a process in which the white dwarf pulls material from a sun-like companion star until it exceeds its weight limit.

"Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging," said co-author Akos Bogdan, also of Max Planck. "This is probably not what many astronomers would expect."

The difference between these two scenarios may have implications for how these supernovae can be used as "standard candles" -- objects of a known brightness -- to track vast cosmic distances. Because white dwarfs can come in a range of masses, the merger of two could result in explosions that vary somewhat in brightness.

Because these two scenarios would generate different amounts of X-ray emission, Gilfanov and Bogdan used Chandra to observe five nearby elliptical galaxies and the central region of the Andromeda galaxy. A Type 1a supernova caused by accreting material produces significant X-ray emission prior to the explosion. A supernova from a merger of two white dwarfs, on the other hand, would create significantly less X-ray emission than the accretion scenario.

The scientists found the observed X-ray emission was a factor of 30 to 50 times smaller than expected from the accretion scenario, effectively ruling it out. This implies that white dwarf mergers dominate in these galaxies.

An open question remains whether these white dwarf mergers are the primary catalyst for Type 1a supernovae in spiral galaxies. Further studies are required to know if supernovae in spiral galaxies are caused by mergers or a mixture of the two processes. Another intriguing consequence of this result is that a pair of white dwarfs is relatively hard to spot, even with the best telescopes.

"To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appeared to exist," said Gilfanov. "Now this path to supernovae will have to be investigated in more detail."

In addition to the X-rays observed with Chandra, other data critical for this result came from NASA's Spitzer Space Telescope and the ground-based, infrared Two Micron All Sky Survey. The infrared brightness of the galaxies allowed the team to estimate how many supernovae should occur.

NASA's Marshall Space Flight Center in Huntsville, Ala., 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.

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Scientists at NASA's Marshall Space Flight Center in Huntsville are studying these recent storms by looking at data from three unique weather monitoring tools to gain a better picture of how storms evolve to produce both heavy rain or large hail, and subsequent strong winds or tornadoes. Researchers are using observations from the Advanced Radar for Meteorological and Operational Research, or ARMOR, radar operated by the University of Alabama Huntsville and NASA Lightning-Mapping Array System and disdrometer data to understand storm precipitation types -- rain, snow or hail -- and how those amounts relate to the amount of lightning produced. This early storm research supports the development of future weather monitoring systems like the Geostationary Operational Environmental Satellite, or GOES-R, that will observe Earth's weather from space.

A better understanding of these storm system could be the difference in a more accurate and timely prediction and would have been useful on Jan. 21. The tornado was later classified as an EF-2 as defined by the Enhanced Fujita Scale, created by Dr. T. Theodore Fujita in 1971, which categorizes each tornado by intensity and area and estimated wind speed associated with the resultant damage. Southern states typically view early spring as tornado season, but they can occur any time of year if the conditions are right. They usually form around violent thunderstorms where there is sufficient instability and wind shear in the lower atmosphere.

"In order to predict severe weather you have to understand how a storm works, and to understand how they work you need to make measurements and observations," said Dr. Walt Petersen, a physical scientist at the Marshall Center. "In terms of societal impacts, we need to be able to reliably measure and predict the occurrence of things like rainfall, lightning and tornadoes to offer more timely warnings."

The ARMOR radar helps scientists understand precipitation processes in the storm from the ground to the top. ARMOR remotely takes precipitation measurements since it is not possible to get to the high-altitude core of these monstrous storms. The analysis of the radar data starts by examining particle size information collected by disdrometers at the surface. Using this information, Petersen and his colleagues can calibrate the ARMOR low-elevation scans and then extend the calibration to look higher in the storm. This enables scientists to diagnose the precipitation particle sizes and shapes at the base and higher into the storm.

The internal precipitation properties of the storm are then compared to lightning production. Lightning data is taken from the Lightning Mapping Array system that collects lightning measurements from a set of 10 antennas positioned around northern Alabama. The lightning array senses the radiation emitted by lightning flashes in the very high frequency bands between the 60 to 100 megahertz range. The resultant data provide scientists with a three-dimensional location for each lightning flash within a given thunderstorm and those locations can be compared to ARMOR radar data collected for the same part of the storm.

ARMOR is a dual-polarimetric Doppler radar meaning that it works by transmitting pulses of microwave energy in vertical and horizontal orientations that are then scattered back to the radar in the same orientations. From the echo, or return of the horizontally and vertically oriented radar pulses, scientists can measure specific properties of the precipitation within a given cloud including the particle size, shape and type, as well as the precipitation rate and the relative velocity of the wind that is moving those precipitation particles either toward or away from the radar, or the Doppler-shifted wind. Disdrometer data is also collected to provide information on individual precipitation particle sizes, shapes and numbers -- they connect what is going on at the particle scale to what scientists observe in the beam of the radar.

"To provide reliable and timely predictions, scientists need to understand the entire storm system by observing and measuring the physics of each process and how they are related," said Petersen. "We are attempting to do this by combining radar, lightning and disdrometer data for analysis. By studying all these data points together we're able to connect the dots between precipitation formation, properties, and movement, and the development of dangerous weather phenomena such as large hail, lightning and tornadoes."

NASA continues to develop advanced satellite platforms that can carry instruments to remotely sense thunderstorms from space. Currently, the joint NASA/Japanese Aerospace Exploration Agency, or JAXA, Tropical Rainfall Measuring Mission satellite launched in 1997 and flying the first and only precipitation radar in space, is able to measure the three-dimensional structure of storms, while NASA’s Aqua satellite provides information on the horizontal structure of precipitation as well as environmental temperature and humidity. However, these satellites get only occasional snapshots of storms rather than the continuous coverage needed.

Scientists need to know what's happening inside the storm systems as they are taking place. NASA is currently working to develop new instruments and techniques to support weather and climate studies for the GOES-R satellite Geostationary Lightning Mapper, a joint effort with the National Oceanic and Atmospheric Administration that will launch in 2015, and the NASA/JAXA Global Precipitation Measurement mission that will launch in 2013.

The ARMOR radar and Northern Alabama lightning-mapping array projects represent collaborative efforts between Marshall and team members and partners from the University of Alabama in Huntsville. These efforts are funded in part by the NOAA/NASA GOES-R satellite program, NASA’s Tropical Rainfall Measuring Mission and Global Precipitation Measurement Mission, all managed by NASA’s Goddard Space Flight Center in Greenbelt, Md.

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Media attendance will be limited to a White House pool spray, but the White House and NASA Television will stream live video of the event online. The online video also can be embedded into sites using the embed code accessible by clicking "share" next to the event video at:

Endeavour and its crew launched Feb. 8 on the STS-130 mission to the space station. During the mission, astronauts installed the Tranquility node and a cupola with seven windows that provide a panoramic view of Earth, celestial objects and visiting spacecraft. Tranquility and its cupola are the final major U.S. portions of the station.

Joining the president are 12 students from Birney Middle School of Detroit, Elkhorn Middle School of Omaha, Neb., St. Thomas the Apostle of Miami and Davidson IB Middle School of Davidson, N.C. These students are in Washington as leaders of four of 39 teams participating in the "Future City" engineering competition hosted by National Engineers Week.

Building on the president's "Educate to Innovate" campaign and his emphasis on inspiring young adults to pursue excellence in science, technology, engineering and math, the students are all leaders of teams that are finalists. The competition included 34,000 seventh and eighth graders from across the nation who produced innovative ideas and designs for a city of tomorrow. The Davidson IB Middle School team was the overall winner of the national competition.

For NASA TV downlink, schedule and streaming video information, visit:

The briefing participants are:
-Expedition 24 Flight Engineer and NASA astronaut Shannon Walker
-Expedition 24 Flight Engineer and NASA astronaut Army Col. Doug Wheelock
-Expedition 24 Flight Engineer and Russian cosmonaut Fyodor Yurchikhin

Following the briefing, the crew members will be available for individual round-robin interviews, in person or by phone. There also will be a photo opportunity. To participate in the interviews, reporters should contact the Johnson newsroom at 281-483-5111, by 4 p.m. on Wednesday, Feb. 24. U.S. and foreign nationals planning to attend the news briefing must contact the Johnson newsroom by 4 p.m., Wednesday, Feb. 24, to arrange credentials.

Walker, Wheelock and Yurchikhin are scheduled to launch to the station aboard a Russian Soyuz spacecraft no earlier than June 14. They will dock to the space station two days later, joining Russian cosmonauts Alexander Skvortsov, Mikhail Kornienko and NASA astronaut Tracy Caldwell Dyson, who are scheduled to arrive at the station in April on another Soyuz spacecraft.

For NASA TV streaming video, schedules and downlink information, visit: