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Expedition 13 preview
International Space Station officials preview the next Expedition mission to the orbiting outpost, which is scheduled for launch March 29. The preview was given during a briefing March 22 from Johnson Space Center.

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STS-41B: Human satellite
One of the iconic moments of the early space shuttle program was astronaut Bruce McCandless floating above the brilliantly blue Earth completely disconnected from his spacecraft. He was testing the Manned Maneuvering Unit, a jet-powered backpack that would enable spacewalkers to travel away from the space shuttle to service satellites. In this post-flight presentation, the crew of Challenger's STS-41B mission of February 1984 narrate the film highlights from their mission that also included the first shuttle landing at Kennedy Space Center.

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Shuttle launch delay
Space Shuttle Program Manager Wayne Hale announces his decision to replace suspect fuel-level sensors inside the liquid hydrogen portion of Discovery's external tank. The three-week job means Discovery will miss its May launch window, delaying the second post-Columbia test flight to the next daylight period opening July 1. Hale made the announcement during a news conference from Johnson Space Center on March 14.

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Stardust science
NASA's Stardust spacecraft returned to Earth in January with the first samples ever retrieved from a comet. This briefing with mission scientists held March 13 from the Johnson Space Center offers an update on the initial research into the comet bits.

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Exploring Enceladus
The Cassini spacecraft orbiting the planet Saturn has found evidence indicating pockets of liquid water may exist near the surface on the icy moon Enceladus, raising the question of whether the small world could support life. This movie includes stunning images of Enceladus taken by Cassini and animation of geysers seen erupting from the moon.

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MRO's orbit insertion explained
The make-or-break engine firing by the Mars Reconnaissance Orbiter to enter orbit around Mars and the subsequent aerobraking to reach the low-altitude perch for science observations are explained by project manager Jim Graf in this narrated animation package.

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Cannibal stars like their food hot, XMM-Newton reveals
Posted: March 26, 2006

The European Space Agency's XMM-Newton observatory has seen vast clouds of superheated gas, whirling around miniature stars and escaping from being devoured by the stars' enormous gravitational fields - giving a new insight into the eating habits of the galaxy's "cannibal" stars.

Artist's impression of a vast cloud of superheated gas whirling around an asteroid-sized cannibal star, part of a low-mass X-ray binary star system. Credit: ESA
The clouds of gas range in size from a few hundred thousand kilometres to a few million kilometres, ten to one hundred times larger than the Earth. They are composed of iron vapour and other chemicals at temperatures of many millions of degrees.

"This gas is extremely hot, much hotter than the outer atmosphere of the Sun," said Maria Diaz Trigo of ESA's European Science and Technology Research Centre (ESTEC), who led the research.

ESA's XMM-Newton x-ray observatory made the discovery when it observed six so-called 'low-mass X-ray binary' stars (LMXBs). The LMXBs are pairs of stars in which one is the tiny core of a dead star.

Measuring just 15-20 kilometres across and comparable in size to an asteroid, each dead star is a tightly packed mass of neutrons containing more than 1.4 times the mass of the Sun.

Its extreme density generates a powerful gravitational field that rips gas from its 'living' companion star. The gas spirals around the neutron star, forming a disc, before being sucked down and crushed onto its surface, a process known as 'accretion'.

The newly discovered clouds sit where the river of matter from the companion star strikes the disc. The extreme temperatures have ripped almost all of the electrons from the iron atoms, leaving them carrying extreme electrical charges. This process is known as 'ionisation'.

The discovery solves a puzzle that has dogged astronomers for several decades. Certain LMXBs appear to blink on and off at X-ray wavelengths. These are 'edge-on' systems, in which the orbit of each gaseous disc lines up with Earth.

In previous attempts to simulate the blinking, clouds of low-temperature gas were postulated to be orbiting the neutron star, periodically blocking the X-rays. However, these models never reproduced the observed behaviour well enough.

XMM-Newton solves this by revealing the ionised iron. "It means that these clouds are much hotter than we anticipated," said Diaz. With high-temperature clouds, the computer models now simulate much better the dipping behaviour.

Some 100 known LMXBs populate our galaxy, the Milky Way. Each one is a stellar furnace, pumping X-rays into space. They represent a small-scale model of the accretion thought to be taking place in the very heart of some galaxies. One in every ten galaxies shows some kind of intense activity at its centre.

This activity is thought to be coming from a gigantic black hole, pulling stars to pieces and devouring their remains. Being much closer to Earth, the LMXBs are easier to study than the active galaxies.

"Accretion processes are still not well understood. The more we understand about the LMXBs, the more useful they will be as analogues to help us understand the active galactic nuclei," says Diaz.

The findings appear in Astronomy & Astrophysics (445, 179-195, 2006). The original article, 'Spectral changes during dipping in low-mass X-ray binaries due to highly-ionized absorbers', is by M. Diaz Trigo and A.N. Parmar (ESA, Noordwijk, The Netherlands), L. Boirin (Observatoire Astronomique de Strasbourg, France), M. Mendez and J.S. Kaastra (SRON, National Institute for Space Research, Utrecht, The Netherlands).



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