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The Discovery crew
The seven astronauts to fly the return to flight space shuttle mission hold a news conference at the Kennedy Space Center runway Jan. 7 to talk about delivery of the external tank, tile/RCC repair options and other issues. (44min 24sec file)

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Shuttle news conference
Senior space shuttle program officials hold a news conference at Kennedy Space Center on Jan. 6 following delivery of the redesigned external fuel tank to be used on the return-to-flight launch. (51min 47sec file)

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External tank arrives
The external tank for space shuttle Discovery's return-to-flight launch arrives at Kennedy Space Center. The tank is offloaded from the barge and moved into the Vehicle Assembly Building. (3min 15sec file)
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Tank leaves New Orleans
The redesigned external fuel tank to be used on the return-to-flight space shuttle launch is rolled out of the Michoud Assembly Facility and place on a barge for shipment from New Orleans to Kennedy Space Center. (1min 29sec file)
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Final touches
Technicians put the final touches on the Lockheed Martin-built space shuttle external fuel tank in advance of its shipment to the Cape. (1min 44sec file)
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Mars rover cake
NASA Administrator Sean O'Keefe is presented with a commemorative birthday cake marking the one-year anniversary of the Mars rover Spirit's landing. (1min 21sec file)
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Rover news briefing
On the one-year anniversary of Spirit's landing on Mars, mission officials hold a status news conference on the twin exploration rovers to discuss the latest findings and future plans for the craft. (31min 20sec file)
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NASA chief speech
During celebrations marking the Mars rover milestone on Jan. 3, NASA Administrator Sean O'Keefe gave this speech at the Jet Propulsion Laboratory. (10min 20sec file)
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The Mars rover story
Storyteller Syd Lieberman presents "Twelve Wheels on Mars" that describes the adventure to build, launch and explore with the Mars rovers Spirit and Opportunity. (54min 57sec file)
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Delta 4-Heavy launch
The Boeing Delta 4-Heavy rocket is launched from Cape Canaveral on its demonstration flight. (4min 35sec file)
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Onboard the Heavy
An onboard camera records the launch of Boeing's Delta 4-Heavy rocket from liftoff through separation of the outer boosters. (4min 40sec file)
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Beyond Einstein: Spacetime wave orbits black hole
Posted: January 10, 2005

Astronomers Jon Miller (Harvard-Smithsonian Center for Astrophysics) and Jeroen Homan (MIT) have seen evidence of hot iron gas riding a ripple in spacetime around a black hole. This spacetime wave, if confirmed, would represent a new phenomenon that goes beyond Einstein's general relativity.

Astronomers have discovered evidence for physics beyond Einstein's general relativity. This artist's conception shows a galactic black hole being orbited by a ripple in spacetime--a distortion in the fabric of space itself. Credit: Dana Berry (CfA/NASA)
These observations confirm one important theory about how a black hole's extreme gravity can stretch light. The data also paint an intriguing image of how a spinning black hole can drag the very fabric of space around with it, creating a choppy spacetime sea that distorts everything falling into the black hole.

Miller and Homan observed the phenomenon with NASA's Rossi X-ray Timing Explorer. They present this result today at a press conference at the 205th meeting of the American Astronomical Society in San Diego, Calif.

"Black holes are such extreme objects that they can actually warp and drag the fabric of spacetime around with them as they spin," said Miller, who is the lead author on an article to be published in The Astrophysical Journal Letters. "Gas whipping around the black hole has no choice but to ride that wave. Albert Einstein predicted this over 80 years ago, and now we are starting to see evidence for it."

A black hole is a region in space where gravitational forces are so great that not even light can escape. Gas and dust funnel towards a black hole in an accretion disk, swirling around and into the void like water down a drain. This process of accretion generates copious amounts of light - predominantly X-ray radiation, particularly in the innermost (hottest) regions of the accretion disk. Near the black hole, gravity is rather intense, but light still can muster an escape by climbing out of the black hole's gravitational "well," losing energy during the climb. Thus, scientists can study black hole activity with X-ray telescopes like the Rossi Explorer.

Miller and Homan, for the first time, found a connection between two important characteristics of black hole observations: quasi-periodic oscillations (QPOs) and the broad iron K line. QPOs refer to the way the X-ray light seems to flicker. The broad iron K line refers to the shape of a spike on a spectrogram (a tool scientists use to analyze light characteristics such as energy). Light from iron atoms emitted at a specific frequency creates a bright line in the spectrogram. The line is broadened, or stretched to lower energies, because the light loses energy as it climbs out of a gravitational well.

This artist's conception shows the binary system GRS1915+105, which shows evidence for a wave of spacetime in its accretion disk. A 10 solar mass black hole at the center of the disk pulls gas from a nearby companion star. The gas spiraling into the black hole heats so much that it emits X-ray radiation. Credit: Dana Berry (CfA/NASA)
Using the Rossi Explorer, Miller and Homan studied a black hole named GRS 1915+105, about 40,000 light years away in the constellation Aquila, the Eagle. They noticed that a low-frequency QPO of 1 to 2 hertz was tied to changes in the broad iron K line, as if the two features knew of each other. The fact that the two signals were in synch and were unaffected by other phenomena-such as black hole jet activity-strongly suggests that both are occurring very close to the black hole. And this, the scientists say, rules out a theory stating that broad iron lines are created in black hole winds far from the black hole itself.

This discovery raised the question of what could be causing the connection. "High-frequency QPOs are likely from matter racing around the black hole, glowing like lightbulbs on a merry-go-round," said Homan. "Of course, matter is moving much faster around a black hole than on any amusement park attraction. We see frequencies of hundreds of hertz, or hundreds of revolutions of the disk per second. That's quite a ride."

Lower-frequency QPOs are a deeper mystery. These are typically 1 to 10 hertz, and they're quite common in many binary star systems with black holes. Miller and Homan say that in the GRS 1915+105 system, the slower QPO could be the frequency of a spacetime warp. In that case, the low-frequency QPO flickering is caused by the fabric of space itself churning around the black hole in a wave. This is known as Lense-Thirring precession, which evolves out of Einstein's theory of general relativity.

Imagine the accretion disk as a music CD. (The Beach Boys come to mind.) The wave produced by the spacetime warp would increase the surface area of the flat disk. The broadness of iron K lines depends on surface area. So, this momentary increase in surface area, "flickering" at a frequency of 1 to 2 hertz, could explain the repetitive changes observed in the iron K line. Each time the hot iron gas encounters the spacetime warp, the light gets a jolt and the broad iron K line changes its appearance.

Miller and Homan caution that this is only one explanation of their observation, and that other explanations may be possible. What is clear, the scientists say, is that they are seeing a connection between QPOs and the broad iron K line, and this in turn means that scientists are probing more closely to black holes than ever before.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.



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