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Changing the way we view the world
by Wallace Tucker
Nov 3, 2011 ::
Thirty years of space shuttle flights came to an end on July 21, 2011 when Space Shuttle Atlantis touched down before dawn at NASA's Kennedy Space Center. "The space shuttle changed the way we view the world, the way we view the universe," Commander Chris Ferguson said soon after landing.
A major part of this change of perspective has been from NASA's Great Observatories: the Hubble Space Telescope, Compton Gamma Ray Observatory, Chandra X-ray Observatory, and Spitzer Space Telescope. Hubble, Compton and Chandra were launched by the space shuttle in 1990, 1991, and 1999, respectively. Spitzer was launched with a Delta II rocket in 2003.
Although the space shuttle was identified as early as 1980 as the launch vehicle for the observatory that would become Chandra, it almost didn't happen that way. In fact, Chandra almost didn't happen. The projected launch date was mid-1987. However, budget difficulties and the Challenger catastrophe pushed development and launch of AXAF further into the future. Then in 1992, a deepening national budget crisis threatened the existence of the AXAF program unless it was drastically revised. NASA officials recommended reducing the number of mirror pairs from the originally proposed number of six to two, and using a unmanned Titan rocket to launch the spacecraft.
For six months an argument raged between scientists, who thought that the scientific productivity would be severely compromised by the reduced mission, and NASA officials, who argued that the original mission was too expensive and literally would not fly. "It's like horse racing," a former NASA official said. "It's not enough to have a horse that you love. You have to have a horse that can win."
In the end a compromise was reached: the observatory would have four sets of mirrors, would be launched aboard the shuttle and then boosted to a high Earth orbit by an upper stage rocket. In this orbit, observing efficiency would be high, and the expense of servicing the observatory by the shuttle would be eliminated, since servicing in a high orbit would be impossible.
Time has proven that this compromise was a good one. It saved the program from cancellation while preserving the capabilities that would make Chandra a major advance over previous X-ray missions. As a bonus, Chandra's large. 64-hour orbit keeps the observatory above Earth's radiation belts for 55 hours at a stretch and enables long science observations. This makes for a high observing efficiency compared to a low-Earth orbit in which the spacecraft orbits the Earth every hour and a half.
Some of the images in this year's releases provide good examples of the advantage of long observing times. An 8-hour observation of the black hole system GRS 1915 revealed that the system pulses in X-ray light every 50 seconds. This heartbeat-like rhythm is likely set up by the interplay between the "pull" of the black hole's gravity and the "push" of radiation from the infalling gas. As the gas falls in, it is heated and compressed, which increases the X-radiation. The radiation pushes back on the gas, stopping its infall. This reduces the X-radiation, allowing the gas to fall toward the black hole again to start another cycle.
The main advantage of long observing times is that, in combination with Chandra's high resolution X-ray mirrors, fainter, more subtle features can be picked up. For example, a series of 9 Chandra observations averaging 23 hours each of the Tycho supernova remnant has revealed a pattern of X-ray "stripes." The spacing of the stripes is an indication of the maximum energy of the accelerated particles by the supernova shock wave. This observation provides some of the best evidence yet that supernova shock waves are a primary source of the high-energy cosmic ray particles that continually bombard the Earth's atmosphere.
Six observations of about 14 hours each over a ten-year interval have allowed astronomers to detect a 4% decline in the temperature of the neutron star in the center of the Cassiopeia A supernova remnant Although this doesn't seem like much of a temperature drop, it is much greater than expected from normal cooling processes and suggests that something unusual is happening. The leading idea is that material inside the neutron star is undergoing a transition to a superfluid state much like that which occurs in laboratories on Earth when liquid helium is cooled to near absolute zero. The difference is that the transition temperature in the neutron star is about a billion degrees Celsius, because the density of matter there is about 6 billion tons per ounce.
Add to these results the discovery from Chandra Deep Field South, that supermassive black holes were already growing in the centers of galaxies when the universe was less than 7 percent of its present age, and it is evident, from this year's accomplishments alone, that the space shuttle has indeed helped to change the way we view the universe.