When Supermassive Supergiants Go Superboom
Article by Phil Plait via Slate
I have long been fascinated by gamma-ray bursts (or GRBs). These are incredibly violent events: It’s like taking the Sun’s entire lifetime energy output and cramming into a single event that lasts for mere seconds! The energy emitted is so intense, so bright, we can see GRBs from a distance of billions of light years.
Gamma rays themselves are just a form of light, like the kind we see, but with huge energy; each photon is packed with millions or billions of times the energy in a single photon of visible light. Only the most energetic events in the Universe can make them, so if we detect a burst of them coming from the sky, we know something literally disastrous has happened.
We know GRBs come in many flavors. Some last literally for milliseconds, while others stretch on for minutes. We also know different events can cause them, too. Short ones seem to come from merging neutron stars, ultra dense compact objects left over after stars explode. The longer ones occur when massive stars explode, leaving their cores to collapse. In both cases, the huge blast of high-energy gamma rays signals the birth of a black hole.
But astronomers were recently surprised to find a third type of GRB, one that lasts not for minutes, but for hours. Whatever these objects are, they don’t just flash with light, they linger, blasting out far, far more gamma rays for far, far longer than was previously thought. What could do such a thing?
Several ideas were put forth, but new observations provided the linchpin: an ultra-long-duration GRB occurred on Christmas Day in 2010, and its distance was found to be a soul-crushing 7 billion light years away, about halfway across the visible Universe! This left only one possible candidate for the progenitor: a hugely massive star, one so big it dwarfs the Sun into insignificance.
Asteroid Threat Collides with Earthly Budget Realities in Congress
In the wake of last month’s meteor strike in Russia and a close asteroid flyby on the same day, members of Congress asked NASA, White House and Air Force officials what they’re doing to combat the threat of near-Earth asteroids during a hearing today (March 19) on Capitol Hill.
By and large, the experts stressed that the two space rock events were a coincidence and that the chance of a catastrophic asteroid impact to Earth any time soon is remote. On Feb. 15, a surprise meteor exploded in the sky over Russia’s Ural Mountains, just hours before the 150-foot-wide (40 meters) asteroid 2012 DA14 flew close by Earth in a pass that had been predicted beforehand by scientists.“The odds of a near-Earth object strike causing massive causalities and destruction of infrastructure are very small, but the potential consequences of such an event are so large that it makes sense to take the risk seriously,” John Holdren, science advisor to President Barack Obama, told the Science, Space and Technology Committee of the U.S. House of Representatives.
Still, Rep. Lamar Smith (R-Texas), chairman of the Science, Space and Technology Committee, said it was “not reassuring” to learn that NASA has so far detected only about 10 percent of the near-Earth objects that are wider than 459 feet (140 meters) across. Holdren estimated that there may be hundreds of thousands of such objects within one-third the distance from Earth to the sun that remain unknown.
In 2005, Congress directed NASA to detect, track and characterize 90 percent of these space rocks —those near-Earth asteroids larger than 459 feet feet (140 m). The space agency’s chief, Charles Bolden, said today that NASA was unlikely to meet that deadline given its current budget.
“Our estimate right now is at the present budget levels it will be 2030 before we’re able to reach the 90 percent level as prescribed by Congress,” Bolden said.
Bolden criticized the lawmakers for slowing NASA down through budget cuts. “You all told us to do something, and between the administration and the Congress, the bottom line is the funding did not come,” Bolden said.
Furthermore, he said the goal of finding a way to respond to asteroid threats has been repeatedly put off by lawmakers who cite a lack of money.
Rep. Bill Posey (R-Fla.) asked what NASA would do if a large asteroid headed on a collision course with Earth was discovered today with only three weeks before impact.
“The answer to you is, ‘if it’s coming in three weeks, pray,’” Bolden said. “The reason I can’t do anything in the next three weeks is because for decades we have put it off.”
Budget concerns also hamper the military’s ability to monitor near-Earth objects and other space threats, such as orbital debris (defunct satellites and spent rocket stages that litter Earth orbit).
“We are clearly less capable under sequestration,” Gen. William Shelton, the current commander of the U.S. Air Force Space Command, told the committee. He said that any further budget cuts could have dire consequences.
“Our dependence on space, not only for our way of life but also for military operations, is very high, so we would sacrifice that,” Shelton said.
This story was updated at 4:56 p.m. ET to correct the size of large near-Earth asteroids (459 feet, or about 140 meters) NASA seeks to identify under its 2005 congressional mandate.
Electrically charged lunar dust near shadowed craters can get lofted above the surface and jump over the shadowed region, bouncing back and forth between sunlit areas on opposite sides, according to new calculations by NASA scientists.
The research is being led by Michael Collier at NASA’s Goddard Space Flight Center, Greenbelt, Md., as part of the Dynamic Response of the Environment At the Moon (DREAM) team in partnership with the NASA Lunar Science Institute (NLSI), managed at NASA’s Ames Research Center, Moffett Field, Calif.
“The motion of an individual dust particle is like a pendulum or a swing,” says Collier. “We predict dust can swarm like bees around a hive over partially shaded regions on the moon and other airless objects in the solar system, such as asteroids. We found that this is a new class of dust motion. It does not escape to space or bounce long distances as predicted by others, but instead stays locally trapped, executing oscillations over a shaded region of 1 to 10 meters (yards) in size. These other trajectories are possible, but we now show a third new motion that is possible.” Collier is lead author of a paper on this research published October 2012 in Advances in Space Research.
This effect should be especially prominent during dusk and dawn, according to the team, as regions become partially illuminated while features like mountains and crater rims cast long shadows.
“The dust is an indicator of unusual surface electric fields,” says William Farrell of NASA Goddard, a co-author on the paper and lead of the NLSI DREAM team. “In these shaded regions, the surface is negatively charged compared to the sunlit regions. This creates a locally complex, larger electric field with separate positively and negatively charged regions, called a dipole field, over the shaded region. The dust performed its swinging motion under the influence of this dipole. Such a surface process occurring on the moon at the line where night transitions to day, called the terminator, might also occur at small bodies like asteroids. It might be a fundamental process occurring at airless rocky bodies.”
There is evidence that dust actually moves this way over the lunar surface. “There are hints for this type of dust swarm in Surveyor images. A twilight was observed over the landed platforms during dusk and dawn. This was surprising at first because the moon does not have a dense enough atmosphere to scatter light when the sun is below the horizon. It was long considered to be light scattered from lifted dust. This model suggests the dust is really leaping or swarming overtop a large number of shaded regions that would exist along the lunar dusk/dawn line, called the lunar terminator. It’s a natural fit. Charged lunar dust transport is also believed responsible for the Apollo 17 Lunar Ejecta and Meteorites (LEAM) experiment’s observation of highly charged dust near the terminator,” adds Collier.
For more information about the leaping lunar dust, visit: http://lunarscience.nasa.gov
