Centauri Dreams continues to maintain that a major justification for interstellar research is the need of our species to protect itself. The record of life on earth is studded with extinction-level events evidently caused by asteroid or cometary impacts, and as technology matures, the danger of a man-made catastrophe cannot be ruled out. We know that life is fragile, as is underscored by the following story.
According to a new study from NASA and the University of Kansas, working with ‘what if’ scenarios and a finely-tuned model of Earth’s atmosphere, a gamma-ray burst from the explosion of a relatively nearby star could destroy up to half the atmosphere’s ozone layer. Remarkably, a burst that hit the Earth for only ten seconds could do the trick, damaging Earth’s only shield against powerful ultraviolet radiation from the Sun. With recovery time of no less than five years, that could have catastrophic effect on all surface species and destroy the food chain.
“A gamma-ray burst originating within 6,000 light years from Earth would have a devastating effect on life,” said Dr. Adrian Melott of the Department of Physics and Astronomy at the University of Kansas. “We don’t know exactly when one came, but we’re rather sure it did come — and left its mark. What’s most surprising is that just a 10-second burst can cause years of devastating ozone damage.”
Image: Gamma-ray bursts are the most powerful explosions known in the Universe, and most originate in distant galaxies. A large percentage of bursts likely arise from the explosion of stars over 15 times more massive than our Sun. Scientists say burst from a nearby star could cause severe damage to the Earth’s protective ozone layer. In this artists conception we see the gamma rays hitting the Earth’s atmosphere. (The expanding shell is pictured as blue, but gamma rays are actually invisible.) Credit: NASA.
Centauri Dreams‘ take: We don’t know enough about the frequency of gamma-ray bursts in the Milky Way, nor do we understand their sources well enough to assume that — in millennial terms — such a burst would not pose a future threat to life on Earth. That makes a responsible, balanced and continuing effort into interstellar research a basic insurance policy for the human future. It also underlines the need for continuing work on gamma ray-bursts and their sources.
What we know now: The bursts seem to arise from the explosions of stars at least 15 times more massive than the Sun, and may signal the birth of a black hole. They are hard to observe even though several occur each day as seen from Earth. The reason: they appear at random and last only a few milliseconds up to a minute.
The scientists estimate that it may have been such a burst that caused the Ordovician extinction 450 million years ago, which killed 60 percent of all marine invertebrates and left most life confined to the sea. That theory is the work of Dr. Bruce Lieberman, a paleontologist at the University of Kansas.
Meanwhile, the NASA-led Swift mission has measured the distance to two gamma-ray bursts coming from opposite parts of the sky, and has found that both were more than 9 billion light years away. These results are the first direct distance measurements of the Swift mission, obtained with the spacecraft’s Ultraviolet/OpticalTelescope (UVOT). Swift has only been operational since November, and it is hoped that such measurements will become routine. The mission has detected 24 bursts so far.
“Swift will detect more gamma-ray bursts than any satellite that has come before it, and now will be able to pin down distances to many of these bursts too,” said Peter Roming, UVOT lead scientist at Penn State. “These two aren’t distance record-breakers, but they’re certainly from far out there. The second of the two bursts was bright enough to be seen from Earth with a good backyard telescope.”
The good thing about Swift is that its three telescopes can study the burst afterglow, although it’s also true that not all bursts produce an afterglow, showing that our knowledge of burst sources is far from complete. As with cometary or asteroid impacts, the danger of an Earth-threatening event from a gamma-ray burst seems tiny, but that threat should be measured against what Tennyson called ‘the long result of time.’ Interstellar migration over the course of the next several thousand years could be the way mankind hedges its bets against the destruction of what is currently its only outpost.
Sources: The extinction story is discussed on this NASA page. You can read more about Swift’s recent detections in a Pennsylvania State University news release. NASA offers a backgrounder on gamma-ray bursts here. Also be aware of the Gamma-Ray Burst Real-Time Sky Map.