Could a Gamma-ray Burst Destroy Life on Earth

Of all the cosmic catastrophes that could affect our planet, an attack by radiation from a gamma-ray burst is certainly one of the most extreme. GRBs, as theyre called, are powerful events that release huge amounts of gamma rays. These are among the most deadly radiation known. If a person happened to be near a gamma-ray producing object, theyd be fried in an instant.  Certainly, a gamma-ray burst could affect lifes DNA, causing genetic damage long after the burst is over. If such a thing happened in Earths history, it could well have altered the evolution of life on our planet. If a gamma-ray burst hit Earth, these regions of the planet would see higher-than-normal to DNA in planets, animals, and humans. NASA/Goddard Space Flight Center Scientific Visualization Studiohttps://svs.gsfc.nasa.gov/3149 The good news is that Earth being blasted by a GRB is a pretty unlikely event. Thats because these bursts occur so far away that the chances of being harmed by one are quite small. Still, they are fascinating events that grab the attention of astronomers whenever they occur.   What are Gamma-ray Bursts?   Gamma-ray bursts are giant explosions in distant galaxies that send out swarms of powerfully energetic gamma rays. Stars, supernovae and other objects in space  radiate away their energy in various forms of light, including visible light, x-rays,  gamma rays, radio waves, and neutrinos, to name a few. Gamma-ray bursts  focus their energy onto a specific wavelength. As a result, they are  some of the most powerful events in the universe, and the explosions that create them are quite bright in visible light, too. This map shows the locations of a thousand gamma-ray bursters across the sky. Nearly all occurred in distant galaxies.   NASA/Swift The Anatomy of a Gamma-ray Burst What causes GRBs? For a long time, they remained quite mysterious. They are so bright that at first people thought they might be very close. It now turns out many are very distant, which means their energies are quite high. Astronomers now know it takes something very weird and massive to create one of these outbursts. They can occur when two highly magnetized objects, like black holes or neutron stars collide, their magnetic fields join together. That action creates huge jets that focus energetic particles and photons streaming out from the collision. The jets extend across many light-years of space. Think of them like Star Trek-like phaser bursts, only a lot more powerful and reaching out on an almost cosmic scale.   An illustration of a gamma-ray burst involving a black hole and a jet of material racing across space. NASA The energy of a gamma-ray burst is focused along a narrow beam. Astronomers say it is collimated. When a supermassive star collapses, it can create a long-duration burst. The collision of two black holes or neutron stars creates short-duration bursts. Oddly enough, short-duration bursts may be less collimated or, in some cases, not highly focused at all. Astronomers are still working to figure out why this might be.   Why We See GRBs   Collimating the energy of the blast means that a lot of it gets focused  into a narrow beam. If Earth happens to be along the line of sight of the focused blast, instruments detect the GRB right away. It actually produces a bright blast of visible light, too.  A long-duration GRB (which lasts more than two seconds) can produce (and focus) the same amount of energy that would be created if 0.05% of the Sun were instantaneously turned into energy. Now, thats a huge blast! Understanding the immensity of that kind of energy is difficult. But, when that much  energy is beamed directly from halfway across the universe, it can be visible to the naked eye here on Earth. Luckily, most GRBs are not that close to us. How Often do Gamma-ray Bursts Occur? In general, astronomers detect about one burst a day. However, they only detect those that beam their radiation in the general direction of Earth. So, astronomers are likely seeing only a small percentage of the total numbers of GRBs that occur in the universe. That raises questions about how GRBs (and the objects that cause them) are distributed in space. They heavily rely on the density of star-forming regions, as well as the age of the galaxy involved (and perhaps other factors as well). While most seem to occur in distant galaxies, they could happen in nearby galaxies, or even in our own. GRBs in the Milky Way seem to be fairly rare, however. Could A Gamma-ray Burst Effect Life on Earth? Current estimates are that a gamma-ray burst will happen in our galaxy, or in a nearby galaxy, about once every five million years. However, its pretty likely that the radiation would not have an impact on Earth. It has to happen pretty close to us for it to have an effect. It all depends on the beaming. Even objects very close to a gamma-ray burst can be unaffected if theyre not in the beam path. However, if an object is in the path, the results can be devastating. There is evidence that suggests that a somewhat nearby GRB could have occurred about 450 million years ago, which might have led to a mass extinction. However, the evidence for this is still sketchy. Standing in the Way of the Beam A nearby gamma-ray burst, beamed directly at Earth, is pretty unlikely. However, if one did occur, the amount of damage would depend on how close the burst is. Assuming one occurs in the Milky Way galaxy, but very far away from our solar system, things might not be too bad. If it happens relatively nearby, then it depends on how much of the beam Earth intersects. With the gamma-rays beamed directly at Earth, the radiation would destroy a significant portion of our atmosphere, specifically the ozone layer. The photons streaming from the burst would cause chemical reactions leading to photochemical smog. This would further deplete our protection from cosmic rays.  Then there are the lethal doses of radiation that surface life would experience. The end result would be mass extinctions of most species of life on our planet. Luckily, the statistical probability of such an event is low. Earth seems to be in a region of the galaxy where supermassive stars are rare, and binary compact object systems arent dangerously close. Even if a GRB happened in our galaxy, the likelihood that it would be aimed right at us is pretty rare. So, while GRBs are some of the most powerful events in the universe, with the power to devastate life on any planets in its path, we are generally very safe. Astronomers observe GRBs with orbiting spacecraft, such as the FERMI mission. It tracks every gamma-ray that is emitted from cosmic sources, both inside our galaxy and in distant reaches of space. It also serves as a sort of early warning of incoming bursts, and measures their intensities and locations. This is what the gamma-ray sky looks like as seen by NASAs Fermi telescope. All the bright sources are emitting gamma rays at strengths greater than 1 GeV (giga-electron-volt). Credit: NASA/DOE/Fermi LAT Collaboration    Edited and updated by Carolyn Collins Petersen.