Gamma ray bursts

In the 1960s, the United States launched the Vela satellites to monitor nuclear weapons tests. On 2 July 1967, two of the satellites detected a burst of radiation unlike that of any known nuclear weapon. Unconcerned, the scientists monitoring the satellites filed the unusual data away for later analysis. The launch of further satellites allowed researchers to confirm that the radiation was not of a terrestrial origin and was labelled a ‘gamma ray burst’.

Each gamma ray burst has a unique intensity spectrum, leading to the astronomers’ joke: “If you’ve seen one gamma ray burst, you’ve seen one gamma ray burst.” The bursts are thought to be the most powerful explosions in the universe and come in two types:

• ‘short-hard’ bursts which are hypothesised to arise from the merger of a binary system of compact objects such as two neutron stars, or a neutron star and a black hole
• ‘long-soft’ bursts which are related to the deaths of short-lived massive stars.

A large, local gamma ray burst could cause a number of devastating impacts on the Earth. The radiation will disassociate N₂ and O₂ molecules in the atmosphere leading to the depletion of the ozone layer and increased irradiation of the planet’s surface in ultraviolet radiation. In addition, the disassociation would lead to increased production of NO₂, increasing atmospheric opacity causing global cooling. The increased NO₂ levels would also increase the acidity of rain, harming aquatic organisms.

Given these devastating effects, gamma ray bursts have been hypothesised to have caused past mass extinctions. It is estimated that a gamma ray burst originating from within 9,800 light years (3 kpc) of the Earth would constitute a serious threat to life and such events are estimated to occur every 170 million years.