Look back: UV view of the cosmic

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  • Published: Apr 1, 2014
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Look back: UV view of the cosmic

Up, up and away...

Astrophysicists are collaborating on an ultraviolet telescope, a cosmic ray telescope, for the International Space Station that will be installed before the end of the decade and will look back at Earth in order observe the collisions of cosmic rays with our atmosphere. Press release image via Wikipedia

Astrophysicists are collaborating on an ultraviolet telescope, a cosmic ray telescope, for the International Space Station that will be installed before the end of the decade and will look back at Earth in order observe the collisions of cosmic rays with our atmosphere.

When Henri Becquerel discovered radioactivity in 1896, scientists generally assumed that elemental radiation was the only cause of ionization of the atmosphere. But, this was not supported by ongoing experimental observations during the first decade of the twentieth century. In 1909 Theodor Wulf, for instance, demonstrated that radiation levels were higher at the top of the Eiffel Tower than at its base, although this discovery was largely ignored and it was not until 1910-1911 when Domenico Pacini observed simultaneous variations of the rate of ionization over a lake, over the sea, and at a depth of 3 metres that there must be an extraterrestrial cause for much of the ionization occurring on Earth.

However, Victor Hess then used three improved Wulf electrometers in 1912 and took a balloon flight to an altitude of 5300 metres during which he demonstrated that the ionization rate was about four times higher at the peak than at ground level. Results obtained during a total solar eclipse also excluded the Sun as the source of the radiation. Today, we know that every second, our bodies and the objects around us are struck by cosmic rays, composed primarily of high-energy protons and atomic nuclei; and so not "rays" at all that is an historical, terminological accident due to Robert A. Millikan's coining of the term in 1928 on the mistaken basis that they were gamma rays. The most energetic of these are much more powerful than anything we can generate on Earth but their origins remain a mystery. Indeed, ultra-high-energy cosmic rays (UHECRs) have an energy of almost 3x10^20 electronvolts (about 50 Joules), which is about 40 million times the energy of particles racing around the Large Hadron Collider. At such energies such a particle has an equivalent kinetic energy to a tennis ball travelling at more than 250 kilometres per hour.

Develop and deploy

Astrophysicist Angela Olinto of the University of Chicago and her international collaborators hope to solve this stubborn puzzle by developing and deploying a cosmic ray telescope on the International Space Station (ISS) late in the second decade of the twenty-first century. They will design the instrument to look back at our planet from its orbit more than 400 kilometres above the Earth and to detect the collisions of cosmic rays with the atmosphere, which could shed light on what produces these enigmatic particles. Our suspicions based on data from the Fermi space telescope obtained in 2013 are that they have their origin in the death throes of massive stars that explode as supernovae. However, it is likely that active galactic nuclei, the supermassive black holes at the centre of galaxies, and other objects in space are also a source. Some theorists suggest that they are remnants of the Big Bang although this is now considered an unlikely explanation.

Olinto has described the mission as being tantamount to using the Earth's atmosphere itself as a giant particle detector. With US $4.4 million in funding from the National Aeronautics and Space Agency (NASA), the US part of the team working with colleagues in 14 other countries plan to build a 2.5-metre ultraviolet telescope, the Extreme Universe Space Observatory, to be installed on the ISS. They do not know what it will find. "The mechanism behind this extreme acceleration challenges our imagination," Olinto says.

Evolution copes with the cosmic

Life on Earth evolved to cope with the daily onslaught of cosmic rays at the levels that reach the planet's surface, but air travel and space travel would expose us to greater levels so understanding their nature is important. Cosmic rays also represent a risk to the functioning of electronic gadgets. Ever had a smart phone or computer crash for no reason? It could well have been an ionising particle from space slamming into the sensitive circuitry.

In the meantime, data from the Telescope Array Project in Utah and the IceCube Neutrino Observatory at the South Pole hint at a possible source of high-energy cosmic rays - a hotspot in the northern sky. Olinto and her colleagues hope that this hotspot persists long enough for them to get the Extreme Observatory up and running on the ISS.

Related Links

Olinto: "Angela V. Olinto at University of Chicago"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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