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Cosmic Fireworks: How to Watch Ultra High-Energy Cosmic Rays Hitting Earth

The best place to watch ultra high-energy cosmic rays hitting the Earth is from space. But building an observatory that can do the trick will be hard, warn physicists

In 1962, a couple of physicists noticed a remarkable phenomenon in the skies above New Mexico. These guys were measuring the light produced by high energy particles called cosmic rays as they smash into the atmosphere. The light gives a good idea of the energy of the ray that generated it.

Cosmic rays are protons or alpha particles that have been accelerated to huge energies by processes that are not entirely understood. However, there are good reasons to think that they cannot have energies above 10^19 eV. That’s because such high energy particles should interact with photons in the cosmic microwave background, losing energy as they travel through space.

So the observation of a particle with 10^20 eV in 1962 was a huge shock. That’s about the same energy as a baseball travelling at 100 km/h. In a single proton!

Since then, physicists have seen dozens of cosmic rays with similar energies but a good explanation of what causes them still eludes scientists.

Part of the problem is that these ultra high-energy cosmic rays are very rare. The best calculations suggest that these particles arrive at a rate of one every thousand years in an area of atmosphere about 1 km square (per steradian).

The Pierre Auger Observatory, which is designed to look for these particles, has an observing area of about 2000 square kilometres. So even this is able to gather limited data.

So the question troubling Marco Pallavicini and pals at the Università di Genova in Italy is how to do better. Their answer is to build a space-based telescope that looks down on Earth, watching for the fiery traces of light that these particles would generate. Today, they outline some of the challenges in building such a device.

The clear advantage of a space-based telescope is the huge area of atmosphere it can monitor. The disadvantage is that such an observatory, at an altitude of about 800 km, would be significantly further away from the particles and the origin of the light they produce. By contrast, ground-based observatories are only a few tens of kilometres away.

In addition, the telescope must be able to distinguish the ultra high energy particles from other light sources that might look similar and from the general background glow that fills the skies at night.

“The design of a space-based telescope aimed at the detection of [ultra high energy cosmic rays] is a very challenging task,” they conclude, adding that they imagine such an observatory could be built and launched by the European Space Agency in the time-frame 2015-2025.

That seems ambitious given the competition for resources at ESA. But it is certainly a worthy goal. Understanding how these particles get their energy and where in the universe they are produced is one of the more important problems in astrophysics today.

Ref: arxiv.org/abs/1105.2129: The Observation Of Extensive Air Showers From An Earth-Orbiting Satellite

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