INTRODUCTION
The history of cosmic ray research is a romantic story of scientific adventure. For three quarters of a century, cosmic ray researchers have climbed mountains, dived into deep mines and long tunnels, ridden hot-air balloons, and traveled to the remote corners of the earth in the tantalising quest to understand these fast-moving particles from space. Their endless explorations have solved scientific mysteries and revealed many more.

We will mention just a few of those pioneers: the intrepid Viktor Hess, with his balloon flights in the beginning of this century; Millikan (and Compton), going around the world measuring the intensity of the radiation; Anderson, the discoverer of the antimatter; Pierre Auger, the discoverer of the big showers of particles; Lattes, Occhialini and Powel, elucidating the mistery of Yukawa's meson; Fermi, proposing a theory for the acceleration mechanism.
  COSMIC RAYS?
Cosmic rays are fast-moving particles from space that constantly bombard the earth from all directions. Each second, about 200 cosmic ray particles with energies of a few million electron volts (106eV) strike every square meter of the earth. While these low-energy cosmic rays are plentiful, cosmic ray at higher energies are far rarer. Above the energy of 1018eV, only one particle each week falls on an area of one square kilometer. Above the energy of 1020eV, only one particle falls on a square kilometer in a century! To find and measure these rare events, a high-energy cosmic ray study needs to wait centuries or to build a truly giant detector.
  THEIR ENERGY
Most of the cosmic-ray particles are either the nuclei of atoms, or electrons. Of the nuclei, most are single protons -the nuclei of hydrogen atoms- but a few are much heavier, ranging up to the nuclei of lead atoms.

Cosmic ray particles travel at nearly the speed of light, which means they have very high energy. Some of them, in fact, are the most energetic of any particles ever observed in nature. The highest-energy cosmic rays have a hundred million times more energy than the particles produced in the world's most powerful particle accelerator.

No one knows the source of the highest-energy cosmic ray particles. Most lower-energy cosmic ray particles that strike the earth come from within our own galaxy, the Milky Way. Many probably come from the exploding stars we call Supernovae. Over time, some cosmic ray particles pick up energy from moving magnetic fields they encouter as they wander around the galaxy.

The great Italian physicist Enrico Fermi first provided an explanation for the acceleration of most cosmic rays. In Fermi's cosmic ray accelerator the protons "bounce" off moving magnetic clouds in space. Despite the random directions of both the cosmic rays and the clouds, over time the cosmic rays gain energy. This process is well understood for low-energy cosmic rays accelerated by magnetic fields produced by the sun. In our galaxy, scientists believe that the strong moving magnetic fields produced in supernovae explosions provide the energy for acceleration.
  PRESENT SITUATION
Presently, there is suggestive evidence from a number of experiments that above 1020 eV the cosmic rays are protons. If so, an origin within our own galaxy can be excluded as the protons would propagate rectilinearly and sources within our galaxy would be readily identified. By contrast, the arrival direction distribution is isotropic, within the limited statistics, even beyond 1020eV where only a handful of events have been detected. This result is extremely paradoxical as sources at large distances (beyond about 30 Mpc) are strongly excluded by the existence of such events. The point is that above 4x1019 eV protons and heavier nuclei interact with the primordial 2.7 K microwave background radiation (from the Big Bang) through well-understood reactions of particle and nuclear physics and are rapidly degraded in energy. The existence of the most energetic event yet detected, 3x1020 eV, implies that it must have originated within 20 Mpc of the earth. However the arrival directions of this event and others of similar energy do not point to any unusually energetic objects within our galaxy or elsewhere and the lack of appropriate electromagnetic acceleration processes has even led to speculation that the particles are created in the collapse of massive cosmic strings which are topological relics of the early universe.
  COLLABORATION
Scientists believe that the very highest-energy cosmic ray particles come from sources beyond the Milky Way - but where?

Something out there -no one knows what and where- is hurling incredibly energetic particles around the universe. Do these particles come from some unknown superpowerful cosmic explosion? From a huge black hole sucking stars to their violent deaths? From the collapse of massive invisible relics from the origin of the universe? We don't yet know the answers, but we do know that solving the mystery of high-energy cosmic rays will take scientists another step forward in understanding the universe.

The results obtained so far derive from the independent efforts of groups in four different nations and are based on only a few hundred records above 1019 eV. To increase this sample significantly requires an international effort in which all the nations historically involved, and others, are eager to participate. In this scenario was born the Pierre Auger Project.

So, a multinational group of physicists presented a detailed proposal for a new cosmic ray observatory, the Pierre Auger Project, named in honor of the discoverer of air showers. The new observatory will use a giant array of detectors to study large numbers of air showers from the very highest-energy cosmic rays, above 1019eV. Tracing high-energy cosmic rays to their unknown source will advance the understanding of the origin and evolution of the universe.
  THE PIERRE AUGER OBSERVATORY
To measure cosmic rays particles directly requires sending detectors to heights above most of the earth's atmospherre, using high-flying balloons and satellites. However, we can also detect cosmic rays indirectly on the surface of the earth by observing the showers of particles they produce in the air. An air shower occurs when a fast-moving cosmic ray particles strikes an air molecule high in the atmosphere, creating a violent collision. Fragments fly out from this collision and collide with more air molecules, in a cascade that continues until the energy of the original particle is spread among millions of particles raining down upon the earth on an area of about 16 km2. The atmosphere absorbs the great part of the energy of the cosmic ray particles in this process and makes it possible to detect and measure them.

By measuring two different features of an air shower, scientist at the Pierre Auger Observatory will be able to determine the direction and energy of the original incoming cosmic ray particle. In the first part of the detection system, 1600 particle detector stations will form a giant regular array, or grid, covering about 3000 km2. The detector stations will be about 1.5 km apart, and each will be about the size of a small one-car garage. Each station will be self-contained and operate on solar power. Instruments in each station will measure the number of particles passing through. Shower particles from a high-energy cosmic ray will reach several stations at the same time. When particles strike a station, a small computer will confer by radio with computers in neighboring stations to decide whether the particles are part of a large shower. If so, information about the shower will be transmitted by radio to a central data center. At the data center, computers will combine the measurements of the number of particles and their time of arrival at each station to determine the direction and energy of the original cosmic ray that set off the shower. The detector will measure about 50 cosmic-ray events a year with energies above 1020eV, along with large numbers of lower-energy ones.

A second detection system will make use of the faint glow caused by the collisions of shower particles with air molecules during cosmic-ray air showers. On dark, moonless nights, in a remote and dry site, finely tuned light sensors can measure this fluorescence. A collection of light sensors pointing around the sky in all directions makes an effective air shower detector, observing an air shower as a trace of light across the sky. The total amount of light depends on the number of particles in the shower and, in turn, on the energy. The shape and direction of the light trace helps determine the cosmic ray's direction and indicates what kind of particle the original cosmic ray might have been. The first fluorescence detector built by the University of Utah was called "fly's eye".

The Pierre Auger Observatory will build two Giant Detectors, one in the Northern and another in the Southern Hemisphere. The detectors will be installed in Argentina (the first to be constructed) and in the USA.