What carries more kinetic energy than a Randy Johnson fastball, yet is only the size of an atom?

The answer is a cosmic ray, the fastest moving bit of matter in the Universe.

Cosmic rays (which are particles, not rays at all) come in a variety of forms, sizes, speeds, and energies. The most energetic, called "ultrahigh-energy cosmic rays," move at nearly the speed of light and have more energy than a major league fastball. Remember, all that energy comes from just one atomic-size particle. Seeing how there are as many atoms in a baseball as there are baseballs that could fill the moon, that's one powerful pitch. These particles are so fast, in fact, that their origin is a major mystery in astronomy.

Cosmic rays are essentially fast-flying particles from space. They largely take the form of electrons, protons, neutrinos, or atomic nuclei (atoms without electrons) such as carbon or iron. Austrian physicist Victor Hess discovered them in 1912 during a balloon flight and thought they constituted a new form of radiation, like X rays and gamma rays discovered just a few years prior to this, hence the name "cosmic rays." He shared the Nobel Prize for this discovery, in 1936.

By far, the most common cosmic rays are of low energy, generated by the Sun. These cosmic rays are also called energetic solar particles, the bulk of which are electrons flung towards the Earth during solar flares, coronal mass ejections, or other solar events. A cosmic ray's energy is measured in electron volts, a tiny unit of energy. Solar cosmic rays are about a million to a billion electron volts. This may sound energetic, but really this is still trillions of times less energetic than Randy Johnson's fastball.

Mid-energy cosmic rays are also relatively common, at an energy of about a billion to a million-trillion electron volts. Cosmic rays of this energy range are among the few samples of matter we have from beyond the solar system. These particles are likely associated with star explosions. The explosion itself may fire these particles from the guts of stars to cosmic-ray energies immediately, like shrapnel; or shockwaves from the explosion may accelerate particles already floating in the interstellar medium.

The ultrahigh-energy cosmic rays, which are rare, are in the Randy Johnson range at over a million trillion electron volts -- or about one joule. (A single watt is defined as one joule of energy per second.) The most energetic cosmic ray recorded is over 10^22 electron volts, or 10 billion trillion electron volts. Scientists have detected several ultrahigh-energy cosmic rays with ground-based instruments in recent years.

A cosmic ray this powerful will collide with particles in the Earth's atmosphere and produce a cascade of secondary particle collisions and a characteristic ultraviolet burst of light. Ground-based instruments track either the secondary collisions or the UV light, and then reconstruct the powerful particle that caused it.

The atmosphere largely protects us from cosmic rays, blocking them from reaching Earth's surface. We do, however, get a small dose when flying in an airplane at high altitudes, particularly on transcontinental flights. Lower-energy cosmic rays hit every inch of the Earth's atmosphere every second; it's a constant bombardment.

An ultrahigh-energy cosmic ray will strike at a rate of about one per square kilometer per century. That's not very often. Ground-based detectors covering several square kilometers have to wait years to catch one. Each new event is hot news, exciting scientists around the globe. Depending on how one measures and defines an "ultrahigh energy" cosmic-ray event, there have only been a couple dozen recorded so far.

This is why ultrahigh-energy cosmic rays are a mystery. Scientists haven't been able to collect enough of them to determine where they come from. Naturally, there are theories. Believe it or not, the most mundane theory involves spinning, supermassive black holes churning out the cosmic particles. In reality, scientists know of no black hole nearby that could produce ultrahigh-energy cosmic rays. Most active, supermassive black holes are very far, and any cosmic rays they may produce would lose energy on the long journey towards Earth.

It's a bit of a Catch-22. The cosmic-ray origin must be close, theory predicts, because distant cosmic rays lose too much energy. Yet nothing close to Earth seems powerful enough to make these cosmic rays. This conundrum places the field of cosmic-ray research at the forefront of new physics. Perhaps either the generation or the movement of these cosmic rays involves physics beyond our current theories.

Some theorists suggest that ultrahigh-energy cosmic rays are generated by topological defects in spacetime, mysterious pockets of space and time left over from the Big Bang that possess energies equivalent to those that existed at the creation of the Universe. Other theorists suggest that ultrahigh-energy cosmic rays involve hidden dimensions or unknown energy sources. These theories are speculative, although seriously considered. It seems that the origin of ultrahigh-energy cosmic rays will be fascinating and new, no matter what it is.

The Auger Observatory, now under construction in Argentina, will be the world's largest cosmic ray detector array, covering an area the size of Rhode Island with 1,600 detectors in hopes of catching more ultrahigh-energy cosmic rays. Scientists are also proposing a space-based mission called OWL, which would monitor the atmosphere from above for cosmic rays colliding with atmospheric gases and producing a characteristic streak of ultraviolet radiation. From its space perch, OWL could monitor thousands of square miles at a time, casting a wide net to catch ultrahigh-energy cosmic rays and determine the direction from where they came.

Human-built particle accelerators on Earth, such as the one at Fermi Lab near Chicago, smash atomic particles moving near light speed to uncover the building blocks of matter and energy. Scientists hope to create energies at the 10^15 eV level, a million-billion electron volts. Nature, as always, humbles us. Its cosmic particle accelerators are a million times more powerful than the one at Fermi Lab. There are valuable lessons to be learned in nature's cosmic-ray fastballs.

Christopher Wanjek in a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.