This is

This is Scientific Americans 60 Second Science.

I'm Lee Billings.

Take any square kilometer of Earth's surface.

About once a year, an extraordinary event occurs in the sky directly above that patch of land or sea.

The hefty nucleus of a heavy element slams into the top of Earth's atmosphere at close to the speed

of light.

Scientists have been unable to tell where exactly these particles come from,

in part because their trajectories can be nudged by galactic magnetic fields.

Another puzzle is how the particles reach such blistering speeds.

Two theories dominate efforts to explain these mysteries.

One posits that the particles mostly come from exploding stars and

other high-energy phenomena in our galaxy.

The other speculates that the particles are produced beyond our galaxy, perhaps in the

active cores of other galaxies surrounding the Milky Way. Now a study in the

journal Science supports that second notion. Amazingly, any of these ultra-high

energy cosmic rays has the kinetic energy of an apple falling from a tree to the ground.

That means that Isaac Newton or you would definitely feel it hit your head.

Luckily that never happens. Instead, when these cosmic speed demons strike our atmosphere,

they create a brief flash of light,

as well as high-altitude air showers of less energetic particles that harmlessly dissipate.

In the new study, an international team of more than 400 researchers analyzed a dozen years

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