Cosmic rays, traveling nearly at the speed of light, bombard Earth from all directions. The electrically charged particles are the most energetic component of cosmic radiation—yet no one knows where they come from.

Astrophysicists speculate that high-energy cosmic rays may have emerged from supermassive black holes in faraway galaxies or possibly from decaying particles from the big bang.

Whatever their origin, these rays crash into Earth’s atmosphere about once per square kilometer per century. The impact produces an air shower of tens of billions of secondary, lower-energy particles that in turn excite nitrogen molecules in the atmosphere. The interactions produce ultraviolet fluorescence that lights up the air shower’s path. Scientists are trying to use such paths to measure the direction and energy of cosmic rays and reconstruct their trajectories back millions of light-years into space to pinpoint their source.

Seeing these extreme events is rare. Earth-based observatories can spot cosmic- ray collisions only if they occur directly above the detectors. The Pierre Auger Observatory in Argentina, which houses the world’s largest cosmic-ray detector and covers an area roughly the size of Rhode Island, records about 20 extreme-energy particle showers a year.

Hoping to improve the odds of observing the rays, a team of scientists from 15 nations came together more than a decade ago and designed a cosmic- ray telescope for the International Space Station (ISS). On the Japanese Experimental Module, the Extreme Universe Space Observatory (JEM- EUSO) will record ultraviolet emissions with a wide-angle, high-speed video camera that points toward Earth. With such a large observation area, the camera will see more air showers. The team originally hoped to launch EUSO in 2006. But troubles on Earth—first the space shuttle Columbia disaster in 2003, then the Fukushima nuclear meltdown in 2011 and now the turmoil in Ukraine—have delayed its deployment until at least 2018.

The science, however, marches onward. In August the team launched a prototype of the telescope 38 kilometers into the stratosphere onboard a helium- filled balloon. For two hours, researchers followed below in a helicopter, shooting a pulsed UV laser and LED into the telescope’s field of view. The test was a success: the prototype detected the UV traces, which are similar to the fluorescence generated by extreme energy cosmic-ray air showers. In 2016 astronauts will transport a shoebox-size prototype called Mini-EUSO to the ISS and see how it fares at the altitude of the full mission.