They come from the Sun. They come from interactions between cosmic rays and Earth's atmosphere. They come from exploding stars in the Milky Way and beyond. They pass through you - a trillion a day - and probably don't even know it. They're called neutrinos.
Neutrinos are tiny subatomic particles. They have almost no mass. They have a neutral electrical charge. They pass through matter undisturbed. Accordingly, they're all but undetectable.
Being able to detect neutrinos, however, could help us answer questions about our cosmos - questions about how stars die and about how our Universe was formed.
Ice Cube spans a square kilometer (about three fifths of a square mile) of Antarctic ice at the National Science Foundation's Amundsen-Scott South Pole Station.
The University of Wisconsin-Madison spearheaded the design and creation of the observatory that cost an estimated $270 million to build. Using a hot-water drill developed at the University, 86 holes were drilled in the ice, each over 2 km (1.25 mi.) deep.
Each hole houses a cable string of 60 soccer-ball sized optical sensors that was lowered to a depth of 1,450 to 2,450 meters (a mile to a mile and a half) below the surface. The holes will gradually fill in with ice until all 5,160 sensors are permanently embedded in the ice, forever on a neutrino hunt.
Most of the trillions of neutrinos passing through the Earth on any given day go on their merry way without deviating from their course at all. Occasionally, however, a neutrino strikes an atom. The collision creates a burst of positively-charged particles called muons that race away from the neutrino. The burst is seen as a tiny flash of blue light called Cherenkov radiation.
The ice makes an ideal medium for observing neutrino-atom encounters. It's 100,000-year-old pure, compressed snow. It's clear and dark. When a neutrino collides with an atom in a water molecule somewhere in the one kilometer range of the Ice Cube observatory, the blue flash will be seen and recorded by one of the sensors.
By tracing the path of the particles in the blue flash, the sensor can find the origin of the neutrino. Since neutrinos have few interactions on their journeys through the cosmos, their paths lead directly back to their origins. Finding their origins will allow scientists to create a map of popular neutrino sources.
In fact, they've already started doing this, getting data from the partially-built observatory since 2005.
The observatory is expected to be fully operational by April once all of the instruments have been calibrated. Then, data that could give us a better picture of supernovae and black holes and tell us more about dark matter and dark energy should start arriving in earnest.
Update: To learn more about neutrinos and muons, listen to this Physics Buzz podcast: