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FRANKFURT, Germany — The European Space Agency on Friday launched three satellites it hopes will help scientists understand why the magnetic field that makes human life possible on Earth appears to be weakening.

The satellites, comprising ESA's Swarm project, were launched from Russia's Plesetsk Cosmodrome on a Rockot vehicle at 7.02 a.m. ET and were placed in near-polar orbit at an altitude of 490 kilometers (304 miles) about 91 minutes later.

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The readings that Swarm is due to collect for the next four years will help improve scientists' relatively blurry understanding of the magnetic field that shields life on Earth from deadly solar radiation and helps some animals migrate.

Scientists say the magnetosphere is weakening and could all but disappear in as little as 500 years as a precursor to flipping upside down.

It has happened before — the geological record suggests the magnetic field has reversed every 250,000 years, meaning that, with the last event 800,000 years ago, another would seem to be overdue.

While the effects are hard to predict, the consequences may be enormous. Satellites, essential among others for communications, could be more exposed to solar wind. Meanwhile, the oil industry uses readings from the magnetic field to guide drills.

"Swarm is an essential mission, not only for Europe but also for the world," ESA Director General Jean-Jacques Dordain told Reuters following the launch. "We cannot live on planet Earth without this shield."

The Swarm mission was developed and built by European aerospace group EADS' Astrium unit.