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Those who have irrational fears of rare but dangerous events should be on full alert on Wednesday: space junk is falling to Earth somewhere and there is an incredibly small chance it could hit someone.

It's coming down in the form of pieces of a dead European Space Agency satellite called European Remote Sensing 2 (ERS-2). The satellite was launched in 1995, retired in 2011, and has been gradually making its way back to Earth since then.

 as of Tuesday afternoon are that the dead satellite will reenter the Earth's atmosphere at 10:32 a.m. on Wednesday, give or take 4 and a half hours.

This uncertainty is due primarily to the influence of unpredictable solar activity, which affects the density of the Earth's atmosphere and therefore the drag experienced by the satellite.

The space agency says the satellite will break into pieces at about 50 miles above the Earth's surface and the vast majority will burn up in the atmosphere. Some fragments could make it to Earth and will likely fall into the ocean. 

The largest satellite fragment that could land would weigh about 115 lbs 

"The odds of a piece of satellite falling on someone's head is estimated at one in a billion," ESA space debris system engineer Benjamin Bastida Virgili said during a press briefing last week, 

Henri Laur from the ESA Earth observation mission said the largest fragment that could reach the ground would weigh about 115 pounds, the website reported. The mass of the whole satellite is roughly 5,000 pounds.

Over its 16-year life, the ERS-2 collected information on climate change and the Earth's atmosphere. Along with its older sister satellite ERS-1, it used: "an imaging synthetic aperture radar, a radar altimeter and other powerful sensors to measure ocean-surface temperature and winds at sea," 

. The ERS-2 also carried another sensor to measure atmospheric ozone.

The space agency said the two satellites gathered information on "diminishing polar ice, changing land surfaces, sea-level rise, warming oceans and atmospheric chemistry."

The ESA decommissioned ERS-2 in 2011 and maneuvered it from an orbit of about 490 miles down to 360 miles above the Earth. It's now out of fuel and out of batteries. The agency isn't sure where exactly it will go.

The goal was to bring it down gradually and prevent it from adding to the big problem of orbital space junk. About 500,000 marble-sized objects are in orbit and there are more than 100 million objects 1mm or smaller, 

Most of the debris comes from satellite explosions and collisions. And when objects hit each other, they can create even smaller pieces of debris. The average impact speed is 

, meaning even tiny objects can be dangerous.

 that on average, an object of similar mass to the ERS-2 reenters the Earth's atmosphere every one to two weeks.  

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James Doubek

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There's a one in a billion chance it could hit somebody on the head. Europe's space agency doesn't know exactly when or where it will land.

Watch out, a dead satellite is falling back to Earth

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