German satellite: What are the odds that it will hit you?

German satellite: The odds of a piece of the retired German ROSAT satellite striking a person are higher than that of the NASA satellite that plunged into the Pacific Ocean in September.

By Natalie Wolchover, Life's Little Mysteries Staff Writer / October 20, 2011

This undated artist rendering shows the scientific satellite Rosat. The German Aerospace Center said the decomissioned satellite is hurtling toward the atmosphere and pieces could crash into the earth during the weekend.

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Not long after re-emerging en masse from our underground bunkers and panic rooms, having successfully avoided being squashed by a falling NASA satellite on Sept. 24, humanity has learned that the sky is falling yet again. Another huge piece of space debris, a 2.6-ton, defunct German telescope called the Roentgen Satellite (ROSAT), will crash back to Earth Saturday or Sunday (Oct. 22 or 23), and the chances it will hit someone are even greater this time around.

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The odds are 1-in-2,000 that a chunk of ROSAT will strike a person. For the UARS satellite that fell into the southern Pacific Ocean in September, the odds were 1-in-3,200. According to Heiner Klinkrad, head of the European Space Agency's Orbital Debris Office, ROSAT poses a higher risk than UARS because more of its mass is expected to survive atmospheric re-entry and reach Earth's surface. [Photos: Germany's ROSAT Satellite Falling to Earth]

"The fact that the ROSAT re-entry risk estimate is higher than for UARS lies in the surviving mass, which, percentage-wise, is considerably higher for ROSAT than for UARS, and hence, the net mass reaching ground is higher for ROSAT than for UARS," Klinkrad told Life's Little Mysteries, a sister site to SPACE.com. "This is due to the ROSAT internal mirror assembly that is very resistant to [heat] during re-entry."

Typically, when a satellite crashes to Earth, only 20 to 40 percent of its mass survives; the rest burns up from heat generated by friction between the satellite and particles in the atmosphere, Klinkrad said. Because ROSAT's mirrors - which collected X-rays and extreme ultraviolet light emitted by celestial objects - resist heat, they reduce the percentage of the spacecraft that will burn up, and over half of the spacecraft's mass, about 1.7 tons of it, is expected to reach the surface. [If a Satellite Falls On Your Home, Who Pays for Repairs?]

According to scientists in NASA's orbital debris office at Johnson Space Center in Houston, calculating the risk of space debris hitting someone requires first working out how much debris makes landfall. Analysts then make a grid of how the human population is distributed around the globe. Oceans, deserts and the North and South poles are largely devoid of people, for example, whereas coastlines are brimming with them. In short, the analysts must figure out which patches of Earth have people standing on them.