CORRECTION: Next Up (Er, Down) — ROSAT

CORRECTION: Next Up (Er, Down) — ROSAT

CORRECTION: The estimate for the likelihood of an individual being hit by debris from the UARS satellite last month has been corrected. See editor’s note at bottom.

For those who enjoyed following the reentry of NASA’s Upper Atmosphere Research Satellite (UARS) last month, another opportunity is coming up next week. Germany’s ROentgen SATellite (ROSAT) is expected to reenter between October 20 and October 25.

Like UARS, ROSAT was launched two decades ago when issues about satellite reentries and space debris were of less concern. Neither satellite had its own propulsion system to allow for a controlled reentry. Both incorporated components that are expected to survive the heat of reentry and reach Earth’s surface.

UARS circled the Earth at a 57 degree inclination, while ROSAT’s inclination is 53 degrees. In both cases, that takes the satellite over the most populated areas of the Earth (between 53 degrees north latitude and 53 degrees south latitude in the case of ROSAT), but the Earth is 70 percent covered with water, so the chance of it hitting a populated area is less than one might initially assume.

ROSAT’s mission was x-ray astronomy. Launched in 1990, it operated through 1999. The United States and the United Kingdom were partners with Germany in the project, and it was launched by a U.S. Delta II from Cape Canaveral, Florida.

Germany’s space agency, DLR, has a website with information about the reentry. It says that 30 individual pieces could survive. The largest fragment is the telescope’s mirror “which is very heat resistant and may weigh up to 1.7 tons.” The debris footprint is estimated at 80 kilometers (50 miles) wide.

As with UARS and any uncontrolled satellite reentry, there is no way to predict with certainty where the reentry will occur other than the boundaries established by the inclination of the orbit. DLR estimates the probability of “someone somewhere on Earth getting injured is about 1 in 2,000.” The comparable number for UARS was 1 in 3,200, which analysts pointed out did not mean that a particular individual had that likelihood of being hit. The chance of “a” person — YOU — being hit by UARS debris was about 1 in 20 trillion according to a September 19 tweet by The Weather Channel. UARS reentered over the Pacific Ocean and there have been no credible reports of damage or injuries.

Editor’s Note: The estimate of the likelihood of a particular person like you being hit by UARS debris as cited by a tweet from the Weather Channel (@twcspacewx) on Sept. 19 was 1 in 20 trillion, not 1 in 20 million as earlier stated in this article.

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