You may have seen news reports about 6 ton Earth observing satellite the size of a school bus deorbiting soon. Upper Atmosphere Research Satellite (UARS) was launched from the Space Shuttle Discovery 10 years ago this month. This 5,900 kg spacecraft studied the Earths atmosphere including the ozone layer completing its mission in December 2005.
The best estimate of when this spacecraft will fall to Earth is the end of this month. More precise estimates will be available as the spacecraft’s orbit continues to be drawn in by Earth’s gravity as it is slowed by Earth’s atompshere. More accurate estimates are not yet available because there are a lot of factors that will control exactly when and where it comes down, mostly due to the constant changes in our atmosphere. It is our atmosphere that is slowing UARS down.
The Joint Space Operations Center of the U.S. Strategic Command at California’s Vandenberg Air Force Base tracks this and every other spacecraft (operational or not) and trackable bit of orbital debris. It will begin issuing reports about 4 days before the anticipated reentry with updates with increasing frequency leading up about 2 hours before where a landing location will be estimated within an accuracy of +/- 10,000 km.
To put it into perspective, something manmade, previously orbiting our Earth reenters our atmosphere about once a day. 75 metric tons reentered last year alone. A moderate sized object, along the lines of the size of a small car such as a spent rocket body, reenters our atmosphere about every week. Something large, like UARS, returns about once a year.
In the nearly 54 years humans have been putting man made objects into orbit around the Earth, not once has one returned to Earth and caused any injury. There hasn’t even been any appreciable damage to structures caused by debris making it to the ground. Why? Most of the vehicle simply burns up upon reentry. Whatever makes it through has a really poor chance of hitting anything because so much of the Earth is covered in water and even if it hit land, so much of that is uninhabited. Pieces of past spacecraft returning have been found in the Australian outback, Canadian tundra or Siberia as well as deserts in the middle east. Odds are very long that they’d make it somewhere where people live.
The odds calculated by the NASA team are 1:3200 that any piece of this spacecraft will hit anyone. This number will likely change as well as data improves as it’s based completely on the current estimate of falling somewhere between 57 degres North and 47 degres South latitude taking into account population density in this huge area.
All remaining fuel was used to lower the orbit of UARS so nothing toxic remains onboard but should anyone find a piece of the spacecraft, they are reminded not to touch it and to instead notify local authorities. It remains property of the U.S. government. Even if it falls outside of the U.S., a U.N. convention allows for the return of debris to the owning country.
This spacecraft was built before the current initiative known as “Design for demise” which encourages Earth orbiting spacecraft to be built in a way that minimizes the mass that survives reentry.
So should you be worried? No, especially if you play the lottery where your odds are much much worse. If you are a bit concerned by those numbers, keep an eye on news about this space craft, the odds of this reentry affecting anyone are likley to get much much larger.