But I read in
this article that NASA are perfectly happy to let 550kg of satellite fall randomly to the earth.
Guardian's article is a little scant on information and detail. Here's a more in-depth article about the satellite, it's re-entry and possible risks involved:
http://www.space.com/12893-dead-nasa-satellite-falling-space-junk.html
The satellite is an old decommisioned observatory Upper Atmosphere Research Satellite (UARS). It was deployed by space shuttle Discovery in 1991 (
STS-48). UARS was decommisioned in 2005 and newer satellites have continued the measurements (Aura and SORCE).
As the re-entry is uncontrolled it is not known when and where exactly it will happen until hours before the event. NASA and U.S. Strategic Command will give updates here:
http://www.nasa.gov/mission_pages/uars/index.html
You can follow the satellite and even see it in the sky using these sites:
http://www.calsky.com/
http://www.heavens-above.com/
They calculate odds of hitting someone at 3200 to 1. This seems a little too probable to me.
It might be a statistical worst case number methinks - as nobody knows where exactly UARS is going to re-enter.
The event itself is not unusual. Smaller spacejunk fall to the Earth every day. Dry mass of UARS is 5668 kg. It's considered a medium-sized object and such re-entrys happen about once a year on average. Most of the space junk falls into oceans and on unpopulated or sparsely polulated areas, and there has never been reported injuries by falling space debris (but it could have happened somewhere). Lottie Williams is the only known person who has been hit by falling space junk. She was not injured:
http://www.wired.com/science/discoveries/news/2009/01/dayintech_0122
Here's a list of some notable re-entrys this year so far (though not as big as UARS):
(
http://reentrynews.aero.org/past.html)
They make
spectacular lightshows if you're lucky enough to see one. You can use the list of
upcoming re-entries,
Calsky or
Heavens Above to see if there might be visible events in your area. Estimating time and place of an uncontrolled re-entry is uncertain business, and the more circular the orbit the more uncertain the estimates will be.
Actually where would the 3200 to one figure have come from anyway?
Here's a (bit older) technical paper of UARS risk assesment:
Analysis of reentry survivability of UARS spacecraft
But as I don't have an access to it, I'll just describe the general principles of the method. Please keep in mind that details may differ.
AFAIK there's three modeling software suites for risk assesment: SCARAB by ESA,
DAS and
ORSAT by NASA. The risk assesment for the upcoming UARS re-entry was done with ORSAT. The general steps in estimating the risks are:
1. Most of the spacecraft is going to burn up in the atmosphere. Use modeling to estimate which parts of the spacecraft are going to survive the re-entry and hit the ground.
Dry mass of UARS is 5886 kg and most of it will burn.
2. Estimate which parts could cause injuries by their kinetic energy upon impact.
It's estimated that 26 surviving UARS' parts could potentially cause injuries. Their size, mass and impact energy varies. The lightest part is a 0,6 kg titanium GF extension and the most massive part is an 158 kg aluminium box structure. Parts' combined mass is 532 kg.
3. For each part estimate the area within which it could cause injuries, debris casualty area (DCA).
DCA depends on the size of the part but is usually very small for each part. The bigger the part the wider the DCA. The DCA for UARS' parts varies, stainless steel reaction wheel rims have a DCA of 0,43 m^2 and the aluminium box structure 2,44 m^2.
4. Add the DCA of each part to get the estimated total DCA for the re-entry. For UARS the total DCA is about 22 m^2, or about the area of a living room (YMMV). Different parts will fall hundreds of kilometers from each other. UARS parts will spread about 800 km downrange but their total DCA will still be the same 22 m^2.
5. Calculate the probability that a person is injured by debris. This could happen if an unsheltered person happens to be within the DCA.
In DAS-software the probability is a function of the year of the re-entry, world population density and distribution in that year, inclination of the orbit, and the total DCA. But I don't know how it's calculated in ORSAT.
For population densities DAS uses public databases like
Gridded Population of the World. Most of the Earth's area is ocean, unpopulated or sparsely populated.
This step is sketchy if the re-entry is uncontrolled as it's not known exactly when and where it will happen. But you can always use the inclination of the orbit as a limit for latitudes. The re-entry will happen somewhere between those latitudes though you don't know where. This is the case with UARS. As we don't know when the re-entry will happen, it's impossible to tell where it's going to happen. The exact time and place will be known only hours before the event. But at the moment it's possible to know the limiting latitudes and use that information to come up with a probability estimate (1:3200).
I don't have an access to the
technical paper so the actual steps might differ. Would be great if someone with an access could provide corrections or fill in missing details. The figures are from UARS briefing material
Re-Entry and Risk Assessment (498 KB PDF).
