Space junk on the ground - ''do not touch it" - why?

[Machine_Elf]

@Khristy

What cracks me up about this whole thing is, in 1969 we were able to land men on the moon 250,000 miles away, and bring them back to earth using computers less powerful than the digital watch on your arm, yet… in 2011 they can’t even figure out where this junk is going to land with all the super computers available to the US government. Really?

In 1969, they were flying a controlled spacecraft, able to make course corrections with on-board rockets, and final decel burns to choose the time/location of reentry into earth’s atmosphere. In contrast, the UARS satellite is a dead hulk, drifting along on its orbit. Instead of controlling it (which they can’t), NASA can only use calculations to estimate when/where it will crash. The calculations are based in large part on:

-actual measurements of its current position and speed
-an assumed coefficient of aerodynamic drag
-assumed density of the atmosphere at its current altitude

Probably not hard to pin down its location and speed fairly accurately at any given moment with actual measurement via radar. However, the drag profile is only an estimate, and it can vary widely depending on the satellite’s (uncontrolled) orientation. Local atmospheric density, important for the drag calculation, also likely varies quite a bit.

Finally, any computational simulation of the reentry event is going to be extremely sensitive to the initial calculations: every sequential step in the simulation amplifies any error present in the conditions used to calculate previous steps, so small errors in estimates of initial conditions are going to result in big errors in the final splashdown time/location. As we get closer and closer to splashdown, the length of the computational simulation decreases, and the uncertainty of the result goes down:

@NASA

As of 9:30 p.m. EDT Sept. 22, 2011, the orbit of UARS was 110 mi by 115 mi (175 km by 185 km). Re-entry is possible sometime during the afternoon or early evening of Sept. 23, Eastern Daylight Time. The satellite will not be passing over North America during that time period. It is still too early to predict the time and location of re-entry with any more certainty, but predictions will become more refined in the next 24 hours.
…
The actual date of re-entry is difficult to predict because it depends on solar flux and the spacecraft’s orientation as its orbit decays. As re-entry draws closer, predictions on the date will become more reliable.

[Francis_Vaughan]

OK, a credible reason not to touch any debris comes to mind. It seems some of the parts expected to make it to the ground are made of Beryllium. That isn’t good. Beryllium metal by itself isn’t so bad, but by the time any remains made of it make to the ground they may well have been heated up enough to be partly oxidised, and there may well be a significant presence of beryllium metal powder and oxide dust. This is dangerous enough that it could kill you, or cause chronic illness, if you inhale it. It is also carcinogenic. It is a pretty disgusting poison. You would probably want a guy in a hazmat suit to take it away, and do some careful assessment and possible decontamination of the site. Even intact metal is not something you want a random person taking home as souvenir.