# Loss of mass of an object falling through air

1. Apr 29, 2015

Hello All,

I am an electrical engineer and hence quite far away from mechanics of masses.

I just came to know that a space craft got crazy and might fall on earth.

I did a simple math using matlab considering no atmosphere with h and m of the object as input and Nagasaki Bomb scale energy release upon contact as output.

But there will be atmosphere which will lead to loss in mass. How do I do that (assuming the falling object is as Iron)

Regards,
Sumit

2. Apr 29, 2015

### Staff: Mentor

There is no man-made object in space that would release as much energy as the Nagasaki bomb. Even the kinetic energy of the ISS is just at ~4000 tons of TNT and it won't fall on earth in an uncontrolled way.

Simulating how things break up into smaller pieces on reentry is very complex.
Satellites reach terminal velocity in the atmosphere before they (or their pieces) hit the ground - might be deadly if something happens to fall onto your head, but without any effect if it does not directly hit anything (and the Earth is huge and mainly empty). Satellites deorbit all the time, so far nothing hit anything. Space stations get deorbited in a controlled way.

3. Apr 30, 2015

I understand that very well.

My question is out of curiosity and not out of scare. You can sat a fun timepass at office :)

4. Apr 30, 2015

### BobG

The spacecraft is already traveling much faster than terminal velocity (around 15,000 mph). As the atmosphere sucks energy out of the orbit, the orbit is getting smaller (losing potential energy), with a lot of that potential energy being turned into kinetic energy (velocity). So, ironically, the spacecraft is speeding up in spite of the thickening, but still thin atmosphere, until, eventually, the atmosphere is thick enough to actually start slowing the spacecraft.

Hence the heating and the break-up of the spacecraft long before reaching the surface. Only the strongest of spacecraft parts have any chance of reaching the surface (titanium spherical fuel tanks from Delta II boosters are notorious for surviving re-entry).

5. May 1, 2015

### Monsterboy

What is the safe altitude to blow up a falling satellite with a missile such that no space debris is created ? I remember the US did it few years ago and China did it too.

6. May 1, 2015

### newjerseyrunner

It's not possible to do it without space debris, the trick is to make that debris stay up there for as short of time as possible. The atmosphere doesn't stop, it just kinda fades away as you get higher up. The lower you are, the more drag and the faster it'll be slowed enough to crash down. Any object in earth orbit will eventually either be sucked into earth or be thrown off into space, any satellite is by default in a death spiral and will come down if left up there long enough. Remember interplanetary space isn't a vacuum, the sun's heliosphere has it's own friction.

7. May 1, 2015

### Staff: Mentor

Just don't blow up deorbiting satellites - there is no point in that (unless you expect some hostile mission of the satellite).

8. May 2, 2015

### Monsterboy

http://edition.cnn.com/2008/TECH/space/02/20/satellite.shootdown/index.html
The danger seems to be only with new satellites with their tanks full.

9. May 2, 2015

### Staff: Mentor

Hmm okay, dangerous chemicals can be an issue if you lose control over the satellite.