Daniel Spain asked:
If there were a large object, say a meteorite falling straight down where I'm standing, what kind of warning would I notice? Would there be an accompanying sound that could warn people on the ground? Or would I not know until it's too late?
Love the show!
Nashville, Tennessee USA
We put this question to Dr Mark Lewney, who gained his PhD in Acoustics from Cardiff University...
Mark - For most of us here on the Earthís surface, the speed of sound is around 340 m/s or 750 miles an hour. That's one mile every 5 seconds which is actually not all that fast. Itís only about twice the speed of the fastest arrows in archery. Sound is also even slower in colder air and the air altitude is both colder and much less abundant.
So when the meteoroid, the lump of rock itself enters the atmosphere 100 km up and becomes a meteor, it will always be too far away to hear. Itís possible that its electromagnetic waves travelling a million times faster than its sound waves will cause a hissing noise in phones or radios, but even this so-called electrophonic effect is debatable.
But Daniel asks not about a meteor which burns up in the atmosphere but a meteorite which actually makes it all the way through the atmosphere to the Earthís surface. Would this give any warning? Sadly, no. It will be travelling at least 11,000 metres a second, 33 times the speed of sound, far too fast to hear its approach. In fact, we probably wouldnít be able to see it either. Only a tiny fraction of the solar systemís asteroids are currently being tracked by just a handful of volunteers worldwide. So unless it reflected the light in just the right way in a small region of the sky that someone happened to be looking at very carefully, it would almost certainly be too small and too fast to see until it entered the atmosphere just a few seconds before impact. I'm afraid Danielís final moment would be rather disappointing.
Hannah - Poor, Daniel. Mark adds that it is possible that the meteorite will burn up just enough to be decelerated by the atmosphere to subsonic speeds. But that this would leave a meteorite so tiny that getting killed by it would be a bit embarrassing.
Assuming the meteorite was travelling at supersonic speeds, there would be no sound until after the impact. So, you would have to rely on either tracking the meteorite before it impacts Earth with our NEO warning system, or watching the flash of light as the meteorite enters our atmosphere.
You can always tell if you're on a collision course with anything by a simple rule known as "constant bearing, decreasing range."
I think you might feel a shock wave a few milliseconds before you are squashed and you might notice a bright light from re-entry.
air compression? CZARCAR, Wed, 7th Mar 2012
Ah. But the air between you and the asteroid hitting the top of the atmosphere will be compressed to a fraction of its size in about 1/100 of a second. This will raise the temperature on top of his head to about 8000K. So, theoretically you will know the meteor will hit you before it does as all your hair will be burnt off just before impact. Of course, you'll be to busy being incinerated and turned into a plasma to bother noticing. steve, Mon, 5th Nov 2012