Via this BBC news article I discovered a neat little tool called Impact Earth (available here) which simulates objects impacting the Earth.

You start off by specifying your impacting object in terms of size, composition and speed and then choose the impact location – land or water. Finally, you choose your distance from the impact site to see what effects you would witness if you were lucky enough (so to speak) to witness your chosen object slam into the ground.

The results are comprehensive and quite enlightening.

Whilst playing with it, I chose a 270m diameter iron based asteroid slamming into the Earth at 35km per second whilst watching from 50km away – whilst outside the norm, not necessarily impossible.

The results are interesting, to say the least :

Thermal Radiation

Time for maximum radiation: 211.7 milliseconds after impact
Visible fireball radius: 7.162 km ( = 4.448 miles )
The fireball appears 32.556 times larger than the sun
Thermal Exposure: 9.19 x 10^6 Joules/m^2
Duration of Irradiation: 1.594 minutes
Radiant flux (relative to the sun): 96.1

Effects of Thermal Radiation:
Clothing ignites.
Much of the body suffers third degree burns.
Newspaper ignites.
Plywood flames.
Deciduous trees ignite.
Grass ignites.

Seismic Effects
The major seismic shaking will arrive approximately 10 seconds after impact.
Richter Scale Magnitude:  7.3
Mercalli Scale Intensity at a distance of 50 km:

General panic.  Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb.  Damage great in substantial buildings, with partial collapse.  Buildings shifted off foundations.  Serious damage to reservoirs.  Underground pipes broken.  Conspicuous cracks in ground.  In alluviated areas sand and mud ejected, earthquake fountains, sand craters.

Most masonry and frame structures destroyed with their foundations.  Some well-built wooden structures and bridges destroyed.  Serious damage to dams, dikes, embankments.  Large landslides.  Water thrown on banks of canals, rivers, lakes, etc.  Sand and mud shifted horizontally on beaches and flat land.  Rails bent slightly.

Airblast

The air blast will arrive approximately 2.525 minutes after impact.
Peak Overpressure: 132117.637 Pa = 1.321 bars = 18.761 psi
Max wind velocity: 213.26 m/s = 477.049 mph
Sound Intensity: 102 dB (May cause ear pain)
Damage Description:
Multistory wall-bearing buildings will collapse.
Wood frame buildings will almost completely collapse.
Highway truss bridges will collapse.
Glass windows will shatter.
Up to 90 percent of trees blown down; remainder stripped of branches and leaves.

So, to summarise, your clothes will catch fire, you get some ear pain and most things around you either collapse or get blown away. And all that at 50km away from the impact site!

For a bit of fun I also tried a London sized iron object traveling at 72 km per second whilst watching from 500km away and I wouldn’t be too happy if it actually happened :

Crater Dimensions

Transient Crater Diameter: 602.703 km ( = 374.278 miles )
Transient Crater Depth: 213.12 km ( = 132.347 miles )
Final Crater Diameter: 1393.262 km ( = 865.216 miles )
Final Crater Depth: 2.616 km ( = 1.624 miles )
The crater formed is a complex crater.
The volume of the target melted or vaporized is 8658910.49 km^3 ( = 2077272.626 miles^3 )
Roughly half the melt remains in the crater, where its average thickness is 30.351 km ( = 18.848 miles ).

Thermal Radiation

Time for maximum radiation: 27.526 seconds after impact
Your position is inside the fireball.
The fireball appears 891.892 times larger than the sun
Thermal Exposure: 1.83 x 10^12 Joules/m^2
Duration of Irradiation: 7.154 hours
Radiant flux (relative to the sun): 71241.007

Seismic Effects

The major seismic shaking will arrive approximately 1.667 minutes after impact.
Richter Scale Magnitude:  12.2 (This is greater than any earthquake in recorded history)
Mercalli Scale Intensity at a distance of 500 km:

Airblast

The air blast will arrive approximately 25.253 minutes after impact.
Peak Overpressure: 209670576.809 Pa = 2096.706 bars = 29773.222 psi
Max wind velocity: 11654.865 m/s = 26071.234 mph
Sound Intensity: 166 dB (Dangerously Loud)

At 500 km, you are inside the fireball, being shaken to buggery and blasted with wind at 26000 mph.

Oh, and deaf too if you are not too crispy or broken at that point to care less.