Destruction by Impact
There has been a lot of discussion in the last few years about
the devasting effects that the impact of a small asteroid on the
Earth would have on our civilisation. But why is this the case?
After all, the asteroids of which we speak might typically be
less than a kilometre in diameter. Even a small hill might be of
similar size and have comparable mass.
The answer lies in the velocity of such an asteroid. The
minimum velocity of an asteroid impact is 11 km/sec. This is
speed acquired simply from the Earth's gravitation. The most
probable velocity is somewhere between 15 and 30 km/sec. These
velocities, which are far greater than anything we usually
experience, are termed hypervelocities.
Now the destructiveness of an impact is proportional to the
energy that the object releases on impact, and this is
essentially the energy that the body possesses by virtue of its
motion - its kinetic energy, which is given by the formula:
K = 1/2 m v2
where m is the mass of the object and v is its velocity. The
mass of an asteroid depends on its density and thus its
composition. The most common asteroids are believed to have
'rock-like' densities of around 2500 kg/m3.
Note that this energy is proportional to the square of the
velocity. This is what makes a hypervelocity impact so
devastating. For every doubling in velocity, the energy, and thus
the destructiveness of the impact, quadruples. Only the explosion
of a nuclear bomb releases energy comparable to a hypervelocity
impact, and even the largest thermonuclear bombs in current
military arsenals never reach the destructiveness of asteroids
more than 100 metres in diameter.
The table below lists the kinetic energy, in megatons TNT
equivalent, of various diameter asteroids with this density:
| Diameter |
Mass |
Velocity
(km/sec) |
(metres)
|
(million - tons)
|
15
|
20
|
25
|
30
|
| |
| 100 |
1.3 |
35 |
62 |
97 |
140 |
| 200 |
10 |
280 |
500 |
780 |
1,100 |
| 300 |
35 |
950 |
1,700 |
2,600 |
3,800 |
| 400 |
84 |
2,200 |
4,000 |
6,200 |
9,000 |
| 600 |
280 |
7,600 |
13,000 |
21,000 |
30,000 |
| 800 |
670 |
18,000 |
32,000 |
50,000 |
72,000 |
| 1,000 |
1,300 |
35,000 |
62,000 |
97,000 |
140,000 |
| 2,000 |
10,000 |
280,000 |
500,000 |
780,000 |
1,100,000 |
| 3,000 |
35,000 |
950,000 |
1,700,000 |
2,600,000 |
3,800,000 |
| 4,000 |
84,000 |
2,200,000 |
4,000,000 |
6,200,000 |
9,000,000 |
| 5,000 |
160,000 |
4,400,000 |
7,800,000 |
12,000,000 |
18,000,000 |
| 6,000 |
280,000 |
7,600,000 |
13,000,000 |
21,000,000 |
30,000,000 |
| 7,000 |
450,000 |
12,000,000 |
21,000,000 |
33,000,000 |
48,000,000 |
| 8,000 |
670,000 |
18,000,000 |
32,000,000 |
50,000,000 |
72,000,000 |
| 9,000 |
950,000 |
26,000,000 |
45,000,000 |
71,000,000 |
100,000,000 |
| 10,000 |
1,100,000 |
35,000,000 |
62,000,000 |
97,000,000 |
140,000,000 |
Thus a small 100 m diameter asteroid with a mass of 1.3
million tons will have a kinetic energy of typically 62 megatons
of TNT explosive. Enough to destroy a city! A 10 kilometre
monster would deliver around 100,000 gigatons of TNT explosive!!
Material Prepared by John Kennewell. © Copyright IPS - Radio and Space Services.
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