Australian Space Academy
Hypervelocity is a velocity in excess of about 4 kilometres per second. An asteroid or comet colliding with the Earth has a minimum velocity of about 11 km/sec, and is thus well in the hypervelocity regime. The important fact about a hypervelocity impactor is that its kinetic energy is in excess of the energy that the body would release in an explosion if its mass were entirely composed of high explosive!
The apparent diameter of a hypervelocity impact crater on Earth is very roughly given by multiplying the impactor diameter by a factor of 10 to 20. Exact crater size varies with impactor density, velocity and other factors. An approximation to crater size can be calculated by first computing the kinetic energy of the impactor in kilotons TNT (1 kT = 4.2 x 1012 Joule), and noting that a surface explosion of 1 kT of TNT creates a crater with an average apparent diameter of 46 metres. Crater diameters for different energies are then observed to scale as the 0.3 power of the energy released.
The tables below provide estimates for crater diameter (in kilometres) as a function of impactor speed for three different densities - typical of comets, stony asteroids and iron asteroids.
| Diameter | Velocity (km/sec) | |||||||||
| (m) | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 55 | 60 |
| 40 | 0.3 | 0.3 | 0.4 | 0.4 | 0.5 | 0.5 | 0.5 | 0.6 | 0.6 | 0.6 |
| 100 | 0.6 | 0.8 | 0.9 | 1.0 | 1.1 | 1.2 | 1.2 | 1.3 | 1.4 | 1.5 |
| 200 | 1.2 | 1.4 | 1.6 | 1.8 | 2.0 | 2.2 | 2.3 | 2.5 | 2.6 | 2.7 |
| 400 | 2.2 | 2.7 | 3.0 | 3.4 | 3.7 | 4.0 | 4.3 | 4.6 | 4.9 | 5.1 |
| 600 | 3.2 | 3.8 | 4.4 | 4.9 | 5.3 | 5.8 | 6.2 | 6.6 | 7.0 | 7.4 |
| 800 | 4.2 | 5.0 | 5.7 | 6.3 | 6.9 | 7.5 | 8.1 | 8.6 | 9.1 | 9.6 |
| 1000 | 5.1 | 6.1 | 6.9 | 7.7 | 8.5 | 9.2 | 9.8 | 10.5 | 11.1 | 11.7 |
| 2000 | 9.5 | 11.3 | 12.9 | 14.4 | 15.8 | 17.1 | 18.4 | 19.6 | 20.7 | 21.8 |
| 4000 | 17.7 | 21.1 | 24.1 | 26.9 | 29.5 | 31.9 | 34.3 | 36.5 | 38.7 | 40.7 |
| 10000 | 40.5 | 48.1 | 55.0 | 61.3 | 67.3 | 72.9 | 78.2 | 83.3 | 88.2 | 92.9 |
| Diameter | Velocity (km/sec) | |||||||||
| (m) | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 55 | 60 |
| 40 | 0.5 | 0.5 | 0.6 | 0.7 | 0.8 | 0.8 | 0.9 | 0.9 | 1.0 | 1.0 |
| 100 | 1.0 | 1.2 | 1.4 | 1.6 | 1.7 | 1.9 | 2.0 | 2.1 | 2.3 | 2.4 |
| 200 | 1.9 | 2.3 | 2.6 | 2.9 | 3.2 | 3.5 | 3.7 | 4.0 | 4.2 | 4.5 |
| 400 | 3.6 | 4.3 | 4.9 | 5.5 | 6.0 | 6.5 | 7.0 | 7.5 | 7.9 | 8.3 |
| 600 | 5.2 | 6.2 | 7.1 | 7.9 | 8.7 | 9.4 | 10.1 | 10.7 | 11.4 | 12.0 |
| 800 | 6.8 | 8.0 | 9.2 | 10.2 | 11.2 | 12.2 | 13.1 | 13.9 | 14.7 | 15.5 |
| 1000 | 8.3 | 9.8 | 11.2 | 12.5 | 13.7 | 14.9 | 16.0 | 17.0 | 18.0 | 19.0 |
| 2000 | 15.4 | 18.3 | 20.9 | 23.3 | 25.6 | 27.7 | 29.8 | 31.7 | 33.6 | 35.4 |
| 4000 | 28.7 | 34.2 | 39.1 | 43.6 | 47.8 | 51.8 | 55.6 | 59.2 | 62.7 | 66.0 |
| 10000 | 65.6 | 77.9 | 89.1 | 99.4 | 109.0 | 118.1 | 126.7 | 135.0 | 143.0 | 150.6 |
| Diameter | Velocity (km/sec) | |||||||||
| (m) | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 55 | 60 |
| 40 | 0.6 | 0.8 | 0.9 | 1.0 | 1.1 | 1.1 | 1.2 | 1.3 | 1.4 | 1.5 |
| 100 | 1.5 | 1.7 | 2.0 | 2.2 | 2.4 | 2.6 | 2.8 | 3.0 | 3.2 | 3.3 |
| 200 | 2.7 | 3.2 | 3.7 | 4.1 | 4.5 | 4.9 | 5.3 | 5.6 | 5.9 | 6.2 |
| 400 | 5.1 | 6.0 | 6.9 | 7.7 | 8.4 | 9.1 | 9.8 | 10.4 | 11.1 | 11.6 |
| 600 | 7.3 | 8.7 | 9.9 | 11.1 | 12.1 | 13.2 | 14.1 | 15.0 | 15.9 | 16.8 |
| 800 | 9.5 | 11.2 | 12.9 | 14.3 | 15.7 | 17.0 | 18.3 | 19.5 | 20.6 | 21.7 |
| 1000 | 11.6 | 13.7 | 15.7 | 17.5 | 19.2 | 20.8 | 22.4 | 23.8 | 25.2 | 26.6 |
| 2000 | 21.6 | 25.7 | 29.3 | 32.7 | 35.9 | 38.9 | 41.7 | 44.5 | 47.1 | 49.6 |
| 4000 | 40.3 | 47.9 | 54.7 | 61.1 | 67.0 | 72.6 | 77.9 | 82.9 | 87.8 | 92.5 |
| 10000 | 91.9 | 109.2 | 124.8 | 139.3 | 152.8 | 165.5 | 177.6 | 189.2 | 200.4 | 211.1 |
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