The orbital velocity of the earth makes a dramatic difference in the kinetic energy of comet impacts. For a comet falling from infinity toward the sun at earth’s orbit, the ratio of kinetic energy between comets hitting earth head-on in its orbit around the sun and those catching up with earth is a factor of 19, but most fall somewhere in between. (This calculation factors in earth’s gravity.)

Earth escape velocity: 11.2 km/s
Earth, orbital velocity: 29.8 km/s
Body falling from infinity towards the sun to a distance of 1 AU: 42.2 km/s (calculated from gravitational potential energy and checked by comparing velocity falling from infinity to the diameter of the sun with the escape velocity of the sun)

Running into the earth head on in its orbit:
42.2 km/s + 29.78 km/s = 71.98 km/s
71.98 * 71.98 + 11.19 * 11.19 = 5181.12 + 125.21 = 5306.33 km^2/s^2 (specific energy)

Catching up with earth in its orbit:
42.2 km/s – 29.78 km/s = 12.42 km/s
12.42 * 12.42 + 11.19 * 11.19 = 154.26 + 125.21 = 279.47 km^2/s^2 (specific energy)

Specific kinetic energy ratio between hitting the earth head-on and catching up with earth in its orbit:
5306.33 / 279.47 = 18.99
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