Characteristics of Impacts

Some physical characteristics of the impactor, such as its mass and its chemical composition, are determinant to evaluate the effects of an impact. Dynamical characteristics such as the speed and the angle at which the body approaches the ground are relevant as well.

Energy of the impactor
The energy of the impactor is the most usual parameter used to evaluate how dangerous an impact is. As long as the impactor is travelling in space, its energy is called kinetic energy, depending both on the mass and on the speed of the object. This energy is calculated with the formula:

 where m is the mass and v is the speed of the impacting body

When the impactor hits the target body, it will suddenly stop, and this kinetic energy will be freed as an explosion. It is therefore very intuitive that the kinetic energy of the impactor is directly proportional to how forceful the impact will be.
Sometimes, instead of measuring the danger of an impact using the kinetic energy of the impactor, the diameter of the impactor is used. This approximation can be accepted (using average values of speed and density) especially when treating high energy impacts.

Composition of the impactor
The composition of the impactor is also very important. NEOs are made fundamentally of 3 kinds of materials: sand (comets without ices), rock (of the kind most meteorites are made of) and iron (a material with high contents of iron, like for asteroids of the M kind). These materials have very different behaviors: for example, they have different cohesion properties and therefore can break up for fragmentation at different levels in the atmosphere, depending on their speeds (the faster they are, the higher in the atmosphere they start to break up). This means that if we consider two impactors of the same mass and with the same speed, one made of sand (a comet ) and one made of rock (an M meteorite) they will produce impacts with different energies.
Anyway, as far as we know, there seem to be no important difference between impacts of asteroids and comets. In fact, while comets normally have higher average speeds than asteroids, they seem to be less dense (and so less heavy). This means that there shouldn't be important differences between the energy involved in an asteroid or in a comet impact.
An interesting remark is that normally, slower objects tend to be more dangerous: for example, for iron bodies, if the object is slower than 12,4 Km/s the body will get to the ground without breaking at all!

Energy of the impact
The energy freed during an impact is measured by a Megaton (MT), which is the energy of almost 100 Hiroshima bombs. To have an idea of the scale of energies: if a 2 meters compact body with a speed of 20 Km/s impacts the Earth, about 1 MT is released. This energy, which seems to be enormous to a human high, is not enough to get through the atmosphere!