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Re: bolide physics

>    It would be difficult to arrange for a lower impact speed than
>that that would result if the object had first somehow been captured
>into low Earth orbit -- like a typical artificial satellite.  The
>speed on entering the atmosphere would then be the same speed that
>such a satellite has on re-entry, about 8 Km/s.

Almost. The speed of an object in low orbit is the escape velocity over the
square root of 2. As the satellite falls in though, it will, in the absence of
air friction, accelerate to the escape velocity. Even with air friction, the
energy released is the same as if it accelerated to escape velocity, but some
of the energy is then released in the atmosphere, not in the crater formation.

>    There's more.  The Earth is not standing still, it is moving in
>its orbit at almost 30 Km/s.  If the colliding object happened to be
>traveling directly opposite to the Earth's orbital motion at the time
>of collision, then its own speed and the Earth's orbital speed would
>add, resulting in a relative speed of about 70 Km/s.
>    There's still more, for the Earth's gravity is accelerating the
>object as it approaches.  To see how much extra speed is obtained, you
>square the relative speed without the effect of Earth's gravity, add
>in the square of the Earth's 11 Km/s escape velocity, and take the
>square root of the result.  For our "bad luck" case of 70 Km/s, this
>results in only about another Km/s of speed.

This is the worst case scenario. It could also approach from behind the Earth's
wake, which would reduce the velocity substantially (you calculate it!).

>    Thus I think it fair to say that the dynamically likely range of
>top-of-the-atmospere impact speeds for a comet or asteroid striking
>the Earth is from roughly 8 to roughly 70 Km/s.  The worst case, 70

Read that as from 11 to 70 km/s and I agree.

>Km/s, is not likely in any case (just from how things work out if
>orbital directions and speeds are distributed randomly).  Furthermore,
>speaking very broadly, asteroids tend to have somewhat less weird
>orbits than do comets, so a worst-case speed for an asteroid impact is
>probably less likely than a worse-case speed for a comet impact.  Were
>I to hazard a guess, I would suspect that speeds of 20 to 30 Km/s would
>be most common.

Right. Asteroids orbit the sun prograde (in the same direction as the Earth),
while comets are half prograde and half retrograde (at least the component in
the x-y plane of the solar system).

>    What happened with the recent Jupiter impact was that the comet
>was first captured into an orbit -- though not a "low-Jupiter" orbit,
>rather one with more energy.  And since Jupiter is much more massive
>                                             --  Jay Freeman

The comet was perturbed by Jupiter to bring it back on another close approach
in its orbit around the SUN (NOT JUPITER).

>The worst scenario is that the comet is perturbed from the (still hypo-
>thetical) Oort cloud, diving down to a perihelion between earth and sun,
>and in a retrograde orientation (counter to earth's motion around the sun).
>In this case the relative velocity at impact could be the SUM of:
><escape velocity w.r.t. the sun> and <escape velocity w.r.t. the earth>.

It is not as simple as adding the two velocities together. See the formula
described by Jay Freeman.

>Wouldn't the energy of impact with the earth be GREATER than if the same
>bolide hit an outer planet, since solar escape velocity is less out there.
>Are you saying solar escape velocity is only a minor component?  Or that
>Jupiter is so much bigger it dwarfs the solar component?
>Mike Bonham        bonham@jade.ab.ca      Jade Simulations International

Yes, the escape velocity of Jupiter (60 km/s) is a greater component than the
solar escape velocity (about 10 km/s) at the distance of Jupiter from the Sun.

Scott Horton
Geophysicist/Computer Programmer