On 6/09/2018 11:37 AM, Alain Fournier wrote:

A small nudge a long way out can be enough to direct it towards the
Earth. But after the aerobraking, you don't have that option. You need
to raise its perigee quite a lot.

But you would probably want to nudge it millions of kilometres for it to
go towards Earth. You only need to raise perigee a few kilometres per
orbit.

Yes, you do have to know what you are doing. And make sure that you can
raise the perigee. But in most cases, raising the perigee should be
easier than sending it to Earth's atmosphere, maybe not all cases, but
most cases. And those who would do this are most likely people who
understand the difficulty of the task and the risks. You don't send an
asteroid towards Earth without understanding orbital mechanics and
rocketry.


Alain Fournier

That's really not how orbital mechanics work. It's not distances that
are a problem, but velocities. A small change in velocity for an object
a long way out can translate into a large change in distance by the time
the object arrives in the vicinity of Earth.

By contrast, raising the perigee requires a significant change in
velocity, and there's no getting around that. For a massive object, that
implies a correspondingly large rocket.

Putting a large rocket into space is difficult and expensive.

Sylvia.