On Sep/6/2018 at 21:04, Sylvia Else wrote :
On 7/09/2018 5:10 AM, Alain Fournier wrote:


Raising the perigee in Earth orbit a few kilometres is much easier
than raising the perigee in Sun orbit by a few million kilometres
(especially that here, for Earth orbit, we are talking about a highly
elliptical orbit). And if you are going to make a plane change to move
a few million kilometres instead of raising perigee it isn't easier.
It is kind of true that you can use highly efficient low thrust
engines to make easier the change in Sun orbit. But keep in mind that
ion engines are fired for long periods of time to move spacecrafts
weighing a few tons. Are you going to fire an ion engine for centuries
to move an asteroid?

Once again, it is possible that one would find an interesting asteroid
that would need only a very small push to do an Earth aerobraking
manoeuvre, it just isn't bloody likely. So if you plan on capturing an
asteroid, be prepared to give it some sizeable delta-V.

Clearly one isn't going to choose a random asteroid. The whole scheme is
only remotely practical if the asteroid already approaches Earth, and
only needs a nudge to bring it into the atmosphere.

You also want it to be passing by not too fast, as your typical Earth
crossing asteroid would do. If you attempt aero-braking a sizeable
asteroid passing by at 25 km/s you risk causing damage on Earth even if
the asteroid performs perfectly as planed its aero-braking manoeuvre.
Asteroids passing close to Earth are rare enough, if you also want them
to pass by at an appropriate speed, you risk waiting an awful long time
for such an asteroid. One would probably be less picky on the choice of
asteroid. You just use a more powerful rocket to nudge an asteroid that
will be available in a timely manner rather than wait centuries for the
asteroid with the perfect trajectory.


Alain Fournier