On Sep/5/2018 at 21:13, Sylvia Else wrote :
On 6/09/2018 6:18 AM, Alain Fournier wrote:
On Sept./4/2018 at 06:44, Jeff Findley wrote :
In article ,
ess says...

On 4/09/2018 9:29 AM, wrote:
"The notion of an asteroid headed for Earth is typically seen as a
bad omen. On
the flip side, some scientists and entrepreneurs increasingly see
this scenario
as a potential opportunity. Deliberately redirecting asteroids to
our planet?s
vicinity could enable us to study them up close, or even mine them.

Given that these objects are packed with valuable resources,
building a
collection of them nearby could spark major advances in
spaceflight, to say
nothing of the scientific research that might result from easy
access to these
extraterrestrial bodies.

A recent paper published in Acta Astronautica suggests that
asteroids could be
captured in Earth?s orbit with aerobraking, a maneuver that uses
atmospheric
drag to decelerate and position objects in stable trajectories
around a planet.
Aerobraking has helped place interplanetary spacecraft in orbit
around Mars and
Venus, and to slow down spacecraft returning to Earth.

Led by Minghu Tan, a PhD student at the University of Glasgow, the
paper
immediately addresses the most obvious concern with this scenario:
What if
there?s some mistake in the redirect process and an asteroid
accidentally
impacts Earth? It?s bad enough that the dinosaurs were oblivious to
their
doomsday space rock, but it would be especially embarrassing if we
humans smack
ourselves in the face with one."

See:

https://motherboard.vice.com/en_us/a...o-earths-orbit



After the aerobraking pass, you need to raise its perigee quite a lot,
or it's just going to come back and burn up, either on the next
pass, or
some subsequent pass.

The article doesn't say how the perigee would be raised.* Unless they're
planning on using a lunar flyby, I don't see how you can do this without
a sufficiently large rocket engine.

If you can nudge it enough to get it to do aero-braking, it shouldn't
be a problem to raise the perigee. One can imagine an extreme case
where the asteroid was going to pass 200 km above Earth's surface and
you can just barely nudge a little lower for aero-braking and then
aren't able to raise the perigee fast enough. But a big asteroid being
on such a trajectory isn't very likely. And those deciding to capture
it being smart enough to be able to nudge into the atmosphere, yet too
stupid to notice the danger isn't likely either. (People too stupid to
see the danger aren't hard to find, it's the combination with smart
enough to be able to nudge it that isn't likely.)

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