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.

So I wouldn't be too scared about that.

Another thing that could be a problem is that you could mess up the
atmosphere if you do this with a big asteroid.

Personally, I would prefer doing a capture by selenic gravity assist.

I'm not an expert in orbital mechanics but I do agree you have to
somehow raise the perigee after the aerobraking pass(es) is(are) done.

I would expect that for a somewhat big asteroid, there would be several
aero-braking passes and that you would do small tweaks to the orbit
after each pass.


Alain Fournier