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Scientists Map Out How to Nudge Small Asteroids into Earth’s Orbit



 
 
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  #11  
Old September 6th 18, 02:37 AM posted to sci.space.policy
Alain Fournier[_3_]
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Posts: 548
Default Scientists Map Out How to Nudge Small Asteroids into Earth?sOrbit

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
  #12  
Old September 6th 18, 02:44 AM posted to sci.space.policy
Sylvia Else
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Posts: 1,063
Default Scientists Map Out How to Nudge Small Asteroids into Earth?sOrbit

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.

  #13  
Old September 6th 18, 11:49 AM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Scientists Map Out How to Nudge Small Asteroids into Earth?s Orbit

In article ,
ess says...

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 is right. That's why this issue was mentioned early on in the
thread. Starting aerobraking is a lot easier, in terms of delta-V, than
raising the perigee such that you're no longer aerobraking. If you
can't raise the perigee fast enough, the orbit will decay turning your
asteroid into a meteorite.

The low thrust engine(s) originally used to nudge the asteroid onto an
intercept course with the earth's atmosphere would likely be woefully
undersized for the task. For example, you could use solar electric
propulsion to nudge the asteroid's orbit over many months or years to
get it on an intercept course. But when trying to raise the perigee,
you need orders of magnitude more thrust so you can perform the perigee
raising maneuver in much less time (i.e. a fraction of an orbit of
earth).

This is why I, only half jokingly, said that dropping Kuiper Belt
Objects directly onto Mars would be easier and safer. For that task,
the low thrust, high ISP engines used to nudge the KBO onto an intercept
course with Mars are truly all you need. In fact, long before the KBO
enters Mars' atmosphere, you'd disconnect the drive unit from the KBO
and put it on a course to miss Mars, so you could reuse the drive unit.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #14  
Old September 6th 18, 08:10 PM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 548
Default Scientists Map Out How to Nudge Small Asteroids into Earth?sOrbit

Le sept./6/2018 Ã* 06:49, Jeff Findley a écritÂ*:
In article ,
ess says...

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 is right. That's why this issue was mentioned early on in the
thread. Starting aerobraking is a lot easier, in terms of delta-V, than
raising the perigee such that you're no longer aerobraking. If you
can't raise the perigee fast enough, the orbit will decay turning your
asteroid into a meteorite.

The low thrust engine(s) originally used to nudge the asteroid onto an
intercept course with the earth's atmosphere would likely be woefully
undersized for the task. For example, you could use solar electric
propulsion to nudge the asteroid's orbit over many months or years to
get it on an intercept course. But when trying to raise the perigee,
you need orders of magnitude more thrust so you can perform the perigee
raising maneuver in much less time (i.e. a fraction of an orbit of
earth).


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.

This is why I, only half jokingly, said that dropping Kuiper Belt
Objects directly onto Mars would be easier and safer.


I would assume that if something goes wrong while you send the KBO to
Mars, and it falls 100 km off its target, the consequences would be less
severe than if an asteroid intended for Earth aerocapture goes 100 km
too low into the atmosphere (lithosphere). So yes, I too would consider
it easier and safer to drop stuff on Mars.


Alain Fournier
  #15  
Old September 7th 18, 02:04 AM posted to sci.space.policy
Sylvia Else
external usenet poster
 
Posts: 1,063
Default Scientists Map Out How to Nudge Small Asteroids into Earth?sOrbit

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.

Sylvia.
  #17  
Old September 8th 18, 12:25 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 548
Default Scientists Map Out How to Nudge Small Asteroids into Earth?sOrbit

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
 




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