Dodging Doomsday With a Space Tug

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Topic:	Science > Physics
User:	"Sam Wormley"
Date:	15 Nov 2005 02:57:31 PM
Object:	Dodging Doomsday With a Space Tug

Dodging Doomsday With a Space Tug
http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3
Preventing an asteroid from slamming into Earth may not require
Hollywood bravado. Two NASA astronauts have hit upon an idea wherein
a spacecraft uses gravity to literally tow a threatening projectile
into safer orbit.
See: http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3
.

User: "G=EMC^2 Glazier"
Title: Re: Dodging Doomsday With a Space Tug	17 Nov 2005 08:03:05 AM

Hi Sam I always liked the way a small tug boat could push a very heavy
barge around. Always liked how I could push a 32 foot cabin cruiser
with one finger. All this is true to over come the inertia(weight) of a
large asteroid. Our real up front problem is seeing an asteroid coming
our way,and having the right tug boat to nudge it away so it misses us.
Some steroids reflect little light. Those are the ones to worry about.
Bert
.

User: "Sam Wormley"
Title: Re: Dodging Doomsday With a Space Tug	17 Nov 2005 08:21:28 AM

G=EMC^2 Glazier wrote:

The key here is pull, not push, as the earth-crossing asteroid's
center of mass is unknown.
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	19 Nov 2005 06:21:14 PM

G=EMC^2 Glazier wrote:

Alligators (or tug-boats) push the water, which in turn pushes the
boat. Floating free, your push on the boat will have neglible effect.
Crash into it at high speed, and its direction will certainly alter
(maybe to down :-( )
Jim G
c'=c+v
.

User: "The Ghost In The Machine"
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 02:00:05 AM

In sci.physics, Sam Wormley
<swormley1@mchsi.com>
wrote
on Tue, 15 Nov 2005 20:57:31 GMT
<%Yref.335296$084.277059@attbi_s22>:

Dodging Doomsday With a Space Tug
http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

Preventing an asteroid from slamming into Earth may not require
Hollywood bravado. Two NASA astronauts have hit upon an idea wherein
a spacecraft uses gravity to literally tow a threatening projectile
into safer orbit.

See: http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

The problem I have with this idea is that the "space tug"
can't simply sit there with the asteroid; the system
becomes a two-plus-body problem (the third body is very
far away, accelerating both of them roughly equally).
To give concrete numbers, assume a spherical iron asteroid
100m in diameter; that's 523600 m^3 or 4.123 megatonnes,
and enough to ruin just about everyone's day if it hits us.
We put a small (well, OK, 220,000 or 100 metric tonnes --
about the size of the NASA shuttle after it reaches orbit)
spacecraft 10m away from this very large rock.
One gets a gravitational force [*] of about 275.2 Newtons.
The rock will accelerate towards the center of mass at
a rate of 66.7 nm/s/s; might be detectable if one had a
ultraviolet laser handy. The spacecraft, having less mass,
will accelerate at 2.75 mm/s/s. In order to compensate it
will have to thrust with a force of -- surprise surprise --
275.2 Newtons. Otherwise it will hit the asteroid.
Might as well attach it to the other side of the rock
and give it a push. Same diff as far as I can tell,
though it's probably easier to do station keeping than
to bolt or weld a shuttle to a piece of rock. (Just
remember that hitting the asteroid with one's exhaust
diminishes the effect.)
Granted, the above is a worst-case scenario though there's
no real good way to know until we have to do it (the
good news is that we have asteroid landing missions
in space already; the NEAR spacecraft comes to mind and
there's a Japanese effort up there now). Presumably,
most rocks aren't going to be quite that dense.
The good news is that, if we detect the rock around the
orbit of Mars and moving towards us at about 33 km/s,
we have 21 day's warning at least. That's enough to
shift it by about 110 km.
(That's the bad news, given my ad hoc parameters.
The more likely scenario is a snowy dirtball, rather
than a lump. Given something light and fluffy,
a gravtug might be more effective. Of course
were we to detect it farther out we'd also be better off.)
I'd have to work out how much reactant the tug will
need to keep it separate; from the looks of it though
it will need 500 million newton-seconds of impulse
to keep firing for 21 days.
[*] This is a Newtonian estimate.
--
#191,

It's still legal to go .sigless.
.

User: "Black Knight"
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 10:25:31 AM

"The Ghost In The Machine" <ewill@sirius.tg00suus7038.net> wrote in message
news:2i3r43-34l.ln1@sirius.tg00suus7038.net...

In sci.physics, Sam Wormley
<swormley1@mchsi.com>
wrote
on Tue, 15 Nov 2005 20:57:31 GMT
<%Yref.335296$084.277059@attbi_s22>:

Dodging Doomsday With a Space Tug
http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

Preventing an asteroid from slamming into Earth may not require
Hollywood bravado. Two NASA astronauts have hit upon an idea wherein
a spacecraft uses gravity to literally tow a threatening projectile
into safer orbit.

See: http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

The problem I have with this idea is that the "space tug"
can't simply sit there with the asteroid; the system
becomes a two-plus-body problem (the third body is very
far away, accelerating both of them roughly equally).

To give concrete numbers, assume a spherical iron asteroid
100m in diameter; that's 523600 m^3 or 4.123 megatonnes,
and enough to ruin just about everyone's day if it hits us.
We put a small (well, OK, 220,000 or 100 metric tonnes --
about the size of the NASA shuttle after it reaches orbit)
spacecraft 10m away from this very large rock.

One gets a gravitational force [*] of about 275.2 Newtons.
The rock will accelerate towards the center of mass at
a rate of 66.7 nm/s/s; might be detectable if one had a
ultraviolet laser handy. The spacecraft, having less mass,
will accelerate at 2.75 mm/s/s. In order to compensate it
will have to thrust with a force of -- surprise surprise --
275.2 Newtons. Otherwise it will hit the asteroid.

Might as well attach it to the other side of the rock
and give it a push. Same diff as far as I can tell,
though it's probably easier to do station keeping than
to bolt or weld a shuttle to a piece of rock. (Just
remember that hitting the asteroid with one's exhaust
diminishes the effect.)

Granted, the above is a worst-case scenario though there's
no real good way to know until we have to do it (the
good news is that we have asteroid landing missions
in space already; the NEAR spacecraft comes to mind and
there's a Japanese effort up there now). Presumably,
most rocks aren't going to be quite that dense.

The good news is that, if we detect the rock around the
orbit of Mars and moving towards us at about 33 km/s,
we have 21 day's warning at least. That's enough to
shift it by about 110 km.

(That's the bad news, given my ad hoc parameters.
The more likely scenario is a snowy dirtball, rather
than a lump. Given something light and fluffy,
a gravtug might be more effective. Of course
were we to detect it farther out we'd also be better off.)

I'd have to work out how much reactant the tug will
need to keep it separate; from the looks of it though
it will need 500 million newton-seconds of impulse
to keep firing for 21 days.

[*] This is a Newtonian estimate.

http://antwrp.gsfc.nasa.gov/apod/ap051110.html
Funny how his conversation started with the arrival
of this page... Coincidence?
Androcles.
.

User: "Mark Martin"
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 10:29:01 AM

Black Knight wrote:

http://antwrp.gsfc.nasa.gov/apod/ap051110.html

Funny how his conversation started with the arrival
of this page... Coincidence?

Uuhhh... Yeah. What about it?
-Mark Martin
.

User: "The Ghost In The Machine"
Title: Re: Dodging Doomsday With a Space Tug	17 Nov 2005 12:00:03 AM

In sci.physics, Mark Martin
<qed100@hotmail.com>
wrote
on 16 Nov 2005 08:29:01 -0800
<1132158541.414139.221500@g47g2000cwa.googlegroups.com>:

Black Knight wrote:

http://antwrp.gsfc.nasa.gov/apod/ap051110.html

Funny how his conversation started with the arrival
of this page... Coincidence?

Uuhhh... Yeah. What about it?

-Mark Martin

Hm. It's worse than I thought. Regrettably, they don't
specify the content of the asteroid, but assuming iron
again, one gets the following.
force = 20000 * (7874 * 4/3 * Pi * (100^3)) * 6.674215*10^-11/100^2
= 4.403 N
where "near" is assumed to be about 100 m. (Note that the diameter
is specified as 200m; therefore 100m radius. I'm not doing
this quite right admittedly since the asteroid isn't a point mass.)
asteroid mass = (7874 * 4/3 * Pi * (100^3)) = 3.298 * 10^10 kg
asteroid acc = 4.403 N / 3.298 * 10^10 kg = 133.5 pm/s/s
ship acc = 4.403 N / 20000 = .22015 mm/s/s
time to ship/asteroid impact: approx. 953 seconds
Say we pick up this asteroid near the orbit of Mars again,
moving 33 km/s, and "tow" it again for 21 days. We'll be
able to deflect it a grand total of 220 meters, expending
at least 8 million newton-seconds of impulse in the process.
If we assume a pure iceball (1000 kg/m^3), we get *exactly
the same asteroid acceleration*, although the ship only has
to spend 1 million newton-seconds of impulse in that case.
To paraphrase a certain chihuahua -- I think we need a bigger ship...
--
#191,

It's still legal to go .sigless.
.

User: "Black Knight"
Title: Re: Dodging Doomsday With a Space Tug	17 Nov 2005 01:01:50 AM

"The Ghost In The Machine" <ewill@sirius.tg00suus7038.net> wrote in message
news:65gt43-o85.ln1@sirius.tg00suus7038.net...

In sci.physics, Mark Martin
<qed100@hotmail.com>
wrote
on 16 Nov 2005 08:29:01 -0800
<1132158541.414139.221500@g47g2000cwa.googlegroups.com>:

Black Knight wrote:

http://antwrp.gsfc.nasa.gov/apod/ap051110.html

Funny how his conversation started with the arrival
of this page... Coincidence?

Uuhhh... Yeah. What about it?

-Mark Martin

Hm. It's worse than I thought. Regrettably, they don't
specify the content of the asteroid, but assuming iron
again, one gets the following.

force = 20000 * (7874 * 4/3 * Pi * (100^3)) * 6.674215*10^-11/100^2
= 4.403 N

where "near" is assumed to be about 100 m. (Note that the diameter
is specified as 200m; therefore 100m radius. I'm not doing
this quite right admittedly since the asteroid isn't a point mass.)

asteroid mass = (7874 * 4/3 * Pi * (100^3)) = 3.298 * 10^10 kg
asteroid acc = 4.403 N / 3.298 * 10^10 kg = 133.5 pm/s/s

ship acc = 4.403 N / 20000 = .22015 mm/s/s

time to ship/asteroid impact: approx. 953 seconds

Say we pick up this asteroid near the orbit of Mars again,
moving 33 km/s, and "tow" it again for 21 days. We'll be
able to deflect it a grand total of 220 meters, expending
at least 8 million newton-seconds of impulse in the process.

If we assume a pure iceball (1000 kg/m^3), we get *exactly
the same asteroid acceleration*, although the ship only has
to spend 1 million newton-seconds of impulse in that case.

To paraphrase a certain chihuahua -- I think we need a bigger ship...

--
#191,

It's still legal to go .sigless.

Or a bigger chihuahua...
Androcles.
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 03:33:51 AM

Sounds like an April Fools to me!
How the hell does the tractor "hover"? The asteroid is pulling on the
rocket too, and for the rocket to pull it via the gravitational rope,
it would need to expend energy (do work) to stop the two coming
together. So fire the rocket engines? ...and look at what they (the
expelled propellant) are pushing on (in the opposite direction).
Someone seems to have forgotten conservation of energy!
Jim G
c'=c+v
.

User: "The Ghost In The Machine"
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 10:00:06 AM

In sci.physics,

<jgreen@seol.net.au>
wrote
on 16 Nov 2005 01:33:51 -0800
<1132133631.144222.290810@g14g2000cwa.googlegroups.com>:

The firing of the engines would be at a splay angle, so that the
expelled exhaust doesn't hit the asteroid. Otherwise you're right;
the gravitational tugging would be negated by the propellant
hitting the asteroid.
Unfortunately this increases various forces on the spacecraft,
which gets squeezed as well as pushed, and reduces efficiency.
Earth would be better off with the spacecraft landing thrusters
up on the other side of the rock, and pushing.
As for conservation -- I think you mean conservation of momentum,
and you're correct in that case.

Jim G
c'=c+v

--
#191,

It's still legal to go .sigless.
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 05:51:37 PM

The Ghost In The Machine wrote:

In sci.physics,

<jgreen@seol.net.au>
wrote
on 16 Nov 2005 01:33:51 -0800
<1132133631.144222.290810@g14g2000cwa.googlegroups.com>:

The firing of the engines would be at a splay angle, so that the
expelled exhaust doesn't hit the asteroid. Otherwise you're right;
the gravitational tugging would be negated by the propellant
hitting the asteroid.

Unfortunately this increases various forces on the spacecraft,
which gets squeezed as well as pushed, and reduces efficiency.

Earth would be better off with the spacecraft landing thrusters
up on the other side of the rock, and pushing.

As for conservation -- I think you mean conservation of momentum,
and you're correct in that case.

Yeh....whatever
Say the rocket expelled half its mass (propellant). Even if this was
directed to miss the asteroid, the center of gravity of that gas would
still exert a pull, and once past the asteroid (from the rocket's
perspective), would pull counter to the rocket.
I feel the end net result (tug on the asteroid) would be nullified.
Just prang the thing at highest speed; it will either deviate from its
course, or hopefully be in small enough pieces to significantly "burn"
in the atmosphere, reducing shock on impact.
Jim G
c'=c+v
.

User: "Mark Martin"
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 06:26:50 PM

wrote:

Say the rocket expelled half its mass (propellant). Even if this was
directed to miss the asteroid, the center of gravity of that gas would
still exert a pull, and once past the asteroid (from the rocket's
perspective), would pull counter to the rocket.
I feel the end net result (tug on the asteroid) would be nullified.

The proposed rocket is ion propelled, meaning the exhaust velocity
will be very high. The reaction mass will be carried away from the
spacecraft/asteroid system way too fast to make any important
difference.
-Mark Martin
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	17 Nov 2005 04:45:23 AM

Mark Martin wrote:

jgreenfield@seol.net.au wrote:

Yep. And the accelleration of the rocket (which provides the "pull"),
is WAY less, so that is insignificant also.
Jim G
c'=c+v
.

User: "Sam Wormley"
Title: Re: Dodging Doomsday With a Space Tug	17 Nov 2005 07:15:15 AM

wrote:

Mark Martin wrote:

wrote:

Yep. And the accelleration of the rocket (which provides the "pull"),
is WAY less, so that is insignificant also.

Jim G
c'=c+v

Acceleration?
The "pull" is provided by F = (G m1 m2)/r^2
The rockets engines are used to maintain a constant r and, therefore,
a constant F on the asteroid.
.

User: "Mark Martin"
Title: Re: Dodging Doomsday With a Space Tug	17 Nov 2005 05:37:50 AM

wrote:

Mark Martin wrote:

wrote:

Yep. And the accelleration of the rocket (which provides the "pull"),
is WAY less, so that is insignificant also.

Acceleration is not the same as speed. The ion thrust will be adding
tiny increments of speed to the spacecraft/asteroid system, whereas the
rocket exhaust will, upon leaving the motor, already have tremendous
speed relative to it, and will be at escape velocity for such a low
mass system. If I stand in my back yard and toss a stone away from
Earth at escape velocity, it means that not only the stone recedes
permanently away from Earth, but also that Earth recedes always from
the stone. It's the same for a rocket and its exhaust. And by allowing
a rocket and an asteroid to be gravitationally bound, the asteroid
becomes part of the spacecraft. Impose a small acceleration upon the
asteroid for a sufficiently long time, and its trajectory WILL be
altered enough to miss Earth.
-Mark Martin
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	19 Nov 2005 06:14:06 PM

Mark Martin wrote:

jgreenfield@seol.net.au wrote:

Mark Martin wrote:

jgreenfield@seol.net.au wrote:

Yep. And the accelleration of the rocket (which provides the "pull"),
is WAY less, so that is insignificant also.

The gravitational tug of the rocket must be VERY small, or the "rope"
will break (rocket will accellerate, leaving the asteroid behing).
Therefore the thrust of the rocket engines must be very small as well
(tiny quantites of exhaust blast per time). If you think that this gas
emmission has negligible effect counter to the pull, if directed past
the asteroid, why waste the momentum it had while travelling fast in
the fuel tank, and just give the thing a quick impact?
Where is Ghost? He's fond of this mathy bit! What is the net effect on
throwing a rock upwards at escape velocity, or down to strike the earth
(at the same initial energy), on the earth's orbit? (ignoring the fact
that the rock was once part of the earth's mass; the rocket or fuel
was never part of the asteroid)
Jim G
c'=c+v
.

User: "Mark Martin"
Title: Re: Dodging Doomsday With a Space Tug	19 Nov 2005 06:55:50 PM

wrote:

Yes, the force of gravity between the tug & asteroid will be very
small, and the thrust from the motor will be extremely small. Keep in
mind that the motor proposed for the tug is an ion accelerator. Ion
engines are very weak, but have extremely high specific impulse and can
impart thrust cheaply for years at a stretch, accumulating large total
changes in velocity.

If you think that this gas
emmission has negligible effect counter to the pull, if directed past
the asteroid, why waste the momentum it had while travelling fast in
the fuel tank, and just give the thing a quick impact?

The momentum of the ion exhaust is *not* wasted by vectoring it past
the asteroid. If the spacecraft were to be anchored directly upon the
rock's surface with the ion engine pointed away so as to push it at the
same thrust level, there would be no difference in the time required to
affect the change. Of course, the exhaust could be directed parallel to
the resultant acceleration, instead of at an angle, which would
increase the efficiency, but we're still talking ion propulsion vs a
massive asteroid, so we're still talking about years.
What's worse, is that asteroids spin. This severly complicates
things. Shall we first null the rotation? If so, that also is a major
job. And how shall we accomplish such a task? Maybe we'd have to mount
ion engines on the surface! And how long will it take? It's bound to
take a very long time, which will steal from the lead time needed to
effectively alter its Earth crossing trajectory.
But it's not impossible to mount engines on the rock and just follow
a schedule of thrust bursts as it rotates. When the engine is pointed
approximately the right direction, it issues a burst. But at that rate,
how long will it take to accumulate the required total delta-V? We're
still talking years.
It's important to remember that the whole idea of the ion powered
gravity tug is that it doesn't matter if the asteroid is spinning, or
what its mass distribution is. All the tug has to do is hover at a
constant radius from the asteroid with its thrust pointed in the right
direction.

What is the net effect on
throwing a rock upwards at escape velocity, or down to strike the earth
(at the same initial energy), on the earth's orbit? (ignoring the fact
that the rock was once part of the earth's mass; the rocket or fuel
was never part of the asteroid)

If you throw a rock upwards at escape velocity it (hypothetically)
recedes to infinity. If the same rock falls towards Earth starting from
infinity with initial velocity of zero, then it accumulates a velocity
upon reaching Earth exactly equal to escape velocity.
Now suppose you throw the rock straight down onto Earth with energy
& velocity equal to that of escape. What happens to Earth's trajectory?
It depends on what happens *after* the collision. If Earth & the rock
are perfectly elastic then they bounce off each other and separate at
escape velocity. The mass center of the two stays on its orbital path
unchanged, while the two objects' individual paths are altered. But if
the two bodies become bound at the time of collision, then *nothing*
happens to Earth's trajectory, and for the same reason. The mass center
of the two objects remains on course.
-Mark Martin
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	20 Nov 2005 03:16:11 AM

Mark Martin wrote:

jgreenfield@seol.net.au wrote:

The momentum of the ion exhaust is *not* wasted by vectoring it past
the asteroid. If the spacecraft were to be anchored directly upon the
rock's surface with the ion engine pointed away so as to push it at the
same thrust level, there would be no difference in the time required to
affect the change. Of course, the exhaust could be directed parallel to
the resultant acceleration, instead of at an angle, which would
increase the efficiency, but we're still talking ion propulsion vs a
massive asteroid, so we're still talking about years.

What's worse, is that asteroids spin. This severly complicates
things. Shall we first null the rotation? If so, that also is a major
job. And how shall we accomplish such a task? Maybe we'd have to mount
ion engines on the surface! And how long will it take? It's bound to
take a very long time, which will steal from the lead time needed to
effectively alter its Earth crossing trajectory.

But it's not impossible to mount engines on the rock and just follow
a schedule of thrust bursts as it rotates. When the engine is pointed
approximately the right direction, it issues a burst. But at that rate,
how long will it take to accumulate the required total delta-V? We're
still talking years.

It's important to remember that the whole idea of the ion powered
gravity tug is that it doesn't matter if the asteroid is spinning, or
what its mass distribution is. All the tug has to do is hover at a
constant radius from the asteroid with its thrust pointed in the right
direction.

Righto! I hadn't thought on the spin. Gravitational tug certainly
avoids that problem.
It is the momentum of the ions thrown in one direction, which is
countered by the momentum of the rocket/asteroid combined system, which
is the basis for the force alterring the joint trajectory, is it not?
It just seems intuitive that a bloody good shove (short duration
strike) would have a more marked effect.

If you throw a rock upwards at escape velocity it (hypothetically)
recedes to infinity. If the same rock falls towards Earth starting from
infinity with initial velocity of zero, then it accumulates a velocity
upon reaching Earth exactly equal to escape velocity.

Now suppose you throw the rock straight down onto Earth with energy
& velocity equal to that of escape. What happens to Earth's trajectory?
It depends on what happens *after* the collision. If Earth & the rock
are perfectly elastic then they bounce off each other and separate at
escape velocity. The mass center of the two stays on its orbital path
unchanged, while the two objects' individual paths are altered. But if
the two bodies become bound at the time of collision, then *nothing*
happens to Earth's trajectory, and for the same reason. The mass center
of the two objects remains on course.

Ok. But you forgot that the rock is thrown by a force ADDITIONAL to
gravity (up or down-----------and I nearly said +/- which would make
Andro react :-) )
Perhaps it would be easier to alter the _earth's_ trajectory, in order
to avoid an asteroid impact ; - (
Thanks
Jim G
c'=c+v
.

User: "Mark Martin"
Title: Re: Dodging Doomsday With a Space Tug	20 Nov 2005 10:11:49 AM

wrote:

Certainly if a a very high thrust is applied to the asteroid then
its course will be significantly altered sooner rather than later.
Hypothetically speaking, the required delta-V could be imparted in a
matter of minutes, if the thrust is high enough. The problem with this
is the state of our technology. Sustained low thrust is something
within our grasp for the near future.
If we just maintain a thorough survey of the asteroid population, in
time we'll know probably every object with any likelihood of a
collision within several decades. This is something which can be done
very affordably. Long lead time is the key. If we were to discover a
doomsday-sized rock scheduled to clobber us anytime in less than a
decade, it's not likely we'd be able to dodge it at all given our
current technology.
-Mark Martin
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	20 Nov 2005 08:44:23 PM

Mark Martin wrote:

jgreenfield@seol.net.au wrote:

Certainly if a a very high thrust is applied to the asteroid then
its course will be significantly altered sooner rather than later.
Hypothetically speaking, the required delta-V could be imparted in a
matter of minutes, if the thrust is high enough. The problem with this
is the state of our technology. Sustained low thrust is something
within our grasp for the near future.

If we just maintain a thorough survey of the asteroid population, in
time we'll know probably every object with any likelihood of a
collision within several decades. This is something which can be done
very affordably. Long lead time is the key. If we were to discover a
doomsday-sized rock scheduled to clobber us anytime in less than a
decade, it's not likely we'd be able to dodge it at all given our
current technology.

-Mark Martin

Perhaps a small one would be a mixed blessing. The initial damage might
be offset by reversing global warming.
Thanks for the input
Jim G
c'=c+v
.

User: "Sam Wormley"
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 06:08:53 PM

wrote:

The Ghost In The Machine wrote:

In sci.physics,

<jgreen@seol.net.au>
wrote
on 16 Nov 2005 01:33:51 -0800
<1132133631.144222.290810@g14g2000cwa.googlegroups.com>:

The firing of the engines would be at a splay angle, so that the
expelled exhaust doesn't hit the asteroid. Otherwise you're right;
the gravitational tugging would be negated by the propellant
hitting the asteroid.

Unfortunately this increases various forces on the spacecraft,
which gets squeezed as well as pushed, and reduces efficiency.

Earth would be better off with the spacecraft landing thrusters
up on the other side of the rock, and pushing.

As for conservation -- I think you mean conservation of momentum,
and you're correct in that case.

You can "feel" all you want.... what do the equations of classical
mechanics say?
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	17 Nov 2005 04:41:00 AM

Sam Wormley wrote:

wrote:

The Ghost In The Machine wrote:

In sci.physics,

<jgreen@seol.net.au>
wrote
on 16 Nov 2005 01:33:51 -0800
<1132133631.144222.290810@g14g2000cwa.googlegroups.com>:

The firing of the engines would be at a splay angle, so that the
expelled exhaust doesn't hit the asteroid. Otherwise you're right;
the gravitational tugging would be negated by the propellant
hitting the asteroid.

Unfortunately this increases various forces on the spacecraft,
which gets squeezed as well as pushed, and reduces efficiency.

Earth would be better off with the spacecraft landing thrusters
up on the other side of the rock, and pushing.

As for conservation -- I think you mean conservation of momentum,
and you're correct in that case.

You can "feel" all you want.... what do the equations of classical
mechanics say?

That you can't do a hand-brake stop, without considering conservation
of momentum/energy. "The Equations of Classical Mechanics" say, "that
Sam W hasn't a clue."
Jim G
c'=c+v
.

User: "Boris Mohar"
Title: Re: Dodging Doomsday With a Space Tug	15 Nov 2005 06:24:53 PM

On Tue, 15 Nov 2005 20:57:31 GMT, Sam Wormley <swormley1@mchsi.com> wrote:

Dodging Doomsday With a Space Tug
http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

Preventing an asteroid from slamming into Earth may not require
Hollywood bravado. Two NASA astronauts have hit upon an idea wherein
a spacecraft uses gravity to literally tow a threatening projectile
into safer orbit.

See: http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

Has anybody proposed spinning in until it flies apart? Sure it is a
cockamamie idea.
Regards,
Boris Mohar
Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca
void _-void-_ in the obvious place

.

User: "Mark Martin"
Title: Re: Dodging Doomsday With a Space Tug	15 Nov 2005 07:26:33 PM

Boris Mohar wrote:

Has anybody proposed spinning in until it flies apart? Sure it is a
cockamamie idea.

It's not a stupid idea, but there are reasons why it may not be
effective. For one, just off the top of my head it'll probably take a
lot more energy to spin an asteroid to destruction than to just impart
enough delta-V to allow it to miss Earth.
But there's an even more interesting problem. Let's start out by
considering a satellite orbiting with Earth. For a given radius the
satellite is subject to a certain gravitational force which accelerates
it towards Earth. (And earth towards the satellite.) If the satellite
is given just the right amount of horizontal speed, it'll balance the
force of gravity and travel in a circular orbit. Now let's give it a
boost somewhere on its orbit. If we give it enough extra velocity
tangent to its circular orbit, it'll have escape velocity and travel
away from Earth indefinitely. But if the increase is less than escape
velocity, it'll just assume an eccentric elliptical orbit. The highest
point on the new orbit will be higher than the original circular path,
but the low point will be right back on that circular orbit.
Now if we spin an asteroid, essentially all the parts are on orbital
paths. They're held on cicular paths by central forces, including
gravity. If we spin it fast enough, the parts could be given escape
velocity, and they'll fly apart and never meet again, in which case
Earth is saved. But what'll happen if we apply torque to increase the
spin rate is that the parts out near the surface will experience
breakaway velocity before the core does. So they'll break away, leaving
the core to continue on its Earth crossing trajectory.
To make it even worse, the outer pieces will necessarily break off
only as they just barely aquire enough velocity to enter into an
ellipical orbit about the whole system's mass center. (It's like being
on a carousel that's steadily spinning faster & faster. If I'm on the
perimeter, once my tangential speed overcomes whatever force is holding
me to a cicular path, I fling off and am no longer subject to the
torque. I fly off with only whatever velocity I had at the moment I
broke free.) The pieces will then all return together and collide.
Overall, the entire asteroid will survive on its Earth crossing path.
-Mark Martin
.

User: ""
Title: Re: Dodging Doomsday With a Space Tug	15 Nov 2005 09:23:29 PM

In article <1132104393.558861.229210@o13g2000cwo.googlegroups.com>, "Mark Martin" <qed100@hotmail.com> writes:

Boris Mohar wrote:

Has anybody proposed spinning in until it flies apart? Sure it is a
cockamamie idea.

It's not a stupid idea, but there are reasons why it may not be
effective. For one, just off the top of my head it'll probably take a
lot more energy to spin an asteroid to destruction than to just impart
enough delta-V to allow it to miss Earth.

But there's an even more interesting problem. Let's start out by
considering a satellite orbiting with Earth. For a given radius the
satellite is subject to a certain gravitational force which accelerates
it towards Earth. (And earth towards the satellite.) If the satellite
is given just the right amount of horizontal speed, it'll balance the
force of gravity and travel in a circular orbit. Now let's give it a
boost somewhere on its orbit. If we give it enough extra velocity
tangent to its circular orbit, it'll have escape velocity and travel
away from Earth indefinitely. But if the increase is less than escape
velocity, it'll just assume an eccentric elliptical orbit. The highest
point on the new orbit will be higher than the original circular path,
but the low point will be right back on that circular orbit.

Now if we spin an asteroid, essentially all the parts are on orbital
paths. They're held on cicular paths by central forces, including
gravity. If we spin it fast enough, the parts could be given escape
velocity, and they'll fly apart and never meet again, in which case
Earth is saved. But what'll happen if we apply torque to increase the
spin rate is that the parts out near the surface will experience
breakaway velocity before the core does. So they'll break away, leaving
the core to continue on its Earth crossing trajectory.

To make it even worse, the outer pieces will necessarily break off
only as they just barely aquire enough velocity to enter into an
ellipical orbit about the whole system's mass center. (It's like being
on a carousel that's steadily spinning faster & faster. If I'm on the
perimeter, once my tangential speed overcomes whatever force is holding
me to a cicular path, I fling off and am no longer subject to the
torque. I fly off with only whatever velocity I had at the moment I
broke free.) The pieces will then all return together and collide.
Overall, the entire asteroid will survive on its Earth crossing path.

Nice thinking but no, it is not going to work like this. A small
piece of rock is held together by cohesive forces (effectively,
chemical forces), the gravitational forces are negligible. So, once
you break the cohesive forces, the pieces will just fly, the
gravitationla forces will be way to weak to pull them back. The model
you present would work for much larger bodies, ones where the
gravitational bounding energy exceeds the cohesive one. This'll be
much larger then little asteroids.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
.

User: "Mark Martin"
Title: Re: Dodging Doomsday With a Space Tug	16 Nov 2005 06:30:46 AM

wrote:

Yes, you're right. Thanks.
-Mark Martin
.

User: "Tice with a J"
Title: Re: Dodging Doomsday With a Space Tug	15 Nov 2005 07:25:14 PM

Boris Mohar wrote:

Has anybody proposed spinning in until it flies apart? Sure it is a
cockamamie idea.

Way too much energy required. It would be easier and cheaper to push it
off course than to spin it like that. Furthermore, if it broke up,
pieces might still hit earth. Not doomsday, but still lethal.
.

User: "David Wimp"
Title: Re: Dodging Doomsday With a Space Tug	15 Nov 2005 06:03:24 PM

Sam Wormley wrote:

Dodging Doomsday With a Space Tug
http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

Preventing an asteroid from slamming into Earth may not require
Hollywood bravado. Two NASA astronauts have hit upon an idea wherein
a spacecraft uses gravity to literally tow a threatening projectile
into safer orbit.

See: http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

"While gravitational towing is not new...". Does this mean the idea is
not new or such towing has actually been done? A search on
"gravitational towing" returns only the cited article.
.

User: "Mark Martin"
Title: Re: Dodging Doomsday With a Space Tug	15 Nov 2005 06:10:36 PM

David Wimp wrote:

Sam Wormley wrote:

Dodging Doomsday With a Space Tug
http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

Preventing an asteroid from slamming into Earth may not require
Hollywood bravado. Two NASA astronauts have hit upon an idea wherein
a spacecraft uses gravity to literally tow a threatening projectile
into safer orbit.

See: http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3

"While gravitational towing is not new...". Does this mean the idea is
not new or such towing has actually been done? A search on
"gravitational towing" returns only the cited article.

It means the idea in general has been thought of before. No such
gravitational towing has been done. (Not as a deliberate act of
engieering that is. Of course, it's occured on a trivial scale each
time a projectile has been launched from Earth.)
-Mark Martin
.

User: "Puppet_Sock"
Title: Re: Dodging Doomsday With a Space Tug	15 Nov 2005 03:22:57 PM

Sam Wormley wrote:

Dodging Doomsday With a Space Tug
http://sciencenow.sciencemag.org/cgi/content/full/2005/1109/3
From the article:
A 20-ton gravitational tractor could tow a 200-meter-wide asteroid
out of an Earth-smacking trajectory if started 20 years beforehand,
the authors calculate 10 November in Nature.

Ok, hands up those who believe we will have 20 years of warning.
Socks
.

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